U.S. patent application number 10/786133 was filed with the patent office on 2004-08-26 for magnetic recording medium, method for producing the same, and magnetic recording apparatus.
This patent application is currently assigned to HITACHI MAXELL, LTD.. Invention is credited to Inaba, Nobuyuki, Kirino, Fumiyoshi, Takeuchi, Teruaki, Wakabayashi, Kouichirou.
Application Number | 20040166376 10/786133 |
Document ID | / |
Family ID | 26465051 |
Filed Date | 2004-08-26 |
United States Patent
Application |
20040166376 |
Kind Code |
A1 |
Kirino, Fumiyoshi ; et
al. |
August 26, 2004 |
Magnetic recording medium, method for producing the same, and
magnetic recording apparatus
Abstract
A magnetic recording medium (300) includes a substrate (1) and
on the substrate (1) a first underlying layer (32), a second
underlying layer (33), a magnetic layer (34), and a protective
layer (35). Because of the existence of the first underlying layer
(32) of Hf, initial growth layer having no specific crystal
structure is prevented from growing in the second underlying layer
(33). The second underlying layer (33) has a structure in which CoO
particles having a cross section of a regular hexagon and separated
by SiO.sub.2 portion are arranged in honeycomb. Since magnetic
particles are epitaxially grown from CoO particles, the size of the
magnetic particles and particle size distribution can be
controlled, and the magnetic interaction between magnetic particles
can be lessened. The underlying layer (33) and the protective layer
(35) are formed by ECR sputtering. Such a magnetic recording medium
is free from noise and thermal fluctuation, and ultrahigh recording
density over 40 Gbit/inch.sup.2 is realized.
Inventors: |
Kirino, Fumiyoshi;
(Suginami-ku, JP) ; Inaba, Nobuyuki; (Hasuda-shi,
JP) ; Takeuchi, Teruaki; (Kitasoma-gun, JP) ;
Wakabayashi, Kouichirou; (Toride-shi, JP) |
Correspondence
Address: |
OLIFF & BERRIDGE, PLC
P.O. BOX 19928
ALEXANDRIA
VA
22320
US
|
Assignee: |
HITACHI MAXELL, LTD.
Ibaraki-shi
JP
|
Family ID: |
26465051 |
Appl. No.: |
10/786133 |
Filed: |
February 26, 2004 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10786133 |
Feb 26, 2004 |
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09959855 |
Nov 9, 2001 |
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6730421 |
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09959855 |
Nov 9, 2001 |
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PCT/JP00/03016 |
May 11, 2000 |
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Current U.S.
Class: |
428/831 ;
428/836.2; G9B/5.28; G9B/5.288; G9B/5.299 |
Current CPC
Class: |
G11B 5/72 20130101; Y10T
428/12854 20150115; G11B 5/667 20130101; G11B 5/7377 20190501; G11B
5/737 20190501; Y10S 428/90 20130101; G11B 5/727 20200801; Y10T
428/265 20150115; Y10T 428/12465 20150115; G11B 5/8404 20130101;
G11B 5/7379 20190501 |
Class at
Publication: |
428/694.0TM ;
428/694.0TS |
International
Class: |
G11B 005/127 |
Foreign Application Data
Date |
Code |
Application Number |
May 11, 1999 |
JP |
11-129758 |
Jun 16, 1999 |
JP |
11-169382 |
Claims
1. A magnetic recording medium comprising: a substrate; an
underlying layer which is formed on the substrate, the underlying
layer being composed of crystal grains substantially formed of
magnesium oxide, and a crystal grain boundary containing at least
one oxide selected from the group consisting of silicon oxide,
aluminum oxide, titanium oxide, tantalum oxide, and zinc oxide; and
a magnetic layer which is formed on the underlying layer and on
which information is recorded.
2. The magnetic recording medium according to claim 1, wherein the
crystal grains are arranged in a honeycomb configuration.
3. The magnetic recording medium according to claim 2, wherein an
average number of the crystal grains deposited around each of the
crystal grains is 5.9 to 6.1.
4. The magnetic recording medium according to claim 1, wherein the
crystal grains are subjected to crystal orientation in a certain
direction.
5. The magnetic recording medium according to claim 1, wherein the
magnetic layer is epitaxially grown on the underlying layer.
6. The magnetic recording medium according to claim 1, wherein a
standard deviation of a grain diameter distribution of the crystal
grains is not more than 8% of an average grain diameter.
7. The magnetic recording medium according to claim 1, wherein the
magnetic layer includes magnetic grains grown corresponding to the
respective crystal grains of the underlying layer; and a boundary
between the magnetic grains.
8. The magnetic recording medium according to claim 7, wherein the
boundary has a width of 0.5 to 2 nm.
9. The magnetic recording medium according to claim 1, wherein a
difference between a lattice constant of the crystal grains of the
underlying layer and a lattice constant of magnetic grains of the
magnetic layer is within .+-.10%.
10. The magnetic recording medium according to claim 1, wherein the
underlying layer is formed by means of an ECR sputtering
method.
11. A magnetic recording medium comprising: a substrate; a first
underlying layer which is formed on the substrate; a second
underlying layer which is formed on the first underlying layer; and
a magnetic layer which is formed on the second underlying layer and
on which information is recorded, wherein: the second underlying
layer is composed of crystal grains substantially formed of at
least one oxide selected from the group consisting of cobalt oxide,
chromium oxide, iron oxide, nickel oxide, and magnesium oxide, and
a crystal grain boundary containing at least one oxide selected
from the group consisting of silicon oxide, aluminum oxide,
titanium oxide, tantalum oxide, and zinc oxide; and the first
underlying layer serves as a layer to prevent the second underlying
layer from initial growth.
12. The magnetic recording medium according to claim 11, wherein
the first underlying layer is an amorphous film, and the amorphous
film includes a metal selected from the group consisting of
hafnium, titanium, tantalum, niobium, zirconium, tungsten,
molybdenum, and an alloy containing at least one element thereof; a
cobalt alloy principally composed of cobalt and containing at least
one element selected from the group consisting of titanium,
tantalum, niobium, zirconium, and chromium; or at least one
inorganic compound selected from the group consisting of silicon
nitride, silicon oxide, and aluminum oxide.
13. The magnetic recording medium according to claim 11, wherein
the crystal grains are arranged in a honeycomb form.
14. The magnetic recording medium according to claim 11, wherein
the first underlying layer is a crystalline film, and the
crystalline film includes at least one selected from the group
consisting of chromium, chromium alloy, vanadium, and vanadium
alloy.
15. The magnetic recording medium according to claim 14, wherein
the alloy is an alloy containing at least one element selected from
the group consisting of titanium, tantalum, aluminum, nickel,
vanadium, and zirconium.
16. The magnetic recording medium according to claim 11, wherein
the first underlying layer has a film thickness of 2 nm to 50
nm.
17. The magnetic recording medium according to claim 11, wherein
the first underlying layer and the second underlying layer are
formed by using an ECR sputtering method.
18. The magnetic recording medium according to claim 11, wherein
the magnetic layer is a granular type magnetic layer.
19. The magnetic recording medium according to claim 18, wherein
the first underlying layer is formed of hafnium, and the second
underlying layer is formed of CoO--SiO.sub.2.
20. The magnetic recording medium according to claim 11, wherein a
difference between a lattice constant of the crystal grains of the
second underlying layer and a lattice constant of magnetic grains
of the magnetic layer is within .+-.10%.
21. A magnetic recording medium comprising: a substrate; an
underlying layer which is formed on the substrate; a control layer
which is formed on the underlying layer and which is formed of at
least one selected from the group consisting of magnesium oxide,
chromium alloy, and nickel alloy; and a magnetic layer which is
formed on the control layer and on which information is recorded,
wherein: the underlying layer is composed of crystal grains
substantially formed of at least one oxide selected from the group
consisting of cobalt oxide, chromium oxide, iron oxide, nickel
oxide, and magnesium oxide, and a crystal grain boundary containing
at least one oxide selected from the group consisting of silicon
oxide, aluminum oxide, titanium oxide, tantalum oxide, and zinc
oxide.
22. The magnetic recording medium according to claim 21, wherein
the control layer is an alloy containing chromium or nickel as a
major component, and at least one element selected from the group
consisting of chromium, titanium, tantalum, vanadium, ruthenium,
tungsten, molybdenum, niobium, nickel, zirconium, and aluminum.
23. The magnetic recording medium according to claim 21, wherein
the control layer is formed of one selected from the group
consisting of chromium-titanium, chromium-tungsten, magnesium
oxide, chromium-ruthenium.
24. The magnetic recording medium according to claim 21, wherein
the control layer has a bcc or B2 structure.
25. The magnetic recording medium according to claim 21, wherein
the control layer is epitaxially grown from the underlying layer,
the control layer has a structure which reflects a crystal
structure of the underlying layer, and the control layer has a
crystalline portion which is constructed by crystal grains
corresponding to the crystal grains of the underlying layer, and a
grain boundary which corresponds to the crystal grain boundary of
the underlying layer.
26. The magnetic recording medium according to claim 21, wherein
the control layer has a film thickness of 2 nm to 10 nm.
27. The magnetic recording medium according to claim 21, wherein a
combination of the underlying layer, the control layer, and the
magnetic layer is at least one combination selected from the group
consisting of CoO--ZnO/Cr--Ti alloy/Co--Cr--Pt alloy,
CoO--SiO.sub.2/MgO/Co--Cr--Pt--Ta alloy, CoO--SiO.sub.2/Cr--W
alloy/Co--Cr--Pt--Ta alloy, CoO--SiO.sub.2/MgO/CoO--SiO.sub.2
granular type magnetic film, CoO--SiO.sub.2/Ni--Al
alloy/Co--Cr--Pt--Ta alloy, CoO--SiO.sub.2/Cr--Ti
alloy/Co--Cr--Pt--Ta alloy, CoO--SiO.sub.2/Ni--Ta
alloy/Co--Pt--SiO.sub.2 granular type magnetic film,
CoO--SiO.sub.2/Ni--Ta alloy/Co--Cr--Pt--Ta alloy,
CoO--SiO.sub.2/Cr--Ru alloy/Co--Cr--Pt--Ta alloy,
CoO--SiO.sub.2/Cr--Ru alloy/Co--Pt--SiO.sub.2 granular type
magnetic film, CoO--SiO.sub.2/Co--Cr--Zr alloy/Co--Pt--SiO.sub.2
granular type magnetic film, CoO--SiO.sub.2/Co--Cr--Zr
alloy/Co--Cr--Pt--Ta alloy, CoO--SiO.sub.2/Cr--Mo
alloy/Co--Cr--Pt--Ta alloy, and CoO--SiO.sub.2/Cr--Mo
alloy/Co--Pt--SiO.sub.2 granular type magnetic film.
28. The magnetic recording medium according to claim 21, wherein
differences between a lattice constant of the crystal grains of the
underlying layer and a lattice constant of crystal grains of the
control layer and between the lattice constant of the crystal
grains of the control layer and a lattice constant of magnetic
grains of the magnetic layer are within .+-.5% respectively.
29. The magnetic recording medium according to claim 21, wherein
the crystal grains of the underlying layer are arranged in a
honeycomb configuration.
30. The magnetic recording medium according to claim 21, wherein
the underlying layer and the control layer are formed by means of
an ECR sputtering method.
31. A magnetic recording medium comprising: a substrate; an
underlying layer which is formed on the substrate; and a magnetic
layer which is formed on the underlying layer and on which
information is recorded, wherein: the underlying layer is composed
of crystal grains and a crystal grain boundary which surrounds the
respective crystal grains, the crystal grains being arranged in a
honeycomb configuration; and the crystal grains protrude at a
height of 3 to 20 nm from a surface of the underlying layer.
32. The magnetic recording medium according to claim 31, wherein
the underlying layer is formed by means of an ECR sputtering
method, the crystal grain is substantially formed of at least one
oxide selected from the group consisting of cobalt oxide, chromium
oxide, iron oxide, nickel oxide, and magnesium oxide, and the
crystal grain boundary is substantially composed of at least one
oxide selected from the group consisting of silicon oxide, aluminum
oxide, titanium oxide, tantalum oxide, and zinc oxide.
33. The magnetic recording medium according to claim 31, wherein
the underlying layer has a film thickness of 10 nm to 100 nm.
34. The magnetic recording medium according to claim 31, further
comprising a protective layer on the magnetic layer, wherein each
of the magnetic layer and the protective layer has projections
which protrude at a height of 3 to 20 nm from a surface of each of
the magnetic layer and the protective layer while reflecting a
surface structure of the underlying layer.
35. The magnetic recording medium according to claim 34, wherein
the projections are used as a texture.
36. The magnetic recording medium according to claim 35, wherein a
distance between the adjoining projections is 10 to 30 nm.
37. The magnetic recording medium according to claim 31, wherein
the crystal grain boundary has a width of 0.5 to 2 nm.
38. A magnetic recording medium comprising: a substrate; an
underlying layer which is formed on the substrate; and a magnetic
layer which is formed on the underlying layer, wherein: the
underlying layer exhibits soft magnetization, and the underlying
layer is composed of crystal grains substantially formed of at
least one oxide selected from the group consisting of cobalt oxide,
chromium oxide, iron oxide, nickel oxide, and magnesium oxide, and
a crystal grain boundary containing at least one oxide selected
from the group consisting of silicon oxide, aluminum oxide,
titanium oxide, tantalum oxide, and zinc oxide.
39. The magnetic recording medium according to claim 38, wherein
the underlying layer has a coercive force of 0.05 Oe to 10 Oe and a
relative permeability of 100 to 10000.
40. The magnetic recording medium according to claim 38, wherein
the crystal grains are arranged in a honeycomb configuration.
41. The magnetic recording medium according to claim 38, wherein
the magnetic recording medium is irradiated with light when
information is recorded or reproduced.
42. The magnetic recording medium according to claim 41, wherein
ECR sputtering is carried out in a reducing atmosphere.
43. The magnetic recording medium according to claim 38, wherein
the underlying layer has in-plane magnetization.
44. A magnetic recording medium comprising: a substrate; an
underlying layer which is formed on the substrate; and a magnetic
layer which is formed on the underlying layer and which has an
easily magnetized direction in a direction perpendicular to a
substrate surface, wherein: the underlying layer is composed of
crystal grains substantially formed of at least one oxide selected
from the group consisting of cobalt oxide, chromium oxide, iron
oxide, nickel oxide, and magnesium oxide, and a crystal grain
boundary containing at least one oxide selected from the group
consisting of silicon oxide, aluminum oxide, titanium oxide,
tantalum oxide, and zinc oxide which surrounds the respective
crystal grains.
45. The magnetic recording medium according to claim 44, wherein
the crystal grains are pillar-shaped in a plane perpendicular to
the substrate surface and hexagonal in a plane parallel to the
substrate surface, and the crystal grains are arranged in a
honeycomb configuration in the plane parallel to the substrate
surface.
46. The magnetic recording medium according to claim 45, wherein
the crystal grains and the crystal grain boundary are
non-magnetic.
47. The magnetic recording medium according to claim 45, wherein
the crystal grains of the underlying layer are oriented in (111)
orientation.
48. The magnetic recording medium according to claim 47, wherein
the magnetic layer is oriented in (00.1) orientation.
49. The magnetic recording medium according to claim 45, further
comprising a soft magnetic layer which is provided between the
substrate and the underlying layer.
50. The magnetic recording medium according to claim 45, further
comprising, on the underlying layer,a control layer which controls
crystalline orientation of the magnetic layer.
51. The magnetic recording medium according to claim 45, wherein
the magnetic layer is a ferromagnetic layer which is composed of an
alloy principally containing Co and containing at least two
elements selected from the group consisting of Cr, Pt, Ta, Nb, Ti,
and Si.
52. The magnetic recording medium according to any one of claims 1,
11, 21, 31, 38, and 44, further comprising a protective film having
a film thickness of 1 to 5 nm.
53. The magnetic recording medium according to claim 52, wherein
the protective film is formed by means of an ECR sputtering
method.
54. A magnetic recording apparatus comprising the magnetic
recording medium as defined in any one of claims 1, 11, 21, 31, 38,
and 44.
55. A method for producing a magnetic recording medium comprising,
on a substrate, a magnetic layer for recording information thereon
and a protective layer, the method comprising: generating plasma by
means of resonance absorption; allowing the generated plasma to
collide with a target so that target particles are sputtered; and
applying a bias voltage between the substrate and the target to
introduce and deposit the sputtered target particles on the
substrate, whereby forming at least one layer of the magnetic layer
and the protective layer.
56. The method for producing the magnetic recording medium
according to claim 55, wherein a microwave is used for the
resonance absorption.
57. The method for producing the magnetic recording medium
according to claim 55, wherein the bias voltage is applied with an
alternating current power source having a radio frequency or a
direct current power source.
58. The method for producing the magnetic recording medium
according to claim 55, wherein the target for the protective layer
is carbon.
59. The method for producing the magnetic recording medium
according to claim 55, wherein when the protective layer is formed,
then the target is carbon, and a mixed gas, which principally
contains argon and which contains at least one of nitrogen and
hydrogen, is used as a plasma gas.
60. A method for producing a magnetic recording medium comprising,
on a substrate, an underlying layer and a magnetic layer for
recording information thereon, the method comprising: generating
plasma by means of resonance absorption; allowing the generated
plasma to collide with a target so that target particles are
sputtered; and applying a bias voltage between the substrate and
the target to introduce and deposit the sputtered target particles
on the substrate, whereby forming the underlying layer.
61. The method for producing the magnetic recording medium
according to claim 60, wherein at least one selected from the group
consisting of cobalt oxide, chromium oxide, iron oxide, nickel
oxide, and magnesium oxide, and at least one selected from the
group consisting of silicon oxide, aluminum oxide, titanium oxide,
tantalum oxide, and zinc oxide are used as the target.
62. The method for producing the magnetic recording medium
according to claim 60, wherein the target particles are sputtered
in a reactive atmosphere containing oxygen.
63. The method for producing the magnetic recording medium
according to claim 60, wherein: the magnetic recording medium
further comprises a protective layer on the magnetic layer; and the
protective layer and the magnetic layer are formed respectively by
generating the plasma by means of the resonance absorption,
allowing the generated plasma to collide with the target so that
the target particles are sputtered, and applying the bias voltage
between the substrate and the target to introduce and deposit the
sputtered target particles on the substrate.
64. The method for producing the magnetic recording medium
according to claim 63, wherein when the protective layer is formed,
then the target is carbon, and a mixed gas, which principally
contains argon and which contains at least one of nitrogen and
hydrogen, is used as a plasma gas.
65. The method for producing the magnetic recording medium
according to claim 60, wherein a microwave is used for the
resonance absorption.
66. The method for producing the magnetic recording medium
according to claim 60, wherein the bias voltage is applied with an
alternating current power source having a radio frequency or a
direct current power source.
Description
TECHNICAL FIELD
[0001] The present invention relates to a magnetic recording medium
which is suitable for high density recording. In particular, the
present invention relates to a magnetic recording medium which
makes it possible to record bit information in an extremely minute
area of a magnetic layer. The present invention also relates to a
method for producing the magnetic recording medium and a magnetic
recording apparatus.
BACKGROUND ART
[0002] Recent development of the advanced information society is
remarkable. The multimedia technology, with which various types of
information can be dealt with, is quickly popularized. A magnetic
recording apparatus, which is installed, for example, to a
computer, is known as one of those based on the multimedia
technology. At present, the development of the magnetic recording
apparatus is advanced along with a course to realize a small-sized
apparatus while improving the recording density.
[0003] In order to realize high density recording with the magnetic
recording apparatus, it is demanded, for example, that (1) the
distance between a magnetic disk and a magnetic head is narrowed,
(2) the coercive force of a magnetic recording medium is increased,
(3) the speed of the signal processing process is increased, and
(4) a medium, which suffers from less thermal fluctuation, is
developed.
[0004] The magnetic recording medium has a magnetic layer in which
magnetic particles or magnetic grains are aggregated on a
substrate. Information is recorded thereon by magnetizing a certain
group of several magnetic grains in an identical direction by the
aid of a magnetic head. Therefore, in order to realize the high
density recording, it is necessary to decrease the minimum area
which may be magnetized in the identical direction at once in the
magnetic layer, i.e., the unit area in which the inversion of
magnetization may occur, in addition to the increase in coercive
force of the magnetic layer. In order to decrease the unit area of
inversion of magnetization, it is necessary that individual
magnetic grains are allowed to have a fine and minute size, or it
is necessary to decrease the number of magnetic grains for
constructing the unit of inversion of magnetization. For example,
in order to achieve a recording density above 40 Gbits/inch.sup.2
(6.20 Gbits/cm.sup.2), it is necessary that the diameter of the
magnetic grain is suppressed to be not more than 10 nm. Further, it
is also necessary to make countermeasures in order to decrease the
dispersion of the grain diameter when the magnetic grain is allowed
to have a fine and minute size, and decrease the thermal
fluctuation. As a trial to realize the demands as described above,
it has been suggested that a seed film is provided between a
substrate and a magnetic layer, as disclosed, for example, in U.S.
Pat. No. 4,652,499.
[0005] However, the method, in which the magnetic layer is provided
on the substrate with the seed film intervening therebetween as
described above, has had a limit to control the magnetic grain
diameter and the distribution thereof in the magnetic layer. For
example, even when the material for the seed film, the film
formation condition, the structure of the seed film, and other
factors were adjusted in order to obtain magnetic grains having a
grain diameter of about 10 nm in the magnetic layer, the grain
diameter distribution was broad, in which considerable amounts of
grains having a size coarsely increased to several tens nm and
grains inversely having a size finely decreased to about a half of
10 nm were present in a mixed manner. As for such magnetic grains,
magnetic grains having a grain diameter larger than the average
cause the increase in noise upon recording/reproduction. On the
other hand, magnetic grains having a grain diameter smaller than
the average cause the increase in thermal fluctuation upon
recording/reproduction. As a result of the presence of the magnetic
grains having a variety of sizes in a mixed manner, the boundary
line between an area in which the inversion of magnetization occurs
and an area in which the inversion of magnetization does not occur
provides a coarse zigzag pattern as a whole. This fact was also a
factor to increase the noise. Further, the inversion of
magnetization hitherto occurred in a unit composed of a number of 5
to 10 individuals of magnetic grains in the magnetic layer of the
conventional magnetic recording medium.
[0006] As for the spacing distance between the magnetic head and
the magnetic layer of the magnetic recording medium for the high
density recording, it is investigated that the spacing distance is
narrowed to be not more than 15 nm. In general, scratches and rough
irregularities exist on the substrate surface. For this reason,
rough irregularities originating from the substrate have hitherto
appeared on the surface of the magnetic recording medium prepared
by stacking a film on the substrate. If the distance between the
magnetic recording medium and the magnetic head is narrowed, it is
impossible for the magnetic head to stably fly due to the rough
irregularities as described above, resulting in the occurrence of
the following problems. That is, the recording and reproducing
characteristics are deteriorated, and the magnetic head collides
with the magnetic recording medium to cause damages of the both.
Therefore, it is demanded to realize a technique for forming a flat
film without being affected by the surface roughness of the
substrate.
[0007] On the other hand, as the spacing distance between the
magnetic head and the magnetic layer is narrowed, it is more
necessary to protect the magnetic layer from the shock exerted by
the magnetic head and the environment of use. Therefore, it is
required to form a protective film for protecting the magnetic film
so that the protective film is more uniform without causing any
deficiency. However, in order to realize the spacing distance of
not more than 15 nm between the magnetic head and the magnetic
layer, it is necessary that the protective film to be formed on the
magnetic layer has a film thickness of not more than 5 nm. Even
when it is intended to form a carbon protective film with a
thickness of not more than 5 nm by using the conventional DC
sputtering method or the magnetron sputtering method, it has been
impossible to completely cover the surface of the magnetic
recording medium with the protective film, because the carbon
protective film is formed only in an island form, or any defect
such as hollow hole and crack occurs in the protective film. If the
surface of the magnetic recording medium is not completely covered
with the protective film, then any corrosion occurs in the magnetic
layer, and the magnetic layer suffers from any physical damage due
to the head crash or the like.
[0008] In order to allow the magnetic head to fly over the magnetic
disk, it is necessary to provide a texture provided with a
concave/convex structure on the surface. However, it has not been
easy to control the concave/convex structure to have an appropriate
size.
[0009] Japanese Patent No. 2704957 discloses a magnetic recording
medium having a keeper layer. The keeper layer is an auxiliary film
having soft magnetization. The keeper layer is arranged so that it
makes tight contact with the surface of a magnetic layer (recording
layer) for performing recording. When the magnetic layer is in a
recording magnetization state, a portion of the keeper layer, which
contacts with a recording magnetization portion of the magnetic
layer, is magnetized in a direction opposite to the magnetic layer,
because the keeper layer has the soft magnetization. An annular
magnetic path is formed by the recording magnetization portion of
the magnetic layer and the portion of magnetization in the opposite
direction of the keeper layer. Even when the film thickness of the
magnetic layer is thinned, the recording magnetization is stably
maintained without being demagnetized. Further, owing to such a
situation, the diamagnetic field, which acts on the magnetic layer
of the recording magnetization portion, is decreased. Therefore,
even when the recording density is increased by allowing the
recording magnetic domain to have a fine and minute size, the
influence of the diamagnetic field is mitigated, because the film
thickness of the magnetic layer can be made thin. The
demagnetization based thereon is also reduced. Simultaneously, the
recording magnetization state is stabilized by the keeper layer.
Therefore, the demagnetization, which would be otherwise caused by
the thermal fluctuation in accordance with the elapse of time, can
be also reduced, giving high stability of storage of recording
information for a long period of time.
[0010] However, when the keeper layer is formed, a problem arises
such that a magnetic resistance effect (MR) magnetic head, which is
widely used for the present hard disk apparatus for recording and
reproduction, cannot be used as it is, because of the following
reason. As clarified from the role of-the keeper layer as described
above, the keeper layer acts as a type of shield on the magnetic
layer. Therefore, the presence of the keeper layer brings about an
obstacle for the reproducing operation in which the leak magnetic
field is read with the magnetic head-and for the recording
operation in which the magnetization of the magnetic layer is
inverted with the magnetic head. That is, the recording magnetic
field is obstructed by the leak magnetic field of the keeper layer
during the recording. Therefore, the effective recording magnetic
field is lowered. During the reproduction, an arrangement is made,
in which the magnetic pole of the keeper layer counteracts the
magnetic pole of the magnetic layer. For this reason, the leak
magnetic field from the area of inversion of magnetization of the
magnetic layer is decreased, and the reproducing sensitivity is
lowered. Therefore, even when the reproducing element of the
magnetic head is operated in this state, the reproduction output is
small. In order to deal with the problem caused upon the recording
and reproduction, the following method is known. That is, a DC bias
current is allowed to flow through the magnetic head, and a DC
magnetic field is applied to an area disposed just under the
magnetic head, in order to eliminate the action of the keeper layer
only when the recording operation and the reproducing operation are
performed. With the DC magnetic field, the keeper layer of the area
, on which the DC magnetic field is exerted, is magnetically
saturated so that a so-called window may be effectively bored
through the shield. On the other hand, as for the reproducing
element of the magnetic head, it is desirable to use a
magnetic-resistance effect (MR) element having a high reproducing
sensitivity or a giant magnetic resistance effect (GMR) element.
However, such an element has no function to apply the DC magnetic
field as described above. Therefore, it is impossible to perform
reproduction on the magnetic recording medium provided with the
keeper layer by using an ordinary MR element or an ordinary GMR
element as it is.
[0011] The perpendicular magnetic recording system attracts
attention as a recording system for realizing the high density
recording for the magnetic recording medium. The perpendicular
magnetic recording system uses a magnetic recording medium
(hereinafter referred to as "perpendicular magnetic recording
medium") including, as a recording layer, a magnetic layer in which
the magnetization-prompt direction is perpendicular to the disk
surface. The perpendicular magnetic recording of this type does not
involve such a problem as caused in the in-plane magnetic recording
that the magnetic field, which is generated from the boundary
between magnetic domains having different magnetization directions,
inhibits the formation of the minute magnetic domain. Therefore, it
is possible to thicken the film thickness of the magnetic layer of
the magnetic recording medium. For this reason, in the case of the
perpendicular magnetic recording medium, it is possible to form a
minute recording magnetic domain on the magnetic layer in order to
achieve the high density recording. The perpendicular magnetic
recording medium is highly resistant to the thermal fluctuation as
compared with the in-plane magnetic recording medium.
[0012] As for the perpendicular magnetic recording medium as
described above, a perpendicular magnetic recording medium of the
monolayer type provided with only one layer of magnetic layer
(recording layer) has been investigated, in which the magnetic
layer of the in-plane magnetization of the in-plane magnetic
recording medium is changed to the magnetic layer of the
perpendicular magnetization. Although the monolayer type
perpendicular magnetic recording medium has a simple structure, it
involves the following problem. That is, the leak magnetic field
generated from the medium is small as compared with the in-plane
magnetic recording medium, and the reproduction output is small. In
order to solve this problem, a two-layered type perpendicular
magnetic recording medium has been suggested, in which an in-plane
magnetizable layer is formed between a substrate and a magnetic
layer. In the case of the two-layered type perpendicular magnetic
recording medium, the magnetic flux, which is generated on the side
of the substrate of the magnetic layer, passes through the in-plane
magnetizable layer, and thus the magnetic path is formed.
Accordingly, the leak magnetic field, which is generated on the
side opposite to the substrate of the magnetic layer, is increased.
Therefore, when the leak magnetic field from the magnetic layer is
detected by using the reproducing head, the reproduction output is
increased.
[0013] However, in the case of the two-layered type perpendicular
magnetic recording medium, the magnetic flux to cause any noise,
which originates from any confused magnetic domain in the area of
inversion of magnetization of the magnetic layer, also passes
through the in-plane magnetizable layer. Therefore, in the case of
the two-layered type perpendicular magnetic recording medium, not
only the reproduced signal but also the noise are increased. As a
result, the signal to noise ratio (S/N) is equivalent to that of
the monolayer type perpendicular magnetic recording medium.
Therefore, as for the two-layered type perpendicular magnetic
recording medium, it has been necessary to decrease the noise from
the viewpoint of SIN.
[0014] The present invention has been achieved in order to solve
the problems involved in the conventional technique as described
above, a first object of which is to provide a magnetic recording
medium having a magnetic layer composed of magnetic particles or
magnetic grains which are allowed to have a fine and minute size
and in which the dispersion of grain diameter is reduced, and to
provide a magnetic recording apparatus installed with the magnetic
recording medium.
[0015] A second object of the present invention is to provide a
magnetic recording medium in which magnetic grains are controlled
to have desired crystal orientation, and to provide a magnetic
recording apparatus installed with the magnetic recording
medium.
[0016] A third object of the present invention is to provide a
magnetic recording medium in which the unit of inversion of
magnetization is small, and to provide a magnetic recording
apparatus installed with the magnetic recording medium.
[0017] A fourth object of the present invention is to provide a
magnetic recording medium in which the noise is low, the thermal
fluctuation is low, and the thermal demagnetization is low and
which is suitable for high density recording, and to provide a
magnetic recording apparatus installed with the magnetic recording
medium.
[0018] A fifth object of the present invention is to provide a
magnetic recording medium which is formed with a texture having a
desired concave/convex structure, and to provide a magnetic
recording apparatus installed with the magnetic recording
medium.
[0019] A sixth object of the present invention is to provide a
method for producing a magnetic recording medium provided with a
protective layer composed of a super thin film to cover, with a
uniform film thickness, a surface of a magnetic layer of the
magnetic recording medium.
[0020] A seventh object of the present invention is to provide a
magnetic recording medium and a magnetic recording apparatus
suitable for high density recording, in which an MR element or a
GMR element having a high reproducing sensitivity can be used for
reproduction, although a layer, which plays a role of a keeper
layer to mitigate recording demagnetization which would be
otherwise caused by the high density recording, is provided.
[0021] An eighth object of the present invention is to provide a
perpendicular magnetic recording medium and a magnetic recording
apparatus in which the noise is reduced, and information can be
reproduced with high S/N.
[0022] A ninth object of the present invention is to provide a
super high density magnetic recording medium having a surface
recording density exceeding 40 Gbits/inch.sup.2, and to provide a
magnetic recording apparatus installed with the magnetic recording
medium.
DISCLOSURE OF THE INVENTION
[0023] According to a first aspect of the present invention, there
is provided a magnetic recording medium comprising:
[0024] a substrate;
[0025] an underlying layer which is formed on the substrate,
and
[0026] a magnetic layer which is formed on the underlying layer and
on which information is recorded, wherein:
[0027] the underlying layer is composed of crystal grains
substantially formed of magnesium oxide, and a crystal grain
boundary or boundaries containing at least one oxide selected from
the group consisting of silicon oxide, aluminum oxide, titanium
oxide, tantalum oxide, and zinc oxide.
[0028] The present inventors have disclosed, in U.S. patent
application Ser. No. 09/478,377 (corresponding to Japanese Patent
Application No. 11-1667), a magnetic recording medium comprising a
non-magnetic substrate; an inorganic compound film including
crystalline first oxide composed of at least one selected from
cobalt oxide, chromium oxide, iron oxide, and nickel oxide, and
second oxide composed of at least one selected from silicon oxide,
aluminum oxide, titanium oxide, tantalum oxide, and zinc oxide, in
which the second oxide exists at a grain boundary of crystal grains
of the first oxide; and a magnetic film formed on the inorganic
compound film. In the magnetic recording medium, the crystal grains
of the first oxide, which constitute the inorganic compound film,
have a honeycomb structure. Magnetic particles or magnetic grains
of the magnetic layer, which are formed on the inorganic compound
film, are epitaxially grown from the crystal grains of the first
oxide. Therefore, the magnetic grains of the magnetic layer also
have a honeycomb structure. Accordingly, the crystal grains of the
magnetic film are allowed to have a fine and minute size, and it is
possible to uniformize the grain diameter. Thus, the magnetic
recording medium is realized, in which the noise is low and the
thermal fluctuation is reduced.
[0029] However, according to an experiment performed by the present
inventors, when the inorganic compound film is formed on the
substrate of the magnetic recording medium described above, an
initial growth layer, which is an aggregate or cluster of
microcrystals having no regular structure, is initially produced.
It has been revealed that the inorganic compound film has to be
grown so that it has a certain degree of film thickness, for
example, a film thickness of not less than 30 nm until the regular
honeycomb structure appears on the inorganic compound film, because
of the presence of the initial growth layer. In the present
invention, it has been revealed that the occurrence of the initial
growth layer portion can be suppressed owing to the use of
magnesium oxide as the material for the underlying layer
corresponding to the inorganic compound film, especially as the
material for constructing the crystal grains, and a satisfactory
honeycomb structure can be formed from the initial stage of film
formation. Accordingly, the thickness of the underlying layer as
well as the thickness of the magnetic recording medium can be made
thin. It is possible to shorten the film formation step, and it is
possible to decrease the production cost.
[0030] When the first oxide, which is disclosed in U.S. patent
application Ser. No. 09/478,377, is used for the crystal grains of
the underlying layer, the standard deviation .sigma. of the crystal
grain diameter distribution is not more than 10% of the average
grain diameter. On the contrary, when magnesium oxide is used for
the crystal grains of the underlying layer in the present
invention, the standard deviation .sigma. of the crystal grain
diameter distribution in the underlying layer has been successfully
made to be not more than 8% of the average grain diameter. It is
approved that the regularity of the honeycomb structure is high as
the number of grains (hereinafter referred to as "number of
coordinated grains") which surround one crystal grain of the
underlying layer is close to 6.0. When magnesium oxide is used for
the material for the crystalline matter, the number of coordinated
grains, which is closer to 6.0, has been successfully obtained.
That is, the following fact has been revealed. When magnesium oxide
is used for the material for the crystalline matter, then the
dispersion of the grain diameter of the underlying layer can be
further decreased, and it is possible to improve the regularity of
the honeycomb structure. Further, the magnetic grains of the
magnetic layer to be formed on the underlying layer can be formed
with the more uniform grain diameter and the more uniform
structure. Therefore, in the magnetic recording medium of the
present invention, the noise is low, the thermal fluctuation is
low, and the thermal demagnetization is low. Further, the magnetic
recording medium may be suitable for the high density
recording.
[0031] The expression "crystal grains substantially formed of
magnesium oxide" used herein means the fact that the crystal grains
may be constructed while containing not only magnesium oxide but
also any impurity including, for example, oxide or element for
constructing the oxide contained in the crystal grain boundary in
an amount of about several %, generally in an amount of not more
than 5%.
[0032] As shown in FIG. 14, the underlying layer may have such a
structure that the shape of one crystal grain may be a regular
hexagon in a plane parallel to the substrate surface, and the
crystal grain may be grown upwardly in a pillar-shaped
configuration in a cross section perpendicular to the substrate
surface. Especially, the pillar-shaped cross section of the crystal
grain is not widened as the underlying layer is grown, having a
structure in which the width of the crystal grain boundary is
uniform. Therefore, the aggregate of the crystal grains each of
which forms a regular hexagonal cylinder forms the honeycomb
structure in which the hexagonal cylinders are regularly arranged.
Mathematically, the aggregate approximately has a fractal feature,
and it can be expressed with the group theory as well. In the
underlying layer, one crystal grain having the regular hexagonal
configuration may be surrounded by 5.9 to 6.1 individuals of the
grains in average.
[0033] As explained in an embodiment as described later on, it has
been revealed that the grains deposited in the underlying layer and
the grain boundary or boundaries therebetween are crystalline and
amorphous respectively, by means of the lattice image observation
based on the X-ray diffraction method. The standard deviation a of
the crystal grain diameter distribution is not more than 8% of the
average grain diameter. Further, the grain diameter distribution is
a normal distribution. Therefore, it is approved that the
regularity of the grain arrangement is extremely high. The crystal
grains in the underlying layer has strong crystalline orientation.
Therefore, when the magnetic layer is formed on the underlying
layer having the structure as described above, for example, it is
possible to grow the ferromagnetic magnetic grains having
crystalline orientation from the crystal grain portion of the
honeycomb structure. On the other hand, it is possible to grow the
non-magnetic boundary portion from the crystal grain boundary of
the honeycomb structure.
[0034] It is preferable that the film thickness of the underlying
layer is 3 nm to 50 nm. If the film thickness of the underlying
layer is less than 3 nm, it is difficult to stably form the film
because of circumstances of a film-forming machine. If the film
thickness exceeds 50 nm, then the thickness of the entire
underlying layer is increased, and it takes a long period of time
to form the film. It is desirable that the spacing distance of the
crystal grains (width of the crystal grain boundary) is 0.5 nm to 2
nm, because the honeycomb structure is obtained in a stable manner,
and it is possible to sufficiently suppress the magnetic
interaction between the magnetic grains. The spacing distance
between the crystal grains can be regulated by controlling the
concentration of the oxide of inorganic compound existing in the
crystal boundary and the composition ratio with respect to
magnesium oxide.
[0035] It is preferable that the underlying layer is formed by
means of the ECR sputtering method which utilizes the resonance
discharge based on the use of the microwave as described later on.
In the sputtering method, the kinetic energy of the target particle
can be uniformized depending on the way of application of the bias
voltage, and it is possible to control the energy more precisely.
Especially, when the underlying layer is formed by using the ECR
sputtering method, the film, which has the desired crystalline
orientation and the satisfactory honeycomb structure, is obtained
without requiring any complicated sputtering condition.
[0036] The magnetic layer, which is formed on the underlying layer,
has a similar honeycomb structure reflecting or replicating the
structure of the underlying layer. The magnetic grains in the
magnetic layer are epitaxially grown in a continuous manner from
the top of the crystal grains in the underlying layer. Therefore,
when the honeycomb structure of the underlying layer is
appropriately adjusted, it is possible to grow the magnetic grains
having the desired grain diameter and the desired crystalline
orientation thereon. That is, the underlying layer serves to
control the grain diameter, the grain diameter distribution, and
the crystalline orientation of the magnetic layer. The structure,
the orientation, the crystal grain diameter, and other factors of
the underlying layer can be controlled, for example, by selecting
the concentration (composition) of the crystal grain boundary
substance and magnesium oxide for forming the crystal grains,
selecting the material for the crystal grain boundary, and
selecting the film formation condition.
[0037] As for the magnetic layer, the magnetic grains of the
magnetic layer can be grown from the crystal grains of the
honeycomb structure of the underlying layer. On the other hand, the
non-magnetic boundary or boundaries can be grown from the boundary
or boundaries of the honeycomb structure of the underlying layer.
Therefore, it is possible to provide the structure in which the
magnetic grains are magnetically separated from each other.
Accordingly, the unit of inversion of magnetization upon recording
and reproduction can be reduced, for example, to be 2 or 3
individuals of magnetic grains. It is possible to realize the super
high density recording. Further, it is possible to avoid the
formation of any zigzag pattern of the boundary between the
adjoining recording magnetic domains in the magnetic film, and thus
it is possible reduce the noise.
[0038] Conventionally, in order to reduce the magnetic interaction
between the magnetic grains, any non-magnetic element has been
subjected to segregation in the vicinity of the crystal grain
boundary in the crystal grain. However, in the present invention,
it is possible to grow the nonmagnetic portion in the magnetic
layer corresponding to the crystal grain boundary which surrounds
the regular hexagonal crystal grains in the underlying layer. In
this case, the distance between the crystal grains in the
underlying layer is controlled to be 0.5 nm to 2 nm, and the
magnetic layer is epitaxially grown while reflecting this
structure. Thus, it is possible to provide the non-magnetic portion
having such a spacing distance in the magnetic layer. The
epitaxially grown magnetic grain portion is ferromagnetic, and it
has crystalline orientation suitable for the high density
recording. On the other hand, the grain boundary, which surrounds
the magnetic grain, resides in random orientation even when it is
amorphous or crystalline. Therefore, the grain boundary exhibits
the non-magnetic or the magnetization different from that of the
magnetic grain portion, making it possible to allow the magnetic
grains to be magnetically independent from each other. Accordingly,
the size of the magnetic domain of the magnetic recording medium
can be decreased to be fine and minute up to the magnetic grain
size.
[0039] It is desirable for the magnetic layer to use an alloy
principally containing cobalt and further containing at least two
elements selected from the group consisting of chromium, platinum,
tantalum, niobium, titanium, and silicon. For example, it is
possible to use a film of CoCrPt or CoCrPtTa. The magnetic grain in
the magnetic layer is composed of a cobalt alloy, and it may be
composed of a crystalline material. The boundary between the
magnetic grains may contain at least one element selected from the
group consisting of chromium, tantalum, niobium, titanium, and
silicon, and it may be composed of a polycrystalline material. The
magnetic layer may be a multilayered film such as Co/Pt.
[0040] It is also preferable for the magnetic layer to use a
magnetic film having a granular structure composed of two phases of
a crystalline phase and an amorphous phase. In this case, the
crystalline phase principally contains cobalt, and it further
contains at least one element selected from the group consisting of
neodymium, praseodymium, yttrium, lanthanum, samarium, gadolinium,
terbium, dysprosium, holmium, platinum, and palladium. As for the
amorphous phase, a phase of at least one compound selected from
silicon oxide, zinc oxide, tantalum oxide, and aluminum oxide may
exist to surround the crystal grains. For example, Co--SiO.sub.2
may be used. When the magnetic layer is formed as a film, cobalt
grains may be grown as oxide corresponding to the crystal grain
boundary on the crystal grains of the underlying layer formed by
means of the ECR method.
[0041] According to a second aspect of the present invention, there
is provided a magnetic recording medium comprising:
[0042] a substrate;
[0043] a first underlying layer which is formed on the
substrate;
[0044] a second underlying layer which is formed on the first
underlying layer; and
[0045] a magnetic layer which is formed on the second underlying
layer and on which information is recorded, wherein:
[0046] the second underlying layer is composed of crystal grains
substantially formed of at least one oxide selected from the group
consisting of cobalt oxide, chromium oxide, iron oxide, nickel
oxide, and magnesium oxide, and a crystal grain boundary containing
at least one oxide selected from the group consisting of silicon
oxide, aluminum oxide, titanium oxide, tantalum oxide, zinc oxide,
and magnesium oxide; and
[0047] the first underlying layer serves as a layer to prevent the
second underlying layer from initial growth.
[0048] The magnetic recording medium according to this aspect is
provided with only the first underlying layer having a minute
thickness of about several nanometers between the substrate and the
second underlying layer including the crystal grain boundary and
the group of crystal grains substantially formed of the oxide as
described above. Thus, it is possible to substantially suppress the
occurrence of the initial growth layer on the second underlying
layer. Accordingly, it is possible to thin the thickness of the
entire underlying layer as well as the thickness of the magnetic
recording medium. It is possible to shorten the step of forming the
film, and it is possible to decrease the production cost. Further,
an advantage is also obtained such that the performance of tight
contact between the substrate and the magnetic layer is enhanced
owing to the provision of the first underlying layer.
[0049] It has been revealed that when the first underlying layer is
provided between the substrate and the second underlying layer, the
second underlying layer grows while reflecting the crystal
structure of the first underlying layer and/or the morphology of
the surface of the first underlying layer. For this reason, if the
second underlying layer is grown on the substrate without providing
the first underlying layer, the standard deviation a of the crystal
grain diameter distribution of the second underlying layer is not
more than 10% of the average grain diameter. On the contrary, in
the case of the present invention, the standard deviation a of the
crystal grain diameter distribution of the second underlying layer
has been successfully not more than 8% of the average grain
diameter. It is approved that the regularity of the honeycomb
structure is high as the number of grains (hereinafter referred to
as "number of coordinated grains") which surround one crystal grain
of the second underlying layer is close to 6.0. When the first
underlying layer is provided, the number of coordinated grains,
which is closer to 6.0, has been successfully obtained. That is,
the following fact has been revealed. The grain size distribution
and the number of coordinated grains can be controlled by forming
the first underlying layer on the substrate. Further, the magnetic
grains of the magnetic layer, which are formed on the second
underlying layer, can be also formed to have a more uniform grain
size and a more uniform structure.
[0050] The phrase "crystal grains substantially formed of at least
one oxide selected from the group consisting of cobalt oxide,
chromium oxide, iron oxide, nickel oxide, and magnesium oxide"
herein means the fact that the crystal grains may be constructed
while containing not only the at least one oxide selected from the
group consisting of cobalt oxide, chromium oxide, iron oxide,
nickel oxide, and magnesium oxide but also any impurity including,
for example, oxide or element for constructing the oxide contained
in the crystal grain boundary in an amount of about several %.
[0051] In the present invention, an amorphous film or a crystalline
film may be used as the first underlying layer. When the amorphous
film is used, those usable for the amorphous film include:
[0052] (1) a metal selected from the group consisting of hafnium,
titanium, tantalum, niobium, zirconium, tungsten, molybdenum, and
an alloy containing at least one element of them;
[0053] (2) a cobalt alloy principally composed of cobalt and
containing at least one element selected from the group consisting
of titanium, tantalum, niobium, zirconium, and chromium; or
[0054] (3) at least one inorganic compound selected from the group
consisting of silicon nitride, silicon oxide, and aluminum oxide.
When the inorganic compound is used, it is allowable to further
contain at least one metal selected from the group consisting of
hafnium, titanium, tantalum, niobium, zirconium, chromium, and
aluminum. When the material as described above is used, the second
underlying layer can be epitaxially grown from the top of the first
underlying layer more appropriately without substantially growing
the initial growth layer as an aggregate of microcrystals.
[0055] On the other hand, when the crystalline film is used for the
first underlying layer, the crystalline film may be composed of at
least one selected from the group consisting of chromium, chromium
alloy, vanadium, and vanadium alloy. In this case, the alloy may
contain at least one element selected from the group consisting of
titanium, tantalum, aluminum, nickel, vanadium, and zirconium. When
the element as described above is added, the lattice constant of
crystalline chromium or vanadium can be controlled to precisely
control the crystalline structure of the second underlying layer to
be formed on the first underlying layer.
[0056] When the first underlying layer is the crystalline film, it
is most preferable to use the hcp (Hexagonal Closest Packing) or
bcc (Body-Centered Cubic) structure, because of the following
reason. That is, such a structure is a structure which is the same
as or similar to the crystal structure of the magnetic layer.
Therefore, it is possible to facilitate the epitaxial growth of the
magnetic grains of the magnetic layer from the top of the crystal
grains of the underlying layer. According to the knowledge of the
present inventors, it has been revealed that the second underlying
layer is grown while reflecting the crystal structure of the first
underlying layer and/or the morphology of the surface of the first
underlying layer. Therefore, it is preferable that the crystal
structure of the first underlying layer is appropriately selected
while considering the crystal structure of the second underlying
layer intended to be formed thereon.
[0057] In the magnetic recording medium of the present invention,
the second underlying layer contains, as the crystal grains, at
least one oxide selected from the group consisting of cobalt oxide,
chromium oxide, iron oxide, nickel oxide, and magnesium oxide. The
grain boundary, which surrounds the crystal grains, is composed of
at least one oxide selected from the group consisting of silicon
oxide, aluminum oxide, titanium oxide, tantalum oxide, and zinc
oxide.
[0058] As shown in FIG. 3, the second underlying layer may have the
following structure. That is, the shape of one crystal grain is a
regular hexagon as viewed in a plane parallel to the substrate
surface. The crystal grains are grown upwardly in a pillar-shaped
configuration as viewed in a cross section perpendicular to the
substrate surface of the second underlying layer. Especially, the
pillar-shaped cross section of the crystal grain is not widened in
a sector form even when the second underlying layer is grown,
giving such a structure that the width of the grain boundary is
uniform. Therefore, the aggregate of the crystal grains, in which
one crystal grain forms a regular hexagonal cylinder, forms a
honeycomb structure in which the hexagonal cylinders are regularly
arranged. In the same manner as in the underlying layer in the
first aspect of the present invention, the honeycomb structure
mathematically has a fractal feature although in an approximate
manner, and it can be expressed with the group theory. One crystal
grain having the regular hexagonal configuration may be surrounded
by 5.8 to 6.2 individuals of the grains in average.
[0059] As explained in embodiments as described later on, according
to the analysis based on the X-ray diffraction method, the grains
deposited in the second underlying layer and the grain boundary
thereof are crystalline and amorphous respectively. The standard
deviation a of the crystal grain diameter distribution is not more
than 8% of the average grain diameter. Further, the grain diameter
distribution is a normal distribution. Therefore, the regularity of
the grain arrangement is extremely high. Further, the crystal
grains in the second underlying layer have strong crystalline
orientation. Therefore, when the magnetic layer is formed on the
second underlying layer having the structure as described above,
for example, the magnetic grains, which are ferromagnetic and
subjected to crystalline orientation, can be grown from the crystal
grain portions of the honeycomb structure. On the other hand, the
non-magnetic boundary portion can be grown from the crystal grain
boundary of the honeycomb structure.
[0060] It is preferable that the first underlying layer has a film
thickness of 2 nm to 50 nm. If the film thickness of the first
underlying layer is less than 2 nm, it is impossible to expect the
effect of the provision of the first underlying layer. If the film
thickness of the first underlying layer exceeds 50 nm, then the
thickness of the entire underlying layer is increased, and it takes
a long period of time to form the film. It is preferable that the
second underlying layer has a film thickness of 3 nm to 100 nm. If
the film thickness of the second underlying layer is less than 3
nm, the good epitaxial growth of the magnetic layer scarcely takes
place from the top of the underlying layer. If the film thickness
of the second underlying layer exceeds 100 nm, then the thickness
of the entire underlying layer is increased, and it takes a long
period of time to form the film. It is preferable that the entire
film thickness of the first and second underlying layers is 3 nm to
100 nm. It is desirable that the spacing distance between the
crystal grains (width of the crystal grain boundary) is 0.5 nm to 2
nm, because such a spacing distance is sufficient to block the
magnetic interaction between the magnetic grains in the magnetic
layer formed on the underlying layer, the bulk density of the
formed magnetic film is appropriate, and the recording density is
improved.
[0061] According to a third aspect of the present invention, there
is provided a magnetic recording medium comprising:
[0062] a substrate;
[0063] an underlying layer which is formed on the substrate;
[0064] a control layer which is formed on the underlying layer and
which is formed of at least one selected from the group consisting
of magnesium oxide, chromium alloy, and nickel alloy; and
[0065] a magnetic layer which is formed on the control layer and on
which information is recorded, wherein:
[0066] the underlying layer is composed of crystal grains
substantially formed of at least one oxide selected from the group
consisting of cobalt oxide, chromium oxide, iron oxide, nickel
oxide, and magnesium oxide, and a crystal grain boundary containing
at least one oxide selected from the group consisting of silicon
oxide, aluminum oxide, titanium oxide, tantalum oxide, and zinc
oxide. In the underlying layer, the respective crystal grains may
have a hexagonal configuration, and they may be arranged in a
honeycomb form.
[0067] According to en experiment performed by the present
inventors, when the magnetic layer is formed on the underlying
layer of the magnetic recording medium described above, the lattice
constants of these layers are deviated from each other depending on
the combination of materials for constructing the underlying layer
and the magnetic layer. For this reason, the magnetic layer failed
to be epitaxially grown on the underlying layer in a well-suited
manner in some cases. Further, when the underlying layer is formed,
it is difficult to effect complete phase separation for the crystal
grains and the amorphous substance existing in the grain boundary
thereof. The crystal grains are mixed with the amorphous substance
in an amount of about 3 to 5% in some cases. For example, in the
case of a CoO--SiO.sub.2 film having the honeycomb structure, it
has been revealed by the .mu.-Auger analysis that several % of
SiO.sub.2 may be contained in CoO in the crystal grain, while CoO
may be contained in SiO.sub.2 of the amorphous substance.
Therefore, even when an appropriate combination of materials is
selected for the underlying layer and the magnetic film, the
amorphous substance exists in the crystal grains in a mixed manner
as described above. For this reason, the lattice constant of the
crystal grain of the actually formed film is deviated from the
original lattice constant which is expected to be obtained when no
impurity exists. As a result, a situation has been probably caused,
in which the lattice match is not obtained sufficiently between the
crystal grains of the underlying layer and the magnetic grains of
the magnetic layer. It has been revealed that if the deviation of
crystal lattice occurs in an amount of not less than .+-.10% as
represented by the difference in lattice constant, then the
coercive force of the magnetic grains of the magnetic layer formed
on the underlying layer is decreased, and it is impossible to
obtain desired magnetic characteristics. It has been also revealed
that if the discrepancy of the crystal lattice is further
increased, then the honeycomb structure of the underlying layer is
not reflected to the magnetic layer, the magnetic grains are not
formed in the magnetic layer, and a polycrystalline structure is
obtained as a whole.
[0068] In the present invention, the control layer (lattice
constant control layer) for adjusting the discrepancy of the
crystal lattices of the layers is provided between the underlying
layer and the magnetic layer. Accordingly, the decrease in coercive
force and the change of magnetic characteristic, which would be
otherwise caused by the discrepancy of the crystal lattice, have
been successfully suppressed in a substantial manner. When the
control layer, in which the material is selected so that the
discrepancy of the crystal lattice between the underlying layer and
the control layer and between the control layer and the magnetic
layer is decreased, for example, the discrepancy is within .+-.5%
as represented by the difference in lattice constant of each of
them, is provided, the magnetic grains of the magnetic layer can be
epitaxially grown while reliably reflecting the honeycomb structure
of the underlying layer. Therefore, the grain diameter of the
magnetic grains of the magnetic layer can be made fine and minute
by reflecting the crystal grain diameter of the underlying layer,
and the magnetic grains can be surrounded by the non-magnetic
boundary portion of the magnetic layer corresponding to the crystal
grain boundary of the underlying layer. Accordingly, it is possible
to reduce the magnetic interaction between the magnetic grains.
Thus, it is possible to produce the magnetic recording medium which
is suitable for the high density recording.
[0069] It is preferable to use at least one selected from the group
consisting of magnesium oxide, chromium alloy, and nickel alloy for
the control layer of the magnetic recording medium of the present
invention. Especially, it is preferable that the control layer is
formed of chromium-titanium, chromium-tungsten, magnesium oxide, or
chromium-ruthenium. In the present invention, it is preferable to
use, for the chromium alloy or the nickel alloy, a material
containing at least one element selected from the group consisting
of chromium, titanium, tantalum, vanadium, ruthenium, tungsten,
molybdenum, niobium, nickel, zirconium, and aluminum, other than
chromium or nickel as the base element.
[0070] It is most preferable to adopt the bcc structure or the B2
structure for the control layer. The structure is closely similar
to the crystal structure of the magnetic layer to be used for the
magnetic recording medium. Therefore, the lattice match is achieved
between the control layer and the magnetic layer. The magnetic
layer can be epitaxially grown from the control layer with ease. It
is preferable to appropriately select the composition of the
control layer while considering the compositions of the underlying
layer and the magnetic layer so that the lattice constant of the
crystal lattice of the control layer simultaneously has an
approximately intermediate value between those of the underlying
layer and the magnetic layer. By doing so, even when the crystal
lattice of the underlying layer is different from that of the
magnetic layer, it is possible to mitigate the difference by the
aid of the control layer.
[0071] When the control layer is formed, it is preferable that the
control layer is epitaxially grown from the underlying layer. As
for the control layer, the crystalline portion is epitaxially grown
from the crystal grain portion of the underlying layer, and the
crystal structure or the polycrystal, which is different from the
crystal grain portion, is grown from the amorphous crystal grain
boundary of the underlying layer. Further, when the magnetic layer
is continuously grown from the control layer, the lattice deviation
between the control layer and the magnetic layer can be decreased
by appropriately selecting the structure and the composition of the
control layer. Therefore, the epitaxial crystal growth is
facilitated, and thus an effect is obtained such that the growth of
the magnetic layer is facilitated. The structure of the magnetic
layer formed as described above reflects the honeycomb structure of
the underlying layer. The magnetic grain diameter and the grain
diameter distribution of the magnetic layer can be made
substantially equal to the crystal grain diameter and the grain
diameter distribution of the underlying layer. Further, the control
layer also has such an effect that the adhesion force between the
substrate and the magnetic layer is improved.
[0072] As described above, the underlying layer has the honeycomb
structure such that the shape of one crystal grain is a regular
hexagon in the plane parallel to the substrate surface of the
underlying layer, and the crystal grain is grown upwardly in a
pillar-shaped configuration in the plane perpendicular to the
substrate surface. The magnetic layer, which is formed on the
underlying layer, has a similar honeycomb structure which reflects
the structure of the underlying layer. Further, the magnetic grains
in the magnetic layer are epitaxially grown in the continuous
manner from the top of the crystal grains in the underlying layer
by the aid of the crystal grains in the control layer. Therefore,
the magnetic grains, which have a desired grain diameter and
crystalline orientation, can be grown in the magnetic layer to be
formed on the underlying layer with the control layer intervening
therebetween, by appropriately adjusting the honeycomb structure of
the underlying layer.
[0073] That is, the underlying layer serves to reduce the magnetic
interaction between the magnetic grains by controlling the magnetic
grain diameter, the magnetic grain diameter distribution, and the
orientation of the magnetic layer to be formed on the underlying
layer with the control layer intervening therebetween, and growing
the non-magnetic boundary portion from the crystal grain boundary
having the uniform width. On the other hand, the control layer has
the following effect. That is, the control layer reliably reflects
the honeycomb structure of the underlying layer to the magnetic
layer to facilitate the epitaxial growth by ensuring the lattice
match between the crystal grains of the underlying layer and the
magnetic grains of the magnetic layer. Thus, the control layer
avoids the decrease in coercive force of the magnetic layer and the
change of magnetic characteristics.
[0074] It is preferable that the underlying layer and the control
layer are formed in accordance with the ECR sputtering method which
utilizes the resonance discharge based on the used of the
microwave. It is preferable that the film thickness of the
underlying layer is 2 nm to 50 nm. If the film thickness of the
underlying layer is less than 2 nm, it is difficult for the
magnetic grains of the magnetic layer to cause the epitaxial growth
in a well-suited manner. If the film thickness of the underlying
layer exceeds 50 nm, then the thickness of the underlying layer is
increased, and it takes a long period of time to form the film. It
is preferable that the film thickness of the control layer is 2 nm
to 10 nm. If the film thickness of the control layer is less than 2
nm, it is impossible to obtain the film which has the good crystal
structure. If the film thickness of the control layer exceeds 10
nm, then the entire thickness is increased, and it takes a long
period of time to form the film. Accordingly, considering the fact
that the two layers are used for the underlying base for forming
the magnetic layer for the magnetic recording medium, it is most
preferable that the film thickness of the two layers is 5 nm to 100
nm.
[0075] It is preferable that the crystal structures of the
underlying layer and the control layer and the crystal structures
of the control layer and the magnetic layer are similar to one
another. That is, it is preferable that any one of crystal forms of
the crystal grains of the underlying layer and the control layer
and any one of crystal forms of the magnetic layer (for example,
the crystal structure, the crystal shape, and the crystal size) are
substantially equal to one another, and the differences in lattice
constant between the underlying layer and the control layer and
between the control layer and the magnetic layer are within .+-.5%
respectively. Accordingly, the magnetic grains of the magnetic
layer can be epitaxially grown in a well-suited manner from the top
of the crystal grains of the underlying layer while reflecting the
honeycomb structure of the underlying layer with the crystal grains
of the control layer intervening therebetween. Therefore, in the
present invention, even when the difference in lattice constant
between the underlying layer and the control layer is not less than
.+-.10%, the uniform and fine magnetic grains can be epitaxially
grown in the magnetic layer while mitigating the difference by
providing the plurality of layers for adjusting the lattice plane
between the underlying layer and the magnetic layer. The number of
control layer is not limited to single. A plurality of control
layers may be provided to disperse the difference in lattice
constant between the underlying layer and the magnetic layer at
boundaries between the respective layers.
[0076] In the present invention, preferred combinations of
materials for constructing the stack of the underlying
layer/control layer/magnetic layer include CoO--ZnO/Cr--Ti
alloy/Co--Cr--Pt alloy, CoO--SiO.sub.2/MgO/Co--Cr--Pt--Ta alloy,
CoO--SiO.sub.2/Cr--W alloy/Co--Cr--Pt--Ta alloy,
CoO--SiO.sub.2/MgO/Co--SiO.sub.2 granular type magnetic film,
CoO--SiO.sub.2/Ni--Al alloy/Co--Cr--Pt--Ta alloy,
CoO--SiO.sub.2/Cr--Ti alloy/Co--Cr--Pt--Ta alloy,
CoO--SiO.sub.2/Ni--Ta alloy/Co--Pt--SiO.sub.2 granular type
magnetic film, CoO--SiO.sub.2/Ni--Ta alloy/Co--Cr--Pt--Ta alloy,
CoO--SiO.sub.2/Cr--Ru alloy/Co--Cr--Pt--Ta alloy,
CoO--SiO.sub.2/Cr--Ru alloy/Co--Pt--SiO.sub.2 granular type
magnetic film, CoO--SiO.sub.2/Co--Cr--Zr alloy/Co--Pt--SiO.sub.2
granular type magnetic film, CoO--SiO.sub.2/Co--Cr--Zr
alloy/Co--Cr--Pt--Ta alloy, CoO--SiO.sub.2/Cr--Mo
alloy/Co--Cr--Pt--Ta alloy, and CoO--SiO.sub.2/Cr--Mo
alloy/Co--Pt--SiO.sub.2 granular type magnetic film. When such a
combination is selected, then the structure and the grain diameter
distribution of the magnetic grains of the magnetic layer can be
controlled more appropriately to produce the magnetic recording
medium which is suitable for the high density recording.
[0077] According to a fourth aspect of the present invention, there
is provided a magnetic recording medium comprising:
[0078] a substrate;
[0079] an underlying layer which is formed on the substrate;
and
[0080] a magnetic layer which is formed on the underlying layer and
on which information is recorded, wherein:
[0081] the underlying layer is composed of crystal grains and a
crystal grain boundary which surrounds the respective crystal
grains, the crystal grains being arranged in a honeycomb
configuration; and
[0082] the crystal grains protrude at a height of 3 to 20 nm from a
surface of the underlying layer.
[0083] As for the texture of the surface desirable for the high
density recording, it is desirable that the distance value, which
ranges from the surface of the magnetic recording medium or the
convex portion (apex) of the magnetic layer to the convex portion
(apex) nearest to the foregoing convex portion, is smaller than the
upper limit value of the control limit of the flying amount of the
magnetic head. Further, it is desirable that the protruding amount
(height) of the convex portion of the magnetic layer or the surface
of the magnetic recording medium, i.e., the distance from the apex
of the magnetic layer or the surface of the magnetic recording
medium to the valley nearest to the apex is smaller than the upper
limit value of the control limit of the flying amount of the
magnetic head. In the magnetic recording medium of the present
invention, as shown in FIG. 8, the underlying layer is provided
with the crystal grains (12) which protrude by the height (16) of 3
to 20 nm from the surface of the underlying layer (surface of the
crystal grain boundary 14). Therefore, the magnetic recording
medium of the present invention satisfies the demand as described
above. The underlying layer having such a structure may be formed
while appropriately controlling the sputtering condition by using
the ECR sputtering method. It is desirable that the crystal grain
of the underlying layer is substantially formed of at least one
oxide selected from the group consisting of cobalt oxide, chromium
oxide, iron oxide, nickel oxide, and magnesium oxide, and the
crystal grain boundary is composed of at least one oxide selected
from the group consisting of silicon oxide, aluminum oxide,
titanium oxide, tantalum oxide, and zinc oxide. It is desirable
that the underlying layer has a film thickness of 10 nm to 100 nm.
The magnetic layer or a protective layer which may be formed
thereon may have projections protruding by a height of 3 to 20 nm
from a surface of the magnetic layer or a surface of the protective
layer while reflecting a surface structure of the underlying layer.
The projections may be used as a texture of the magnetic recording
medium. Considering the size of the crystal grains, it is desirable
that a distance between the adjoining projections is 10 to 30 nm.
When the distance is within the range as described above, the
projections also function as the texture in a well-suited
manner.
[0084] According to a fourth aspect of the present invention, there
is provided a magnetic recording medium comprising:
[0085] a substrate;
[0086] an underlying layer which is formed on the substrate;
and
[0087] a magnetic layer which is formed on the underlying layer,
wherein:
[0088] the underlying layer has soft magnetization, and the
underlying layer is composed of crystal grains substantially formed
of at least one oxide selected from the group consisting of cobalt
oxide, chromium oxide, iron oxide, nickel oxide, and magnesium
oxide, and a crystal grain boundary containing at least one oxide
selected from the group consisting of silicon oxide, aluminum
oxide, titanium oxide, tantalum oxide, and zinc oxide.
[0089] The magnetic recording medium according to this aspect is
provided with the underlying layer having the two features as
described below between the substrate and the magnetic layer. The
first feature of the underlying layer is that the underlying layer
has the structure in which the plurality of crystal grains are
surrounded by the crystal grain boundary respectively and they are
regularly arranged in the honeycomb configuration as described
above. In the present invention, when the magnetic layer is stacked
on the underlying layer as described above, the regular honeycomb
structure of the magnetic grains can be brought about for the
magnetic layer by reflecting the honeycomb structure of the
underlying layer.
[0090] The second feature of the underlying layer is that the
underlying layer is provided with the soft magnetization. The
underlying layer having the soft magnetization functions as a
keeper layer as described above. Therefore, it is possible to
suppress the demagnetization which would be otherwise caused by the
diamagnetic field of the recording magnetization area of the
magnetic layer, and it is possible to stably retain the recorded
magnetization state of the magnetic recording medium. Further, it
is also possible to thin the film thickness of the magnetic layer.
Therefore, when the underlying layer as described above is
provided, it is possible to realize the magnetic recording medium
which is excellent in long term storage performance.
[0091] In order to allow the underlying layer to have the soft
magnetization, the composition of the crystal grain in the
underlying layer may be appropriately changed. For example, in the
case of a CoO--SiO.sub.2 film used in an embodiment described later
on, the soft magnetization can be generated by progressively
deviating the composition of the crystal grain principally composed
of CoO from the stoichiometric composition (X.noteq.0 in the case
of expression with CoO.sub.1-X), i.e., by allowing Co to exist in
the CoO film. This can be achieved, for example, by using a
sputtering gas of a mixed gas (reducing atmosphere) including Ar
gas mixed with H.sub.2 in an amount of about 1%, when the
underlying layer is formed by means of sputtering. The metal Co
atom having magnetization is generated in CoO as described above,
and thus the soft magnetization is successfully brought about for
the underlying layer. As for the soft magnetization of the
underlying layer, it is preferable that the underlying layer has a
coercive force of 0.05 (Oe) to 10 (Oe) (about 3.95 A/m to about 790
A/m) and a relative permeability of 500 to 10000, in order that the
underlying layer functions as the keeper layer.
[0092] When the keeper layer is provided for the magnetic recording
medium, problems arise concerning the recording sensitivity and the
reproducing sensitivity as described above. In the present
invention, taking notice of the fact that the underlying layer
having the soft magnetization has a relatively low Curie
temperature, the area, on which information is to be recorded, is
irradiated with a convergent light beam when the magnetic recording
medium is subjected to recording (or reproduction) to apply a
recording magnetic field to the area in a state in which the
temperature of the area is locally raised. In this case, the
coercive force is lowered in the area irradiated with the light
beam due to the increase in temperature. When the temperature
exceeds the Curie temperature, the magnetization of the area
disappears. In this state, the magnetic recording medium is
effectively equivalent to a magnetic recording medium in which the
underlying layer as the keeper layer does not exist. That is, in
the present invention, although the underlying layer exists, it is
enough to use a small recording magnetic field in order to generate
the inversion of magnetization. Also during the reproduction, a
reproducing magnetic field is applied while irradiating an
information-reproducing area with light. When such a reproducing
method (light assist reproducing method) is used, it is possible to
detect the magnetization information at a high sensitivity for the
leak magnetic field from the magnetic layer without being inhibited
by the keeper layer. When the recording or the reproduction is
completed, and the temperature of the area in which the information
is recorded or reproduced is lowered, then the coercive force is
gradually increased, and the underlying layer restores the soft
magnetization. At the room temperature, the direction of
magnetization of the underlying layer is directed in a direction
opposite to the direction of magnetization of the magnetic layer
due to the leak magnetic field generated from the boundary of the
area of inversion of magnetization of the magnetic layer, i.e., the
area having the different direction of magnetization, and the
annular magnetic path is formed through the magnetic layer and the
underlying layer. Therefore, the recording magnetization state is
stabilized.
[0093] According to a sixth aspect of the present invention, there
is provided a magnetic recording medium comprising:
[0094] a substrate;
[0095] an underlying layer which is formed on the substrate;
and
[0096] a magnetic layer which is formed on the underlying layer and
which has a magnetization-prompt direction in a direction
perpendicular to a substrate surface, wherein:
[0097] the underlying layer is composed of crystal grains
substantially formed of at least one oxide selected from the group
consisting of cobalt oxide, chromium oxide, iron oxide, nickel
oxide, and magnesium oxide, and a crystal grain boundary containing
at least one oxide selected from the group consisting of silicon
oxide, aluminum oxide, titanium oxide, tantalum oxide, and zinc
oxide.
[0098] The magnetic recording medium of this aspect is provided
with the underlying layer composed of the crystal grains
substantially formed of the specified oxide and the crystal grain
boundary containing the specified oxide for surrounding the
respective crystal grains, the underlying layer being disposed
between the substrate and the magnetic layer (recording layer) for
recording information thereon. As for the underlying layer, it is
possible to freely control the diameter and the distribution of the
crystal grains of the material for constructing the magnetic layer.
Therefore, it is possible to reduce the noise generated from the
magnetic recording medium. In general, the noise, which is
generated from the magnetic recording medium, includes a component
which is generated even after the direct current demagnetization
irrelevant to the recording density, and a component which is
increased in accordance with the increase in recording density. It
has been revealed for the perpendicular magnetic recording medium
that the noise, which is generated even after the direct current
demagnetization, is decreased by strengthening the perpendicular
magnetic anisotropy of the magnetic layer having the perpendicular
magnetization to increase the horny ratio of the magnetization
curve in the vertical direction. Accordingly, investigation has
been made for the component as the other noise component which is
increased in accordance with the increase in recording density, in
a state in which the noise, which is generated even after the
direct current demagnetization, is reduced by means of the method
as described above. As a result, it has been revealed that the
latter noise is principally generated in the area of inversion of
magnetization (boundary between adjoining recording magnetic
domains).
[0099] The noise, which is generated in the area of inversion of
magnetization, results from the fact that the crystal grains of the
material for constructing the magnetic layer are large. That is,
when the size of the crystal grain is large, then the area of
inversion of magnetization is decreased in the circumferential
direction of the disk-shaped recording medium, and the area of
inversion of magnetization has a zigzag form. Therefore, in order
to reduce the noise generated in the area of inversion of
magnetization, it is desirable that the size of the crystal grain
is small. However, if the crystal grain diameter is extremely small
to be about several nm, the crystal grains undergo the diamagnetic
field for a long period of time when the magnetic recording medium
is stored for the long period of time. As a result, the
magnetization is decreased due to the demagnetization action of
thermal fluctuation, and the reproduction output is decreased when
information is reproduced. Therefore, it is necessary that the
crystal grain diameter has an appropriate size, and it is desirable
that the distribution thereof is as small as possible as well. Even
when the size of the crystal grain is decreased, if the magnetic
interaction between the crystal grains is large, then the same
state is magnetically given as the state in which large crystal
grains exist. Therefore, in order to reduce the noise, it is
desirable that the crystal grains are magnetically isolated.
[0100] Accordingly, in the present invention, in order to realize
the state as described above, the underlying layer is provided
between the substrate and the magnetic layer. The crystal grains
are composed of at least one of cobalt oxide, chromium oxide, iron
oxide, nickel oxide, and magnesium oxide. The crystal grain
boundary is composed of at least one oxide of silicon oxide,
aluminum oxide, titanium oxide, tantalum oxide, and zinc oxide. In
the underlying layer as described above, the size of the crystal
grains composed of deposited cobalt oxide, chromium oxide, iron
oxide, or nickel oxide is constant in accordance with the film
formation condition when the underlying layer is formed as the film
on the substrate. Further, the crystal grains may be arranged in a
honeycomb configuration. That is, the respective deposited crystal
grains have the regular hexagonal shape in a plane parallel to the
substrate surface and they are pillar-shaped in a cross section
perpendicular to the substrate surface of the underlying layer. The
crystal grains, each of which forms the regular hexagonal cylinder,
may be gathered into an aggregate to form a honeycomb structure in
which the regular hexagonal cylinders are regularly arranged.
[0101] When the magnetic layer is formed on the underlying layer as
described above, the honeycomb structure, which is similar to that
of the underlying layer, is formed on the formed magnetic layer
while reflecting the structure of the underlying layer. The-crystal
grains in the magnetic layer are epitaxially grown continuously
from the top of the crystal grains in the underlying layer.
Therefore, the magnetic grains, which have the desired grain
diameter and the desired crystalline orientation, can be grown on
the underlying layer by appropriately adjusting the honeycomb
structure of the underlying layer. As described above, the
underlying layer has the action to control the grain diameter, the
grain diameter distribution, and the crystalline orientation of the
magnetic layer. Therefore, it is possible to realize the fine and
minute crystal grain diameter of the magnetic layer, and it is
possible to eliminate the dispersion of the grain diameter. It is
possible to decrease the thermal fluctuation and the noise of the
magnetic recording medium resulting from the above. Further, the
area of inversion of magnetization in the magnetic layer is
prevented from formation of the zigzag pattern. Therefore, it is
possible to reduce the noise. In order to control, for example, the
structure, the orientation, and the crystal grain diameter of the
underlying layer, for example, selection may be made appropriately
and preferably, for example, for the concentration (composition) of
the oxide for forming the crystal grains and the crystal grain
boundary substance, the material for the crystal grain boundary,
and the film formation condition.
[0102] In the present invention, it is desirable that the crystal
grains of the underlying layer are oriented in (111) orientation,
because of the following reason. That is, when the magnetic layer
is formed on the underlying layer oriented in the orientation
described above, the magnetic layer can be easily oriented in
(00.1) orientation. The magnetic layer, which is oriented in (00.1)
orientation, exhibits the perpendicular magnetization.
[0103] In the magnetic recording medium of the present invention,
it is desirable that the magnetic layer is composed of, for
example, an alloy principally containing Co and containing at least
two elements selected from Cr, Pt, Ta, Nb, Ti, and Si. It is
desirable that the alloy as described above exhibits the
ferromagnetic property. The alloy as described above has a large
leak magnetic field because the saturation magnetization is large,
making it possible to increase the obtained reproduced signal.
[0104] It is preferable that the magnetic recording medium of the
present invention is provided with a control layer in order to
reinforce the orientation of the magnetic layer, the control layer
being disposed between the underlying layer and the magnetic layer.
The control layer may be composed of Ti or an alloy principally
containing Ti. When such a control layer is provided, it is
possible to give a desired magnetization-prompt direction to the
magnetic layer. Therefore, it is possible to obtain better
characteristics. In this case, the shape, the size, and the
arrangement of the crystal grains of the control layer follow those
of the underlying layer. Therefore, the shape, the size, and the
arrangement of the crystal grains of the underlying layer are also
inherited by the magnetic layer which is formed on the control
layer.
[0105] It is preferable that the magnetic recording medium of the
present invention comprises a soft magnetic layer which is provided
between the substrate and the underlying layer. It is preferable
that the soft magnetic layer is composed of an amorphous material.
Those preferably usable for such a material include, for example,
CoNbZr, FeCoZrB, and FeCoSiB. It is desirable that the material for
constructing the soft magnetic layer has the following magnetic
characteristics. That is, the magnetic characteristics make it
possible to allow the magnetic flux to pass sufficiently and avoid
any change of the recording state of the recording layer, which
would be otherwise caused such that the external magnetic field is
amplified by the magnetic path formed by the magnetic head and the
magnetic recording medium. For example, it is desirable that the
coercive force is 5 (Oe) (about 400 A/m), and the magnetic
permeability is not less than 100 and not more than 10000.
[0106] According to an eighth aspect of the present invention,
there is provided a magnetic recording apparatus comprising the
magnetic recording medium in accordance with any one of the first
to seventh aspects of the present invention.
[0107] According to a ninth aspect of the present invention, there
is provided a method for producing a magnetic recording medium
comprising, on a substrate, a magnetic layer for recording
information thereon and a protective layer, the method
comprising:
[0108] generating plasma by means of resonance absorption;
[0109] allowing the generated plasma to collide with a target so
that target particles are sputtered; and
[0110] applying a bias voltage between the substrate and the target
to introduce and deposit the sputtered target particles on the
substrate so that at least one layer of the magnetic layer and the
protective layer is formed.
[0111] In the present invention, the particles such as electrons
are accelerated by means of the resonance absorption of the energy
of the electromagnetic wave such as a microwave, and they collide
with the gas to cause ionization of the gas. Thus, the plasma
having high energy is generated. The particles, which constitute
the plasma, have the high energy, and the energy of each of the
particles is uniform as compared with ordinary plasma generated by
electric discharge or the like. The plasma, in which the energy
distribution is narrow, is obtained. The plasma collides with the
target by the aid of the bias voltage to drive out the target
particles. During this process, the kinetic energy of the plasma to
collide with the target, and consequently the kinetic energy of the
target particles driven out by the plasma can be precisely
controlled by changing the bias voltage. The target particles, in
which the energy is controlled as described above, are directed
toward the substrate as the flow of the target particles, and they
are successfully deposited on the substrate uniformly with an
equivalent film thickness. When this method is used to produce the
magnetic recording medium, it is possible to make control and
obtain a desired value for any one the density of the thin film to
be formed, the flatness of the surface, the crystalline
orientation, the orientation of crystal growth, the crystal
structure, and the crystal grain diameter by appropriately
selecting the material and the film formation condition. Further,
when this method is used to form thin films of two or more layers,
it is possible to suppress the mutual substrate diffusion between
the thin films. For example, when the protective layer is formed on
the magnetic layer by means of this method, it is possible to avoid
any damage of magnetic characteristics of the magnetic layer, which
would be otherwise caused by the diffusion of the component of the
magnetic grain to the protective layer. Additionally, when this
technique is used, it is possible to reduce the crystalline
deficiency in the thin film. Accordingly, it is possible to form a
dense film, and it is possible to obtain crystals which is strongly
oriented in a constant orientation.
[0112] When the electrons are excited by means of the resonance
absorption, it is also possible to use an electromagnetic wave in a
region other than the region of the microwave. However, it is
preferable to use the microwave. Further, it is preferable to use
an alternating current power source having a radio frequency (RF)
or a direct current power source (DC) as a bias power source which
is used to control the kinetic energy of the plasma and the target
particles to have a constant value. For example, when a conductive
material such as carbon is subjected to ECR sputtering, it is
possible to use the DC power source. On the other hand, when a
nonconductive material such as silicon oxide is subjected to ECR
sputtering, it is possible to use the RF power source. The
selection of the use of either the DC power source or the RF power
source as the bias power source is determined depending on the
characteristics and the structure of the thin film intended to be
obtained.
[0113] In this specification, the term "resonance absorption"
refers to a phenomenon in which the particle, which performs the
periodic motion, absorbs the energy of the electromagnetic wave,
and the amplitude in the periodic motion of the particle, i.e., the
energy possessed by the particle is remarkably increased, when the
angular frequency of the particle which receives the action of the
external force and which performs the periodic motion at the
specified angular frequency is approximately coincident with the
frequency of the electromagnetic wave incoming from the
outside.
[0114] When the protective layer is formed as described above, then
the target may be carbon, and a mixed gas, which principally
contains argon and which contains at least one of nitrogen and
hydrogen, may be used as a plasma gas. When the protective film is
formed by using the method of the present invention, it is possible
to-obtain the protective film having a uniform film thickness of 1
to 5 nm. Further, it is possible to obtain the protective film
having a density of 60% of the theoretical density.
[0115] According to a tenth aspect of the present invention, there
is provided a method for producing a magnetic recording medium
comprising, on a substrate, an underlying layer and a magnetic
layer for recording information thereon, the method comprising:
[0116] generating plasma by means of resonance absorption;
[0117] allowing the generated plasma to collide with a target so
that target particles are sputtered; and
[0118] applying a bias voltage between the substrate and the target
to introduce and deposit the sputtered target particles on the
substrate so that the underlying layer is formed.
[0119] In the production method according to the tenth aspect of
the present invention, the underlying layer is formed by using the
so-called ECR sputtering method prior to the formation of the
magnetic layer. When the magnetic layer is formed on the underlying
layer, it is possible to more precisely control at least one of the
parameters of the crystal structure of the magnetic layer, the
crystalline orientation, the crystal grain diameter, the grain
diameter distribution, and the density and the surface flatness of
the formed film. Accordingly, it is possible to produce the
magnetic recording medium which makes it possible to perform the
super high density recording.
[0120] The underlying layer is composed of an amorphous portion and
a crystalline portion comprising crystal grains having a uniform
grain diameter, by forming the film by using the ECR sputtering
method described above, making it possible to provide a structure
in which the crystal grains are separated from each other by the
amorphous portion (crystal grain boundary) having a uniform width.
Further, when the ECR sputtering method described above is used,
the crystal grains of the underlying layer can be oriented in a
constant crystalline orientation. As for the structure of the
underlying layer, when the ECR sputtering method described above is
used, it is possible to control, for example, the crystal grain
diameter, the width of the grain boundary, and the crystalline
orientation by changing the material and the film formation
condition. It is possible to form a desired structure. For example,
in an embodiment described later on, an underlying layer has been
successfully formed, in which hexagonal crystal grains are
regularly arranged in a honeycomb configuration with a crystal
grain boundary intervening therebetween. The number of crystal
grains deposited around one crystal grain is 5.9 to 6.1. The
underlying layer, which has an extremely regular honeycomb
structure, has been successfully formed. The underlying layer as
described above can be formed such that the crystal grains are
formed with cobalt oxide, chromium oxide, magnesium oxide, iron
oxide, nickel oxide, or a combination of these oxides, and the
crystal grain boundary is formed with silicon oxide, aluminum
oxide, titanium oxide, tantalum oxide, zinc oxide, or a combination
of these oxides. It is preferable that such an underlying layer has
a film thickness of 2 to 50 nm.
[0121] When the underlying layer is formed, the target particles
may be sputtered in a reactive atmosphere containing oxygen
(reactive ECR sputtering method). The velocity of film formation
can be improved to be 2-fold to 3-fold as compared with a case in
which the atmosphere is not the reactive atmosphere, for example,
by using a mixed gas containing oxygen in argon as a sputtering
gas.
[0122] A thin film, which has the bcc structure or the B2
structure, may be used other than the underlying layer constructed
with the compound described above. Especially, it is possible for
the thin film to use an inorganic compound such as magnesium oxide,
chromium, nickel, chromium alloy, or nickel alloy. It is preferable
that such an alloy contains solid solution of chromium, titanium,
tantalum, vanadium, ruthenium, tungsten, molybdenum, vanadium,
niobium, nickel, zirconium, aluminum, or a combination of these
element, other than chromium or nickel as the base element.
Further, it is preferable that the underlying layer is oriented in
a certain orientation. It is preferable that such a thin film has a
film thickness of 2 to 10 nm.
[0123] When the magnetic layer is formed, it is preferable that the
magnetic layer is epitaxially grown on the underlying layer with
the structure controlled as described above. As a result, the
obtained magnetic layer reflects the structure of the underlying
layer, and it has such a structure that the magnetic grains grown
on the crystal grains of the underlying layer are uniformly
separated by the non-magnetic portion grown on the crystal grain
boundary of the underlying layer. Accordingly, it is possible to
reduce the magnetic interaction between the magnetic grains, and it
is possible to decrease the unit of inversion of magnetization.
Further, the grain diameter of the magnetic grain is equal to the
crystal grain diameter of the underlying layer, making it possible
to miniaturize the magnetic grain diameter and reduce the
dispersion of the grain diameter. Therefore, it is possible to
obtain the magnetic recording medium in which the thermal
fluctuation and the thermal demagnetization are small. In an
embodiment described later on, the standard deviation (.sigma.) in
the magnetic grain diameter distribution is not more than 8% of the
average grain diameter, and thus the dispersion of the grain
diameter has been successfully decreased. Further, the crystalline
orientation can be made in the constant orientation by epitaxially
growing the magnetic grains on the crystal grains of the underlying
layer. In an embodiment described later on, the crystalline
orientation of (11.0), which is preferable for the high density
recording, has been successfully obtained for Co in the magnetic
layer. Further, when the magnetic layer is formed as the film by
means of the sputtering method based on the use of the resonance
absorption described above, then it is possible to more precisely
control the structure and the orientation of the magnetic layer,
and the epitaxial growth of the magnetic layer from the underlying
layer is facilitated. Therefore, the obtained film is preferred for
the high density recording as compared with a conventional film
formed by the DC sputtering method or the magnetron sputtering
method.
[0124] The material for the magnetic layer is an alloy principally
containing cobalt. It is preferable to contain cobalt as well as
chromium, platinum, tantalum, niobium, titanium, silicon, boron,
phosphorus, palladium, vanadium, terbium, gadolinium, samarium,
neodymium, dysprosium, holmium, europium, or a combination of these
elements. When chromium, tantalum, niobium, titanium, silicon,
boron, phosphorus, or a combination of them is contained in the
magnetic layer, such an element is deposited (segregated) at the
grain boundary or in the vicinity of the crystal grain boundary of
the crystal grains of cobalt. It is also possible to decrease the
magnetic interaction between the magnetic grains by means of the
segregation.
[0125] Other than the above, it is also possible for the magnetic
layer to use a magnetic film having the granular structure in which
crystal grains of metal exist while being surrounded by an
amorphous phase. It is preferable that the crystal grains are
composed of cobalt or alloy principally containing cobalt,
containing neodymium, praseodymium, yttrium, lanthanum, samarium,
gadolinium, terbium, dysprosium, holmium, platinum, palladium, or a
combination of these elements. It is preferable that the amorphous
portion, which exists to surround the metal crystal grains, is
composed of silicon oxide, aluminum oxide, titanium oxide, zinc
oxide, silicon nitride, or a combination of these compounds. In the
case of the magnetic film having the granular structure, it is
possible to reduce the magnetic interaction between the magnetic
grains owing to the presence of the amorphous portion, in the same
manner as in the segregation described above. Alternatively, the
magnetic layer may be an artificial lattice multilayered film
obtained by alternately stacking Co and Pt. Such a film may be
formed by co-sputtering targets of Co and Pt.
[0126] A plurality of underlying layers may be provided, if
necessary. For example, when the difference in lattice constant is
large between the underlying layer and the magnetic layer, it is
possible to facilitate the epitaxial growth of the magnetic layer
in a well-suited manner by inserting therebetween a layer (control
layer) having an intermediate lattice constant between those of the
two layers. In this case, in addition to the underlying layer
described above, the magnetic layer may be formed with a magnesium
oxide layer or an alloy layer principally containing chromium or
nickel intervening therebetween.
[0127] Further, in order to isolate the magnetic layer from the
atmospheric air and protect the magnetic layer from any shock
received from the magnetic head, it is possible to form a
protective layer on the magnetic layer (on the side to make contact
with the magnetic head) by using the ECR sputtering method
described above. An upper limit exists for the film thickness of
the protective layer because of the high density recording as
described above. It is desirable that the film thickness of the
protective layer is not more than 5 nm. When the ECR sputtering
method is used, it is possible to control the kinetic energy of the
target particles. Therefore, even when the film thickness is thin,
it is possible to form the protective layer which is dense and
which coats the magnetic layer with the uniform thickness. In order
to stably form the protective layer by means of this method, it is
enough to provide a film thickness of not less than 1 nm which is
extremely thin as compared with the conventional film thickness of
10 nm. When the ECR sputtering method is used for a carbon film of
the protective layer, the density of the carbon film is not less
than 60% of the theoretical density (density obtained in a state in
which carbon atoms are accumulated without any gap). It is possible
to form a denser film as compared with the conventional film of 40
to 50%. Further, the hardness of the film is not less than two-fold
as compared with a film formed by means of the conventional
sputtering method (for example, the RF magnetron method).
[0128] It is preferable that the protective layer is composed of a
carbon thin film. The protective layer may be formed in an electric
discharge gas atmosphere principally containing argon. It is
preferable that the protective layer is formed in a mixed gas
atmosphere containing argon as well as at least one gas selected
from nitrogen and hydrogen. When the film is formed by using the
mixed gas, nitrogen and hydrogen are contained in the obtained thin
film. Accordingly, it is possible to facilitate the dense feature
of the carbon thin film as the protective layer.
[0129] As indicated by a result of measurement performed with an
interatomic force electron microscope (AFM) in an embodiment as
described later on, when the underlying layer is formed by means of
the ECR sputtering method, it is possible to realize the flat
feature of the film surface without being affected by scratches and
rough irregularities on the substrate surface. On the other hand, a
minute and regular concave/convex pattern, which results from the
slight difference in growth speed between those grown on the
crystal grain boundary and those grown on the crystal grains of the
underlying layer, appears on the surface of the magnetic layer by
epitaxially growing the magnetic layer on the underlying layer.
Further, when the protective layer is formed on the magnetic layer
by means of the ECR sputtering method, the protective layer covers
the surface of the magnetic layer with the uniform film thickness.
Therefore, the surface of the protective layer has a shape which
reflects the regular concave/convex pattern of the magnetic layer.
The minute concave/convex pattern is useful as a texture for
allowing the magnetic head to stably fly over the magnetic
recording medium.
[0130] According to the present invention, it is possible to
produce the magnetic recording medium capable of performing the
high density recording in which the surface recording density
exceeds 40 Gbits/inch.sup.2.
BRIEF DESCRIPTION OF THE DRAWINGS
[0131] FIG. 1 shows a sectional view conceptually illustrating a
structure of a magnetic disk produced in the first embodiment of
the present invention.
[0132] FIG. 2 shows an X-ray diffraction profile illustrating a
crystal structure of an underlying layer of the magnetic disk
produced in the first embodiment.
[0133] FIG. 3 shows a plan view conceptually illustrating the
structure of the underlying layer of the magnetic disk produced in
the first embodiment of the present invention.
[0134] FIG. 4 conceptually illustrates the fractal feature of the
underlying layer of the magnetic disk produced in the first
embodiment of the present invention.
[0135] FIG. 5 conceptually illustrates a structure of an ECR
sputtering apparatus used in the embodiment of the present
invention.
[0136] FIG. 6 shows a sectional view conceptually illustrating a
structure of a magnetic disk produced in the second embodiment of
the present invention.
[0137] FIG. 7 shows an X-ray diffraction profile illustrating a
crystal structure of a magnetic layer of the magnetic disk produced
in the second embodiment.
[0138] FIG. 8 conceptually illustrates a surface structure of an
underlying layer of a magnetic disk produced in the third
embodiment.
[0139] FIG. 9 shows an X-ray diffraction profile illustrating a
crystal structure of a magnetic layer of the magnetic disk produced
in the third embodiment.
[0140] FIG. 10 shows a sectional view conceptually illustrating a
structure of a magnetic disk produced in the fifth embodiment.
[0141] FIG. 11 shows a top view illustrating a magnetic recording
apparatus produced in the embodiment of the present invention.
[0142] FIG. 12 shows a sectional view illustrating the magnetic
recording apparatus taken along a broken line A-A' shown in FIG.
11.
[0143] FIG. 13 shows a sectional view conceptually illustrating a
structure of a magnetic disk produced in the sixth embodiment.
[0144] FIG. 14 shows a plan view conceptually illustrating a
structure of an underlying layer of the magnetic disk produced in
the sixth embodiment.
[0145] FIG. 15 shows an X-ray diffraction profile illustrating a
crystal structure of the underlying layer of the magnetic disk
produced in the sixth embodiment.
[0146] FIG. 16 shows an X-ray diffraction profile illustrating a
crystal structure of a magnetic layer of the magnetic disk produced
in the sixth embodiment.
[0147] FIG. 17 shows a sectional view conceptually illustrating a
structure of a magnetic disk produced in the seventh
embodiment.
[0148] FIG. 18 shows an X-ray diffraction profile of a magnetic
film of a magnetic disk concerning Reference Example provided with
no control film.
BEST MODE FOR CARRYING OUT THE INVENTION
[0149] Specified embodiments of the magnetic recording medium of
the present invention and the method for producing the same will be
described in detail below with reference to the drawings.
Description of ECR Sputtering Apparatus
[0150] At first, explanation will be made for an ECR (Electron
Cyclotron Resonance) sputtering apparatus used to produce the
magnetic recording medium of the present invention. FIG. 5
conceptually shows the ECR sputtering apparatus 80. The ECR
sputtering apparatus 80 principally includes a first chamber 81 in
which plasma is generated, an annular target 70 which is connected
to an upper portion of the first chamber 81, and a second chamber
83 which is connected at a position over the target 70. The first
chamber 81 is a cylindrical tube made of quartz. A pair of coils
64, 66 are provided in a circumscribing manner respectively at
upper and lower positions in the axial direction. A microwave
generator 74 is connected via an introducing tube to the first
chamber 81. The introducing tube is connected to a portion between
the coils 64, 66 of the first chamber 81. The second chamber 83 is
a vacuum chamber made of metal. A substrate 68, on which particles
driven out from the target 70 are deposited, is installed at the
top of the second chamber 83. Further, a coil 62, which is used to
converge (suppress divergence of) the plasma induced by an applied
bias voltage, is provided on the second chamber 83. The target 70
and the substrate 68 installed in the second chamber 83 are
connected to a power source 90 so that the bias voltage is
successfully applied.
[0151] The interior of the first chamber 81, the inside of the
target 70, and the interior of the second chamber are communicated
with each other, and they are closed from the outside. When the
apparatus is operated, then an unillustrated vacuum pump is used to
reduce the pressure of the space shared by the interior of the
first chamber 81, the inside of the target 70, and the interior of
the second chamber 83, and a gas (for example, Ar) is introduced
into the first chamber 81 via an unillustrated gas supply port.
Subsequently, a constant magnetic field is applied to the inside of
the apparatus by using the coils 64, 66. Free electrons, which
exist at the inside of the apparatus, make the cyclotron motion in
the clockwise direction about the magnetic field axis in accordance
with the magnetic field. The angular frequency of the cyclotron
motion of the electron is, for example, about 10.sup.9 Hz, when the
electron density is about 10.sup.10 cm.sup.-3, giving the angular
frequency in the microwave region. When the generated microwave is
introduced from the microwave generator 74 into the magnetic field,
the microwave is resonant with the cyclotron motion of the electron
to cause the resonance absorption in which the energy of the
microwave is absorbed by the electron. As a result of the resonance
absorption, the electron acquires the high energy, and it is
accelerated. The electron collides with the gas to cause the
ionization of the gas. Thus, the ECR plasma 76 having the high
energy is generated in the first chamber 81. Since the energy at a
constant level is given to the electron by means of the resonance
absorption, the energy state of the electron is also at a constant
high energy level. The plasma is generated by allowing such
electrons to collide with the gas. Therefore, the particles, which
constitute the plasma, have high energy. Further, the energy is
uniform for the respective particles as compared with the ordinary
plasma which is generated by electric discharge or the like. Thus,
it is possible to obtain the plasma having a narrow energy
distribution.
[0152] The bias voltage is applied between the substrate 68 and the
annular target 70 disposed over the position at which the plasma is
generated. Therefore, the generated plasma is introduced toward the
target 70. The plasma collides with the target 70 to drive out the
target particles. When the bias voltage is changed during this
process, it is possible to precisely control the kinetic energy of
the plasma to collide with the target 70, and consequently the
kinetic energy of the target particles driven out by the plasma.
The target particles, which has the energy that is controlled as
described above, are directed as the flow 72 of the target
particles toward the substrate 68. The target particles are
deposited on the substrate 68 while giving a uniform and equivalent
film thickness.
[0153] When the ECR sputtering method as described above is used to
produce the magnetic recording medium, it is possible to make
control and obtain a desired value for any one of the density of
the thin film to be formed, the surface flatness,the crystalline
orientation, the orientation of crystal growth, the crystal
structure, and the crystal grain diameter, by appropriately
selecting the material and the film formation condition. When the
ECR sputtering method is used, it is possible to suppress the
mutual substrate diffusion between thin films when the thin films
of two or more layers are formed. For example, when a protective
layer is formed on a magnetic layer by means of the ECR sputtering
method as described later on, it is possible to avoid any damage of
magnetic characteristics of the magnetic layer, which would be
otherwise caused by the diffusion of the components of magnetic
grains to the protective layer. Additionally, when the ECR
sputtering method is used, it is possible to reduce any crystal
deficiency in the formed thin film. Therefore, it is possible to
form the dense film, and it is possible to obtain the crystals
which are strongly orientated in a constant direction.
[0154] An electromagnetic wave, which is in a region other than
that of the microwave, can be also used to excite the electron by
means of the resonance absorption. However, it is preferable to use
the microwave. Further, it is preferable to use an alternating
current power source having a radio frequency (RF) or a direct
current power source (DC) as the bias power source for controlling
the kinetic energy of each of the plasma and the target particles
to have a constant value. For example, when a conductive material
such as carbon is subjected to the ECR sputtering, it is possible
to use the DC power source. On the other hand, when a
non-conductive material such as silicon oxide is subjected to the
ECR sputtering, it is possible to use the RF power source.
First Embodiment
[0155] In this embodiment, a magnetic disk 10 comprising an
inorganic compound thin film 2, a magnetic film 3, and a protective
film 4 on a disk substrate 1 as shown in FIG. 1 was produced as
follows.
[0156] Formation of Inorganic Compound Thin Film
[0157] A glass substrate having a diameter of 2.5 inches (6.25 cm)
was prepared as the substrate 1. A CoO--SiO.sub.2 film was formed
as the inorganic compound thin film 2 on the substrate 1 by means
of the ECR sputtering apparatus described above. A CoO--SiO.sub.2
mixture (mixing ratio of CoO:SiO.sub.2=3:1) was used for a
sputtering target (target 80 in FIG. 5), and Ar was used for a
sputtering gas. The gas pressure of Ar was 0.3 mTorr. The
introduced electric power of the microwave was 0.7 kW. In order to
introduce the plasma excited by the microwave in a direction toward
the target, an RF bias voltage of 500 W was applied to the
target.
[0158] Structural Analysis for Inorganic Compound Thin Film
[0159] The crystal structure of the inorganic compound thin film 2
formed by the ECR sputtering method was investigated by means of
the X-ray diffraction method. An obtained result is shown in a
chart shown in FIG. 2. As shown in FIG. 2, a peak appeared in the
vicinity of 2.theta.=62.5.degree.. This peak was identified to be
(220) of CoO.
[0160] Further, the planar structure of the inorganic compound thin
film 2 was observed with a transmission electron microscope (TEM).
FIG. 3 shows a schematic illustration of the observed structure of
the thin film. As shown in FIG. 3, the thin film 2 is an aggregate
of regular hexagonal grains 12. The grains were regularly arranged
two-dimensionally with a grain boundary 14 intervening
therebetween. That is, as viewed in a plan view, the thin film 2
exhibited the honeycomb structure in which the grains 12 were
arranged in a honeycomb configuration. When the sectional structure
of the thin film was observed, it was revealed that the regular
hexagonal grains 12 had grown in a pillar-shaped configuration in
the direction perpendicular to the substrate surface. Further, it
was revealed that the size of the grain 12 (crystal grain diameter)
and the width of the grain boundary 14 (distance between the
crystal grains) in the honeycomb structure were successfully
regulated by controlling the film formation condition and the
composition. In the case of the observed thin film 2, the crystal
grain diameter was about 10 nm, and the distance between the
crystal grains was 1.5 nm.
[0161] The lattice image of the thin film 2 was observed. As a
result, it was revealed that the grains 12 were crystalline, and
the grain boundary 14 was amorphous. When the lattice constant of
the grains 12 was determined, it was approximately equal to a value
of cobalt. Further, the crystal grains 12 and the grain boundary 14
were analyzed by means of the energy dispersion type X-ray analysis
(.mu.-EDX) for the extremely minute area of the thin film 2. As a
result, it was revealed that the crystal grains 12 were composed of
CoO, and the substance existing in the grain boundary 14 was
SiO.sub.2.
[0162] Next, the previous TEM observation result for the thin film
surface of the underlying base film was used to analyze the crystal
grain diameter distribution and the number of crystal grains
existing around one crystal grain (hereinafter referred to as
"number of coordinated grains"). At first, as for the crystal grain
diameter, the investigation was made for the grains existing in a
randomly selected square having one side of 200 nm. As a result,
the average crystal grain diameter was 10 nm. The grain diameter
distribution was a normal distribution. The standard deviation was
determined to be 0.5 nm. As for the number of coordinated grains,
the investigation was made for randomly selected 500 individuals of
crystal grains. As a result, the number of coordinated grains was
6.01 in average. This fact indicates that the grain diameter of the
crystal grain is scarcely dispersed, and the regular hexagons of
crystal grains are arranged extremely regularly in the honeycomb
configuration in the plane parallel to the substrate surface.
[0163] The number of coordinated grains is varied depending on the
spacing distance between the crystal grains. In this viewpoint, it
has been revealed that SiO.sub.2 plays an important role to allow
the structure to have the regularity, and the concentration of
SiO.sub.2 determines the spacing distance between the formed
crystal grains. A desired value can be selected easily and
arbitrarily for the spacing distance between the crystal grains by
changing the composition of the target (for example, the ratio
between Co and Si or the ratio between CoO and SiO.sub.2). For
example, when the SiO.sub.2 concentration in the CoO--SiO.sub.2
film is increased, the distance between the crystal grains is
lengthened. However, if SiO.sub.2 exists in a large amount, the
deposition and the growth of CoO are suppressed. Therefore, it is
desirable that the distance does not exceed 2 nm at the maximum.
The distance between the crystal grains can be changed by
optimizing the film formation process, for example, by raising the
substrate temperature. On the other hand, when the SiO.sub.2
concentration is lowered, the distance between the crystal grains
is narrowed (crystal grains approach to one another).
Simultaneously, any disorder has been observed for the grain shape.
In this situation, the number of coordinated grains was about seven
in which the number of grains was large in some cases. On the
contrary, the number of coordinated grains was four to five in
which the number of existing grains was small in other cases. In
general, the dispersion was large. Further, it was observed that
the two-dimensional arrangement was disordered, and the honeycomb
structure was collapsed. An appropriate range of the spacing
distance between the crystal grains was 0.5 to 2 nm. In this
embodiment, the SiO.sub.2 concentration was controlled so as to be
included within this range.
[0164] For the purpose of comparison, a CoO--SiO.sub.2 film was
formed in accordance with the RF magnetron sputtering method in
place of the ECR sputtering method. The structure of the
CoO--SiO.sub.2 film formed by means of the magnetron sputtering
method was analyzed by using an observation image based on TEM in
the same manner as for the film formed by means of the ECR
sputtering method. As a result, the average grain diameter was 10
nm, and the grain diameter distribution was a normal distribution.
However, the standard deviation (.sigma.) was 1.2 nm, and the
dispersion of the grain diameter was large as compared with the
value of 0.7 nm of the film formed by means of the ECR sputtering
method. The number of coordinated grains was investigated for 500
individuals of crystal grains. As a result, the number was 6.30 in
average. It was revealed that the regularity was lowered as
compared with the value of 6.01 individuals obtained for the
CoO--SiO.sub.2 film formed by means of the ECR sputtering method.
According to the comparative experiment, it has been revealed that
the regularity of the thin film 2 can be greatly improved when the
inorganic compound thin film 2 is formed by using the ECR
sputtering method.
Analysis of Fractal Feature
[0165] Taking notice of the arrangement of the inorganic compound
thin film 2, the following feature is appreciated. That is, the
inorganic compound thin film 2 is composed of an aggregate of
regular hexagonal crystal grains 12. Geometrically, a self-similar
figure is constructed as shown in FIG. 4. As shown in the left half
of FIG. 4, one individual of regular hexagonal crystal grain is
surrounded by six individuals of the same regular hexagonal crystal
grains. Seven individuals of crystal grains form one unit. Further,
as shown in the right half drawing, the unit, which is composed of
seven crystal grains, is surrounded by six individuals of the same
units to form a group composed of seven individuals of units. This
form will be overviewed from a fractal viewpoint. When the phase
dimension dim.sub.T and the Hausdrorff dimension (dimensional
function of real number) dim.sub.H are used for the fractal
feature, the relationship of numerical values is represented by
dim.sub.T<dim.sub.H. In this case, dim.sub.T is 2 because it
resides in the two-dimensional plane. As for dim.sub.H, there is
given dim.sub.H.ltoreq.n (in this case, n=2). It is understood that
dim.sub.T<dim.sub.H is given. In this manner, the aggregate of
the crystal grains 12 has the fractal feature in the inorganic
compound thin film 2. The width of the grain boundary 14
(amorphous) existing to surround the crystal grains 12, i.e., the
distance (spacing distance) between the crystal grains is 1.5 nm
which is constant. Therefore, the structure of the inorganic
compound thin film 2 and the fractal feature of the organization
are not lost. If the distance between the crystal grains is less
than 0.5 nm, or if the distance exceeds 2 nm, then the structure of
the inorganic compound thin film 2 and the fractal feature of the
organization may be lost. Therefore, it is preferable that the
width of the crystal grain boundary 12 is controlled to have a
constant value within a range of 0.5 to 2 nm as described above. It
is believed that the fractal feature has been obtained because the
thin film 2 has been formed by means of the ECR sputtering method
as described above.
Formation of Magnetic Film
[0166] Subsequently, a Co.sub.69Cr.sub.19Pt.sub.12 film as the
magnetic film 3 was formed to have a film thickness of 15 nm on the
inorganic compound thin film 2 by using the DC sputtering method.
During the DC sputtering, the substrate 1 was heated to 300.degree.
C. A Co--Cr--Pt alloy was used for the sputtering target, and pure
Ar was used for the sputtering gas. The gas pressure during the
sputtering was 0.3 mTorr. The introduced DC electric power was 1
kW/150 mm.phi..
Structure and Magnetic Characteristics of Magnetic Film
[0167] Subsequently, the structure of the obtained magnetic film 3
was investigated by means of TEM observation. As a result, the
magnetic film 3 had a honeycomb structure reflecting the structure
and the shape of the inorganic compound thin film 2. The
investigation was made for 250 individuals of crystal grains by
means of the observation for the plane based on an electron
microscope. As a result, the average grain diameter of grains was
10 nm. When the grain diameter distribution was determined, it was
determined as represented by a standard deviation a of not more
than 1 nm. The grain size distribution had a value close to the
limit of the resolution of TEM. As described above, it was revealed
that the grains of the magnetic film were finely minute, the size
distribution was small, and thus these features were the same as
those of the inorganic compound thin film 2. The grain
configuration of the magnetic film 3 also had the honeycomb
structure in which the grains were regularly arranged
two-dimensionally in the same manner as in the inorganic compound
thin film. The form of the magnetic film also had the fractal
feature. This fact indicates that the magnetic crystal grains each
having a hexagonal shape with a uniform size are regularly arranged
two-dimensionally (in the honeycomb structure) continuously from
the inorganic compound thin film 2. The cross section was observed
by means of TEM. As a result, the connection of lattice was found
between the inorganic compound thin film 2 and the magnetic film 3.
It was revealed that the magnetic film 3 was epitaxially grown from
the inorganic compound thin film 2. It was also revealed that the
growth mechanism of the magnetic film differed between the
crystalline phase and the grain boundary phase, giving different
metallic organizations. Especially, a good pillar-shaped
organization was grown from the crystal grains of the inorganic
compound, but no clear organization was observed from the grain
boundary phase. It is known that such an organization exhibits
non-magnetic behavior. According to the X-ray diffraction, a peak
was observed in the vicinity of 2.theta.=62.5.degree.. Considering
this result in view of the result of the TEM observation in
combination, it was revealed that (102) of Co was strongly oriented
for this peak.
[0168] Subsequently, magnetic characteristics of the magnetic film
3 were measured. The obtained magnetic characteristics were as
follows. That is, the coercive force was 3.5 kOe, Isv was
2.5.times.10.sup.-6 emu, S as the index for the squareness or the
angular property of hysteresis in M-H loop was 0.8, and Swas 0.86.
Accordingly, magnetic film 3 had the good the magnetic
characteristics. The reason why the index to indicate the angular
property is large (approximate to the angular or square form) is
that the interaction between the magnetic crystal grains is reduced
because of the different growth mechanism of the magnetic film
while reflecting the crystal grain boundary layer of the inorganic
compound thin film. The obtained magnetic film was observed with a
scanning magnetic force microscope (MFM). As a result, it was
revealed that the magnetic characteristics were different between
the grain boundary portion and the crystal grain portion of the
magnetic film. In the crystal grain boundary portion, the coercive
force and the magnetic anisotropy were suddenly decreased.
Formation of Protective Film
[0169] Subsequently, a C film was formed as the protective film 4
to have a film thickness of 5 nm on the magnetic film 3 by means of
the DC sputtering. The sputtering condition was as follows. That
is, the introduced DC electric power density was 1 kW/150 mm.phi.,
and the electric discharge gas pressure was 5 mTorr.
Production and Evaluation of Magnetic Disk Apparatus
[0170] Subsequently, a lubricant was applied to the surface of the
obtained magnetic disk to complete the magnetic disk. A plurality
of magnetic disks were produced in accordance with the same
process, and they were incorporated into a magnetic recording
apparatus. The schematic arrangement of the magnetic recording
apparatus is shown in FIGS. 11 and 12. FIG. 11 shows a top view of
the magnetic recording apparatus 60. FIG. 12 shows a sectional view
illustrating the magnetic recording apparatus 60 taken along a
broken line A-A' shown in FIG. 11. A thin film magnetic head, which
was based on the use of a soft magnetic film having a high
saturated magnetic flux density of 2.1 T, was used as a recording
magnetic head. The gap length of the magnetic head was 0.12 .mu.m.
A magnetic head having the giant magneto-resistive effect (dual
spin valve type magnetic head) was used for reproduction. The
recording magnetic head and the reproducing magnetic head were
integrated into one unit, and they are shown as a magnetic head 53
in FIGS. 11 and 12. The. integrated type magnetic head 53 is
controlled by a driving system 54 for the magnetic head. The
plurality of magnetic disks 10 are coaxially rotated by means of a
spindle 52 of a rotary driving system 51. The distance between the
magnetic head surface and the magnetic disk 10 was kept to be 20
nm. A signal corresponding to 40 Gbits/inch.sup.2 was recorded on
the disk to evaluate S/N of the disk. As a result, a reproduction
output of 32 dB was obtained.
[0171] In this case, a magnetic force microscope (MFM) was used to
measure the unit of inversion of magnetization during recording of
information. In any of the magnetic disks of the embodiment, two to
three individuals of magnetic grains were subjected to inversion of
magnetization at once with respect to a recording magnetic field
applied when 1 bit of data was recorded. This value is sufficiently
small as compared with a conventional unit of inversion of
magnetization of five to ten individuals. Accordingly, the portion
(zigzag pattern), which corresponded to the boundary between
adjoining units of inversion of magnetization, was remarkably small
as compared with the conventional magnetic disk. This result
indicates the fact that the boundary line of the area of inversion
of magnetization was smoothened, because the magnetic grains were
fine and minute, and the unit of inversion of magnetization was
small as well. Neither thermal fluctuation nor demagnetization due
to heat was caused. The error rate of the magnetic disk was
measured. As a result, the value was not more than
1.times.10.sup.-5 when the signal processing was not performed.
[0172] In this embodiment, CoCrPt was used for the magnetic film.
Alternatively, CoCrPtTa may be used. Further alternatively, an
intermediate layer composed of CrTi or the like may be provided
between the inorganic compound thin film and the magnetic film to
regulate the difference in lattice constant between the inorganic
compound thin film and the magnetic film.
Second Embodiment
[0173] This embodiment is illustrative of a case of production of a
magnetic disk including a lattice constant control film 40 provided
between a magnetic film 3 and an inorganic compound thin film 2
formed on a substrate 1 as shown in FIG. 6.
[0174] The inorganic compound thin film 2 was formed on the glass
substrate 1 by using the ECR sputtering method in the same manner
as in the first embodiment. However, a mixture of CoO and ZnO in a
ratio of 3:1 was used as the sputtering target. The film thickness
of the inorganic compound thin film 2 was 30 nm considering, for
example, the generation of the internal stress of the entire
magnetic disk and the peeling off of the thin film 2 from the
substrate 2.
[0175] The surface of the inorganic compound thin film 2 was
observed by means of TEM. As a result, the structure was equivalent
to the honeycomb structure obtained in the first embodiment.
Regular hexagonal crystal grains 12 of CoO, which were uniform in
size, were arranged in a honeycomb configuration with a grain
boundary 14 of ZnO as shown in FIG. 3. The crystal grain diameter
of the regular hexagonal crystal grain was about 10 nm.
Subsequently, the number of crystal grains existing around one
crystal grain was determined. The investigation was made for 250
individuals of crystal grains. As a result, the number was 6.01
which was appropriately coincide with the value obtained for the
inorganic compound thin film of the embodiment.
[0176] Subsequently, a thin film of Cr.sub.85Ti.sub.15 alloy was
formed as the lattice constant control film 40 by means of the DC
sputtering to have a film thickness of 50 nm on the inorganic
compound thin film 2. A Cr--Ti alloy was used for the target, and
pure Ar was used for the sputtering gas. The sputtering gas
pressure was 3 mTorr. The introduced DC electric power was 1 kW/150
mm.phi..
[0177] Further, a film of Co.sub.69Cr.sub.19Pt.sub.12 was formed as
the magnetic film 3 by means of the DC sputtering to have a film
thickness of 12 nm on the Cr.sub.85Ti.sub.15 alloy thin film. A
Co--Cr--Pt alloy was used for the target, and pure Ar was used for
the sputtering gas. The pressure during the sputtering was 3 mTorr.
The introduced DC electric power was 1 kW/150 mm.phi..
[0178] Finally, a film of C was formed as the protective film 4 by
means of the DC sputtering to have a film thickness of 5 nm. As for
the sputtering condition, the introduced DC electric power density
was 1 kw/150 mm.phi., and the sputtering gas pressure was 5
mTorr.
[0179] In this magnetic disk, the difference between the lattice
constant of the magnetic film 3 and the lattice constant of the
inorganic compound thin film 2 was not less than 20%. However,
owing to the control film 40 allowed to intervene therebetween, the
difference in lattice constant between the inorganic compound thin
film 2 and the control film 40 and the difference in lattice
constant between the magnetic film 3 and the control film 40 were
successfully less than 10% respectively.
[0180] The lattice constant of the control film 40 can be changed
at an arbitrary ratio starting from the lattice constant of Cr by
controlling the Ti concentration of the Cr--Ti alloy material for
forming the lattice constant control film 40. Therefore, the
epitaxial growth can be achieved from the inorganic compound thin
film 2 to the magnetic film 3 with the control film 40 intervening
therebetween by selecting the composition of the control film so
that the lattice constant of the control film 40 is between the
lattice constants of the inorganic compound thin film 2 and the
magnetic film 3, especially the difference in lattice constant
between the inorganic compound thin film 2 and the control film 40
and the difference in lattice constant between the magnetic film 3
and the control film 40 are less than 10% respectively.
[0181] Subsequently, the structure of the magnetic film 3 was
investigated in accordance with the X-ray diffraction method. An
obtained result is shown in FIG. 7. As shown in FIG. 7, it is
understood that (11.0) of Co is strongly oriented. According to the
observation for the surface of the magnetic disk, the average grain
diameter of the magnetic film 3 was 10 nm, which was approximately
the same value as that of the crystal grain diameter of the
inorganic compound thin film 3. When the grain diameter
distribution of the grains was determined, it was expressed by
.sigma. of not more than 0.8 nm. As described above, it is
appreciated that the crystal grains of the magnetic film are
extremely fine, and the grain diameter distribution is small.
According to the observation for the cross section, it was revealed
that the inorganic compound thin film and the magnetic film were
epitaxially grown. It was also revealed that the inorganic compound
thin film and the magnetic film were organized to have an
appropriate pillar-shaped structure, and the size of the crystal
grain was not varied from the surface of the substrate to the
surface of the magnetic film.
[0182] Subsequently, the magnetic characteristics of the magnetic
film 3 were measured. The following magnetic characteristics were
obtained. That is, the coercive force was 3.0 kOe, Isv was
2.5.times.10.sup.-16 emu, S as the index for the angular property
of hysteresis in M-H loop was 0.81, and Swas 0.85. Therefore, the
magnetic film 3 had the good magnetic characteristics.
[0183] The lattice constant control film 40 formed in this
embodiment was introduced between the magnetic film 3 and the
inorganic compound thin film 2 in order to adjust the discrepancy
in lattice constant between the magnetic film 3 and the inorganic
compound thin film 2. However, it has been revealed that the
lattice constant control film 40 also has a function to prevent the
magnetic film from oxidation which would be otherwise caused by the
influence of the underlying layer composed of oxide.
[0184] The magnetic recording apparatus shown in FIGS. 11 and 12
was constructed in the same manner as in the first embodiment by
using the magnetic disk obtained as described above. A signal
corresponding to 40 Gbits/inch.sup.2 was recorded on the disk to
evaluate S/N of the disk. As a result, a reproduction output of 32
dB was obtained. The unit of inversion of magnetization during the
recording of information, which was measured with a magnetic force
microscope (MFM), included 2 to 3 individuals of magnetic grains.
When the error rate of the magnetic disk was measured, it was not
more than 1.times.10.sup.-5 as a value obtained when no signal
processing was performed.
[0185] In this embodiment, a sintered mixture of CoO and
Fe.sub.2O.sub.3 may be used in place of the CoO target for forming
the inorganic compound thin film. It has been revealed that the
lattice constant of CoO can be adjusted by regulating the mixing
ratio of Fe.sub.2O.sub.3. Alternatively, for example, a sintered
mixture of SiO.sub.2 and ZnO in a ratio of 1:3 may be used in place
of the SiO.sub.2 target. It has been revealed that the distance of
the crystal grain boundary can be adjusted by regulating the mixing
ratio of SiO.sub.2/ZnO.
[0186] Further, an NiO--Al.sub.2O.sub.3 thin film was also
successfully formed by using a mixture of NiO and Al.sub.2O.sub.3,
for example, in a ratio of 1:3 in order to form the inorganic
compound thin film. This thin film also had a honeycomb structure
in the same manner as the thin film obtained in this
embodiment.
Third Embodiment
[0187] In this embodiment, a magnetic disk having the same stacking
structure as that described in the first embodiment was produced.
The concave/convex structure of the magnetic disk will be explained
in detail. An inorganic compound thin film 2 was formed on a
substrate 1 in the same manner as in the first embodiment except
that a mixture of CoO and SiO.sub.2 in a ratio of 2:1 was used as a
target for the ECR sputtering for the inorganic compound thin film
2.
[0188] The surface structure of the inorganic compound thin film 2
obtained as described above was equivalent to the structure
obtained in the first embodiment. The magnetic disk had a honeycomb
structure as shown in FIG. 3. According to the observation with
TEM, the size of the regular hexagonal crystal grain was 10 nm, and
the distance between crystal grains was 2 nm. The other structural
features were the same as those of the inorganic compound thin film
2 obtained in the first embodiment.
[0189] It was revealed that concave/convex portions corresponding
to the honeycomb structure existed on the surface of the
CoO--SiO.sub.2 film. According to the analysis for the crystalline
feature based on the X-ray diffraction, it was revealed that the
convex portion was a crystalline phase corresponding to the crystal
grains 12, and the concave portion was amorphous corresponding to
the crystal grain boundary 14. The concave/convex configuration in
cross section of the CoO--SiO.sub.2 film was measured with the
interatomic force microscope (AFM). As shown in FIG. 8, taking
notice of one peak (convex portion: crystal grain) as the height 16
of the convex-portion, the difference in height was measured
between the apex of the peak and the valley (concave portion:
crystal grain boundary) nearest to the peak. As a result, the
difference was 10 nm in average. This value was determined as an
average value by performing the measurement for randomly selected
500 places. This value was not more than the measurement limit of
AFM, and the standard deviation of measured values obtained for the
500 places was not more than 0.5 nm. According to this fact, it was
revealed that one convex portion itself was relatively small, the
entire CoO--SiO.sub.2 film was flat, and the dispersion of the
concave/convex configuration was remarkably small as well.
[0190] On the other hand, the distance in the direction parallel to
the substrate surface, which ranged from the apex (center of convex
portion) of a certain peak (convex portion) to the apex (center of
convex portion) of the nearest peak, was measured as the distance
18 between the convex portions shown in FIG. 8. As a result, the
distance 18 was 12 nm. Further, the distance between the peak
(center of convex portion) and the valley (center of concave
portion) was about 5.5 nm. It was revealed that the crystal grains
having the convex portions are useful for the texture-equipped
substrate for the magnetic disk, because such crystal grains were
regularly arranged in the honeycomb configuration on the
CoO--SiO.sub.2 film, and the shape of the concave/convex portions
and the distribution thereof were satisfactory. The concave/convex
shape can be controlled by controlling the film formation
temperature, the velocity of the sputtering, and the sputtering gas
pressure during the sputtering. Further, the concave/convex
structure may serve as a pinning site for the magnetic wall, and
the concave/convex structure makes it possible to fix the edge
position of the magnetic domain. The concave/convex structure
effectively serves for the high density recording owing to the
functions as described above.
[0191] The concave/convex structure on the CoO--SiO.sub.2 film 2
was compared with the concave/convex structure existing on the
surface of the substrate 1 itself. The measurement was performed
with AFM for concave/convex configuration of the used glass
substrate 1 in the same manner as the measurement for the
concave/convex configuration of the underlying layer. In this case,
the investigation was made for randomly selected squares each
having one side of 300 .mu.m at several places, and the measurement
was performed for randomly selected about 500 places for each of
the distance between convex portions and the height of convex
portion. As a result, the distance between convex portions in the
direction parallel to the substrate surface was 50 nm in average,
and the height of convex portion in the direction perpendicular to
the substrate surface was 60 nm. When the comparison was made for
the size of the concave/convex structure obtained when the
CoO--SiO.sub.2 film 2 was formed on the substrate, it was revealed
that the concave/convex structure of the underlying base film was
different from the concave/convex structure of the substrate
surface, and the concave/convex structure of the underlying base
film was small. Accordingly, a CoO--SiO.sub.2 film 2 was formed on
another substrate having a large-sized concave/convex structure
(distance between convex portions: 30 nm, height of convex portion:
100 nm). In this case, the concave/convex structure on the
CoO--SiO.sub.2 film gave the same results as those of the case in
which the previous substrate was used. The distance between convex
portions was 12 nm, and the height of convex portion was not more
than 10 nm (not more than the lower measurement limit of AFM).
However, it was observed that a concave/convex structure, which was
6 .mu.m in the horizontal direction and which was not more than 10
nm in the vertical direction, was repeatedly formed as a
macroscopic concave/convex structure of the CoO--SiO.sub.2 film 2.
Therefore, the following fact has been revealed. That is, when the
CoO--SiO.sub.2 film 2 having the honeycomb structure is formed by
using the ECR sputtering method, the minute concave/convex
portions, which are useful as the texture of the disk, are provided
in a microscopic viewpoint, and the substantially flat surface is
obtained irrelevant to the concave/convex structure of the
substrate surface in a macroscopic viewpoint.
[0192] Subsequently, a magnetic film having a composition of
Co.sub.69Cr.sub.12Pt.sub.19 was formed as the magnetic film 3 on
the inorganic compound thin film 2 by means of the DC sputtering
method. Pure Ar was used for the electric discharge gas, and a
Co.sub.69Cr.sub.12Pt.su- b.19 alloy target was used for the target.
The pressure during the sputtering was 3 mTorr. The introduced DC
electric power was 1 kW/150 mm.phi.. During the film formation
process, the substrate was heated to 300.degree. C. The film
thickness of the magnetic film was 20 nm. The structure of the
surface of the obtained magnetic film was observed with TEM. As a
result, the structure of the inorganic compound 2 was reflected to
the structure of the formed magnetic film. That is, Co was
deposited as crystal grains corresponding to the crystal grains 12
of the inorganic compound thin film 2, and the obtained shape was
regular hexagonal. The size distribution was also equivalent to
that of the inorganic compound thin film 2. When the
cross-sectional structure was investigated, a pillar-shaped
structure (magnetic grain portion) was observed, in which the
magnetic film 3 was grown with crystalline continuity with respect
to the inorganic compound thin film 2. Accordingly, the crystal
grain size of the magnetic film 3 can be controlled by epitaxially
growing the magnetic film 3 from the crystal grains 12 of the
inorganic compound as described above. The portion of the magnetic
film (magnetic grain boundary portion) existing on the crystal
grain boundary 14 had a structure different from that of the
magnetic grain portion of the magnetic film 3, in which no
pillar-shaped structure was observed unlike the magnetic grain
portion. The magnetic grain boundary portion had magnetic
characteristics different from those of the magnetic grain portion,
and it magnetically exhibited semi-hard magnetization. Owing to the
magnetic grain boundary portion, it is possible to reduce the
magnetic coupling between the magnetic grains. Therefore, the
existence of the magnetic grain boundary portion is advantageous
for the high density recording as well as for the reduction of the
grain size distribution.
[0193] Finally, a C film was formed as the protective film 4 to
have a film thickness of 5 nm by means of the DC sputtering method.
The introduced electric power density was 1 kW/150 mm.phi., and the
sputtering gas pressure was 5 mTorr.
[0194] The structure of the magnetic recording medium produced as
described above was investigated by means of the X-ray diffraction
method. An obtained result is shown in FIG. 9. The diffraction peak
observed in the vicinity of 2.theta.=62.5.degree. corresponds to
(220) of CoO of the crystal grain in the inorganic compound film.
The peak observed in the vicinity of 2.theta.=73.degree.
corresponds to (11.0) of Co of the magnetic film. The orientation
of Co as described above is caused by the epitaxial growth from the
crystal grains 12 in the inorganic compound film 2, and it is the
result of reflection of the orientation thereof. When the inorganic
compound thin film 2 was absent, then the (11.0) plane of Co was
not observed, but (002) of Co was observed. According to this fact,
it is appreciated that the inorganic compound film 2 greatly
contributes to the control of orientation of the magnetic film
3.
[0195] Subsequently, the magnetic film plane was observed with the
electron microscope. According to the observation, the average
grain diameter of the magnetic film was 10 nm. When the grain
diameter distribution was determined, it was represented by a
standard deviation .sigma. of not more than 1.5 nm, in which the
grain diameter distribution was remarkably small. The distribution
had the same value as that in the inorganic compound film 2. The
values obtained for the inorganic compound film are reflected to
the size of the crystal grain and the distribution thereof in the
magnetic film. The structure of the magnetic film was a honeycomb
structure with two-dimensional regular arrangement reflecting the
crystal form of the magnetic grains and the crystal form of the
inorganic compound. The cross-sectional structure was observed. As
a result, convex portions, in which the height of convex portion
was about 10 nm and the distance between convex portions was about
12 nm, were formed over the entire surfaces of the magnetic film
and the protective film reflecting the structure of the inorganic
compound thin film.
[0196] Subsequently, the magnetic characteristics of the magnetic
film 3 were measured. The obtained magnetic characteristics were as
follows. That is, the coercive force was 3.5 kOe, Isv was
2.5.times.10.sup.-6 emu, S as the index for the angular property of
hysteresis in M-H loop was 0.85, and Swas 0.91. Thus, the magnetic
characteristics were satisfactory. It is appreciated that this fact
means the small size of the crystal grain of the magnetic film and
the small dispersion thereof, reflecting such a result that the
magnetic interaction between the crystal grains is reduced.
Further, it is appreciated that the medium is excellent in thermal
fluctuation resistance, because minute crystal grains scarcely
exist.
[0197] The magnetic recording apparatus shown in FIGS. 11 and 12
was constructed in the same manner as in the first embodiment by
using the magnetic disk obtained as described above. A signal
corresponding to 40 Gbits/inch.sup.2 was recorded on the disk to
evaluate S/N of the disk. As a result, a reproduction output of 32
dB was obtained. When the spacing distance between the head surface
and the disk surface was 15 nm, the head successfully flied in a
stable manner. The unit of inversion of magnetization during the
recording of information, which was measured with a magnetic force
microscope (MFM), included 2 to 3 individuals of magnetic grains.
The zigzag pattern, which was present in the magnetization
transition region, was also extremely decreased as compared with
the conventional magnetic disk. When the error rate of the magnetic
disk was measured, it was not more than 1.times.10.sup.-5 as a
value obtained when no signal processing was performed.
[0198] In this embodiment, the following fact was revealed. That
is, the honeycomb structure was obtained, and the crystalline
orientation and the structure of the magnetic film was successfully
controlled even when. Fe.sub.2O.sub.3--Al.sub.2O.sub.3 was formed
for the inorganic compound thin film 2 in place of
CoO--SiO.sub.2.
Fourth Embodiment
[0199] This embodiment is illustrative of a case of production of a
magnetic disk based on the use of a Co--SiO.sub.2-based granular
type magnetic film as the magnetic film.
[0200] A CoO--SiO.sub.2 film was formed on the glass substrate with
the same materials and the same condition as those used in the
third embodiment. Subsequently, a Co--SiO.sub.2-based magnetic film
having the granular structure was produced as the magnetic film 3
by using the ECR sputtering apparatus shown in FIG. 5. A
Co--SiO.sub.2-based mixture (mixing ratio: Co:SiO.sub.2=1:1) target
was used for the target, and Ar was used for the sputtering gas.
The sputtering gas pressure was 0.3 mTorr, and the introduced
microwave electric power was 0.7 kW. An RF bias voltage of 500 W
was applied in order to introduce the microwave. During the film
formation, the substrate was heated to 300.degree. C. The film
thickness of the formed granular magnetic film was 10 nm.
[0201] The surface and the cross section of the film were observed
with a high resolution transmission electron microscope. As a
result, in the same manner as in the third embodiment, Co of the
magnetic film 3 was epitaxially grown from the crystal phase of the
inorganic compound thin film 2, and SiO.sub.2 was grown from the
grain boundary which surrounded the crystal grains. The cross
section of the granular magnetic film had a pillar-shaped
structure. Co was surrounded by SiO.sub.2, the grains were
separated from each other, and the magnetic interaction was greatly
reduced. This provides a structure of the magnetic film effective
for the high density magnetic recording. The macroscopic
concave/convex structure of the surface of the magnetic recording
medium was such that the size in the horizontal direction was 6
.mu.m and the size in the vertical direction was not more than 10
nm (not more than the lower limit of the measurement with AFM). The
concave/convex structure reflects the concave/convex structure of
the inorganic compound film.
[0202] In this embodiment, the ECR sputtering method was used to
produce the magnetic film. However, for example, the magnetron
sputtering method may be used with a mixture (or composite) target
of Co--SiO.sub.2. However, in this case, the shape of crystal grain
is slightly deteriorated in some cases as compared with the case in
which the ECR sputtering method is used.
[0203] A protective film 4 was produced on the magnetic film in
accordance with the ECR sputtering method by using the apparatus
shown in FIG. 5. Ar was used for the sputtering gas, and a
ring-shaped carbon target was used for the target. The gas pressure
during the sputtering was 0.3 mTorr, and the introduced microwave
electric power was 0.7 kW. An RF bias voltage of 500 V was applied
to the target in order that the plasma exited by the microwave was
introduced in the direction toward the target. The thickness of the
protective film was 2 nm. The surface of the magnetic disk after
the formation of the protective film was observed with TEM. As a
result, the appearance was equivalent to that of the magnetic film
surface. Further, the magnetic film surface was completely covered
with the protective film.
[0204] The magnetic characteristics of the magnetic recording
medium produced as described above were measured. The following
magnetic characteristics were obtained. That is, the coercive force
was 4.0 kOe, Isv was 2.5.times.10.sup.-16 emu, S as the index for
the angular property of hysteresis in M-H loop was 0.85, and Swas
0.90. Therefore, the magnetic recording medium had the good
magnetic characteristics. It is appreciated that this fact means
the small size of the crystal grain of the magnetic film and the
small dispersion thereof, reflecting such a result that the
magnetic interaction between the crystal grains is reduced.
Further, it is appreciated that the medium is excellent in thermal
fluctuation resistance, because minute crystal grains scarcely
exist.
[0205] As Reference Example, a sample was prepared, in which a
protective film 4 was formed by using the magnetron type RF
sputtering method in place of the ECR sputtering method. As for the
magnetic characteristics of the magnetic film of this sample, the
coercive force was lowered to be 2.5 to 1.8 kOe. The magnitude of
the coercive force was distributed on one sheet of the magnetic
disk. According to Reference Example, it is understood that when
the ECR sputtering method is used, then the coverage of the film
and the density of the film are improved, and it is possible to
suppress the damage on the magnetic film as well.
[0206] The magnetic recording apparatus shown in FIGS. 11 and 12
was constructed in the same manner as in the first embodiment by
using the magnetic disk obtained as described above. A signal
corresponding to 40 Gbits/inch.sup.2 was recorded on the disk to
evaluate S/N of the disk. As a result, a reproduction output of 32
dB was obtained. The unit of inversion of magnetization during the
recording of information, which was measured with a magnetic force
microscope (MFM), included 2 to 3 individuals of magnetic grains.
The zigzag pattern, which was present in the magnetization
transition region, was also extremely decreased as compared with
the conventional magnetic disk. Further, neither thermal
fluctuation nor demagnetization due to heat was caused. This
results from the fact that the distribution of the size of the
crystal grains of the magnetic film is small. When the error rate
of the magnetic disk was measured, it was not more than
1.times.10.sup.-12 as a value obtained when no signal processing
was performed. When the distance between the head and the surface
of the magnetic disk was 15 nm, the head stably flied. On the other
hand, when the magnetic disk, which had the inorganic compound film
produced by the magnetron sputtering method in place of the ECR
sputtering method, was driven under the same condition, then no
stable reproduction signal was obtained, and the head crash
occurred. The reason of the failure to obtain the stable
reproduction signal is as follows. That is, the concave/convex
structure of the surface of the magnetic disk exceeds the range in
which the distance between the magnetic head and the disk surface
can be controlled to be constant, and the distance between the head
and the surface of the magnetic disk is not constant.
Fifth Embodiment
[0207] In this embodiment, explanation will be made for a method
for producing a magnetic disk 300 comprising, on a substrate 31, a
first underlying layer 32, a second underlying layer 33, a magnetic
layer 34, and a protective layer 35 in this order as shown in FIG.
10, and the evaluation of the obtained respective layer and the
obtained magnetic disk.
Formation of First Underlying Layer and Second Underlying layer
[0208] An Hf film was formed as the first underlying layer 32 on
the glass substrate 31 having a diameter of 2.5 inches (6.25 cm) by
using the ECR sputtering apparatus shown in FIG. 5. Ar was used for
the sputtering gas, and the gas pressure during the sputtering was
0.3 mTorr (about 39.9 Pa). The introduced microwave electric power
was 1 kw. In order to introduce the plasma excited by the microwave
in the direction toward the target, a DC bias voltage of 500 V was
applied to the target. The Hf film having a film thickness of 3 nm
was formed by means of the ECR sputtering.
[0209] Subsequently, a CoO--SiO.sub.2 film was formed as the second
underlying layer (inorganic compound thin film) 33 by means of the
ECR sputtering method by using the ECR sputtering apparatus shown
in FIG. 5. A Co--Si alloy was used for the target. Ar--O.sub.2
mixed gas was used for the sputtering gas to perform the reactive
sputtering. The gas pressure during the sputtering was 0.3 mTorr,
and the introduced microwave electric power was 0.7 kW. In order to
introduce the plasma excited by the microwave in the direction
toward the target, an RF bias voltage of 500 V was applied to the
target. The formed CoO--SiO.sub.2 film had a film thickness of 20
nm.
Observation and Measurement for First Underlying Layer and Second
Underlying Layer
[0210] (1) Observation for first underlying layer with SEM and
X-ray diffraction method
[0211] After the Hf film 2 was formed, the film was observed with a
scanning electron microscope (SEM). As a result, it was revealed
that the film was extremely flat, and no defect such as pin hole
existed. The crystalline property of the Hf film was analyzed by
means of the X-ray diffraction method. As a result, the Hf film was
amorphous.
[0212] (2) Measurement of composition of second underlying layer
and observation with TEM and .mu.-EDX
[0213] The composition of the CoO--SiO.sub.2 film formed on the Hf
film was investigated. It was revealed that CoO and SiO.sub.2 were
in a ratio of 2:1 by means of the quantitative analysis for Co and
Si based on the use of fluorescent X-ray.
[0214] The planar structure of the CoO--SiO.sub.2 film 33 was
observed in the bright field with a high resolution transmission
electron microscope (TEM). The observed image was similar to the
structure shown in FIG. 3. The CoO--SiO.sub.2 thin film 33 was an
aggregate of regular hexagonal crystal grains 12, and the crystal
grains 12 were regularly arranged two-dimensionally with the
crystal grain boundary 14 intervening therebetween. Subsequently,
the cross section of the thin film was observed. As a result, it
was observed that the regular hexagonal crystal grains 12 had a
pillar-shaped structure grown in the direction perpendicular to the
substrate surface. That is, it was revealed that the entire
CoO--SiO.sub.2 film 33 had a honeycomb structure. It was also
revealed that the pillar-shaped structure was epitaxially grown
with a uniform grain diameter in the growth direction.
[0215] The crystal grain 12 and the crystal grain boundary 14 of
the CoO--SiO.sub.2 film were analyzed by means of the energy
dispersion type X-ray analysis for an extremely minute area
(.mu.-EDX analysis). As a result, the crystal grain 12 was composed
of CoO, and the crystal grain boundary 14 was composed of
SiO.sub.2. The distance between the crystal grains, i.e., the width
of the crystal grain boundary 14 was 0.5 to 1.0 nm.
[0216] The lattice image of the CoO--SiO.sub.2 film was observed.
As a result, it was confirmed that the interior of the regular
hexagonal cylinder of the crystal grain was crystalline, and the
crystal grain boundary was amorphous. The lattice constant was
determined. As a result, the lattice constant had a value
approximately equal to the value of cobalt.
[0217] It is noted that SiO.sub.2 plays an important role to allow
the structure to have the regularity, and SiO.sub.2 determines the
spacing distance between the crystal grains to be formed. It is
possible for the spacing distance to select a desired value easily
and arbitrarily by changing the composition of the target (for
example, the ratio between Co and Si or the ratio between COo and
SiO.sub.2), i.e., the SiO.sub.2 concentration. However, if the
SiO.sub.2 concentration is lowered, then the spacing distance
between the grains is narrowed (crystal grains make approach to one
another), and any disorder is simultaneously observed in the shape
of grain. On the contrary, if SiO.sub.2 exists in a large amount,
the deposition and the growth of CoO are suppressed. Therefore, it
is desirable that the spacing distance is not more than 2 nm. An
appropriate range of the spacing distance between crystal grains is
0.5 to 2 nm. The SiO.sub.2 concentration was controlled so that the
appropriate range was realized.
[0218] The result of observation with TEM was used to analyze the
crystal grain diameter of the CoO--SiO.sub.2 film (spacing distance
between the opposite sides of the regular hexagon), the
distribution thereof, and the number of grains located to. surround
one crystal grain (hereinafter referred to as "number of
coordinated grains"). At first, for the crystal grain diameter, the
grains existing in a randomly selected square area having one side
of 200 nm were investigated. As a result, the average grain
diameter was 10 nm. The grain diameter distribution was a normal
distribution. The standard deviation (.sigma.) was determined to be
0.5 nm. When the number of coordinated grains was investigated for
300 individuals of crystal grains, it was 6.01 in average. This
fact indicates that the dispersion of the grain diameter of the
crystal grain is small, and the regular hexagons of the crystal
grains are arranged extremely regularly in the honeycomb
configuration in the plane parallel to the substrate surface.
[0219] The structure in the vicinity of the interface between the
CoO--SiO.sub.2 layer and the Hf layer was investigated in detail by
means of the lattice image observation. As a result, the crystal
grains having the honeycomb configuration as described above were
grown from the top of the Hf layer, and the initial growth layer,
which was an aggregate of microcrystals with no honeycomb structure
as found in Comparative Example as described later on, was not
observed.
[0220] (3) Observation for second underlying layer with X-ray
diffraction method
[0221] The crystal structure of the CoO--SiO.sub.2 thin film was
observed by means of the X-ray diffraction method. According to an
obtained profile, only a diffraction peak of (220) of CoO appeared
in the vicinity of 2.theta.=62.5.degree. in the same manner as in
the chart shown in FIG. 2. This fact indicates that CoO is
subjected to crystalline orientation in only one direction in the
thin film. This crystal structure can be changed by controlling the
film formation condition and the composition as described later
on.
Formation of Magnetic Layer
[0222] Subsequently, a Co--SiO.sub.2 magnetic film having a
granular structure was formed as a magnetic layer on the
CoO--SiO.sub.2 film as the second underlying layer by means of the
ECR sputtering method. A Co--SiO.sub.2 mixture (mixing ratio was
CO:SiO.sub.2=1:1) was used for the target, and Ar was used for the
sputtering gas. The gas pressure of the sputtering gas was 3 mTorr,
and the introduced microwave electric power was 0.7 kW. The RF bias
voltage was 500 W, which was applied to the target in order that
the plasma excited by the microwave was introduced in the direction
toward the target. The glass substrate was heated to 300.degree. C.
during the ECR sputtering. The granular type Co--SiO.sub.2 magnetic
film was formed to have a film thickness of 10 nm by means of the
ECR sputtering method performed under the condition as described
above.
[0223] The CoO--SiO.sub.2 film as the second underlying layer and
the granular type Co--SiO.sub.2 film as the magnetic layer were
observed with TEM. As a result of the observation for the surface,
it was revealed that the granular type Co--SiO.sub.2 film of the
magnetic layer also had a honeycomb structure reflecting the
honeycomb structure of the CoO--SiO.sub.2 film as the underlying
base. The cross sections of the two layers were observed. As a
result, it was revealed that the magnetic grains of Co of the
granular type Co--SiO.sub.2 film were epitaxially grown from the
top of the crystal grains of the CoO--SiO.sub.2 film. The magnetic
grains were grown in a pillar-shaped configuration from the crystal
grains of the second underlying layer in the direction
perpendicular to the substrate surface while maintaining a constant
grain diameter of regular hexagon. SiO.sub.2 of the magnetic layer
was grown on the top of the crystal grain boundary for surrounding
the crystal grains of the second underlying layer. It is understood
that the magnetic interaction between the magnetic grains is
greatly reduced owing to the separation of the individual magnetic
grains by the boundary of SiO.sub.2 having a uniform width, because
Co of magnetic grain is surrounded by SiO.sub.2 in the magnetic
layer. The structure of the granular type Co--SiO.sub.2 film is
preferred to realize a magnetic recording medium having a high
density, because the structure of the granular type Co--SiO.sub.2
film makes it possible to decrease the unit of inversion of
magnetization.
[0224] As a result of the observation with an interatomic force
electron microscope (AFM), it was found that the concave/convex
structure was present on the surface of the granular type
Co--SiO.sub.2 film. The concave/convex structure was 6 .mu.m in the
direction parallel to the substrate surface, and it was not more
than 10 nm in the vertical direction (not more than the measurement
lower limit of AFM). This value is small as compared with scratches
and irregularities on the substrate surface, indicating the fact
that the surface of the magnetic layer is smooth, and it is
possible to avoid any influence of the roughness of the substrate
surface on the surface of the magnetic layer. As a result of
comparison with the result obtained by observing the CoO--SiO.sub.2
film as the second underlying layer with AFM, it was found that the
concave/convex structure of the magnetic layer reflected the
morphology of the second underlying layer.
[0225] The magnetic characteristics of the granular type
Co--SiO.sub.2 film as the magnetic layer were measured. The
obtained magnetic characteristics were as follows. That is, the
coercive force was 4.0 kOe, Isv was 2.5.times.10.sup.-16 emu, S as
the index for the angular property of hysteresis in M-H loop was
0.85, and Swas 0.90. Thus, the magnetic characteristics were good.
This results from the fact that the magnetic grain diameter of the
granular type Co--SiO.sub.2 film is small, the dispersion thereof
is also small, and the magnetic interaction between the magnetic
grains is reduced. The large coercive force was obtained, because
Co was pillar-shaped and it was oriented.
Formation of Protective Film
[0226] A carbon film as a protective layer was formed on the
granular type Co--SiO.sub.2 film formed as described above, by
using the ECR sputtering method. Ar was used for the sputtering
gas, and a ring-shaped carbon target was used for the target. The
gas pressure during the sputtering was 3 mTorr, and the introduced
microwave electric power was 1 kW. A DC bias voltage of 500 V was
applied to the target in order that the plasma excited by the
microwave was introduced in the direction toward the target. The
carbon protective film was formed to have a film thickness of 2
nm.
[0227] After the carbon film was formed on the granular type
Co--SiO.sub.2 film as described above, the surface was observed
with TEM. As a result, it was revealed that a small concave/convex
structure, which reflected the honeycomb structure in the same
manner as the surface of the granular type Co--SiO.sub.2 film as
the magnetic layer, was present, and the surface of the magnetic
film was completely covered with the protective film.
COMPARATIVE EXAMPLE
[0228] In this example, magnetic disks were produced in the same
manner as in the embodiment described above for a case in which a
first underlying layer was provided with a thickness of 3 nm and
for a case in which no first underlying layer was provided, in
order to confirm the effect of the first underlying layer. However,
in any case, the magnetic layer was composed of a
Co.sub.69Cr.sub.19Pt.sub.12 film. The surface and the cross section
of the underlying layer formed as described above were observed
with TEM. In any of the magnetic disks, a planar structure was
observed, in which regular hexagonal crystal grains having a grain
diameter of 9 nm were regularly arranged to form a honeycomb
structure. However, when the cross section perpendicular to the
substrate surface was observed, the following feature was observed
for the magnetic disk provided with no first underlying layer. That
is, the initial growth layer having no specified structure was
grown over a range of not less than about 20 nm, and the regular
honeycomb structure observed in this embodiment was observed
thereon. On the other hand, in the case of the magnetic disk
provided with the first underlying layer, the initial growth layer
was not present in the second underlying layer.
[0229] When the first underlying layer was not provided, the
surface of the Co.sub.69Cr.sub.19Pt.sub.12 film as the magnetic
layer was observed with the electron microscope to determine the
magnetic grain diameter distribution of magnetic grains. As a
result, .sigma. was about 2 nm. On the other hand, when the first
underlying layer was provided, the magnetic grain diameter
distribution of magnetic grains was determined. As a result,
.sigma. was about 0.7 nm. That is, when the underlying layer was
provided as the two layers, then the magnetic grain diameter in the
magnetic layer was successfully controlled more precisely, the
grain diameter of the magnetic grains was more uniform, and the
dispersion was successfully decreased.
[0230] As understood from Comparative Example, when the first
underlying layer is not provided, it is necessary to grow the
underlying layer up to a film thickness of 30 nm until the
honeycomb structure appears. On the other hand, when the first
underlying layer was provided, the second underlying layer having
the honeycomb structure was successfully grown directly from the
first underlying layer owing to the presence of the first
underlying layer having the thickness of only 3 nm. That is, in the
magnetic recording medium according to the present invention, it is
possible to avoid the appearance of the initial growth layer which
is not subjected to the crystalline orientation. Therefore, it is
possible to reduce the thickness of the underlying layer, and
consequently the thickness of the entire recording medium.
[0231] According to the fact described above, the first underlying
layer has such an effect that the formation of the initial growth
layer of the second underlying layer having no specified crystal
structure is suppressed by stacking the second underlying layer on
the first underlying layer. Therefore, the film thickness of the
portion corresponding to the initial growth layer can be decreased.
Accordingly, it is possible to shorten the time required to form
the film, and it is possible to reduce the production cost as well.
Further, when the first underlying layer is provided, then the
adhesive force between the substrate and the magnetic layer is
improved, and the magnetic layer can be scarcely peeled off.
Therefore, it is possible to provide the magnetic recording medium
which scarcely undergoes any physical damage. When the crystalline
material is used for the first underlying layer, it is possible to
more precisely control the crystal structure of the second
underlying layer to be formed thereon.
[0232] The magnetic recording apparatus shown in FIGS. 11 and 12
was constructed by using the magnetic disk obtained in the fifth
embodiment in the same manner as in the first embodiment. A signal
corresponding to 50 Gbits/inch.sup.2 was recorded on the disk to
evaluate S/N of the disk. As a result, a reproduction output of 30
dB was obtained. The unit of inversion of magnetization during the
recording of information, which was measured with a magnetic force
microscope (MFM), included 1 to 2 individuals of magnetic grains.
It was revealed that this value was sufficiently small as compared
with the unit of inversion of magnetization of 5 to 10 individuals
of the conventional magnetic disk. Accordingly, the portion (zigzag
pattern), which corresponded to the boundary between adjoining
units of inversion of magnetization, was remarkably small as
compared with the conventional magnetic disk. This result indicates
the fact that the boundary line of the area of inversion of
magnetization is smoothened, because the magnetic grains are fine
and minute, and the unit of inversion of magnetization is small as
well. Neither thermal fluctuation nor demagnetization due to heat
was caused. This is because of the fact that the grain diameter
distribution of the granular type Co--SiO.sub.2 film as the
magnetic layer was decreased as compared with the conventional
magnetic layer. The error or defect rate of the disk was measured.
As a result, the value was not more than 1.times.10.sup.-12 when
the signal processing was not performed. The distance between the
magnetic head and the magnetic disk surface was 12 nm. The magnetic
recording apparatus succeeded in allowing the magnetic head to flow
in a stable manner. However, when the magnetic disk having neither
first underlying layer nor second underlying layer was driven under
the same condition, then no stable reproduction signal was obtained
in some cases, and the head crash was caused in other cases. The
reason of the failure to obtain the stable signal is as follows.
The surface irregularity of the disk having neither first
underlying layer nor second underlying layer is large, exceeding a
range in which the magnetic recording apparatus successfully makes
control so that the distance between the magnetic head and the
magnetic disk surface is constant.
[0233] In this embodiment, hafnium was used for the first
underlying layer. However, other than the above, it is allowable to
use any one of an alloy thin film principally containing cobalt and
further containing at least one element selected from titanium,
tantalum, niobium, zirconium, and chromium, and an inorganic
compound thin film composed of at least one selected from silicon
nitride, silicon oxide, and aluminum oxide. Also in this case, it
was confirmed that the honeycomb structure of the second underlying
layer was successfully grown from the first underlying layer.
Especially, those usable as the alloy principally containing cobalt
include, for example, Co--Ta--Zr, Co--Nb--Zr, Co--Ti--Zr,
Co--Cr--Zr, Co--Nb, Co--Ta, Co--Ti, Co--Nb, Co--Zr, and Co--Cr. In
this case, in order to allow the cobalt alloy to be non-magnetic,
it is possible to contain an additive element in an amount of about
30%.
[0234] In this embodiment, when the CoO--SiO.sub.2 film as the
second underlying layer was formed, the reactive sputtering was
executed, in which the Co--Si alloy was used for the target, and
the Ar--O.sub.2 mixed gas was used for the sputtering gas. Other
than the above, a sintered mixture of CoO and SiO.sub.2 in a ratio
of 2:1 may be used for the target, and Ar may be used for the
sputtering gas. However, the reactive sputtering used in the
embodiment has the high film formation velocity, and hence it is
advantageous in view of productivity. Further, a second underlying
layer having a honeycomb structure is also obtained in accordance
with the reactive sputtering by using targets of single sintered
materials of Co and Si respectively and forming a film by means of
the two-target simultaneous sputtering. In any of the sputtering
methods, it is important to precisely control the energy of
sputtering particles. It has been revealed that more precise
control is performed with ease by using the ECR sputtering
method.
REFERENCE EXAMPLE
[0235] As Reference Example, a CoO--SiO.sub.2 layer as a second
underlying layer was formed by means of the ordinary magnetron
sputtering method. The structure of the CoO--SiO.sub.2 film
obtained by the magnetron sputtering method was analyzed. As a
result, the average grain diameter was 10 nm, and the grain
diameter distribution was a normal distribution. However, .sigma.
was 1.2 nm, and the dispersion of the grain diameter was large.
When 300 individuals of crystal grains were investigated, the
number of coordinated grains was 6.30 in average. The regularity
was slightly lowered. According to this fact, it is understood that
the regularity of the structure of the inorganic compound film is
more improved when the ECR sputtering method is used.
[0236] The ECR sputtering method is also effective for the Hf film
as the first underlying layer. When the Hf film was formed by using
the DC sputtering method or the RF sputtering method, then the
surface irregularity of the CoO--SiO.sub.2 film formed thereon was
increased, and the deficiency of the crystal growth of the
CoO--SiO.sub.2 film was suddenly increased. When 300 individuals of
crystal grains were investigated, the number of coordinated grains
was 6.08 in average, and the regularity was slightly lowered. If it
is assumed that a magnetic layer is formed thereon, it is easy to
postulate the decrease in regularity of the crystal structure of
the magnetic layer. Therefore, it is more desirable to use the ECR
sputtering method when the first underlying layer and the second
underlying layer are formed.
[0237] In this embodiment, the ECR sputtering method was used to
form the carbon film as the protective layer. As Reference Example,
a protective film was formed by using the magnetron type RF
sputtering method to produce a magnetic disk. The magnetic
characteristics of the magnetic disks were compared. The coercive
force was lowered to be 2.5 to 1.8 kOe in the case of the magnetic
disk having the carbon film formed by means of the magnetron type
RF sputtering method, as compared with the case in which the carbon
film was formed by means of the ECR sputtering method.
Simultaneously, the coercive force was greatly uneven over one
sheet of the magnetic disk. As described above, it has been
revealed that when the ECR sputtering method is used for the
formation of the protective layer, then the magnetic layer can be
uniformly covered with the carbon film, the formed carbon film is
dense, and it is also possible to suppress the damage on the
magnetic layer during the formation of the film.
Sixth Embodiment
[0238] In this embodiment, explanation will be made for a method
for producing a magnetic disk comprising an underlying base film
22, a control film 23, a magnetic film 24, and a protective film 25
stacked on a substrate 21 in this order as shown in FIG. 13, and
evaluation of the obtained respective films and the obtained
magnetic disk. Especially, in this embodiment, an MgO--SiO.sub.2
film was used for the underlying base film 22, and a Co-based alloy
was used for the magnetic film 24.
Formation of Underlying Layer
[0239] The MgO--SiO.sub.2 film was formed as the underlying base
film 22 on the glass substrate 21 having a diameter of 2.5 inches
(6.35 cm) by means of the ECR sputtering method by using the
apparatus shown in FIG. 5. A material, which was obtained by mixing
MgO and SiO.sub.2 in a ratio of 3:1 followed by being sintered to
have a ring-shaped configuration, was used for the target. Ar was
used for the sputtering gas. The gas pressure during the sputtering
was 0.3 mTorr (about 39.9 Pa), and the introduced microwave
electric power was 1 kW. An RF bias voltage of 500 W was applied to
the target in order that the plasma excited by the microwave was
introduced in the direction toward the target. The MgO--SiO.sub.2
film was formed to have a film thickness of 20 nm by means of the
ECR sputtering.
Observation for Underlying Base Film with TEM, Analysis by X-Ray
Diffraction Method, and Measurement of Composition
[0240] After the MgO--SiO.sub.2 film 22 was formed as the
underlying base film as described above, the surface of the film
was observed with a high resolution transmission electron the
MgO--SiO.sub.2 film 22 as described later on, it was revealed that
the oxide of magnesium was crystalline, and the crystal grain
boundary was amorphous. When the lattice constant of the crystal
grain was determined, it was approximately equal to the value of Co
for constructing the magnetic grains as described later on.
[0241] Subsequently, the TEM observation result for the surface of
the MgO--SiO.sub.2 film 22 was used to analyze the crystal grain
diameter (spacing distance between the opposite sides of the
regular hexagon), the crystal grain diameter distribution, and the
number of grains (number of coordinated grains) located to surround
one crystal grain. Crystal grains of 500 individuals were extracted
as samples for the analysis. At first, the crystal grain diameter
was determined. As a result, the crystal grain diameter was 10 nm
in average. The grain diameter distribution was a normal
distribution. When the standard deviation (.sigma.) was determined,
it was 0.5 nm (5% of the average grain diameter). The number of
coordinated grains was 6.02 in average. This fact indicates that
the crystal grain diameter is scarcely dispersed, and the crystal
grains of the regular hexagonal cylinders are arranged extremely
regularly in a honeycomb configuration in the plane parallel to the
substrate surface.
[0242] The crystal structure of the MgO--SiO.sub.2 film 22 was
analyzed by means of the X-ray diffraction. An obtained diffraction
profile is shown in FIG. 15. According to this result, a
diffraction peak of MgO was observed in the vicinity of
2.theta.=62.5.degree., and no other peak was observed. This fact
indicates that MgO is subjected to crystalline orientation in only
one direction in the thin film. No deviation from the
stoichiometric composition was found in the MgO--SiO.sub.2 thin
film 22 formed in this case, owing to the use of the ECR sputtering
method. Therefore, it is indicated that the metal film such as the
magnetic film, which is formed on the MgO--SiO.sub.2 film, is not
oxidized, because no free oxygen exists in the MgO--SiO.sub.2 film.
Therefore, when this film is used for a magnetic recording medium,
it is possible to produce the medium which ensures high reliability
for a long period of time.
Formation of Control Film
[0243] A Cr.sub.90Ru.sub.10 film was formed as the control film 23
on the MgO--SiO.sub.2 film 22 by means of the ECR sputtering
method. A CrRu alloy was used for the target, and Ar was used for
the sputtering gas. The gas pressure during the sputtering was 3
mTorr, and the introduced microwave electric power was 1 kW. An RF
bias voltage of 500 V was applied to the target in order that the
plasma excited by the microwave (2.93 GHz) was introduced in the
direction toward the target. The Cr.sub.90Ru.sub.10 film 23 was
formed to have a film thickness of 5 nm by means of the ECR
sputtering method.
Formation of Magnetic Film
[0244] A Co.sub.68Cr.sub.17Pt.sub.12Ta.sub.3 film was formed as the
magnetic film 24 on the control film 23 formed as described above,
by means of the DC sputtering method. A Co--Cr--Pt--Ta alloy having
the same composition as the composition of the objective film was
used for the target, and Ar was used for the sputtering gas. The
gas pressure during the sputtering was 3 mTorr. The introduced DC
electric power was 1 kW/150 mm.phi.. The substrate was heated to
300.degree. C. during the formation of the
Co.sub.68Cr.sub.17Pt.sub.12Ta.sub.3 film 3. The
Co.sub.68Cr.sub.17Pt.sub.12Ta.sub.3 film 3 was formed to have a
film thickness of 10 nm by means of the DC sputtering method under
the condition as described above.
Observation for Magnetic Film with TEM. Analysis by X-Ray
Diffraction Method, and Measurement of Composition
[0245] The surface of the Co.sub.68Cr.sub.17Pt.sub.12Ta.sub.3 film
(magnetic film 24) formed as described above was observed with TEM.
According to the observation, the
Co.sub.68Cr.sub.17Pt.sub.12Ta.sub.3 film also had a honeycomb
structure reflecting the honeycomb structure of the MgO--SiO.sub.2
film (underlying base film 22). The average grain diameter
determined by the observation for the surface was 10 nm, and
.sigma. in the grain diameter distribution was not more than 0.6
nm. As described above, it was revealed that the grains of the
magnetic film 24 were fine and minute, the grain diameter
distribution was small, and the film having the same form as that
of the underlying base film 22 was obtained. Subsequently, the
number of coordinated grains coordinated with one crystal grain was
determined. As a result of investigation for 500 individuals of
crystal grains, the number of coordinated grains was 6.01 in
average which was coincident with the number of coordinated grains
of crystal grains in the MgO--SiO.sub.2 film 22 of the underlying
base film described above. This fact indicates that the magnetic
grains are continuously grown upwardly in a regular hexagonal
cylinder configuration from the underlying base film, giving a
structure (honeycomb structure) in which regular hexagons are
regularly arranged in the plane parallel to the substrate surface
as shown in FIG. 14.
[0246] Further, according to the results of the lattice image
observation and the X-ray diffraction described later on, it was
revealed that the magnetic grains in the
Co.sub.68Cr.sub.17Pt.sub.12Ta.sub.3 film were crystalline, while
the boundary between the magnetic grains (crystal grains) had a
polycrystalline form. The cross section of the
Co.sub.68Cr.sub.17Pt.sub.12Ta.sub.3 film was observed with TEM. As
a result, it was revealed that the crystalline magnetic grains were
grown from the top of the regular hexagonal crystal grains of the
MgO--SiO.sub.2 film as the underlying base film, and the boundary
of the polycrystalline form corresponded to the crystal grain
boundary of the MgO--SiO.sub.2 film. That is, the following fact
has been revealed. The continuity of the crystal lattice was found
between the MgO--SiO.sub.2 film as the underlying base film and the
Co.sub.68Cr.sub.17Pt.sub.12Ta.su- b.3 film as the magnetic film.
The magnetic grains in the Co.sub.68Cr.sub.17Pt.sub.12Ta.sub.3 film
were epitaxially grown from the crystal grains in the
MgO--SiO.sub.2 film.
[0247] It is known that the boundary (polycrystalline form) in the
Co.sub.68Cr.sub.17Pt.sub.12Ta.sub.3 film as described above behaves
as a non-magnetic member unlike the magnetic grain portion. The
boundary exists in a width of 0.5 to 1.0 nm between the magnetic
grains. Accordingly, the magnetic interaction between the adjoining
magnetic grains is weakened. Therefore, it is easy for the
individual magnetic grains (crystal grains) to behave independently
upon the inversion of magnetization during the recording and the
erasing. It is possible to decrease the number of magnetic grains
for constructing the unit of inversion of magnetization, i.e., the
area of the magnetic film.
Formation of Protective Film
[0248] Finally, a carbon film was formed as the protective film 25
by means of the ECR sputtering method. A ring-shaped carbon target
was used for the target, and Ar was used for the sputtering gas.
The gas pressure during the sputtering was 0.3 mTorr, and the
introduced microwave electric power was 0.7 kW (frequency: 2.93
GHz). The substrate temperature during the film formation was
300.degree. C. An RF bias voltage of 500 W was applied to the
target in order that the plasma excited by the microwave was
introduced in the direction toward the target. The carbon film 25
was formed to have a film thickness of 3 nm by means of the ECR
sputtering method as described above. Thus, the magnetic disk 400
having the structure shown in FIG. 13 was obtained.
COMPARATIVE EXAMPLE
[0249] A magnetic disk was produced in the same manner as in the
embodiment described above except that the control film was not
provided unlike the embodiment.
Analysis of Magnetic Films of Embodiment and Comparative Example by
X-Ray Diffraction Method and Evaluation of Magnetic
Characteristics
[0250] The crystal structure was analyzed by means of the X-ray
diffraction method at the stage at which the magnetic film, i.e.,
the Co.sub.68Cr.sub.17Pt.sub.12Ta.sub.3 film 24 was formed on the
Cr.sub.90Ru.sub.10 film 23 as the control film of the magnetic disk
of this embodiment. An obtained X-ray diffraction profile is shown
in FIG. 16. As shown in FIG. 16, a peak indicating Co in the
magnetic film was observed in the vicinity of 2.theta.=72.5.degree.
in addition to a peak in the vicinity of 2.theta.=62.5.degree.
indicating MgO in the MgO--SiO.sub.2 film as the underlying base
film. In view of the results of the structural analysis for the
underlying base film and the TEM observation in combination, the
peak located in the vicinity of 2.theta.=72.5.degree. resides in
(11.0) of Co in the Co.sub.68Cr.sub.17Pt.sub.12Ta.sub.3 film as the
magnetic film, indicating that Co is strongly oriented in this
direction. That is, it has been revealed that the desired
crystalline orientation suitable for the high density recording is
intensely obtained in the magnetic film.
[0251] FIG. 18 shows an X-ray diffraction profile of the magnetic
film of the magnetic disk of Comparative Example provided with no
control film. When the X-ray diffraction profile of the magnetic
film of the magnetic disk of Comparative Example was compared with
the X-ray diffraction profile of the embodiment of the present
invention, then the intensity of the peak of Co was increased, the
half value width was narrowed, and the peak shape was further
sharpened in the embodiment of the present invention. This
indicates the fact that the deviation of the crystal lattice can be
adjusted between the underlying base film and the magnetic film,
and the desired crystalline orientation of Co is obtained more
intensely in the magnetic film, owing to the provision of the
Cr.sub.90Ru.sub.10 film as the control film on the underlying base
film.
[0252] Subsequently, the magnetic characteristics of the magnetic
film were measured. The obtained magnetic characteristics were as
follows. That is, the coercive force was 4.0 kOe, Isv was
2.5.times.10.sup.-16 emu, S as the index for the angular property
of hysteresis in M-H loop was 0.89, and Swas 0.95. Thus, the
magnetic characteristics were satisfactory. The reason why the
coercive force is increased and the index to indicate the angular
property is large (approximate to the angular form) as described
above is that the magnetic film is grown while reflecting the
honeycomb structure and the interaction between the magnetic grains
is reduced, owing to the provision of the underlying base film
having the honeycomb structure and the provision of the control
film, i.e., the Cr.sub.90Ru.sub.10 film.
Evaluation of Magnetic Disk
[0253] Further, the magnetic disk 400 was completed by applying a
lubricant onto the carbon film 25 as the protective film formed as
described above. A plurality of magnetic disks were produced in
accordance with the same process. The magnetic disks were coaxially
attached to the spindle of the magnetic recording apparatus. The
magnetic recording apparatus was constructed in the same manner as
in the first embodiment, having the structure shown in FIGS. 11 and
12. The distance between the magnetic head surface and the magnetic
disk was maintained to be 11 nm. A signal corresponding to 40
Gbits/inch.sup.2 was recorded on the disk to evaluate S/N of the
disk. As a result, a reproduction output of 36 dB was obtained. The
unit of inversion of magnetization during the recording of
information was measured with a magnetic force microscope. As a
result, 2 to 3 individuals of magnetic grains were subjected to
inversion of magnetization at once with respect to the recording
magnetic field applied when 1 bit data was recorded. The portion
(zigzag pattern), which corresponded to the boundary between
adjoining units of inversion of magnetization, was remarkably small
as compared with the conventional magnetic disk. This indicates the
fact that the boundary line of the area of inversion of
magnetization is smoothened, because the magnetic grains are fine
and minute, and the unit of inversion of magnetization is small as
well. Neither thermal fluctuation nor demagnetization due to heat
was caused. This is an effect owing to the small distribution of
the magnetic grain diameter of the
Co.sub.68Cr.sub.17Pt.sub.12Ta.sub.3 film 24 as the magnetic film.
Further, the error rate of the disk was measured. As a result, the
obtained value was not more than 1.times.10.sup.-5 when the signal
processing was not performed.
[0254] In this embodiment, the Cr--Ru alloy was used for the
control layer. However, other than the above, at least one element
selected from molybdenum, vanadium, nickel, niobium, aluminum,
titanium, tantalum, and zirconium may be added depending on the
lattice constant of the magnetic layer to be formed on the control
layer. The lattice constant of the control layer can be changed by
adding such an element.
[0255] The DC sputtering method was used to form the magnetic
layer. However, the ECR sputtering method may be used. When the ECR
sputtering method is used, the magnetic layer can be formed at a
low substrate temperature or at the room temperature. Further, the
respective sputtering particles have equal energy. Accordingly, it
is possible to highly accurately control the magnetic grain
diameter and the grain diameter distribution in the magnetic layer,
which is more preferred.
[0256] In this embodiment, the RF voltage was applied when the
carbon film as the protective layer was formed. However, a carbon
film can be formed in the same manner as in the case of application
of the RF voltage, even when a DC voltage is applied, because
carbon is a conductive material.
Seventh Embodiment
[0257] In this embodiment, as shown in FIG. 17, a magnetic disk 500
is produced, the magnetic disk 500 comprising an in-plane
magnetizable layer 42, an underlying layer 43, a magnetic layer 44,
a protective layer 45, and a lubricant layer 46 which are arranged
on a substrate 41 in this order.
Formation of In-Plane Magnetizable Layer
[0258] A CoNbZr layer having an amorphous structure was formed as a
film having a film thickness of 100 nm as the in-plane magnetizable
layer 42 on the glass substrate 41 having a diameter of 2.5 inches
(6.35 cm). When the in-plane magnetizable layer 42 is formed, the
DC sputtering method was used. The substrate 41 was not heated
during the sputtering. A CoNbZr alloy was used for the target, and
Ar gas was used for the electric discharge gas.
Formation of Underlying Layer
[0259] Subsequently, a CoO--SiO.sub.2 layer was formed as the
underlying layer 43 on the in-plane magnetizable layer 42 by means
of the reactive ECR sputtering method by using the ECR apparatus
shown in FIG. 5. A Co--Si alloy was used for the target, and an
Ar--O.sub.2 mixed gas was used for the electric discharge gas. The
gas pressure during the sputtering was 0.3 mTorr (about 39.9 mPa),
and the introduced microwave electric power was 1 kW. An RF bias
voltage of 500 W was applied between the target and the substrate
in order that the plasma excited by the microwave (2.98 GHz) was
introduced in the direction toward the target and the driven-out
sputtering particles were simultaneously introduced in the
direction toward the substrate. Thus, the CoO--SiO.sub.2 film as
the underlying layer 43 was formed to have a film thickness of 20
nm.
Analysis of Composition of Underlying Layer, Observation with TEM,
.mu.-EDX Analysis, and Lattice Image Observation
[0260] The composition of the CoO--SiO.sub.2 layer as the
underlying layer 43 was analyzed. As a result of the analysis, it
was revealed that CoO and SiO.sub.2 were contained in a ratio
(molar ratio) of 2:1.
[0261] Subsequently, the surface of the underlying layer 43 was
observed in the bright field with TEM. The observed image gave the
same structure as that shown in FIG. 3. The underlying layer 43 had
a honeycomb structure in which regular hexagonal crystal grains 12
(magnetic grains) with a distance between opposite sides of 10 nm
were regularly arranged in a honeycomb (comb-shaped) configuration
with a crystal grain boundary 14 having a uniform width intervening
therebetween.
[0262] After that, the underlying layer 43 was analyzed by means of
the energy dispersion type X-ray analysis (.mu.-EDX analysis) for
the extremely minute area. As a result, it was revealed that the
crystal grains were composed of oxide of cobalt, and the substance
existing in the crystal grain boundary was SiO.sub.2. The distance
between the crystal grains (width of the crystal grain boundary)
was 0.5 to 1.0 nm.
[0263] According to the results of the lattice image observation
and the observation with TEM for the structure of the underlying
layer 43, it was revealed that the crystal grains of CoO having the
uniform size were regularly arranged in the honeycomb configuration
with SiO.sub.2 intervening therebetween, in the same manner as in
the second underlying layer described in the fifth embodiment. The
crystal grain diameter distribution of the crystal grains and the
number of coordinated grains were evaluated in accordance with the
same method as that used in the fifth embodiment. As a result, the
grain diameter distribution was a normal distribution, and the
standard deviation (.sigma.) was 0.5 nm. The number of coordinated
grains was 6.01 in average.
[0264] For the purpose of comparison, a CoO--SiO.sub.2 film for
constructing an underlying layer was formed in accordance with an
ordinary magnetron sputtering method other than the ECR sputtering
method. The structure of the CoO--SiO.sub.2 film obtained by the
magnetron sputtering method was analyzed with TEM. As a result, the
average grain diameter was 10 nm. Although the grain diameter
distribution was a normal distribution, the standard deviation
.sigma. was determined to be 1.2 nm, in which the dispersion of the
grain diameter was large. The number of coordinated grains was
investigated for 500 individuals of crystal grains. As a result,
the number of coordinated grains was 6.30 in average. Therefore,
the regularity of the honeycomb structure was lowered. According to
this fact, it was revealed that when the CoO--SiO.sub.2 film as the
underlying layer was formed by using the ECR sputtering method, the
regularity of the structure of the film was successfully improved
to a great extent. Further, the structure of the underlying layer
was analyzed by means of the X-ray diffraction method. As a result,
it was revealed that the crystals of CoO were oriented in an
orientation of (111).
Formation of Magnetic Layer, Protective Layer, and Lubricant
Layer
[0265] A CoCrPt layer was formed as the magnetic layer 44 to have a
film thickness of 15 nm on the underlying layer 43 formed as
described above, by means of the DC sputtering method. A CoCrPt
alloy was used for the target, and Ar was used for the electric
discharge gas. The gas pressure during the sputtering 0.3 mTorr
(about 39.9 mPa), and the introduced DC electric power was 0.7
kW/150 mm.phi.. The substrate was heated to 300.degree. C. during
the formation of the magnetic layer 44.
[0266] Subsequently, a carbon film was formed as the carbon film 45
to have a film thickness of 5 nm on the magnetic layer 44. When the
carbon film was formed, the ECR sputtering method based on the use
of the microwave was used. A ring-shaped carbon material was used
for the target, and Ar was used for the sputtering gas. The gas
pressure during the sputtering was 0.3 mTorr (about 39.9 mPa), and
the introduced microwave electric power was 1 kW. A DC bias voltage
of 500 V was applied between the target and the substrate in order
that the plasma excited by the microwave was introduced in the
direction toward the target and the target particles driven out by
the plasma were simultaneously introduced in the direction toward
the substrate. Alternatively, an RF voltage may be used as the bias
voltage in place of the DC voltage.
[0267] Finally, the lubricant layer 46 composed of
perfluoropolyether was formed on the carbon film 45. Thus, the
magnetic disk 500 having the structure shown in FIG. 17 was
produced.
Observation for Magnetic Layer with TEM and Analysis by X-Ray
Diffraction Method
[0268] The surface structure of the magnetic layer 44 was observed
with TEM after the CoCrPt film as the magnetic layer 44 was formed
in the production steps described above. As a result of the
observation, it was revealed that the magnetic layer 44 also had a
honeycomb structure reflecting the structure of the underlying
layer 43. The average grain diameter of crystal grains, which was
determined by the surface observation with TEM, was 10 nm. The
standard deviation a in the grain diameter distribution was 0.6 nm.
Accordingly, it was revealed that the magnetic grains of the
magnetic layer 44 were fine and minute, the dispersion of the grain
diameter was small, and the magnetic layer 44 had the same form as
that of the underlying layer.
[0269] Subsequently, the number of crystal grains (number of
coordinated grains) to surround one crystal grain in the magnetic
layer 44 was investigated for 500 individuals of crystal grains. As
a result, the number was 6.01 in average, which was well coincident
with the number of coordinated grains in the underlying layer 43.
This result means that the crystal grains having the hexagonal
configuration were regularly arranged two-dimensionally
continuously from the underlying layer 43. Further, the structure
in the vicinity of the grain boundary of the crystal grains was
investigated by means of the lattice image observation. As a
result, the structure of the magnetic layer 44 differs between the
crystal grains and the grain boundary existing therearound.
Especially, it was revealed that the disorder of the lattice was
observed at the grain boundary in which the orientation was
different from that of the crystalline portion. As a result, of the
structural observation for the underlying layer 43 described above,
the portion, at which the lattice was disordered, corresponded to
the grain boundary portion of the underlying layer 43.
[0270] The cross-sectional structure of the magnetic layer 44 was
observed with TEM. As a result of the observation, the continuity
of the lattice was observed between the underlying layer 43 and the
magnetic layer 44. It was revealed that the magnetic layer 44 was
epitaxially grown from the underlying layer 43. It was also
revealed that the mode of growth of the magnetic layer 44 was
different between the crystalline phase and the grain boundary
phase, giving different metallic structures. Especially, an
appropriate pillar-shaped structure was grown from the crystal
grains of the underlying layer 43 up to the magnetic layer 44. On
the other hand, the magnetic layer portion disposed on the grain
boundary phase did not exhibit any distinct structure. According to
this fact, it is considered that the magnetic layer portion
disposed on the grain boundary phase is an aggregate of
polycrystalline matters, exhibiting non-magnetic behavior. Further,
the magnetic layer 44 was investigated by means of the X-ray
diffraction. As a result, it was revealed that the magnetic layer
44 was oriented in an orientation of (00.1), and the c-axis was
perpendicular to the film surface. This fact indicates that the
magnetic layer 44 is a perpendicularly magnetizable film.
[0271] In this embodiment, a control layer for improving the
crystalline orientation of the magnetic layer 44 may be provided
between the underlying layer 43 and the magnetic layer 44 of the
magnetic recording medium. For example, a TiCr film may be formed
as the orientation control layer on the underlying layer 43 by
means of the DC sputtering method.
Eighth Embodiment
[0272] In this embodiment, a magnetic disk is produced, in which an
underlying layer possesses soft magnetization. The magnetic disk to
be produced in this embodiment comprises, on a substrate 1, an
underlying layer (inorganic compound thin film) 2, a magnetic layer
3, and a protective layer 4 in the same manner as in the structure
shown in FIG. 1. However, the underlying layer 2 exhibits the soft
magnetization.
Production of Magnetic Disk
[0273] A CoO--SiO.sub.2 film 2 was formed as the underlying layer
on the glass substrate 1 having a circular shape with a diameter of
2.5 inches (6.35 cm) by means of the ECR sputtering method. A
sintered mixture containing CoO and SiO.sub.2 in a ratio of 2:1 was
used for the target, and Ar containing 1% hydrogen (reducing
atmosphere) was used for the sputtering gas. The reason why the
underlying layer 2 was formed in the reducing atmosphere is that a
part of CoO for constructing the underlying layer 2 is reduced to
deposit Co so that cobalt oxide has the soft magnetization. The
pressure of the sputtering gas was 0.5 mTorr (about 66.5 mPa), and
the introduced microwave electric power was 1 kW. An RF bias
voltage of 500 W was applied between the target and the substrate
in order that the plasma excited by the resonance absorption was
introduced in the direction toward the target, and the particles
driven out from the target by the plasma were introduced in the
direction toward the substrate. The CoO--SiO.sub.2 film 2 as the
underlying layer was formed to have a film thickness of 20 nm by
means of the ECR sputtering method as described above.
Evaluation of Underlying Layer
[0274] The surface structure of the CoO--SiO.sub.2 film 2 as the
underlying layer obtained as described above was observed with a
high resolution transmission electron microscope (TEM). An obtained
observation image resulted in the same structure as that shown in
FIG. 3. An aggregate of regular hexagonal crystal grains (12) was
present, in which the spacing distance between opposite sides was
10 nm. The crystal grains (12) were regularly arranged in a
honeycomb configuration. The distance between the crystal grains
was 0.7 nm.
[0275] According to the energy dispersion type X-ray analysis
(.mu.-EDX) for the extremely minute area, it was revealed that the
crystal grain was composed of oxide of cobalt, and silicon oxide
existed at the crystal grain boundary 14. According to the lattice
image observation for the underlying layer, it was revealed that
the cobalt oxide was crystalline and the silicon oxide was
amorphous. The lattice constant was determined. As a result, the
lattice constant had a value approximately equal to that of Co for
forming the magnetic grains of the magnetic layer to be formed on
the underlying layer. The lattice constant can be precisely
controlled by changing the film formation condition and/or adding a
metal (for example, chromium, iron, and nickel) having a different
ion radius or oxide of such a metal to CoO. In the CoO grains
formed by the ECR sputtering method, a part of COo was reduced as
described above, and Co was deposited in CoO. It was revealed that
the ratio of Co in the mixture of CoO and Co was about 1 atomic
%.
[0276] As for the magnetic recording medium of the present
invention, the ratio of CoO:Co can be determined as follows from
the value of the relative permeability of the underlying layer. At
first, known amounts of CoO and Co are mixed to prepare mixtures
having a variety of mixing ratios. Subsequently, the mixture is
introduced into a tablet-forming machine to form a tablet while
reducing the pressure. In this state, the relative permeability of
the mixture is determined. Table 1 depicted below shows the
actually obtained values of the mixing ratio and the relative
permeability. According to this table, it is appreciated that the
relative permeability is increased as the ratio of Co is increased.
The ratio of Co in the formed underlying layer can be determined by
determining an expression of relation between the two values, and
using the value of the relative permeability of the underlying
layer. In order to determine the relative permeability, a
convenient technique was used as follows. At first, impedances were
determined respectively for a case in which a sample was placed on
a ]-shaped (U-shaped) coil and for a case in which a sample was not
placed. The relative permeability was determined from the
difference between the impedances by means of calculation.
1 TABLE 1 Ratio of Co (at %) Relative permeability Sample 1 0.3 20
to 30 Sample 2 0.5 2000 to 3000 Sample 3 1.0 5000
[0277] The cross section of the thin film was observed with TEM. As
a result, a pillar-shaped structure was observed in the direction
perpendicular to the substrate. The pillar-shaped structure
indicates the fact that the regular hexagonal crystal grains were
grown upwardly from the top of the substrate while maintaining the
crystal grain diameter. Subsequently, the result of the observation
with TEM for the surface of the underlying layer was used to
analyze the grain diameter distribution of the crystal grains and
the number of coordinated grains. The grain diameter distribution
was a normal distribution. The standard deviation (.sigma.) was
determined to be not more than 0.6 nm. The number of coordinated
grains was investigated for randomly selected 280 individuals of
crystal grains. As a result, the number of coordinated grains was
6.02 in average. This fact indicates that the crystal grains having
the hexagonal shape with the uniform size are arranged extremely
regularly in the honeycomb configuration. It has been revealed that
the number of coordinated grains changes depending on the distance
between the crystal grains in the same manner as in other
embodiments.
[0278] The crystal structure of the CoO--SiO.sub.2 film as the
underlying layer was analyzed by means of the X-ray diffraction
method. Only a diffraction peak of (220) of CoO was observed in the
vicinity of 2.theta.=62.5.degree. in an obtained diffraction
profile.
[0279] Subsequently, the magnetic characteristics of the
CoOSiO.sub.2 film as the underlying layer were measured. The
saturated magnetic flux density was Bs=1.8 T, the coercive force
was Hc=0.10 e (about 7.9 A/m), the magnetostrictive constant was
.lambda.=4.times.10.sup.-7, the relative permeability was .mu.=5000
(5 MHz). Thus, the CoO--SiO.sub.2 film had the good soft magnetic
characteristics. The magnetization-prompt axis of the film was in
the direction parallel to the substrate. The underlying layer was a
so-called in-plane magnetizable film.
Formation of Magnetic Layer
[0280] A Co.sub.69Cr.sub.17Pt.sub.11Ta.sub.3 film was formed as the
magnetic layer on the CoO--SiO.sub.2 film as the underlying layer
formed as described above, by means of the DC sputtering method. A
Co--Cr--Pt--Ta alloy was used for the target, and Ar was used for
the sputtering gas. The pressure of the sputtering gas was 0.3
mTorr (about 39.9 mPa), and the introduced DC electric power was 1
kW/150 mm.phi.. The substrate was heated to 300.degree. C. during
the formation of the magnetic layer. In this way, the magnetic
layer was formed to have a film thickness of 15 nm.
Evaluation of Magnetic Layer
[0281] Subsequently, the structure of the
Co.sub.69Cr.sub.17Pt.sub.11Ta.su- b.3 film as the magnetic layer
was observed with TEM. As a result, it was revealed that the
magnetic layer had a honeycomb structure reflecting the structure
of the underlying layer, in the same manner as the underlying layer
2. The average grain diameter of the magnetic grains, which was
obtained from the observation for the surface of the magnetic
layer, was 10 nm. The grain diameter distribution had .sigma. of
not more than 0.8 nm. As described above, it was revealed that the
magnetic grains of the magnetic layer were fine and minute, and the
dispersion of the grain diameter was small. Subsequently, the
number of coordinated grains was investigated for randomly selected
250 individuals of crystal grains. As a result, the number of
coordinated grains was 6.01 in average which was well coincident
with the value obtained for the underlying layer 2 as described
above. This fact indicates that the magnetic grains having the
hexagonal shape with the uniform size are arranged extremely
regularly in the honeycomb configuration.
[0282] The cross-sectional structure of the
Co.sub.69Cr.sub.17Pt.sub.11Ta.- sub.3 film as the magnetic layer
was observed with TEM. As a result, it was revealed that the
continuity of the lattice was observed between the underlying layer
and the magnetic layer, and the magnetic layer was epitaxially
grown from the top of the underlying layer. Further, it was
revealed that the growth mechanism differed between the magnetic
grain portion grown from the top of the crystal grains of the
underlying layer and the boundary portion between the magnetic
grains grown from the top of the crystal grain boundary of the
underlying layer, in the magnetic layer in the same manner as in
the underlying layers of the other embodiments, giving different
metallic structures.
[0283] The analysis was performed by means of the X-ray diffraction
method after forming the Co.sub.69Cr.sub.17Pt.sub.11Ta.sub.3 film
as the magnetic layer. According to an X-ray chart, a peak was
observed in the vicinity of 2.theta.=72.5.degree. in addition to a
peak indicating (220) of Co in the underlying layer 2 in the
vicinity of 2.theta.=62.5.degree.. In view of this fact in
combination with the observation result with TEM, the peak
indicates (11.0) of Co. It was revealed that Co was strongly
oriented in this orientation. As well-known, (11.0) of Co is the
orientation preferable for the high density recording.
[0284] The magnetic characteristics of the
Co.sub.69Cr.sub.17Pt.sub.11Ta.s- ub.3 film were as follows. That
is, the coercive force was 3.6 kOe (about 284.4 kA/m), Isv was
2.5.times.10.sup.-16 emu, S as the index for the angular property
of hysteresis in M-H loop was 0.9, and Swas 0.93. Thus, the
Co.sub.69Cr.sub.17Pt.sub.11Ta.sub.3film had the good magnetic
characteristics.
Formation of Protective Layer
[0285] Finally, a carbon film was formed as the protective film by
means of the DC sputtering method. A carbon target was used for the
target, and Ar was used for the sputtering gas. The sputtering
condition was as follows. That is, the introduced DC electric power
density was 1 kW/150 mm.phi.. The gas pressure of the sputtering
gas was 5 mTorr (about 665 mPa). In this way, the protective layer
4 was formed to have a film thickness of 5 nm.
Production and Evaluation of Magnetic Disk Apparatus
[0286] The magnetic disk was completed by further applying a
lubricant onto the carbon film as the protective film formed as
described above. A plurality of magnetic disks were produced in
accordance with the same process. The magnetic disks were coaxially
attached to the spindle of the magnetic recording apparatus. The
magnetic recording apparatus was constructed in the same manner as
in the first embodiment, having the structure shown in FIGS. 11 and
12. The distance between the magnetic head surface and the magnetic
disk was maintained to be 14 nm.
[0287] A converged laser beam was radiated so that the laser beam
was collected onto the magnetic layer during recording and
reproduction. The reason whey the laser beam was radiated is that
it was intended to extinguish the magnetization of the underlying
layer by using the heat generated by the laser beam. During the
recording, the temperature of the area opposed to the
light-irradiated area on the surface of the magnetic disk was
raised to 170.degree. C. as a result the irradiation with light.
The Curie temperature of the underlying layer is about 130.degree.
C. Therefore, as the temperature is raised, the coercive force is
lowered, and the underlying layer is non-magnetic at a portion just
under the light-irradiated area. That is, this situation is
magnetically equivalent to a situation in which no underlying layer
exists. The recording operation was performed by applying a
magnetic field to the magnetic layer in this state. When the
irradiation with light is stopped, or when the area subjected to
recording is deviated from the light-irradiated area, then the
temperature is in the vicinity of the room temperature. Therefore,
the underlying layer has the soft magnetization, and it behaves as
an in-plane magnetizable layer. In the in-plane magnetizable layer,
the direction of magnetization is opposite to the direction of
magnetization of the magnetic layer by the aid of the leak magnetic
field generated from the area of inversion of magnetization in the
magnetic layer. The arrangement of magnetization as described above
lowers the diamagnetic field of the magnetic layer. Therefore, the
stability of the recording is high even in the case of storage over
a long period of time. During the reproduction, the temperature of
the area opposed to the light-irradiated area on the magnetic disk
surface was locally raised to about 180.degree. C. by radiating the
light to perform the reproduction. The increase in temperature
allows the underlying layer to be non-magnetic at a portion just
under the light-irradiated area. In this area, the situation is
magnetically equivalent to a situation in which no underlying layer
exists. In this state, a GMR head was used to detect the leak
magnetic field generated from the boundary of inversion of
magnetization in the magnetic layer disposed just over the
underlying layer in which the magnetization disappeared. A signal
(700 kFCI) corresponding to 40 Gbits/inch.sup.2 (6.20
Gbits/cm.sup.2) was recorded on the magnetic disk 10 to evaluate
S/N of the disk. As a result, a reproduction output of 34 dB was
obtained.
[0288] The recording and reproduction characteristics of the
magnetic disk were measured. As a result, when the recording was
performed with a linear recording density of 700 kFCI, the
attenuation amount of the reproduction output after 100 hours was
2% of the initial reproduction output. For the purpose of
comparison, the same measurement was performed for a magnetic disk
provided with no underlying layer. As a result, the attenuation
amount of the reproduction output after 100 hours was 4% of the
initial reproduction output. Therefore, the recording and
reproduction characteristics after the passage of a long period of
time were improved as a result of the suppression of
demagnetization due to the thermal fluctuation.
[0289] The unit of inversion of magnetization was measured with a
magnetic force microscope (MFM). As a result, 2 to 3 individuals of
magnetic grains were inverted at once with respect to the recording
magnetic field applied when 1 bit date was recorded. This value is
sufficiently small as compared with 5 to 10 individuals of the
conventional unit of inversion of magnetization. Accordingly, the
portion (zigzag pattern) corresponding to the boundary between the
adjoining units of inversion of magnetization was remarkably small
as compared with the conventional magnetic disk. Neither thermal
fluctuation nor demagnetization due to heat was caused. This result
is based on the small dispersion of the grain diameter of the
magnetic grain of the Co.sub.69Cr.sub.17Pt.sub.11Ta.sub.3 film 3 as
the magnetic layer. The error rate of the disk was measured. As a
result, an obtained value was not more than 1.times.10.sup.-5 when
no signal processing was performed.
[0290] In the embodiments described above, the compound for
constructing the thin film (underlying base film), for example, the
mixture of CoO and SiO.sub.2 was sintered and used as the target
during the formation of the thin film (underlying base film).
However, also in the first embodiment and other embodiments,
compounds for constructing the thin film, for example, CoO and
SiO.sub.2 may be singly sintered respectively and used as targets
so that the film may be formed by means of the two-target
simultaneous sputtering (co-sputtering).
[0291] In the embodiments described above, the glass was used for
the substrate. However, a metal substrate composed of, for example,
Al or Al alloy, or a plastic substrate composed of, for example,
polycarbonate or amorphous polyolefin may be used. The embodiments
described above are illustrative of the case in which the
underlying layer (thin film) is formed on the glass substrate.
However, a substrate may be prepared by using a material for
forming the underlying layer, and a predetermined film such as the
magnetic film may be formed thereon.
[0292] The same or equivalent effect is obtained even when iron
oxide or nickel oxide is used in place of cobalt oxide in the
embodiment in which cobalt oxide is used for the crystal grains of
the underlying layer (inorganic compound thin film). Alternatively,
the lattice constant of the crystal grain may be controlled by
adding, to cobalt oxide, a metal having a different ion radius, for
example, chromium, iron, nickel, or oxide thereof.
[0293] The same or equivalent effect is obtained even when aluminum
oxide, titanium oxide, tantalum oxide, zinc oxide, or a combination
thereof is used in place of silicon oxide in the embodiment in
which silicon oxide is used as the oxide existing in the crystal
grain boundary. Especially, when the crystal grain boundary is
formed with a mixture of silicon oxide and zinc oxide, the spacing
distance between crystal grains can be easily controlled by
changing the mixing ratio thereof.
[0294] In the embodiments described above, the film having the
granular structure is used for the magnetic layer, in which
Co--Cr--Pt, CoCrPtTa-based material, or oxide exists to surround
the crystal grains. However, other than the above, a three-element
material such as Co--Cr--Ta or a five-element material such as
Co--Cr--Pt--Ta--Si may be used. In the embodiment based on the use
of the Co--SiO.sub.2-based granular type magnetic layer, it is also
possible to add, to cobalt, an element such as platinum, palladium,
gadolinium, samarium, praseodymium, neodymium, terbium, dysprosium,
holmium, yttrium, and lanthanum. Up to the present, the granular
type magnetic film has not been used for the magnetic layer of the
magnetic recording medium because of the small coercive force.
However, it is possible to improve the magnetic anisotropy of the
magnetic grains in the granular type magnetic film by adding the
element as described above. Actually, when a system obtained by
adding platinum to cobalt was used, then the magnetic anisotropy of
the magnetic grains was increased, and the coercive force was
increased as well.
[0295] In the embodiments described above, Ar was used for the
sputtering gas when the protective film was formed. However, a gas
containing nitrogen or a gas containing nitrogen and hydrogen may
be used. When the gas containing nitrogen or the gas containing
nitrogen and hydrogen is used, then an obtained film is dense, and
it is possible to improve the protecting performance, because the
grains are fine and minute.
Industrial Applicability
[0296] The magnetic recording medium of the present invention has
the magnetic layer composed of the magnetic grains which are fine
and minute, in which the dispersion of the grain diameter is
reduced, and which have the desired crystalline orientation
advantageous for the high density recording, owing to the presence
of the underlying layer. As for the magnetic layer, the unit of
inversion of magnetization of the magnetic grains is small, the
noise is low, the thermal fluctuation is low, and the thermal
demagnetization is low. Therefore, the magnetic layer is suitable
for the high density recording. The magnetic recording medium of
the present invention is provided with the specified layer in order
to avoid the growth of the layer having no desired crystal
structure at the initial stage of the growth of the underlying
layer. Therefore, it is possible to thin the thickness of the
entire magnetic recording medium. Owing to the provision of the
control layer, the magnetic layer can be epitaxially grown from the
underlying layer via the control layer while mitigating the
difference in lattice constant between the underlying layer and the
magnetic layer. The texture having the desired concave/convex
structure is formed on the surface of the magnetic recording medium
of the present invention. Therefore, the magnetic recording medium
of the present invention is advantageous for the high density
recording and the reproduction therefor. The magnetic recording
medium of the present invention makes it possible to mitigate the
recording demagnetization which would be otherwise caused by the
achievement of high density recording. The MR element or the GMR
element, which has the high reproducing sensitivity, can be used
for the reproduction. The perpendicular magnetic recording medium
of the present invention makes it possible to reproduce information
at high S/N. The magnetic recording medium and the magnetic
recording apparatus installed with the magnetic recording medium of
the present invention make it possible to perform the super high
density recording having a surface recording density exceeding 40
Gbits/inch.sup.2.
[0297] When the method for producing the magnetic recording medium
of the present invention is used, it is possible to obtain the
magnetic recording medium provided with the underlying layer in
which the grain diameter is minute and the uniform crystal grains
are arranged in the honeycomb configuration. Since the fractal
feature of the crystal grains of the underlying layer is
appropriate, the magnetic layer is formed while reflecting the
feature, comprising the magnetic grains which are arranged finely
in a well-regulated manner. When the reactive. ECR sputtering
method is used, then the time required for the film formation can
be further shortened, and it is possible to improve the
productivity. The protective film of the magnetic recording medium,
which is obtained by the production method of the present
invention, uniformly covers the entire magnetic film, although the
film thickness is only about 5 nm. Therefore, it is possible to
protect the magnetic film in a well-suited manner, and it is
possible to shorten the spacing distance between the magnetic head
and the magnetic recording medium. Thus, it is possible to improve
the recording density.
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