U.S. patent application number 10/308119 was filed with the patent office on 2003-05-01 for magnetic recording medium and magnetic storage apparatus.
This patent application is currently assigned to Fujitsu Limited. Invention is credited to Abarra, E. Noel, Kaitsu, Isatake, Mizoshita, Yoshifumi, Okamoto, Iwao, Sato, Hisateru.
Application Number | 20030082410 10/308119 |
Document ID | / |
Family ID | 30773298 |
Filed Date | 2003-05-01 |
United States Patent
Application |
20030082410 |
Kind Code |
A1 |
Sato, Hisateru ; et
al. |
May 1, 2003 |
Magnetic recording medium and magnetic storage apparatus
Abstract
A magnetic recording medium is constructed to include at least
one exchange layer structure and a magnetic layer provided on the
exchange layer structure. The exchange layer structure includes a
ferromagnetic layer and a non-magnetic coupling layer provided on
the ferromagnetic layer. At least one of the ferromagnetic layer
and the magnetic layer has a granular layer structure in which
ferromagnetic crystal grains are uniformly distributed within a
non-magnetic base material.
Inventors: |
Sato, Hisateru;
(Kawasaki-shi, JP) ; Kaitsu, Isatake;
(Kawasaki-shi, JP) ; Abarra, E. Noel;
(Kawasaki-shi, JP) ; Okamoto, Iwao; (Kawasaki-shi,
JP) ; Mizoshita, Yoshifumi; (Kawasaki-shi,
JP) |
Correspondence
Address: |
Patrick G. Burns
Greer, Burns & Crain, Ltd.
Suite 2500
300 South Wacker Drive
Chicago
IL
60606
US
|
Assignee: |
Fujitsu Limited
Kawasaki
JP
|
Family ID: |
30773298 |
Appl. No.: |
10/308119 |
Filed: |
December 2, 2002 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10308119 |
Dec 2, 2002 |
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09588451 |
Jun 6, 2000 |
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09588451 |
Jun 6, 2000 |
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09425788 |
Oct 22, 1999 |
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Current U.S.
Class: |
428/827 ;
G9B/5.241; G9B/5.289 |
Current CPC
Class: |
G11B 5/66 20130101; G11B
5/74 20130101; G11B 2005/0002 20130101; Y10T 428/12861 20150115;
Y10T 428/12465 20150115; G11B 5/82 20130101; Y10S 428/90 20130101;
Y10T 428/265 20150115 |
Class at
Publication: |
428/694.0EC ;
428/694.00R; 428/694.0TM; 428/694.0BM |
International
Class: |
G11B 005/66 |
Foreign Application Data
Date |
Code |
Application Number |
Jun 8, 1999 |
JP |
11-161329 |
Apr 7, 2000 |
JP |
2000-107072 |
Claims
What is claimed is:
1. A magnetic recording medium comprising: at least one exchange
layer structure; and a magnetic layer provided on the exchange
layer structure, said exchange layer structure including a
ferromagnetic layer and a non-magnetic coupling layer provided on
the ferromagnetic layer, at least one of said ferromagnetic layer
and said magnetic layer having a granular layer structure in which
ferromagnetic crystal grains are uniformly distributed within a
non-magnetic base material.
2. The magnetic recording medium as claimed in claim 1, wherein
said ferromagnetic crystal grains are made of a material selected
from a group of Co, Ni, Fe, Ni-based alloys, Fe-based alloys, and
Co-based alloys including CoCrTa, CoCrPt and CoCrPt--M, where M=B,
Mo, Nb, Ta, W, Cu and alloys thereof.
3. The magnetic recording medium as claimed in claim 1, wherein
said non-magnetic base material is made of a material selected from
a group of ceramic materials and oxide materials.
4. The magnetic recording medium as claimed in claim 1, wherein
said non-magnetic coupling layer is made of a material selected
from a group of Ru, Rh, Ir, Ru-based alloys, Rh-based alloys. and
Ir-based alloys.
5. The magnetic recording medium as claimed in claim 1, wherein
magnetization directions of the ferromagnetic layer and the
magnetic layer are mutually antiparallel.
6. The magnetic recording medium as claimed in claim 5, wherein
said non-magnetic coupling layer is made of a material selected
from a group of Ru, Rh, Ir, Ru-based alloys, Rh-based alloys and
Ir-based alloys, and has a thickness in a range of approximately
0.4 to 1.0 nm.
7. The magnetic recording medium as claimed in claim 1, wherein
magnetization directions of the ferromagnetic layer and the
magnetic layer are mutually parallel.
8. The magnetic recording medium as claimed in claim 7, wherein
said non-magnetic coupling layer is made of a material selected
from a group of Ru, Rh, Ir, Ru-based alloys, Rh-based alloys and
Ir-based alloys, and has a thickness in a range of approximately
0.2 to 0.4 nm and 1.0 to 1.7 nm.
9. The magnetic recording medium as claimed in claim 1, wherein
said non-magnetic coupling layer is added with a ceramic material
or an oxide material.
10. The magnetic recording medium as claimed in claim 1, which
further comprises: an underlayer provided above said substrate; a
ferromagnetic layer having a granular layer structure and provided
on the underlayer; and a non-magnetic intermediate layer provided
between the underlayer and the ferromagnetic layer, said
non-magnetic intermediate layer being made of a CoCr--M alloy
having a hcp structure and having a thickness of approximately 1 to
5 nm, where M=B, Mo, Nb, Ta, W, Cu or alloys thereof.
11. The magnetic recording medium as claimed in claim 10, which
further comprises: a NiP layer provided between said substrate and
said underlayer, said NiP layer being mechanically textured or
oxidized.
12. The magnetic recording medium as claimed in claim 10, wherein
said underlayer is made of an alloy having a B2 structure and
selected from a group of NiAl and FeAl.
13. The magnetic recording medium as claimed in claim 1, which
comprises at least a first exchange layer structure and a second
exchange layer structure provided between the first exchange layer
structure and the magnetic layer, said first and second exchange
layer structures having a granular layer structure, said second
exchange layer structure having a granular layer with a magnetic
anisotropy smaller than that of a granular layer of the first
exchange layer structure, and the granular layers of said first and
second exchange layer structures having magnetization directions
which are mutually antiparallel.
14. The magnetic recording medium as claimed in claim 1, which
comprises at least a first exchange layer structure and a second
exchange layer structure provided between the first exchange layer
structure and the magnetic layer, said first and second exchange
layer structures having a granular layer structure, said second
exchange layer structure having a granular layer with a remanence
magnetization and thickness product smaller than that of a granular
layer of the first exchange layer structure, and the granular
layers of said first and second exchange layer structures having
magnetization directions which are mutually antiparallel.
15. A magnetic storage apparatus comprising: at least one magnetic
recording medium including at least one exchange layer structure,
and a magnetic layer provided on the exchange layer structure, said
exchange layer structure including a ferromagnetic layer and a
non-magnetic coupling layer provided on the ferromagnetic layer, at
least one of said ferromagnetic layer and said magnetic layer
having a granular layer structure in which ferromagnetic crystal
grains are uniformly distributed within a non-magnetic base
material.
Description
[0001] This application is a Continuation-In-Part Application of a
U.S. patent application Ser. No. 09/425,788 filed Oct. 22,
1999.
BACKGROUND OF THE INVENTION
[0002] 1. Field of the Invention
[0003] The present invention generally relates to magnetic
recording media and magnetic storage apparatuses, and more
particularly to a magnetic recording medium and a magnetic storage
apparatus which are suited for high-density recording.
[0004] 2. Description of the Related Art
[0005] Due to the development of the information processing
technology, there are increased demands for high-density magnetic
recording media. Characteristics required of the magnetic recording
media to satisfy such demands include low noise, high coercivity,
high remanence magnetization, and high resolution in the case of a
hard disk, for example.
[0006] The recording density of longitudinal magnetic recording
media, such as magnetic disks, has been increased considerably, due
to the reduction of medium noise and the development of
magnetoresistive and high-sensitivity spin-valve heads. A typical
magnetic recording medium is comprised of a substrate, an
underlayer, a magnetic layer, and a protection layer which are
successively stacked in this order. The underlayer is made of Cr or
a Cr-based alloy, and the magnetic layer is made of a Co-based
alloy.
[0007] Various methods have been proposed to reduce the medium
noise. For example, Okamoto et al., "Rigid Disk Medium For 5
Gbit/in.sup.2 Recording", AB-3, Intermag '96 Digest proposes
decreasing the grain size and size distribution of the magnetic
layer by reducing the magnetic layer thickness by the proper use of
an underlayer made of CrMo, and a U.S. Pat. No. 5,693,426 proposes
the use of an underlayer made of NiAl. Further, Hosoe et al.,
"Experimental Study of Thermal Decay in High-Density Magnetic
Recording Media", IEEE Trans. Magn. Vol.33, 1528 (1997), for
example, proposes the use of an underlayer made of CrTiB. The
underlayers described above also promote c-axis orientation of the
magnetic layer in a plane which increases the remanence
magnetization and the thermal stability of written bits. In
addition, proposals have been made to reduce the thickness of the
magnetic layer, to increase the resolution or to decrease the width
of transition between written bits. Furthermore, proposals have
been made to decrease the exchange coupling between grains by
promoting more Cr segregation in the magnetic layer which is made
of the CoCr-based alloy.
[0008] However, as the grains of the magnetic layer become smaller
and more magnetically isolated from each other, the written bits
become unstable due to thermal activation and to demagnetizing
fields which increase with linear density. Lu et al., "Thermal
Instability at 10 Gbit/in.sup.2 Magnetic Recording", IEEE Trans.
Magn. Vol.30, 4230 (1994) demonstrated, by micromagnetic
simulation, that exchange-decoupled grains having a diameter of 10
nm and ratio K.sub.uV/k.sub.BT-60 min 400 kfci di-bits are
susceptible to significant thermal decay, where K.sub.u denotes the
magnetic anisotropy constant, V denotes the average magnetic grain
volume, k.sub.B denotes the Boltzmann constant, and T denotes the
temperature. The ratio K.sub.uV/k.sub.BT is also referred to as a
thermal stability factor.
[0009] It has been reported in Abarra et al., "Thermal Stability of
Narrow Track Bits in a 5 Gbit/in.sup.2 Medium", IEEE Trans. Magn.
Vol.33, 2995 (1997) that the presence of intergranular exchange
interaction stabilizes written bits, by MFM studies of annealed 200
kfci bits on a 5 Gbit/in.sup.2 CoCrPtTa/CrMo medium. However, more
grain decoupling is essential for recording densities of 20
Gbit/in.sup.2 or greater.
[0010] The obvious solution has been to increase the magnetic
anisotropy of the magnetic layer. But unfortunately, the increased
magnetic anisotropy places a great demand on the head write field
which degrades the "overwrite" performance which is the ability to
write over previously written data.
[0011] In addition, the coercivity of thermally unstable magnetic
recording medium increases rapidly with decreasing switching time,
as reported in He et al., "High Speed Switching in Magnetic
Recording Media", J. Magn. Magn. Mater. Vol.155, 6 (1996), for
magnetic tape media, and in J. H. Richter, "Dynamic Coervicity
Effects in Thin Film Media", IEEE Trans. Magn. Vol.34, 1540 (1997),
for magnetic disk media. Consequently, the adverse effects are
introduced in the data rate, that is, how fast data can be written
on the magnetic layer and the amount of head field required to
reverse the magnetic grains.
[0012] On the other hand, another proposed method of improving the
thermal stability increases the orientation ratio of the magnetic
layer, by appropriately texturing the substrate under the magnetic
layer. For example, Akimoto et al., "Relationship Between Magnetic
Circumferential Orientation and Magnetic Thermal Stability", J.
Magn. Magn. Mater. (1999), in press, report through micromagnetic
simulation, that the effective ratio K.sub.uV/k.sub.BT is enhanced
by a slight increase in the orientation ratio. This further results
in a weaker time dependence for the coercivity which improves the
overwrite performance of the magnetic recording medium, as reported
in Abarra et al., "The Effect of Orientation Ratio on the Dynamic
Coercivity of Media for >15 Gbit/in.sup.2 Recording", EB-02,
Intermag '99, Korea.
[0013] Furthermore, keepered magnetic recording media have been
proposed for thermal stability improvement. The keeper layer is
made up of a magnetically soft layer parallel to the magnetic
layer. This soft layer can be disposed above or below the magnetic
layer. Oftentimes, a Cr isolation layer is interposed between the
soft layer and the magnetic layer. The soft layer reduces the
demagnetizing fields in written bits on the magnetic layer.
However, coupling the magnetic layer to a continuously-exchanged
coupled soft layer defeats the purpose of decoupling- the grains of
the magnetic layer. As a result, the medium noise increases.
[0014] Various methods have been proposed to improve the thermal
stability and to reduce the medium noise. However, there was a
problem in that the proposed methods do not provide a considerable
improvement of the thermal stability of written bits, thereby
making it difficult to greatly reduce the medium noise. In
addition, there was another problem in that some of the proposed
methods introduce adverse effects on the performance of the
magnetic recording medium due to the measures taken to reduce the
medium noise.
[0015] More particularly, in order to obtain a thermally stable
performance of the magnetic recording medium, it is conceivable to
(i) increase the magnetic anisotropy constant K.sub.u, (ii)
decrease the temperature T or, (iii) increase the grain volume V of
the magnetic layer. However, measure (i) increases the coercivity,
thereby making it more difficult to write information on the
magnetic layer. In addition, measure (ii) is impractical since in
magnetic disk drives, for example, the operating temperature may
become greater than 60.degree. C. Furthermore, measure (iii)
increases the medium noise as described above. As an alternative
for measure (iii), it is conceivable to increase the thickness of
the magnetic layer, but this would lead to deterioration of the
resolution.
SUMMARY OF THE INVENTION
[0016] Accordingly, it is a general object of the present invention
to provide a novel and useful magnetic recording medium and
magnetic storage apparatus, in which the problems described above
are eliminated.
[0017] Another and more specific object of the present invention is
to provide a magnetic recording medium and a magnetic storage
apparatus, which can improve the thermal stability of written bits
without increasing the medium noise, so as to enable a reliable
high-density recording without introducing adverse effects on the
performance of the magnetic recording medium, that is,
unnecessarily increasing the magnetic anisotropy.
[0018] Still another object of the present invention is to provide
a magnetic recording medium comprising at least one exchange layer
structure and a magnetic layer provided on the exchange layer
structure, where the exchange layer structure includes a
ferromagnetic layer and a non-magnetic coupling layer provided on
the ferromagnetic layer, at least one of the ferromagnetic layer
and the magnetic layer has a granular layer structure in which
ferromagnetic crystal grains are uniformly distributed within a
non-magnetic base material. According to the magnetic recording
medium of the present invention, it is possible to provide a
magnetic recording medium which can improve the thermal stability
of written bits, so as to enable reliable high-density recording
without degrading the overwrite performance. By employing the
granular layer structure which is effective in reducing noise for
at least the ferromagnetic layer of the exchange layer structure
and the magnetic layer which is provided on the exchange layer
structure, it is possible to further reduce the medium noise while
further improving the thermal stability of the written bits.
[0019] The magnetic recording medium may comprise at least a first
exchange layer structure and a second exchange layer structure
provided between the first exchange layer structure and the
magnetic layer, where the first and second exchange layer
structures have a granular layer structure, the second exchange
layer structure has a granular layer with a magnetic anisotropy
smaller than that of a granular layer of the first exchange layer
structure, and the granular layers of the first and second exchange
layer structures have magnetization directions which are mutually
antiparallel.
[0020] The magnetic recording medium may comprise at least a first
exchange layer structure and a second exchange layer structure
provided between the first exchange layer structure and the
magnetic layer, where the first and second exchange layer
structures have a granular layer structure, the second exchange
layer structure has a granular layer with a remanence magnetization
and thickness product smaller than that of a granular layer of the
first exchange layer structure, and the granular layers of the
first and second exchange layer structures have magnetization
directions which are mutually antiparallel.
[0021] A further object of the present invention is to provide a
magnetic storage apparatus comprising at least one magnetic
recording medium of any of the types described above. According to
the magnetic storage apparatus of the present invention, it is
possible to provide a magnetic recording medium which can improve
the thermal stability of written bits, so as to enable reliable
high-density recording without degrading the overwrite
performance.
[0022] Other objects and further features of the present invention
will be apparent from the following detailed description when read
in conjunction with the accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0023] FIG. 1 is a cross sectional view showing an important part
of a first embodiment of the magnetic recording medium according to
the present invention;
[0024] FIG. 2 is a cross sectional view showing an important part
of a second embodiment of the magnetic recording medium according
to the present invention;
[0025] FIG. 3 is a diagram showing an in-plane magnetization curve
of a single CoPt layer having a thickness of 10 nm on a Si
substrate;
[0026] FIG. 4 is a diagram showing an in-plane magnetization curve
of two CoPt layers separated by a Ru layer having a thickness of
0.8 nm;
[0027] FIG. 5 is a diagram showing an in-plane magnetization curve
of two CoPt layers separated by a Ru layer having a thickness of
1.4 nm;
[0028] FIG. 6 is a diagram showing an in-plane magnetization curve
two CoCrPt layers separated by a Ru having a thickness of 0.8
nm;
[0029] FIG. 7 is a cross sectional view showing an important part
of a third embodiment of the magnetic recording medium according to
the present invention;
[0030] FIG. 8 is a cross sectional view showing an important part
of an embodiment of the magnetic storage apparatus according to the
present invention; and
[0031] FIG. 9 is a plan view showing the important part of the
embodiment of the magnetic storage apparatus.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0032] First, a description will be given of the operating
principle of the present invention.
[0033] The present invention submits the use of layers with
antiparallel magnetization structures. For example, S. S. P.
Parkin, "Systematic Variation of the Strength and Oscillation
Period of Indirect Magnetic Exchange Coupling through the 3d, 4d,
and 5d Transition Metals", Phys. Rev. Lett. Vol.67, 3598 (1991)
describes several magnetic transition metals such as Co, Fe and Ni
that are coupled through thin non-magnetic interlayers such as Ru
and Rh. On the other hand, a U.S. Pat. No. 5,701,223 proposes a
spin-valve which employs the above described layers as laminated
pinning layers to stabilize the sensor.
[0034] For a particular Ru or Ir layer thickness between two
ferromagnetic layers, the magnetizations can be made parallel or
antiparallel. For example, for a structure made up of two
ferromagnetic layers of different thickness with antiparallel
magnetizations, the effective grain size of a magnetic recording
medium can be increased without significantly affecting the
resolution. A signal amplitude reproduced from such a magnetic
recording medium is reduced due to the opposite magnetizations, but
this can be rectified by adding another layer of appropriate
thickness and magnetization direction, under the laminated magnetic
layer structure, to thereby cancel the effect of one of the layers.
As a result, it is possible to increase the signal amplitude
reproduced from the magnetic recording medium, and to also increase
the effective grain volume. Thermally stable written bits can
therefore be realized.
[0035] The present invention increases the thermal stability of
written bits by exchange coupling the magnetic layer to another
ferromagnetic layer with an opposite magnetization or, by a
laminated ferrimagnetic structure. The ferromagnetic layer or the
laminated ferrimagnetic structure is made up of exchange-decoupled
grains as the magnetic layer. In other words, the present invention
uses an exchange pinning ferromagnetic layer or a ferrimagnetic
multilayer to improve the thermal stability performance of the
magnetic recording medium.
[0036] FIG. 1 is a cross sectional view showing an important part
of a first embodiment of a magnetic recording medium according to
the present invention.
[0037] The magnetic recording medium includes a non-magnetic
substrate 1, a first seed layer 2, a NiP layer 3, a second seed
layer 4, an underlayer 5, a non-magnetic intermediate layer 6, a
ferromagnetic layer 7, a non-magnetic coupling layer 8, a magnetic
layer 9, a protection layer 10, and a lubricant layer 11 which are
stacked in the order shown in FIG. 1.
[0038] For example, the non-magnetic substrate 1 is made of Al, Al
alloy or glass. This non-magnetic substrate 1 may or may not be
mechanically textured. The first seed layer 2 is made of Cr or Ti,
for example, especially in the case where the non-magnetic
substrate 1 is made of glass. The NiP layer 3 is preferably
oxidized and may or may not be mechanically textured. The second
seed layer 4 is provided to promote a (001) or a (112) texture of
the underlayer 5 when using a B2 structure alloy such as NiAl and
FeAl for the underlayer 5. The second seed layer 4 is made of an
appropriate material similar to that of the first seed layer 2.
[0039] In a case where the magnetic recording medium is a magnetic
disk, the mechanical texturing provided on the non-magnetic
substrate 1 or the NiP layer 3 is made in a circumferential
direction of the disk, that is, in a direction in which tracks of
the disk extend.
[0040] The non-magnetic intermediate layer 6 is provided to further
promote epitaxy, narrow the grain distribution of the magnetic
layer 9, and orient the anisotropy axes of the magnetic layer 9
along a plane parallel to the recording surface of the magnetic
recording medium. This non-magnetic intermediate layer 6 is made of
a hcp structure alloy such as CoCr--M, where M=B, Mo, Nb, Ta, W or
alloys thereof, and has a thickness in a range of 1 to 5 nm.
[0041] The ferromagnetic layer 7 is made of Co, Ni, Fe, Co-based
alloy, Ni-based alloy, Fe-based alloy or the like. In other words,
alloys such as CoCrTa, CoCrPt and CoCrPt--M, where M=B, Mo, Nb, Ta,
W, Cu or alloys thereof may be used for the ferromagnetic layer 7.
This ferromagnetic layer 7 has a thickness in a range of 2 to 10
nm. The non-coupling magnetic layer 8 is made of Ru, Ir, Rh,
Rubased alloy, Ir-based alloy, Rh-based alloy or the like. This
non-magnetic coupling layer 8 preferably has a thickness in a range
of 0.4 to 1.0 nm, and preferably approximately 0.8 nm. For this
particular thickness range of the non-magnetic coupling layer 8,
the magnetizations of the ferromagnetic layer 7 and the magnetic
layer 9 are antiparallel. The ferromagnetic layer 7 and the
non-magnetic coupling layer 8 form an exchange layer structure.
[0042] The magnetic layer 9 is made of Co or a Co-based alloys such
as CoCrTa, CoCrPt and CoCrPt--M, where M=B, Mo, Nb, Ta, W, Cu or
alloys thereof. The magnetic layer 9 has a thickness in a range of
5 to 30 nm. Of course, the magnetic layer 9 is not limited to a
single-layer structure, and a multi-layer structure may be used for
the magnetic layer 9.
[0043] The protection layer 10 is made of C, for example. In
addition, the lubricant layer 11 is made of an organic lubricant,
for example, for use with a magnetic transducer such as a
spin-valve head. The protection layer 10 and the lubricant layer 11
form a protection layer structure on the recording surface of the
magnetic recording medium.
[0044] Obviously, the layer structure under the exchange layer
structure is not limited to that shown in FIG. 1. For example, the
underlayer 5 may be made of Cr or Cr-based alloy and formed to a
thickness in a range of 5 to 40 nm on the substrate 1, and the
exchange layer structure may be provided on this underlayer 5.
[0045] Next, a description will be given of a second embodiment of
the magnetic recording medium according to the present
invention.
[0046] FIG. 2 is a cross sectional view showing an important part
of the second embodiment of the magnetic recording medium. In FIG.
2, those parts which are the same as those corresponding parts in
FIG. 1 are designated by the same reference numerals, and a
description thereof will be omitted.
[0047] In this second embodiment of the magnetic recording medium,
the exchange layer structure includes two non-magnetic coupling
layers 8 and 8-1, and two ferromagnetic layers 7 and 7-1, which
form a ferrimagnetic multilayer. This arrangement increases the
effective magnetization and signal, since the magnetizations of the
two non-magnetic coupling layers 8 and 8-1 cancel. each other
instead of a portion of the magnetic layer 9. As a result, the
grain volume and thermal stability of magnetization of the magnetic
layer 9 are effectively increased. More bilayer structures made up
of the pair of ferromagnetic layer and non-magnetic coupling layer
may be provided additionally to increase the effective grain
volume, as long as the easy axis of magnetization are appropriately
oriented for the subsequently provided layers.
[0048] The ferromagnetic layer 7-1 is made of a material similar to
that of ferromagnetic layer 7, and has a thickness range selected
similarly to the ferromagnetic layer 7. In addition, the
non-magnetic coupling layer 8-1 is made of a material similar to
that of the non-magnetic coupling layer 8, and has a thickness
range selected similarly to the non-magnetic coupling layer 8.
Within the ferromagnetic layers 7-1 and 7, the c-axes are
preferably in-plane and the grain growth columnar.
[0049] In this embodiment, the magnetic anisotropy of the
ferromagnetic layer 7-1 is preferably higher than that of the
ferromagnetic layer 7. However, the magnetic anisotropy of the
ferromagnetic.layer 7-1 may be the same as or, be higher than that
of, the magnetic layer 9.
[0050] Furthermore, a remanence magnetization and thickness product
of the ferromagnetic layer 7 may be smaller than that of the
ferromagnetic layer 7-1.
[0051] FIG. 3 is a diagram showing an in-plane magnetization curve
of a single CoPt layer having a thickness of 10 nm on a Si
substrate. In FIG. 3, the ordinate indicates the magnetization
(emu), and the abscissa indicates the magnetic field (Oe).
Conventional magnetic recording media show a behavior similar to
that shown in FIG. 3.
[0052] FIG. 4 is a diagram showing an in-plane magnetization curve
of two CoPt layers separated by a Ru layer having a thickness of
0.8 nm, as in the case of the first embodiment of the magnetic
recording medium. In FIG. 4, the ordinate indicates the
magnetization (Gauss), and the abscissa indicates the magnetic
field (Oe). As may be seen from FIG. 4, the loop shows shifts near
the magnetic field which indicate the antiparallel coupling.
[0053] FIG. 5 is a diagram showing an in-plane magnetization curve
of two CoPt layers separated by a Ru layer having a thickness of
1.4 nm. In FIG. 5, the ordinate indicates the magnetization (emu),
and the abscissa indicates the magnetic field (Oe). As may be seen
from FIG. 5, the magnetizations of the two CoPt layers are
parallel.
[0054] FIG. 6 is a diagram showing an in-plane magnetization curve
for two CoCrPt layers separated by a Ru having a thickness of 0.8
nm, as in the case of the second embodiment of the magnetic
recording medium. In FIG. 6, the ordinate indicates the
magnetization (emu/cc), and the abscissa indicates the field (Oe).
As may be seen from FIG. 6, the loop shows shifts near the field
which indicate the antiparallel coupling.
[0055] From FIGS. 3 and 4, it may be seen that the antiparallel
coupling can be obtained by the provision of the exchange layer
structure. In addition, it may be seen by comparing FIG. 5 with
FIGS. 4 and 6, the non-magnetic coupling layer 8 is desirably in
the range of 0.4 to 0.9 nm in order to achieve the antiparallel
coupling.
[0056] Therefore, according to the first and second embodiments of
the magnetic recording medium, it is possible to effectively
increase the apparent grain volume of the magnetic layer by the
exchange coupling provided between the magnetic layer and the
ferromagnetic layer via the non-magnetic coupling layer, without
sacrificing the resolution. In other words, the apparent thickness
of the magnetic layer is increased with regard to the grain volume
of the magnetic layer so that a thermally stable medium can be
obtained, and in addition, the effective thickness of the magnetic
layer is maintained since cancellation of signals especially from
the bottom layers is achieved. This allows higher linear density
recording that is otherwise not possible for thick media. As a
result, it is possible to obtain a magnetic recording medium with
reduced medium noise and thermally stable performance.
[0057] Next, a description will be given of a third embodiment of
the magnetic recording medium according to the present invention.
In this third embodiment, at least one of the ferromagnetic layer
and the magnetic layer of the first or second embodiment described
above has a granular layer structure. The granular layer structure
employed in this third embodiment has ferromagnetic crystal grains
uniformly distributed within a non-magnetic base material, so as to
further isolate the magnetic grains.
[0058] In a case where both the ferromagnetic layer and the
magnetic layer have the granular layer structure, the magnetization
directions of the granular layers can be made mutually parallel or
mutually antiparallel, similarly to the first and second
embodiments described above, by making the non-magnetic coupling
layer which is made of Ru or the like and disposed between the
granular layers to have a predetermined thickness. As a result, it
is possible to increase the effective volume, thereby improving the
thermal stability of written bits and reducing the medium
noise.
[0059] It is not essential for both the ferromagnetic layer and the
magnetic layer to have the granular layer structure, and the
granular layer structure may be employed for only one of the
ferromagnetic layer and the magnetic layer. When using only one
granular layer, it is desirable to make the magnetic layer, which
forms the recording layer, to have the granular layer
structure.
[0060] In this embodiment, the granular layer is magnetically
exchange coupled in an opposite magnetization direction
(antiparallel) to that of the other granular layer or the
CoCr-based magnetic layer, so as to improve the thermal stability
of the written bits. In other words, this embodiment is provided
with a pinning structure for improving the thermal stability
performance of the magnetic recording medium, and is also provided
with the granular layer structure for further reducing the medium
noise.
[0061] The granular layer structure refers to a layer structure in
which ferromagnetic crystal grains are uniformly distributed within
a non-magnetic base material, as taught in a Japanese Laid-Open
Patent Application No. 10-92637. A granular medium is obtained by
applying this granular layer structure to the recording medium of
the magnetic storage apparatus. In the conventional recording
medium which uses a CoCr-based magnetic material for the magnetic
recording layer, the Co and Cr segragations are used to promote
isolation of the magnetic grains and to reduce the noise. But in
the conventional recording medium, it was difficult to obtain a
desired isolation state of the magnetic grains.
[0062] On the other hand, in the granular medium according to the
present invention, the ferromagnetic crystal grains are positively
isolated by uniformly distributing the ferromagnetic crystal grains
(metal) within the base material such as SiO.sub.2 (ceramic
material), and thus, it is possible to realize a medium with
extremely low noise.
[0063] FIG. 7 is a cross sectional view showing an important part
of the third embodiment of the magnetic recording medium according
to the present invention.
[0064] The magnetic recording medium includes a non-magnetic
substrate 101, a first seed layer 102, a NiP layer 103, a second
seed layer 104, an underlayer 105, a non-magnetic intermediate
layer 106, a ferromagnetic layer 107, a non-magnetic coupling layer
108, a magnetic layer 109, a protection layer 110, and a lubricant
layer 111 which are stacked in this order as shown in FIG. 7.
[0065] For example, the non-magnetic substrate 101 is made of Al,
Al alloy or glass. The non-magnetic substrate 101 may or may not be
mechanically textured.
[0066] The first seed layer 102 is made of NiP, for example,
especially in the case where the non-magnetic substrate 101 is made
of glass. The NiP layer 103 may or may not be oxidized and may or
may not be mechanically textured. The second seed layer 104 is
provided to promote a (001) or a (112) texture of the underlayer
105 when the underlayer 105 is made of an alloy having the B2
structure, such as NiAl and FeAl. The second seed layer 104 is made
of a material similar to that of the first seed layer 102.
[0067] In a case where the magnetic recording medium is a magnetic
disk, the mechanical texturing provided on the non-magnetic
substrate 101 or the NiP layer 103 is made in a circumferential
direction of the disk, that is, in a direction in which tracks of
the disk extend.
[0068] The non-magnetic intermediate layer 106 is provided to
further promote epitaxy, narrow the grain distribution width of the
magnetic layer 109, and orient the anisotropy axes of the magnetic
layer 109 along a plane parallel to the recording surface of the
magnetic recording medium. However, it is not essential to provide
this non-magnetic intermediate layer 106. This non-magnetic
intermediate layer 106 is made of a hcp structure alloy such as
CoCr--M, where M=B, Mo, Nb, Ta, W, Cu or alloys thereof, and has a
thickness in a range of 1 to 5 nm.
[0069] The ferromagnetic layer 107 may be made of a granular layer
which is formed by uniformly distributing ferromagnetic crystal
grains into a non-magnetic base material. In this case, the
ferromagnetic crystal grains may be made of Co, Ni, Fe, Ni-based
alloys, Fe-based alloys, or Co-based alloys such as CoCrTa, CoCrPt
and CoCrPt--M, where M=B, Mo, Nb, Ta, W, Cu or alloys thereof. It
is preferable that the grain diameter of the ferromagnetic crystal
grain is in a range of approximately 2 to 30 nm. Further, the
non-magnetic base material may be made of a ceramic material such
as SiO.sub.2, Al.sub.2O.sub.3 and MgO or an oxide material such as
NiO. On the other hand, the ferromagnetic layer 107 may be made of
a CoCr-based magnetic material if not employing the granular layer
structure.
[0070] The granular layer structure changes form depending on
fundamental physical constants or properties, such as cohesive
energy, surface energy and elastic strain energy of the
ferromagnetic crystal grains and the non-magnetic base material.
Accordingly, an extremely large number of combinations of the
magnetic material used for the ferromagnetic crystal grains and the
ceramic or oxide material used for the non-magnetic base material
exist, and the combination may be appropriately adjusted to suit
the needs.
[0071] It is preferable that the granular layer structure is used
with priority for the magnetic layer 109, in which case the
ferromagnetic layer 107 may be made of a CoCr-based magnetic
material as described above. The reason for the preferable use of
the granular layer structure for the magnetic layer 109 is because,
due to the exchange coupling caused by the provision of the
non-magnetic coupling layer 108, it is the uppermost. magnetic
layer 109 which contributes most to the noise reduction.
[0072] Of course, the ferromagnetic layer 107 and the magnetic
layer 109 are not limited to a single-layer structure, and a
multi-layer structure may be used for each of the ferromagnetic
layer 107 and the magnetic layer 109.
[0073] The non-magnetic coupling layer 108 is made of Ru, Rh, Ir,
Ru-based alloys, Rh-based alloys, Ir-based alloys, or the like.
For-example, the non-magnetic coupling layer 108 may be added with
a ceramic material such as SiO.sub.2 and Al.sub.2O.sub.3 or an
oxide material such as NiO which are used for the granular layer
proposed in a Japanese Laid-Open Patent Application No. 10-149526.
The addition of the ceramic or oxide material to the non-magnetic
coupling layer 108 promotes the epitaxial growth of the
non-magnetic coupling layer 108 and the magnetic layer 109, thereby
further improving the signal-to-noise (S/N) ratio of the magnetic
recording medium.
[0074] The protection layer 110 and the lubricant layer 111 are
similar to those of the first and second embodiments described
above.
[0075] The ferromagnetic layer 107 may have a thickness in a range
of approximately 2 to 10 nm, and the magnetic layer 109 may have a
thickness in a range of approximately 5 to 30 nm.
[0076] In addition, the magnetization directions of the
ferromagnetic layer 107 and the magnetic layer 109 may be mutually
antiparallel or mutually parallel.
[0077] When making the magnetization directions of the
ferromagnetic layer 107 and the magnetic layer 109 mutually
antiparallel, the non-magnetic coupling layer 108 desirably is made
of a material selected from a group of Ru, Rh, Ir, Ru-based alloys,
Rh-based alloys and Ir-based alloys, and has a thickness in a range
of approximately 0.4 to 1.0 nm.
[0078] When making the magnetization directions of the
ferromagnetic layer 107 and the magnetic layer 109 mutually
parallel, the non-magnetic coupling layer 108 desirably is made of
a material selected from a group of Ru, Rh, Ir, Ru-based alloys,
Rh-based alloys and Ir-based alloys, and has a thickness in a range
of approximately 0.2 to 0.4 nm and 1.0 to 1.7 nm. Ru is desirably
used for the non-magnetic coupling layer 108.
[0079] The number of exchange layer structures having the granular
layer structure described above is of course not limited to one,
and first and second exchange layer structures of the second
embodiment described above may be provided with the granular layer
structure. In this case, it is preferable that the magnetic
anisotropy of the granular layer in the second exchange layer
structure is set smaller than that of the granular layer in the
first exchange layer structure which is disposed under the second
exchange layer structure. Furthermore, it is preferable that the
remanence magnetization and thickness product of the granular layer
in the second exchange layer structure is set smaller than that of
the granular layer in the first exchange layer structure which is
disposed under the second exchange layer structure.
[0080] Next, a description will be given of an embodiment of a
magnetic storage apparatus according to the present invention, by
referring to FIGS. 8 and 9. FIG. 8 is a cross sectional view
showing an important part of this embodiment of the magnetic
storage apparatus, and FIG. 9 is a plan view showing the important
part of this embodiment of the magnetic storage apparatus.
[0081] As shown in FIGS. 8 and 9, the magnetic storage apparatus
generally includes a housing 13. A motor 14, a hub 15, a plurality
of magnetic recording media 16, a plurality of recording and
reproducing heads 17, a plurality of suspensions 18, a plurality of
arms 19, and an actuator unit 20 are provided within the housing
13. The magnetic recording media 16 are mounted on the hub 15 which
is rotated by the motor 14. The recording and reproducing head 17
is made up of a reproducing head such as a MR or GMR head, and a
recording head such as an inductive head. Each recording and
reproducing head 17 is mounted on the tip end of a corresponding
arm 19 via the suspension 18. The arms 19 are moved by the actuator
unit 20. The basic construction of this magnetic storage apparatus
is known, and a detailed description thereof will be omitted in
this specification.
[0082] This embodiment of the magnetic storage apparatus is
characterized by the magnetic recording media 16. Each magnetic
recording medium 16 has the structure of the first through third
embodiments of the magnetic recording medium described above in
conjunction with FIGS. 1, 2 and 7. Of course, the number of
magnetic recording media 16 is not limited to three, and only one,
two or four or more magnetic recording media 16 may be
provided.
[0083] The basic construction of the magnetic storage unit is not
limited to that shown in FIGS. 8 and 9. In addition, the magnetic
recording medium used in the present invention is not limited to a
magnetic disk.
[0084] Further, the present invention is not limited to these
embodiments, but various variations and modifications may be made
without departing from the scope of the present invention.
* * * * *