U.S. patent application number 10/891199 was filed with the patent office on 2004-12-23 for magnetic recording medium and method of producing the same.
Invention is credited to Wako, Hitoshi, Yoshida, Shinya.
Application Number | 20040258964 10/891199 |
Document ID | / |
Family ID | 19136180 |
Filed Date | 2004-12-23 |
United States Patent
Application |
20040258964 |
Kind Code |
A1 |
Yoshida, Shinya ; et
al. |
December 23, 2004 |
Magnetic recording medium and method of producing the same
Abstract
A magnetic recording medium having high C/N ratio
characteristics particularly in a short wavelength range and
capable of attaining a further higher-density recording as a
magnetic recording tape produced by forming a magnetic layer by a
vapor deposition method and other magnetic recording media of the
next generation and a method of producing the same, wherein a
magnetic layer is formed by a vapor deposition method on a
nonmagnetic supporting body made of a polymer substrate, which has
a configuration of comprising a nonmagnetic supporting body, an
under layer formed on the nonmagnetic supporting body, containing
Co and O and having an atomic ratio of O/Co of 0.4 or more and a
magnetic layer containing Co and O, wherein the film thickness of
the under layer is made 50 nm or less and the maximum incident
angle is made 70.degree. or less in the vapor deposition
method.
Inventors: |
Yoshida, Shinya; (Miyagi,
JP) ; Wako, Hitoshi; (Miyagi, JP) |
Correspondence
Address: |
SONNENSCHEIN NATH & ROSENTHAL LLP
P.O. BOX 061080
WACKER DRIVE STATION, SEARS TOWER
CHICAGO
IL
60606-1080
US
|
Family ID: |
19136180 |
Appl. No.: |
10/891199 |
Filed: |
July 14, 2004 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
10891199 |
Jul 14, 2004 |
|
|
|
10271172 |
Oct 15, 2002 |
|
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Current U.S.
Class: |
428/833 ;
427/128 |
Current CPC
Class: |
G11B 5/7368 20190501;
G11B 5/73923 20190501; G11B 5/656 20130101; G11B 5/85 20130101 |
Class at
Publication: |
428/694.0TS ;
427/128; 428/694.0SC |
International
Class: |
B05D 005/12 |
Foreign Application Data
Date |
Code |
Application Number |
Oct 16, 2001 |
JP |
P2001-318485 |
Claims
1-3. (cancelled)
4. A method of producing a magnetic recording medium produced by
forming a magnetic layer by a vapor deposition method on a
nonmagnetic supporting body made of a polymer substrate, comprising
the steps of: forming on a nonmagnetic supporting body an under
layer containing Co and O and having an atomic ratio of O/Co of 0.4
or more; and forming a magnetic layer containing Co and O on said
under layer.
5. A method of producing a magnetic recording medium as set forth
in claim 4, wherein an under layer having a film thickness of 50 nm
or less is formed in the step of forming said under layer.
6. A method of producing a magnetic recording medium as set forth
in claim 4, wherein the under layer component is deposited with the
maximum incident angle of 70.degree. or less with respect to said
nonmagnetic supporting body by the vapor deposition method for
forming in the step of forming said under layer.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The present invention relates to a magnetic recording medium
and a method of producing the same, particularly relates to a
magnetic tape medium made by forming a magnetic layer comprised of
a metal magnetic thin film on a nonmagnetic supporting body and
other metal thin film type magnetic recording media and a method of
producing the same.
[0003] 2. Description of the Related Art
[0004] In a magnetic recording field, strong demands have been made
on higher-density recording every year along with an increase in
the amount of recording information.
[0005] In accordance with the above, a magnetic recording medium
produced by forming a thin film of ferromagnetic metal by a thin
film forming method using plating, vacuum deposition method,
sputtering method, ion-plating method and other vacuum thin film
forming methods (hereinafter, also referred to as a thin film type
medium) has been becoming a main stream instead of those produced
by a widely used method of dispersing magnetic crystal grain in a
binder to coat (hereinafter, also referred to as a coating type
medium).
[0006] The thin film type medium having a ferromagnetic metal thin
film has excellent coercive force and squareness ratio, etc. and
does not require mixing of a binder which is not a magnetic
material in its magnetic layer as in the coating type medium, so it
is possible to heighten the filling density of a magnetic material
(in other words, a magnetization amount per unit volume) and to
make a film thickness of the magnetic layer remarkably thin
compared with that of the coating type medium. Thus, it has an
excellent electromagnetic conversion characteristic in a short
wavelength range expected to be broadly used in the future.
[0007] Furthermore, the above thin film type medium has a
characteristic that its recording degauss is remarkably small.
[0008] From the above advantages, the thin film type medium having
a ferromagnetic metal thin film will be a main stream in magnetic
recording media for high-density recording in the future without
doubt.
[0009] In a magnetic recording system using a magnetic recording
tape, a kind of thin film type media, so-called obliquely
evaporated tape has been put into practice so as to improve
electromagnetic conversion characteristic and attain a higher
output in the short wavelength range.
[0010] FIG. 1 is a cross-sectional view of the above obliquely
evaporated tape.
[0011] A magnetic layer 3 as a ferromagnetic metal thin film is
formed on a nonmagnetic supporting body 1, a protective film 4 made
of carbon, etc. is formed thereon, a top coat layer 5 made of
lubricant, etc. is formed thereon.
[0012] On the other hand, a back coat layer 6 is formed on the back
surface of the nonmagnetic supporting body 1.
[0013] The magnetic recording medium having the above configuration
is cut to be a tape shape to form the obliquely evaporated
tape.
[0014] The nonmagnetic supporting body 1 is comprised of a high
polymer film such as a polyester film, polyamide film, polyimide
film, etc.
[0015] The magnetic layer 3 is a ferromagnetic metal thin film
formed by the so-called oblique evaporation method of moving the
nonmagnetic supporting body 1 in a predetermined direction and
depositing a magnetic metal on the surface of the nonmagnetic
supporting body 1 from an oblique direction by a vacuum deposition
method.
[0016] As the magnetic metal composing the above magnetic layer 3,
Co and Ni are widely used.
[0017] To form the above magnetic layer 3 on the nonmagnetic
supporting body 1 by the vapor deposition method, a method of
using, for example, Co and Ni as a vapor deposition source and
spraying an oxygen gas to the moving nonmagnetic supporting body is
widely used.
[0018] When forming a film as the above, the magnetic layer 3
becomes to have a configuration in which magnetic crystal grain of
.alpha.-Co (or Co--Ni) and nonmagnetic CoO (or CoNiO) exist
together.
[0019] Here, an object of introducing oxygen into the film is to
improve magnetic characteristics by introducing nonmagnetic crystal
grain and making crystal grain finer and to reduce medium noise by
interrupting magnetic bonding between magnetic crystal grain.
[0020] In the thus obtained obliquely evaporated tape currently in
practical use, an inclination angle of an easy axis of
magnetization of the magnetic layer 3 is about 20 to
30.degree..
[0021] Since magnetic crystal grain is oriented obliquely with
respect to the surface of the nonmagnetic supporting body in the
magnetic tape produced by the above oblique evaporation method,
higher-density recording becomes possible compared with magnetic
tapes of the related art wherein magnetic crystal grain is oriented
in the longitudinal direction of the supporting body of the tape
shape.
[0022] In the fields of VTR and computer storage, however, a tape
having a larger capacity, more compact body and lighter weight by
further higher-density recording are desired for the above
obliquely evaporated tape.
[0023] To realize higher-density recording than that of currently
used magnetic recording tapes, a higher output and lower noise of
the medium, that is, a higher C/N ratio is essential, particularly,
a higher C/N ratio in the short wavelength range is
significant.
SUMMARY OF THE INVENTION
[0024] An object of the present invention is to provide a magnetic
recording medium having high C/N ratio characteristics also in a
short wavelength range so as to be able to attain further
higher-density recording as a large capacity magnetic recording
medium of the next generation, such as a magnetic recording tape
produced by forming a magnetic layer by a deposition method, etc.
and a method of producing the same.
[0025] To attain the above object, according to the present
invention, there is provided a magnetic recording medium produced
by forming a magnetic layer by a vapor deposition method on a
nonmagnetic supporting body made of a polymer substrate, comprising
a nonmagnetic supporting body; an under layer formed on the
nonmagnetic supporting body, containing Co and O and having an
atomic ratio of O/Co of 0.4 or more; and a magnetic layer formed on
the under layer and containing Co and O.
[0026] Preferably, the under layer has a film thickness of 50 nm or
more in the above magnetic recording medium of the present
invention.
[0027] Preferably, the under layer is a film deposited by a maximum
incident angle of an under layer component of 70.degree. or less
with respect to the nonmagnetic supporting body in the vapor
deposition method in the above magnetic recording medium of the
present invention.
[0028] In the above magnetic recording medium of the present
invention, the under layer containing Co and O and having the
atomic ratio of O/Co of 0.4 or more is formed between the
nonmagnetic supporting body and the magnetic layer.
[0029] By forming the under layer, magnetic characteristics of the
medium, particularly coercive force Hc is largely improved. By
heightening the Hc, recording magnetization capable of overcoming a
demagnetizing field in the medium can be formed and the
demagnetization becomes larger in a short wavelength range wherein
a length of recording magnetization is short. Therefore, an
increase of output and reduction of noise particularly in the short
wavelength range can be obtained and further higher-density
recording becomes possible.
[0030] It is considered that since the Co/CoO magnetic layer is
stacked on the nonmagnetic supporting body via the under layer, the
magnetic layer can grow under a condition where crystallizing
orientation is good from an initial stage of the growth.
[0031] Furthermore, to decrease noise of a medium, it is efficient
to make crystal grain of the magnetic layer fine and uniform and an
improvement of tape surface nature is also significant.
[0032] Generally, fine crystal grain called filler is added inside
the nonmagnetic supporting body of the evaporated tape or coated as
an undercoat thereof so as to make the surface of the nonmagnetic
supporting body rough and improve durability of the tape. However,
when forming a film on the nonmagnetic supporting body having a
rough surface as such with a maximum incident angle of 90.degree.
at the time of deposition, a surface shape of the magnetic layer
reflects the surface nature of the nonmagnetic supporting body due
to a shadowing effect of deposition crystal grain. Namely, since
deposition crystal grain easily adhere to the places where the
filler exists while hard to adhere to the places in the shadow of
the filler, on the surface of the magnetic layer, places where the
filler exists become convex while the shadow thereof becomes
concave. The noise of the medium largely affects the surface nature
of the medium, so poorness of the surface nature leads to an
increase of the noise.
[0033] Furthermore, due to the shadowing effect, at the initial
stage of growth of the magnetic layer, sizes of grain vary much and
holes and other disadvantages easily arise, becoming the noise
source of the medium at the time of recording and reproducing.
[0034] By adopting the configuration in which an under layer is
formed between the nonmagnetic supporting body and the magnetic
layer, and furthermore, by forming on the nonmagnetic supporting
body a film of the under layer with a maximum incident angle of
70.degree. or less at the time of deposition, the above shadowing
effect is suppressed and a magnetic layer having a smoother surface
and homogeneous configuration from an initial stage of its growth
can be produced in the above magnetic recording medium of the
present invention.
[0035] As explained above, according to the magnetic recording
medium of the present invention, particularly high C/N ratio
characteristics can be attained even in a short wavelength range
and further higher-density recording becomes possible,in a magnetic
recording medium produced by forming a magnetic layer by a vapor
deposition method.
[0036] Also, to attain the above object, there is provided a method
of producing a magnetic recording medium produced by forming a
magnetic layer by a vapor deposition method on a nonmagnetic
supporting body made of a polymer substrate, comprising the steps
of forming on a nonmagnetic supporting body an under layer
containing Co and O and having an atomic ratio of O/Co of 0.4 or
more; and forming a magnetic layer containing Co and O on the under
layer.
[0037] Preferably, an under layer having a film thickness of 50 nm
or less is formed in the step of forming the under layer.
[0038] Preferably, the under layer component is deposited with the
maximum incident angle of 70.degree. or less with respect to the
nonmagnetic supporting body by the vapor deposition method for
forming in the step of forming the under layer.
[0039] In the above production method of the magnetic recording
medium of the present invention, the under layer containing Co and
O and having an atomic ratio of O/Co of 0.4 or more is formed on
the nonmagnetic supporting body and the magnetic layer containing
Co and O is formed on the under layer in the method of producing
the magnetic recording medium produced by forming a magnetic layer
by a vapor deposition on a nonmagnetic supporting body.
[0040] According to the above production method of the magnetic
recording medium of the present invention, since the under layer
containing Co and O and having an atomic ratio of O/Co of 0.4 or
more is formed between the nonmagnetic supporting body and the
magnetic layer, magnetic characteristics of the medium,
particularly a coercive force Hc can be largely improved.
[0041] Also, particularly by forming on the nonmagnetic supporting
body a film of the under layer with a maximum incident angle of
70.degree. or less at the time of depositing, the above shadowing
effect can be suppressed and a magnetic layer having a smooth
surface and homogeneous configuration from an initial stage of its
growth can be produced.
[0042] Therefore, according to the production method of a magnetic
recording medium of the present invention, particularly high C/N
ratio characteristics can be attained even in a short wavelength
range and further higher-density recording becomes possible in the
magnetic recording medium produced by forming a magnetic layer by
the vapor deposition method.
BRIEF DESCRIPTION OF THE DRAWINGS
[0043] These and other objects and features of the present
invention will become clearer from the following description of the
preferred embodiments given with reference to the attached
drawings, in which:
[0044] FIG. 1 is a cross-sectional view of an obliquely evaporated
tape according to the related art;
[0045] FIG. 2 is a cross-sectional view of an obliquely evaporated
tape as a magnetic recording medium according to an embodiment of
the present invention;
[0046] FIG. 3A is a schematic view of an overall oblique
evaporation apparatus used for forming a film of an under layer and
a magnetic layer in a production method of a magnetic recording
medium according to an embodiment of the present invention and FIG.
3B is a schematic view of a key portion; and
[0047] FIG. 4 is a view of a coercive force of a sample plotted
with respect to a film thickness of an under layer in an
example.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0048] Below, a magnetic recording medium and a method of producing
the same according to the present embodiment will be explained.
[0049] FIG. 2 is a cross-sectional view of an obliquely evaporated
tape as a magnetic recording medium according to the present
embodiment.
[0050] An under layer 2 is formed on a nonmagnetic supporting body
1, a magnetic layer 3 as a ferromagnetic metal thin film is formed
thereon, a protective film 4 made of carbon, etc. is formed
thereon, and a topcoat layer 5 made of lubricant, etc. is formed
thereon.
[0051] On the other hand, a back coat layer 6 is formed on the back
surface of the nonmagnetic supporting body 1.
[0052] The magnetic recording medium having the above configuration
is cut to be a tape shape to form the obliquely evaporated
tape.
[0053] As the nonmagnetic supporting body 1, a polymer supporting
body, etc. formed by polymer materials, represented by polyesters
such as polyethylene terephthalate, polyethylene-2,6-naphthalate,
polyolefins such as polypropylene, and cellulose derivatives such
as cellulose triacetate and cellulose diacetate, vinyl-base resin,
polyimides, polyamides, polycarbonate, etc. can be mentioned. Also,
Cu, Al, Zn and other metals and glass, boron nitride, Si carbide
and other ceramics can be used.
[0054] Fine crystal grain called filler is added inside the above
nonmagnetic supporting body 1 or coated as an undercoat thereof so
as to make the surface of the nonmagnetic supporting body rough and
improve durability of the tape.
[0055] The under layer 2 is a film containing Co and O and having
an atomic ratio of O/Co of 0.4 or more. The film thickness is about
10 to 100 nm, preferably equal to or less than 50 nm.
[0056] For example, the nonmagnetic supporting body 1 can be formed
by the so-called oblique evaporation method, wherein the
nonmagnetic supporting body 1 is moved in a predetermined direction
and an under layer component is deposited on its surface from an
oblique direction by a vacuum deposition method. At this time, it
is preferable that the maximum incident angle with respect to the
nonmagnetic supporting body 1 is 70.degree. or less for depositing
to form.
[0057] The magnetic layer 3 is a film containing Co and O, having
an atomic ratio of O/Co of, for example, about 0.2 to 0.4 and a
film thickness of, for example, about 30 to 200 nm.
[0058] The magnetic layer 3 can be formed by the oblique
evaporation method in the same way as the under layer 2.
[0059] In the obliquely evaporated tape obtained as the above, an
inclination angle of an easy axis of magnetization of the magnetic
layer 3 is about 20 to 30.degree..
[0060] As the protective film 4, carbon, Al.sub.2O.sub.3, Ti--N,
Mo--C, Cr--C, SiO, SiO.sub.2, SiN, etc. can be mentioned, but it is
not limited to those and any of conventionally well known materials
can be used.
[0061] The top coat layer 5 is comprised of an antirust or
lubricant and any of those normally used as its material for
magnetic recording media of this kind can be used.
[0062] Also, on a surface of an opposite side of the magnetic layer
of the nonmagnetic supporting body, a back coat layer 6 comprised
of nonmagnetic powder (for example, silica and carbon black) and a
binder can be provided for improving cursoriality of the
medium.
[0063] As a recording/reproducing system of the magnetic recording
medium according to the above present embodiment, not to mention a
conventional system using an inductive head, a system of the next
generation using an MR head or GMR head as a reproduction head can
be applied.
[0064] The magnetic recording medium according to the above present
embodiment is capable of largely improving magnetic
characteristics, particularly a coercive force, of the magnetic
layer, attaining high C/N ratio characteristics particularly even
in a short wavelength range and realizing further higher-density
recording by being provided with an under layer containing Co and O
made of a film formed with a maximum incident angle of 70.degree.
or less between the nonmagnetic supporting body and the magnetic
layer in the obliquely evaporated tape.
[0065] Next, a method of producing the magnetic recording medium
according to the above present embodiment will be explained.
[0066] FIG. 3A is a schematic view of an overall oblique
evaporation apparatus used for forming a film of an under layer and
a magnetic layer in a production method of a magnetic recording
medium according to an embodiment of the present invention and FIG.
3B is a schematic view of a key portion.
[0067] Inside the apparatus, a polymer base film 11 to be a
nonmagnetic supporting body fed from a feeding roll 10 is guided to
an outer circumference of a cylindrical cooling can 12 and wound by
a winding roll 13.
[0068] Here, an electron beam 15 emitted from an electron gun 14 is
irradiated on a crucible 16 filled with Co, etc. to evaporate,
then, adhered substances vapor 17 such as Co is shot obliquely to a
surface of the base film 11 so that the adhered substances are
deposited on the surface of the base film 11.
[0069] At this time, the upper limit (a maximum incident angle
.theta..sub.max) and the lower limit (a minimum incident angle
.theta..sub.min) of the incident angle of the adhered substance
vapor 17 to the surface of the base film 11 are controlled by an
opening position of a block plate 18 and, for example in a process
of forming an under layer, the maximum incident angle of the
adhered substance vapor 17 to be the under layer is preferably
70.degree. or less with respect to the base film 11 for
deposition.
[0070] Also, the adhered substances are deposited while spraying an
oxygen gas on the moving nonmagnetic supporting body from an oxygen
supplier 19 near the surface to be deposited.
[0071] When forming a film while spraying an oxygen gas, for
example in a process of forming a film of the magnetic layer 3,
magnetic grain of .alpha.-Co and nonmagnetic CoO exist together.
Thus, by making crystal grain fine by introducing the nonmagnetic
crystal grain, magnetic characteristics are improved and magnetic
bonding between magnetic grain is interrupted whereby the medium
noise can be reduced.
[0072] Here, by adjusting the amount of oxygen gas to be supplied,
a content ratio of oxygen in the film to be deposited, for example
the O/Co atomic ratio, can be controlled.
[0073] For example, at the time of forming the under layer 2, an
oxygen gas is supplied by 0.8 to 1.5 litter/minute and the atomic
ratio of O/Co is, for example, set to be about 0.6 to 1.0. While
when forming the magnetic layer 3, an oxygen gas is supplied by 0.4
to 0.6 litter/minute and the atomic ratio of O/Co is, for example,
set to be about 0.2 to 0.4.
[0074] The protective layer 4, the topcoat layer 5, and the back
coat layer 6 may be formed by using a well known method.
[0075] A predetermined magnetic recording medium can be obtained by
processing the thus obtained laminated body to be a predetermined
shape and size. As the shape of the magnetic recording medium, any
of the shapes normally used as magnetic recording media may be
adopted, such as a tape shape, film shape, sheet shape, card shape,
disk shape, and drum shape.
[0076] Thus, according to the method of producing the magnetic
recording medium of the present embodiment, a magnetic recording
medium produced by forming a magnetic layer by a deposition method
capable of attaining high C/N ratio characteristics particularly in
a short wavelength range and further high-density recording can be
produced.
EXAMPLE
[0077] A sample of an obliquely evaporated tape of a configuration
shown in FIG. 2 was produced by the following process.
[0078] Polyethylene naphthalate (PEN) was used for a polymer base
film 11 to be a nonmagnetic supporting body 1.
[0079] In the oblique evaporation apparatus shown in FIG. 3, the
base film 11 runs on a cooling can 12 after vacuum pumping a
chamber. A part of the cooling can 12 was opened by a block plate
18, a crucible 16 containing a Co ingot was placed beneath the
opening portion, and the Co was fused by an electron beam 15 to
form a Co thin film on the base film 11.
[0080] By changing a positional relationship of the opening portion
made by the block plate 18 and the crucible 16, an incident angle
of the vapor deposition was changed.
[0081] Also, at the time of vapor deposition, oxygen was introduced
from around the minimum incident angle and maximum incident angle
of the opening portion to appropriately oxidize the Co.
[0082] Common conditions in forming a film of the under layer 2 and
forming the magnetic layer 3 are shown below.
[0083] Film Forming Conditions
[0084] ultimate degree of vacuum: 2.times.10.sup.-3 (Pa)
[0085] degree of vacuum at vapor deposition: 3.times.10.sup.-2
(Pa)
[0086] ingot: Co100
[0087] Note that a thickness of each layer was controlled by a line
speed at the time of vapor deposition.
[0088] First, the under layer 2 was formed. The film was formed by
changing an incident angle within a range of 0 to 50.degree. and a
flow amount of oxygen introduction within a range of 0.5 to 2.0
litter/minute.
[0089] Furthermore, the magnetic layer 3 comprised of Co/CoO was
formed on the under layer 2.
[0090] The vapor deposition incident angle was 45 to 90.degree. and
the a flow amount of oxygen introduction was 0.5 litter/minute.
[0091] Furthermore, a protective film 4 comprised of carbon C was
formed on the magnetic layer 3, a back coat layer 6 was formed on
the back surface of the nonmagnetic supporting body 1, a topcoat
layer 6 was formed by coating a lubricant on the protective film 4,
then the result was cut to complete as a tape.
[0092] Here, a film thickness of the under layer 2 was made to be 5
to 100 nm, the magnetic layer 3 to be 30 to 200 nm and the
protective film 4 to be constant at 10 nm.
[0093] [Test on Dependency on under Layer Thickness]
[0094] Next, changes of magnetic characteristics of the sample at
the time of changing a film thickness of the under layer 2 in a
range of 10 to 100 nm were examined under conditions that a film
thickness of the magnetic layer was constant at 100 nm, the atomic
ratio of O/Co of the under layer was constant at 0.9 and the
maximum incident angle at forming the under layer was constant at
50.degree..
[0095] FIG. 4 is a view wherein a coercive force Hc of the sample
is plotted with respect to the film thickness of the under layer
2.
[0096] It is understood from FIG. 4 that the Hc largely rises due
to the under layer having a thickness of only 10 nm compared with
the case without an under layer.
[0097] When the thickness of the under layer is made further
thicker, the Hc of the medium increases up to around the thickness
of 50 nm, while when the thickness of the under layer is thicker
than that, the Hc does not change much.
[0098] Next, samples in which the magnetic layer 3 had a constant
film thickness of 100 nm, the under layer 2 had a constant atomic
ratio of O/Co of 0.9 and a constant maximum incident angle of
50.degree. at forming the under layer, and a film thickness of the
under layer 2 was changed from 5 nm (sample 1), 10 nm (sample 2),
25 nm (sample 3), 50 nm (sample 4), 75 nm (sample 5) and 100 nm
(sample 6) and a sample not having the under layer 2 (comparative
sample 1) were prepared and a relationship of the atomic of O/Co,
medium noise, and C/N ratio was measured.
[0099] A recording wavelength of a signal to be recorded was set to
about 0.3 .mu.m, noise was made to be an average of values when a
signal frequency was .+-.1 MHz and the comparative sample 1 was
made to be 0 dB.
[0100] The results are shown in Table 1.
1 TABLE 1 Under layer Medium C/N thickness noise ratio [nm] [dB]
[dB] Sample 1 5 -1.6 +2.3 Sample 2 10 -2.3 +3.2 Sample 3 25 -2.2
+3.0 Sample 4 50 -2.0 +3.0 Sample 5 75 -1.9 +2.7 Sample 6 100 -1.8
+2.8 Comparative No under layer 0 0 Sample 1
[0101] From Table 1, the C/N ratio increases up to the under layer
film thickness of 50 nm in the same way as Hc, but the rising pitch
is not very much large. In a range where the under layer thickness
is 50 nm or more, the Hc starts to peak and the C/N ratio also
starts to peak.
[0102] In terms of a volume recording density, the thinner a
thickness of a medium, the better.
[0103] A large improvement of the C/N ratio is attained with an
under layer of 10 nm while improvements of Hc and C/N ratio are not
observed in a range of 50 nm or more, so the thickness of the under
layer is preferably 50 nm or less.
[0104] [Test of Dependency on Maximum Incident Angle at Forming
Under Layer]
[0105] Next, samples in which the magnetic layer 3 had a constant
film thickness of 100 nm, the under layer 2 had a constant film
thickness of 30 nm and a constant atomic ratio of O/Co of 0.9 and a
constant minimum incident angle of 30.degree. at forming the under
layer, and the maximum incident angle at forming the under layer 2
was changed to 40.degree. (sample 7), 50.degree. (sample 8),
60.degree. (sample 9), 70.degree. (sample 10), 80.degree. (sample
11), and 90.degree. (sample 12) and a sample having no under layer
2 (comparative sample 2) were prepared and a relationship of the
maximum incident angle, medium noise, and C/N ratio was
measured.
[0106] A recording wavelength of a signal to be recorded was set to
about 0.3 .mu.m, noise was made to be an average of values when a
signal frequency was .+-.2 MHz and the comparative sample 2 was
made to be 0 dB.
[0107] A recording head was a MIG head having a gap length of 0.2
.mu.m and a reproduction head was a multilayer type head having a
gap length of 0.18 .mu.m.
2 TABLE 2 Maximum incident angle Medium C/N at forming under noise
ratio layer [dB] [dB] Sample 7 40.degree. -2.0 +2.5 Sample 8
50.degree. -2.1 +3.2 Sample 9 60.degree. -2.3 +2.7 Sample 10
70.degree. -2.0 +2.8 Sample 11 80.degree. -1.0 +1.9 Sample 12
90.degree. -0.7 +1.5 Comparative No under layer 0 0 sample 2
[0108] From Table 2, there was a tendency that the medium noise
decreases when the maximum incident angle at forming the under
layer was 70.degree. or less.
[0109] A cause of decreasing the medium noise is assumed as
below.
[0110] Namely, it is assumed that by lowering the maximum incident
angle at forming the under layer, the shadowing effect of vapor
deposition crystal grain is decreased and an under layer having a
smooth surface and a uniformed configuration is formed.
Consequently, the surface nature of the magnetic layer formed
thereon is improved and, moreover, the film becomes to have a
uniformed configuration even from an initial stage of its film
growth.
[0111] Also, since the medium Hc is improved as the medium noise is
reduced, an output in the short wavelength range is improved and
high C/N ratio can be obtained.
[0112] [Test of Dependency on O/Co atomic ratio of under Layer]
[0113] Next, a relationship of O/Co atomic ratio of the under layer
2, magnetic characteristics, and electromagnetic conversion
characteristic was examined.
[0114] The Hc was measured for the magnetic characteristics. The Hc
widely increased when the O/Co atomic ratio was 0.4 or more. It is
conceivable that this composition is a composition by which the
under layer is demagnetized.
[0115] Next, samples in which the magnetic layer 3 had a constant
film thickness of 100 nm, the under layer 2 had a constant film
thickness of 30 nm, the maximum incident angle was constant at
50.degree. at forming the under layer 2, and the O/Co atomic ratio
in the under layer 2 was changed by changing an oxygen introduction
amount at forming the under layer 2 to obtain the atomic ratio of
0.4 (sample 13), 0.6 (sample 14), 1.0 (sample 15), 1.2 (sample 16),
0.2 (comparative sample 4) and 0.3 (comparative sample 5) and a
sample having no under layer 2 (comparative sample 3) were prepared
and a relationship of the O/Co atomic ratio, medium noise, and C/N
ratio was measured.
[0116] A recording wavelength of a signal to be recorded was set to
about 0.3 .mu.m and noise was made to be an average of values when
a signal frequency was .+-.2 MHz.
[0117] The results are shown in Table 3.
3 TABLE 3 Medium O/Co Atomic Ratio Noise C/N Ratio of under Layer
[%] [dB] {dB] Sample 13 0.4 -1.3 +2.5 Sample 14 0.6 -2.0 +3.2
Sample 15 1.0 -2.1 +3.4 Sample 16 1.2 -1.9 +3.0 Comparative No
under layer 0 0 sample 3 Comparative 0.2 +0.8 -0.4 sample 4
Comparative 0.3 -0.2 +0.2 sample 5
[0118] It was learned that when the O/Co atomic ratio became less
than 0.4 and the under layer 2 remained magnetized, the medium
noise was liable to increase.
[0119] When the medium noise is not increased, it is considered
that the ratio of O and Co becomes O/Co=0.4 and there was almost no
magnetization of the under layer remained.
[0120] Also, the C/N ratio is largely increased when the O/Co
atomic ratio is 0.4 or more.
[0121] As the medium noise decreases, the under layer becomes
demagnetized in this composition range, so an output becomes
improved in a short wavelength range due to an improvement of the
Hc of the magnetic layer.
[0122] Due to the above results, the O/Co ratio of the under layer
is made to be 0.4 or more in the present invention.
[0123] The present invention was explained by the above embodiment
and examples as above, but the present invention is not limited to
those.
[0124] For example, a film thickness of the under layer is not
limited to 50 nm or less but may be made thicker than 50 nm. Also,
the maximum incident angle at forming the under layer is not
limited to 70.degree. or less but an incident angle of larger than
70.degree. is also possible.
[0125] Other than the above, a variety of modifications can be made
within the scope of the present invention.
[0126] According to the magnetic recording medium of the present
invention, high C/N ratio characteristics is attained particularly
in a short wavelength range and a further higher-density recording
is possible in a magnetic recording medium produced by forming a
magnetic layer by a vapor deposition method.
[0127] According to the production method of a magnetic recording
medium of the present invention, it is possible to produce a
magnetic recording medium capable of attaining high C/N ratio
characteristics particularly in a short wavelength range and a
further higher-density recording in a magnetic recording medium
produced by forming a magnetic layer by a vapor deposition
method.
* * * * *