U.S. patent application number 11/042793 was filed with the patent office on 2005-08-25 for perpendicular magnetic recording medium and method for manufacturing same.
This patent application is currently assigned to Fuji Electric Device Technology Co., Ltd.. Invention is credited to Sakai, Yasushi, Takenoiri, Shunji.
Application Number | 20050186450 11/042793 |
Document ID | / |
Family ID | 34857602 |
Filed Date | 2005-08-25 |
United States Patent
Application |
20050186450 |
Kind Code |
A1 |
Takenoiri, Shunji ; et
al. |
August 25, 2005 |
Perpendicular magnetic recording medium and method for
manufacturing same
Abstract
A perpendicular magnetic recording medium is disclosed in which
each magnetic crystal grain in the magnetic recording layer has a
multilayer structure and has a configuration like a truncated cone
shape, in which the crystal grain of the final layer deposited in
the film surface side at the final stage is smaller than the
diameter of the crystal grain in the initial layer deposited on the
substrate side at the initial stage. The invention improves S/N
(signal output to noise ratio) by enhancing signal output and
reducing noises. The medium is produced by a simple manufacturing
method suitable for mass production, and provides a medium of high
recording density by improving recording resolution.
Inventors: |
Takenoiri, Shunji; (Nagano,
JP) ; Sakai, Yasushi; (Nagano, JP) |
Correspondence
Address: |
ROSSI & ASSOCIATES
P.O. BOX 826
ASHBURN
VA
20146-0826
US
|
Assignee: |
Fuji Electric Device Technology
Co., Ltd.
Tokyo
JP
|
Family ID: |
34857602 |
Appl. No.: |
11/042793 |
Filed: |
January 25, 2005 |
Current U.S.
Class: |
428/827 ;
427/548; G9B/5.236; G9B/5.304 |
Current CPC
Class: |
G11B 5/656 20130101;
G11B 5/65 20130101; G11B 5/64 20130101; G11B 5/851 20130101 |
Class at
Publication: |
428/827 ;
427/548 |
International
Class: |
B05D 005/12; G11B
005/64 |
Foreign Application Data
Date |
Code |
Application Number |
Jan 28, 2004 |
JP |
JP 2004 - 020427 |
Claims
What is claimed is:
1. A perpendicular magnetic recording medium comprising a magnetic
recording layer that includes a plurality of magnetic crystal
grains formed on a nonmagnetic substrate, wherein each magnetic
crystal grain has a laminated multilayer structure, layers of which
vary in diameter such that layers at the nonmagnetic substrate side
of the magnetic recording layer have a larger diameter than layers
at the opposite side of the magnetic recording layer.
2. The medium according to claim 1, wherein the magnetic crystal
grains are formed in a truncated cone shape that is smaller at a
top face of the magnetic recording layer than at a bottom face of
the magnetic recording layer that faces the nonmagnetic
substrate.
3. The medium according to claim 1, wherein the magnetic recording
layer comprises magnetic materials containing ferromagnetic
particles that are dispersed in oxides or nitrides.
4. The medium according to claim 3, wherein the magnetic recording
layer is composed of a granular structure including a CoCrPt alloy
dispersed in SiO.sub.2.
5. The medium according to claim 1, wherein the magnetic crystal
grains exhibit saturation magnetization that varies from the layer
that faces the nonmagnetic substrate side towards the layer at the
opposite side of the magnetic recording layer.
6. The medium according to claim 5, wherein the magnetic crystal
grains exhibit saturation magnetization that increases from the
layer that faces the nonmagnetic substrate side towards the layer
at the opposite side of the magnetic recording layer.
7. The medium according to claim 1, comprising a soft magnetic
backing layer, a nonmagnetic intermediate layer, and the magnetic
recording layer comprising the magnetic crystal grains sequentially
laminated on the nonmagnetic substrate.
8. A method for manufacturing a perpendicular magnetic recording
medium that comprises a magnetic recording layer formed on a
nonmagnetic substrate, the magnetic recording layer including a
plurality of magnetic crystal grains, the method including a
process for forming the magnetic recording layer that comprises:
conducting a plurality of film depositions, from a first deposition
conducted at an initial stage of the process at a substrate side to
a final deposition conducted at a film surface side, and varying
concentration of oxygen or nitrogen that is contained in a gas used
in the film deposition steps, during the plurality of film
depositions, wherein the plurality of film depositions while
concentration is varied produces a plurality of magnetic crystal
grains each having a laminated multilayer structure, in which each
layer corresponds to one of the film depositions, and a diameter of
the magnetic crystal grain deposited in the final deposition is
less than a diameter of the magnetic crystal grain deposited in the
first deposition.
9. The method according to claim 8, wherein the concentration of
oxygen or nitrogen increases with progression of the film
depositions.
10. A method for manufacturing a perpendicular magnetic recording
medium that comprises a magnetic recording layer formed on a
nonmagnetic substrate, the magnetic recording layer including a
plurality of magnetic crystal grains, the method comprising a
process for forming the magnetic recording layer that comprises:
conducting a plurality of film depositions, from a first deposition
conducted at an initial stage of the process at a substrate side to
a final deposition conducted at a film surface side, and exposure
of a surface formed during each of said film depositions to a gas
containing oxygen or nitrogen before conducting the next film
deposition, wherein the plurality of film depositions and
intervening exposures to oxygen or nitrogen produces a plurality of
magnetic crystal grains each having a laminated multilayer
structure, in which each layer corresponds to one of the film
depositions, and a diameter of the magnetic crystal grain deposited
in the final deposition is less than a diameter of the magnetic
crystal grain deposited in the first deposition.
11. The method according to claim 10, wherein the concentration of
oxygen or nitrogen increases with progression of the exposures.
12. The method according to claim 8, wherein each film deposition
from the first to the final is conducted in a gas containing oxygen
using a granular material including a CoCrPt alloy dispersed in
SiO.sub.2.
13. The method according to claim 10, wherein each film deposition
from the first to the final is conducted in a gas containing the
oxygen using a granular material including a CoCrPt alloy dispersed
in SiO.sub.2.
Description
CROSS REFERENCE TO RELATED APPLICATIONS
[0001] This application claims priority from Japanese application
Serial No. JP PA 2004-010427, filed on Jan. 23, 2004, the contents
of which are incorporated herein in their entirety.
BACKGROUND OF THE INVENTION
[0002] A. Field of the Invention
[0003] The invention relates generally to a perpendicular magnetic
recording medium mounted on various magnetic recording apparatuses
and a method for manufacturing such a medium.
[0004] B. Description of the Related Art
[0005] In technologies for achieving high density magnetic
recording, perpendicular magnetic recording is drawing attention as
a substitute for conventional longitudinal magnetic recording.
[0006] Alloy materials of CoCrPt, CoCrTa, and the like have been
used for a magnetic recording layer material of a perpendicular
magnetic recording medium employing perpendicular magnetic
recording. In these alloy materials, the nonmagnetic substance
chromium segregates into a grain boundary and magnetically isolates
each crystal grain for a magnetic recording medium so that it
exhibits required characteristics such as high coercivity (Hc). The
chromium segregation into the grain boundary has been promoted in
the in-plane medium by controlling the deposition process,
including heating and substrate bias voltage application.
[0007] In a perpendicular magnetic recording medium, however, the
heating or substrate bias voltage application as conducted in the
in-plane medium can segregate only a small amount of chromium,
resulting in high media noise. To solve this problem, a granular
medium has been proposed in which magnetic isolation of crystal
grains is promoted by segregation of oxides or nitrides into the
grain boundary. In a granular film of CoCrPt--SiO.sub.2, for
example, SiO.sub.2 segregates surrounding the CoCrPt crystal grain.
The granular film does not use phase separation (magnetic phase
separation) of an alloy material, but features addition of an
amorphous material such as an oxide or nitride that hardly makes a
solid solution with an alloy material. The granular medium has been
confirmed to reduce media noise in comparison with a conventional
medium comprising a magnetic recording layer of CoCr alloy
material. Therefore, the granular medium is deemed promising for
magnetic recording media.
[0008] In order to further raise the recording density of a
magnetic recording medium, magnetic isolation of the magnetic
crystal grains in the magnetic recording layer must be promoted and
magnetization reversal unit must be decreased. For media using a
continuous film such as a granular medium, magnetic isolation of
magnetic crystal grains in the magnetic recording layer is enhanced
by promoting segregation of the nonmagnetic substance at the grain
boundary. For discrete media such as patterned media the known
methods attempt to decrease magnetization reversal unit by
processing the magnetic recording layer using a technique such as
etching employed in a semiconductor process. It has been difficult,
however, to promote segregation in the granular magnetic recording
layer by a process that is suitable for mass production.
[0009] To achieve a high density recording medium requires
increased signal output at the high recording density in addition
to noise reduction by promoting segregation. Perpendicular magnetic
recording media are known to produce stable bits in high density
recording. At the same time this means that taking out magnetic
flux in high density recording is difficult. While a medium
structure is required that achieves low noise and high signal
output even at high recording density, such a medium structure has
never been proposed.
[0010] There have been various problems in achieving the desired
result. Because deposition of a granular film with substrate
heating raises problems of mixing a nonmagnetic phase with an alloy
phase and/or oxidation or nitridation of cobalt, the film must be
deposited without substrate heating. However, film deposition
without substrate heating cannot achieve sufficient isolation
between the alloy phase and the nonmagnetic phase, which raises a
problem of insufficient noise reduction. To solve this problem,
heat treatment has been conducted at a high temperature from
400.degree. C. to 600.degree. C. after depositing the layers up to
the magnetic recording layer (or to a protective film) without
heating. Thus, proposals have been made to obtain a granular medium
that has enough isolation of magnetic particles from nonmagnetic
matrix and allows high density recording by carrying out heat
treatment at a temperature not lower than 400.degree. C. for 5 to
60 minutes in Japanese Unexamined Patent Application Publication
No. 2000-306228 and at a temperature from 250.degree. C. to
500.degree. C. for 0.1 to 10 hours in Japanese Unexamined Patent
Application Publication No. 2000-311329, for example. Heat
treatment at such a high temperature and for relatively long time
is not suitable for mass production.
[0011] Japanese Unexamined Patent Application Publication No.
2001-202611 proposes a technique to promote magnetic isolation of
crystal grains in the magnetic recording layer, in which grain size
of the magnetic layer is continuously varied along the film
thickness direction. This document teaches varying the grain size
of the magnetic layer along the film thickness direction by varying
a ratio of magnetic material to nonmagnetic material in the
magnetic recording layer. Specifically, separate targets are
prepared for the magnetic material and the nonmagnetic material and
power supplied in the sputtering process is changed in several
(typically five) steps to vary mixing ratio of the magnetic
material to the nonmagnetic material and to control the grain size.
It is unfortunately very difficult to obtain a film that is
homogeneous over a disk surface using an arrangement that places
two targets in equipment for mass production. Therefore, this
technique is not suitable for mass production.
[0012] There are further known examples: patterned media (Japanese
Unexamined Patent Application Publication No. H10-233015, for
example) and use of a self-organized film in which fine particles
are arranged two-dimensionally. (Japanese Unexamined Patent
Application Publication No. H10-320772 and Japanese Unexamined
Patent Application Publication No. 2002-334414, for example.)
However, none of these techniques is suitable for mass
production.
[0013] The present invention is directed to overcoming or at least
reducing the effects of one or more of the problems set forth
above.
SUMMARY OF THE INVENTION
[0014] Therefore, an object of the present invention is to provide
a perpendicular magnetic recording medium and a manufacturing
method that allows reduction of noise and enhancement of S/N
(output signal to noise ratio).
[0015] Another object of the invention is to provide a
perpendicular magnetic recording medium and a manufacturing method
that is suited to mass production and performs high density
recording.
[0016] A perpendicular magnetic recording medium according to the
present invention comprises at least a magnetic recording layer
formed on a nonmagnetic substrate. The magnetic recording layer
includes a plurality of magnetic crystal grains. Each magnetic
crystal grain in the magnetic recording layer has a multilayer
structure laminating a plurality of layers deposited at an initial
stage to a final stage of forming the magnetic recording layer, and
each magnetic crystal grain has a multilayer structure in which a
diameter of the crystal grain is smaller in a final layer deposited
at the final stage in a film surface side than the diameter in an
initial layer deposited at the initial stage in a substrate side.
Advantageously, the magnetic crystal grain having a multilayer
structure is formed in a truncated cone shape that is smaller in a
top face of the final layer deposited at the final stage than in a
bottom face of the initial layer formed at the initial stage.
[0017] Preferably, the magnetic recording layer is composed by
dispersing magnetic particles containing ferromagnetic materials in
oxides or nitrides, and more preferably it is composed of a
granular structure including a CoCrPt alloy dispersed in
SiO.sub.2.
[0018] The magnetic crystal grain having the multilayer structure
preferably exhibits saturation magnetization varying from the
initial layer deposited at the initial stage towards the final
layer at the film surface. More particularly, the magnetic crystal
grain having the multilayer structure exhibits increasing
saturation magnetization from the initial layer deposited at the
initial stage towards the final layer at the film surface.
[0019] The perpendicular magnetic recording medium according to the
invention further comprises a soft magnetic backing layer, a
nonmagnetic intermediate layer, and the magnetic recording layer
comprising the above-mentioned magnetic crystal grains sequentially
laminated on the nonmagnetic substrate.
[0020] A method of the present invention for manufacturing a
perpendicular magnetic recording medium that comprises a magnetic
recording layer formed on a nonmagnetic substrate in which the
magnetic recording layer includes at least a plurality of magnetic
crystal grains, is provided. The method includes a process for
forming the magnetic recording layer, the process comprising a
plurality of separate film deposition steps from a step conducted
at an initial stage of the process in a substrate side to a step
conducted at a final stage of the process in a film surface side in
varied conditions. There is a concentration variation step that
varies a concentration of oxygen or nitrogen contained in a gas for
use in the film deposition steps. The concentration variation step
is conducted during the plurality of film deposition steps, wherein
the plurality of film deposition steps and the concentration
variation step form the plurality of magnetic crystal grains each
having a multilayer structure laminating layers, each layer
corresponding to each of the film deposition steps from a step at
an initial stage to a step at a final stage of the process for
forming the magnetic recording layer. Each magnetic crystal grain
has a diameter smaller in a final layer deposited at the final
stage in the film surface side than a diameter in an initial layer
deposited at the initial stage in a substrate side. Advantageously,
the concentration of oxygen or nitrogen in the concentration
variation step corresponding to each film deposition step increases
with progression of the film deposition steps. Preferably each film
deposition step from the step for depositing the initial layer at
the initial stage to the step for depositing the final layer at the
final stage is conducted in a gas containing the oxygen using a
granular material including a CoCrPt alloy dispersed in
SiO.sub.2.
[0021] Another method of the invention for manufacturing a
perpendicular magnetic recording medium that comprises at least a
magnetic recording layer formed on a nonmagnetic substrate, the
magnetic recording layer including a plurality of magnetic crystal
grains, is provided. The method comprises a process for forming the
magnetic recording layer, the process comprising a plurality of
separate film deposition steps from a step conducted at an initial
stage of the process in a substrate side to a step conducted at a
final stage of the process in a film surface side in varied
conditions, and exposure steps exposing a surface to a gas
containing oxygen or nitrogen. Each exposure step is carried out as
a pre-process to each film deposition step. The plurality of film
deposition steps and exposure steps form the plurality of magnetic
crystal grains each having a multilayer structure laminating
layers. Each layer corresponds to each of the film deposition steps
from a step at an initial stage to a step at a final stage of the
process for forming the magnetic recording layer, and each magnetic
crystal grain has a diameter smaller in a final layer deposited at
the final stage in the film surface side than a diameter in an
initial layer deposited at the initial stage in a substrate side.
Advantageously, the exposure steps each corresponding to a film
deposition step are conducted having the concentration of the
oxygen or the nitrogen increase with progression of the exposure
steps. Preferably each film deposition step from the step for
depositing the initial layer at the initial stage to the step for
depositing the final layer at the final stage is conducted in a gas
containing the oxygen using a granular material including a CoCrPt
alloy dispersed in SiO.sub.2.
[0022] In a perpendicular magnetic recording medium according to
the invention, each magnetic crystal grain in the magnetic
recording layer has a multilayer structure and has a configuration
like a truncated cone shape in which the diameter of the crystal
grain of final layer deposited in the film surface side at the
final stage is smaller than the diameter of the crystal grain in
the initial layer deposited in the substrate side at the initial
stage. As a result, signal output is enhanced and noises are
reduced to improve S/N (signal output to noise ratio).
[0023] According to the invention, magnetic crystal grains having
multilayer structure can be mass produced by a simple manufacturing
method. A perpendicular magnetic recording medium of the invention
attains high recording resolution and achieves high density
recording.
BRIEF DESCRIPTION OF THE DRAWINGS
[0024] The foregoing advantages and features of the invention will
become apparent upon reference to the following detailed
description and the accompanying drawings, of which:
[0025] FIG. 1 is a schematic sectional view of a layer structure of
a perpendicular magnetic recording medium of a first embodiment
according to the invention.
[0026] FIG. 2 is a perspective view showing planar and sectional
structures of the magnetic recording layer.
[0027] FIG. 3 illustrates magnetic flux generated by various shapes
of magnetic crystal grains.
[0028] FIG. 4 illustrates a process in a method for manufacturing a
perpendicular magnetic recording medium in a second embodiment
according to the invention.
[0029] FIG. 5 illustrates a film deposition process repeating
oxygen exposure and film deposition steps in another method for
manufacturing a perpendicular magnetic recording medium in a third
embodiment according to the invention.
[0030] FIG. 6 illustrates in detail a film deposition process
comprising oxygen exposure and film deposition steps being
different from the process of FIG. 4.
[0031] FIG. 7 shows planar TEM images of magnetic recording layers
with different thicknesses in perpendicular magnetic recording
media.
[0032] FIG. 8 shows a cross-sectional TEM image of the
perpendicular magnetic recording medium of manufacturing example
1.
DETAILED DESCRIPTION OF SPECIFIC EMBODIMENTS
[0033] Some preferred examples of embodiment according to the
present invention will be described in detail in the following with
reference to accompanying drawings.
First Embodiment
[0034] This embodiment is described with reference to FIGS. 1
through 3, which disclose the structure of the medium.
[0035] FIG. 1 is a schematic sectional view of a structure of
perpendicular magnetic recording medium 100 of the invention.
Perpendicular magnetic recording medium 100 comprises nonmagnetic
substrate 1, soft magnetic backing layer 2, nonmagnetic
intermediate layer 3, magnetic recording layer 4, protective film
5, and liquid lubricant layer 6. A seed layer or a soft magnetic
intermediate layer can be appropriately inserted between soft
magnetic backing layer 2 and nonmagnetic intermediate layer 3.
[0036] Nonmagnetic substrate 1 can be any substrate having a smooth
surface commonly used in a magnetic recording; medium. The
substrate can be composed of NiP plated aluminum alloy,
strengthened glass, crystallized glass, for example.
[0037] Soft magnetic backing layer 2 can be composed of crystalline
FeTaC, Sendust alloy (FeSiAl), or amorphous cobalt alloy of CoZrNb
or CoTaZr. Optimum thickness of soft magnetic backing layer 2 is in
the range of 10 nm to 500 nm depending on the structure and
characteristics of a magnetic head used for recording, taking
productivity into consideration.
[0038] A seed layer, when provided on soft magnetic backing layer
2, can be composed of Ta, Zr, Ni.sub.3Al or the like. The seed
layer, being nonmagnetic, is favorably as thin as possible from the
view point of effective concentration of magnetic flux generated by
the recording head into the soft magnetic backing layer, and is
preferably in a range of 0.2 nm to 10 nm.
[0039] A soft magnetic intermediate layer, if provided, can be made
of a permalloy such as NiFeAl, NiFeSi, NiFeNb, NiFeB, NiFeNbB,
NiFeMo, or NiFeCr. Thickness of the permalloy underlayer is
adjusted to attain optimum magnetic performance and read-write
performance characteristics of the magnetic recording layer, and is
preferably in a range of 3 nm to 50 nm, balancing between media
performances and productivity.
[0040] Nonmagnetic intermediate layer 3 can be composed of
ruthenium, a ruthenium-based alloy containing one or more additives
selected from C, Cu, W, Mo, Cr, Ir, Pt, Re, Rh, Ta, and V, or a
metal selected from Pt, Ir, Re, and Rh, though not limited to these
materials. The thickness of nonmagnetic intermediate layer 3 needs
to be as thin as possible to achieve high density recording without
degrading magnetic performance or read-write performance
characteristics of the magnetic recording layer, and is preferably
in a range of 1 nm to 20 nm.
[0041] Magnetic recording layer 4 preferably comprises a
ferromagnetic alloy material containing at least cobalt and
platinum. The c-axis in the hexagonal closest packed structure of
the material must align perpendicular to the film surface of the
medium for a perpendicular magnetic recording medium. Magnetic
recording layer 4 can be composed of a granular material, such as
CoPt--SiO.sub.2, CoCrPtO, CoCrPt--SiO.sub.2,
CoCrPt--Al.sub.2O.sub.3, CoPt--AlN, and CoCrPt--Si.sub.3N.sub.4.
Magnetic recording layer 4 does not necessarily take a granular
structure in a narrow meaning containing an oxide or a nitride, but
can also be composed of an alloy material represented by
CoCrPt.
[0042] Protective film 5 can be a thin film of substantially
carbon. Other thin films that are commonly used for a protective
film in a magnetic recording medium can also be used for protective
film 5.
[0043] Liquid lubricant layer 6 can be composed of
perfluoropolyether lubricant, for example. Other lubricants that
are commonly used for a liquid lubricant layer in a magnetic
recording medium can also be used.
[0044] Each of the layers laminated on nonmagnetic substrate 1 can
be formed by a method selected from the film forming techniques
that are generally used in the field of magnetic recording media.
The layers with the exception of liquid lubricant layer 6 can be
formed by DC magnetron sputtering, RF magnetron sputtering, or
vacuum evaporation, for example. The liquid lubricant layer can be
formed by dipping or spin-coating, for example.
[0045] FIG. 2 illustrates planar and sectional structures of the
magnetic recording layer. Magnetic recording layer 4 consists of
magnetic crystal grains 10 and nonmagnetic grain boundary 11.
Magnetic crystal grain 10 has a configuration like a truncated cone
shape. This configuration is simply referred to as a "truncated
cone shape."
[0046] The following describes magnetic properties of magnetic
recording layer 4 including magnetic crystal grains 10 that have a
special configuration of a truncated cone shape.
[0047] FIG. 3 shows different magnetic flux generated by different
shapes of magnetic crystal grain, in comparison with conventional
examples. Various patterns of flux flow are illustrated for
conventional columnar magnetic crystal grains 20 in FIG. 3(a), for
a shape of magnetic crystal grains 21 disclosed in Japanese
Unexamined Patent Application Publication No. 2001-202611 in FIG.
3(b), and for magnetic crystal grains 10 with a configuration of a
truncated cone shape according to the invention in FIG. 3(c).
[0048] In general, magnetic flux that flows out of a magnetic
crystal grain flows back into a neighboring magnetic crystal grain
having an inversely directed magnetic pole. Assuming equal volume
of magnetic crystal grains, the larger the area of a magnetic pole
is, the lower the magnetic flux density around the magnetic pole.
The nearer the neighboring magnetic crystal grain is located, the
smaller the loop through which the magnetic flux flows.
[0049] For columnar magnetic crystal grains 20 in FIG. 3(a) and
magnetic crystal grains 21 with the shape disclosed in Japanese
Unexamined Patent Application Publication No. 2001-202611 shown in
FIG. 3(b), magnetic flux density tends to be low and further,
magnetic flux easily flows back into the neighboring magnetic
crystal grain and is difficult to be picked up at a position far
from the grain surface. In contrast, for magnetic crystal grains 10
with the truncated cone shape according to the invention, magnetic
flux tends to concentrate because of the small magnetic poles on
the film surface and further, since a magnetic pole is distant from
a neighboring magnetic pole on the film surface, the magnetic flux
hardly flows back in a small loop and can be picked up with higher
density and at a remote position.
[0050] The physical reason for higher saturation magnetization of
magnetic crystal grain 10 around the film surface than at the
initial stage of film deposition is as follows. Film deposition
process according to the invention is conducted in such a way that
an amount of oxygen or nitrogen (only a case of oxygen is described
below) increases from the initial stage towards film surface. The
increasing amount of oxygen initially oxidizes unreacted silicon,
and subsequently oxidizes chromium, and the oxides of these
elements precipitate at the grain boundary. Consequently, the
(unreacted) silicon and chromium decrease in the magnetic crystal
grain approaching the film surface. As a result, the saturation
magnetization increases in the magnetic crystal grain as the film
surface is approached.
[0051] Because magnetism is carried by cobalt in crystal grain 10
(CoCrPt+unreacted silicon), the saturation magnetization varies
depending on the quantity of other nonmagnetic additive elements
(Cr, Pt, Si). The saturation magnetization of the magnetic crystal
grain increases with a decrease of the amount of additive elements.
In the mechanism involved in the invention, the saturation
magnetization of the magnetic crystal grain itself increases by
discharging the nonmagnetic elements in magnetic crystal grains 10
to grain boundary 111 as oxides.
[0052] Because higher density magnetic flux can be picked up at a
farther position from the film surface of magnetic recording layer
4 comprising the magnetic crystal grain as described above, low
noise and high output can be simultaneously attained, and thus high
density recording can be achieved. Since magnetic crystal grains 10
having a truncated cone shape can be readily obtained by
controlling the deposition conditions of magnetic recording layer
4, this manufacturing method of the invention is suitable for mass
production of perpendicular magnetic recording media 100.
Second Embodiment
[0053] The second embodiment of the invention is described below
with reference to FIG. 4. Corresponding parts from the first
embodiment are given the same symbol, and their description is
omitted.
[0054] The second embodiment describes a manufacturing method for
perpendicular magnetic recording medium 100 comprising magnetic
recording layer 4 that includes magnetic crystal grains 10 with a
configuration like a truncated cone shape, in which a grain
diameter on the film surface is smaller than a grain diameter at
the initial stage. In the manufacturing method, the amount of
oxygen or nitrogen introduced in the process of depositing magnetic
recording layer 4 is adjusted to vary the amount of oxidized or
nitrided metals in the magnetic recording material and vary
diameter of magnetic crystal grains 10, thereby forming a truncated
cone shape. More particularly, the amount of oxygen introduced in
the process of depositing the magnetic recording layer is adjusted
to vary the amount of oxidized metals in the magnetic recording
material and vary the diameter of magnetic crystal grain 10,
thereby forming a truncated cone shape.
[0055] In the example disclosed in Japanese Unexamined Patent
Application Publication No. 2001-202611, the ratio of compositions
of magnetic material and nonmagnetic material are varied. In the
method of the invention, the amount of oxidized metals contained in
the magnetic material varies along the thickness direction. As a
result, the composition itself of the magnetic material varies
continuously along the thickness direction.
[0056] Since magnetic crystal grain 10 has a truncated cone shape
according to the invention, magnetic isolation of magnetic crystal
grains 10 is promoted due to physical separation. In addition,
owing to the effect of the shape of crystal grain, magnetic flux
hardly diverges and signal intensity is enhanced, which results in
improved recording resolution.
[0057] By including chromium or the like that is more readily
oxidized than cobalt carrying magnetism in magnetic recording layer
4, the chromium or the like is preferentially oxidized when oxygen
is introduced, thereby continuously varying saturation
magnetization of magnetic crystal grain 10 along the thickness
direction. By making the saturation magnetization larger around the
surface than at the initial stage of growth of magnetic crystal
grain 10, the effect of magnetic flux concentration is further
enhanced.
MANUFACTURING EXAMPLE 1
[0058] A chemically reinforced glass substrate (N-10 glass
substrate made by HOYA Corporation, for example) having a smooth
surface was used for nonmagnetic substrate 1. After cleaning,
nonmagnetic substrate 1 was introduced into a sputtering apparatus.
A CoZrNb amorphous soft magnetic backing layer 2 with a thickness
of 200 nm was deposited using a target of 87 at % Co-5 at % Zr-8 at
% Nb. Then, a NiFeSi underlayer 11 nm thick was deposited using a
permalloy target of 82 at % Ni-12 at % Fe-6 at % Si. Subsequently,
nonmagnetic intermediate layer 3 of ruthenium having a thickness of
10 nm was deposited under an argon gas pressure of 4.0 Pa using a
ruthenium target.
[0059] Then magnetic recording layer 4 of CoCrPt--SiO.sub.2 having
a thickness of 10 nm was deposited under a gas pressure of 5.3 Pa
using a target of 90 mol % (74 at % Co-12 at % Cr-14 at % Pt)-10
mol % (SiO.sub.2). In this step, the initial deposition from the
start of deposition to a film thickness of 0.5 nm was conducted in
an atmosphere of only argon without addition of oxygen. Once the
film thickness reached about 0.5 nm, oxygen addition was started
and oxygen content in the argon gas was gradually increased to 2%
at the time of completion of the deposition step. The deposition of
magnetic recording layer 4 of CoCrPt--SiO.sub.2 was conducted in
4.0 seconds. Oxygen was not added in 0.2 sec from the start of
deposition. After 0.2 sec from the start of deposition, oxygen gas
flow rate in argon gas was varied at a rate of 0.53%/sec.
[0060] Finally, protective film 5 of carbon having a thickness of 7
nm was deposited using a carbon target, then the laminated
substrate was taken out from the vacuum chamber. The deposition
steps of all layers other than nonmagnetic intermediate layer 3 of
ruthenium and magnetic recording layer 4 of CoCrPt--SiO.sub.2 were
conducted by DC magnetron sputtering under an argon gas pressure of
0.67 Pa. Next, liquid lubricant layer 6 of perfluoropolyether
having thickness of 2 nm was formed by a dipping method. Thus,
perpendicular magnetic recording medium 100 was manufactured.
[0061] A medium comprising magnetic recording layer 4 having a film
thickness of 0.5 nm was also manufactured in order to compare
diameters of crystal grains at an initial stage and on the film
surface in magnetic recording layer 4.
[0062] The present method differs in several respects from the
conventional manufacturing method. FIG. 4 illustrates a method for
manufacturing a perpendicular magnetic recording medium according
to the invention. A target of CoCrPt--SiO.sub.2 is solely used. The
amount of oxygen introduced in the atmosphere of the process is
adjusted to control the amount of oxides.
[0063] In the case oxygen is introduced as shown in FIG. 4(a),
unreacted silicon is first oxidized, and then chromium is oxidized.
These nonmagnetic oxides deposit in the grain boundary. Thus, first
layer 5a (initial layer at the initial stage of film deposition)
composed of CoCrPt--SiO.sub.2 is first formed. Next, increasing the
amount of oxygen over the previous process, a film deposition
process similar to FIG. 4(a) is conducted to deposit second layer
4b composed of CoCrPt--SiO.sub.2 as shown in FIG. 4(b). Similar
deposition processes are conducted while gradually increasing the
amount of oxygen, resulting in magnetic crystal grain 10 having a
"truncated cone" shape as shown in FIG. 2.
[0064] Since the oxidation proceeds in the order of Si, Cr, to Co
because of affinity for the oxygen, degradation of magnetism due to
oxidation of cobalt does not occur unless excessive oxygen is
added. In the example of Japanese Unexamined Patent Application
Publication No. 2001-202611, targets of CoPt and SiO.sub.2 are
prepared and discharged at the same time (a technique called
co-sputtering), adjusting electric power supplied to each target
(to adjust the depositing rate of each target) to control ratio of
compositions in the deposited film. Thus, the method in Japanese
Unexamined Patent Application Publication No. 2001-202611 uses the
targets of CoPt and SiO.sub.2 and controls the composition by
adjusting the supplied power as described above, while the
manufacturing method of the invention controls the composition
according to the amount of the introduced oxygen. This point is a
difference between the two methods. The same effect can be obtained
using a reaction gas of nitrogen as well as the oxygen.
[0065] The manufacturing method described above makes magnetic
crystal grains 10 in magnetic recording layer 4 have a
configuration like a truncated cone shape, in which the diameter of
the crystal grain is smaller at the film surface than the diameter
of the crystal grain at the initial stage. As a result, the
magnetic isolation of the magnetic crystal grains is promoted and
the magnetic flux generated by magnetic crystal grains 10 reaches
farther from the film surface of magnetic recording layer 4.
Therefore, higher signal intensity and signal to noise ratio (S.N)
are attained even in high density recording.
Third Embodiment
[0066] The third embodiment of the invention is described with
reference to FIG. 5 and FIG. 6. The same symbols as in the previous
examples are given to the corresponding parts in this example and
their description is omitted.
[0067] This embodiment is a manufacturing method with repeating
cycles, where each cycle comprises a film deposition step and an
exposure step. The film deposition steps separately deposit each of
the layers from a layer in a substrate side at an initial stage to
a layer in a film surface side at a final stage in varied
conditions. The exposure steps expose a surface to a gas containing
oxygen or nitrogen, and each exposure step is carried out as a
pre-process to each film deposition step. This manufacturing method
differs from the method of second embodiment described previously.
The exposure steps corresponding to each film deposition step can
be conducted having concentrations of the oxygen or the nitrogen
increased with progression of the exposure steps.
MANUFACTURING EXAMPLE 2
[0068] Next, a manufacturing example of perpendicular magnetic
recording medium 10 is described. The layers up to nonmagnetic
intermediate layer 3 of ruthenium were formed in the same manner as
in the manufacturing example 1.
[0069] Magnetic recording layer 4 of CoCrPt--SiO.sub.2 having a
thickness of 10 nm was formed using a target of 90 mol % (74 at %
Co-12 at % Cr-14 at % Pt)-10 mol % (SiO.sub.2). A step of
depositing 0.5 nm of magnetic recording layer 4 and a step of
exposing the surface to oxygen subsequent to each deposition step
were conducted alternately. The oxygen concentration to which the
layer surface was exposed was gradually varied from 0.2% to 2%.
Specifically, a process to deposit 0.5 nm (corresponding to 0.2
sec) of magnetic recording layer 4 under an argon gas pressure of
5.3 Pa and a step to introduce an argon gas containing oxygen for
0.2 sec were repeated. The gas pressure in the surface exposure
step to the argon plus oxygen gas was 1.3 Pa. The total number of
cycles of oxygen introduction was 19. Initially introduced oxygen
concentration was 0.2%, increasing by 0.1% at every introduction
step, and the oxygen concentration reached 2% at the final
introduction. Then, carbon protective film 5 and liquid lubricant
layer 6 were formed in the same manner as in the manufacturing
example 1, to complete perpendicular magnetic recording medium
100.
[0070] A medium comprising magnetic recording layer 4 having a film
thickness of 0.5 nm was also manufactured in order to compare
diameters of a layer of a crystal grain at an initial stage (the
initial layer) and a layer on the film surface (the final layer) in
magnetic crystal grain 10 of magnetic recording layer 4.
[0071] As described above, magnetic crystal grain 10 having a
multilayer structure in magnetic recording layer 4 has a truncated
cone shape in which the diameter of crystal grain in the final
layer on the film surface is smaller than the diameter of crystal
grain in the initial layer at the initial stage of growth.
Consequently, magnetic isolation of magnetic crystal grains 10 is
promoted, and noise reduction and signal output enhancement are
achieved.
[0072] Next, the conditions for oxygen exposure of the magnetic
recording layer are described. These include conditions in the film
deposition step, layer construction in the case of oxygen exposure,
and oxidation process of grain boundaries. FIG. 5 shows the film
deposition process using repeating oxygen exposure and film
deposition. Concerning conditions in oxygen exposure, a mechanism
of "oxygen exposure" in this embodiment is described as compared to
"oxygen addition" in the manufacturing method of second
embodiment.
[0073] Condition in the Film Deposition Step
[0074] A film deposition condition in the case of oxygen exposure
is compared. In the oxygen exposure in this embodiment, film
deposition is conducted under a constant power supply and constant
gas composition (basically, pure argon without oxygen
addition).
[0075] As shown in manufacturing example 2 and in FIG. 5, first
layer 5a (initial layer), second layer 4b, . . . , a layer on the
film surface (final layer) are sequentially laminated by
alternately repeating steps of film deposition and oxygen exposure
in a sequence like (a) deposition, (b) exposure, (c) deposition,
(d) exposure, (e) deposition . . . . In the exposure steps, oxygen
concentration in the exposure gas is gradually changed as the film
thickness increases. Because an actual thickness of one layer
deposited in one deposition step is extraordinarily small (0.5 nm
for example), adsorbed oxygen generates sufficient oxide to expand
the grain boundary.
[0076] In the case of oxygen addition in the second embodiment, as
shown in manufacturing example 1 and in FIG. 4, the electric power
supplied in the film deposition step is constant, while oxygen
content in the gas is continuously varied.
[0077] Layer Construction in the Case of Oxygen Exposure
[0078] A layer structure in the case of oxygen exposure is
compared. In the case of oxygen exposure in this third embodiment,
oxygen is adsorbed on the grain boundary in precedence. In the
deposition step after the oxygen exposure, silicon and chromium are
oxidized by the oxygen adsorbed on the grain boundary and
precipitate on the grain boundary as oxides. By varying the amount
of oxygen added in the exposure steps, the proportion of magnetic
crystal grains 10 (metallic substance) to grain boundary 11 (oxide)
can be varied. The intermittent deposition that repeats the
exposure and deposition steps according to the invention suppresses
growth of magnetic crystal grain 10, decreasing diameter of the
crystal grain as the film surface is approached.
[0079] In the case of the film deposition step with oxygen addition
in the second embodiment described previously, silicon and chromium
included in CoCrPt containing unreacted silicon are oxidized by
oxygen contained in the processing gas of the film deposition step,
and precipitate in the grain boundary. The proportion of magnetic
crystal grain 10 (metallic substance) to grain boundary 11 (oxide)
varies depending on the content of oxygen in the process gas in the
film deposition step.
[0080] Oxidation Process of Grain Boundary
[0081] An oxidation process of grain boundary 11 is compared. FIG.
6 shows the process of oxygen exposure followed by film deposition
of FIG. 5 more in detail. The process of FIG. 6 is different from
the process of FIG. 4.
[0082] In the case of oxygen exposure plus film deposition, silicon
and chromium react and are oxidized by the oxygen adsorbed on grain
boundary 11 after reaching the film surface. In the sputtering
process shown in FIG. 6(b) after the oxygen exposure step shown in
FIG. 6(a), the material of the target is bombarded and scattered
not in a form of a particle of CoCrPt--SiO.sub.2 but in a form of
each separate atom and runs towards the substrate. Then, the atoms
become an alloy or an oxide again on or around the substrate
surface. Every element has an order of possibility of oxidation;
among Co, Cr, Pt, and Si, the order of readiness of oxidation is
Si, Cr, Co, to Pt. Consequently, on addition of oxygen to these
elements, silicon is oxidized first. After all of the silicon is
oxidized, then chromium is oxidized. Cobalt is not oxidized
generally unless all of the chromium is oxidized. When nitrogen is
added to Co, Cr, Pt, and Si, the order of nitridation is the same
as in the case of oxygen. Nitrides are formed in a process in which
the separately scattered silicon and chromium react on the film
surface with nitrogen adsorbed on grain boundary 11 and nitrides
deposit there, and the layer structure of laminated crystal grain
11 results as shown in FIG. 6(c).
[0083] Unlike this situation, in the case of the film deposition
with oxygen addition in the second embodiment shown in FIG. 4, the
silicon and chromium react and are oxidized with the oxygen mainly
in the deposition process gas before reaching the substrate. Oxides
are formed in the process in which the separately scattered silicon
and chromium react with the added oxygen in the gas phase and then
deposit, and the layer structure of laminated grain boundary 11
results as shown in FIG. 4(b).
Fourth Embodiment
[0084] The fourth embodiment according to the invention is
described with reference to Tables 1 and 2, and FIGS. 7 and 8. The
same symbols are given to parts corresponding to the parts in the
previous examples and description is omitted.
[0085] In this embodiment, a comparative perpendicular magnetic
recording medium was manufactured, and perpendicular magnetic
recording media 100 of manufacturing example 1 of the second
embodiment and the manufacturing example 2 of the third embodiment
were evaluated. The comparative perpendicular magnetic recording
medium (referred to as comparative example 1) was manufactured in
the same manner as in the manufacturing example 1 except that
magnetic recording layer 4 was deposited in pure argon gas at a
pressure of 5.3 Pa.
[0086] Perpendicular magnetic recording medium 100 comprising
magnetic recording layer 4 having a thickness of 0.5 nm was also
manufactured as in the manufacturing example 1 to compare the
diameter of magnetic crystal grain 10 in magnetic recording layer 4
at the layer at the initial stage of growth (an initial layer) and
at the layer in the film surface (a final layer).
[0087] Signal output, media noise, signal output to noise ratio
(S/N), and D50 (the linear recording density at which the output
decreases to half of the maximum value) were measured on
perpendicular magnetic recording media 100 were obtained as
described above using a read/write tester, and compared.
[0088] Read-Write Performance Characteristics
[0089] Table 1 shows the measured results of read-write performance
characteristics of the perpendicular magnetic recording media of
manufacturing examples 1 and 2 and comparative example 1. More
particularly, Table 1 shows read-write performance characteristics
of truck average signal output (TAA), normalized media noise, S/N,
which were measured at a linear density of 367 kfci, and D50.
1 TABLE 1 manufacturing manufacturing comparative example 1 example
2 example 1 TAA at 367 kfci 1.04 1.12 0.84 (mVp-p) media noise at
367 24.5 25.4 29.1 kfci (.mu.Vrms/mVp-p) S/N at 367 kfci 21.0 21.1
20.2 (dB) D.sub.50 423 438 379 (kfci)
[0090] In manufacturing examples 1 and 2, TAA increases, media
noise decreases, and, as a result, S/N is improved by about 1 dB as
compared to example 1. The D.sub.50 represents recording resolution
of a perpendicular magnetic recording medium. Large value of this
quantity means possibility of high density recording. The D.sub.50
is improved by 12% to 16% in manufacturing examples 1 and 2 over
comparative example 1, which demonstrates that high density
recording has been achieved.
[0091] Mean Diameter of Crystal Grains
[0092] To find the reason for the improvement in the
electromagnetic conversion characteristics, images on a
transmission electron microscope (TEM) were compared on the
perpendicular magnetic recording medium of manufacturing examples 1
and 2 and comparative example 1. Table 2 shows mean diameters of
crystal grains calculated from the TEM images shown in FIG. 7.
[0093] FIG. 7 shows planar TEM images at a film thickness of 0.5 nm
and planar TEM images at a film thickness of 10 nm of magnetic
recording layer 4 in the perpendicular magnetic recording media of
manufacturing examples 1 and 2 and comparative example 1.
2TABLE 2 film thickness of mean diameter of crystal grain (nm)
magnetic recording manufacturing manufacturing comparative layer
example 1 example 2 example 1 0.5 nm 9.3 9.2 9.2 10 nm 6.6 6.1
9.8
[0094] In the planar TEM images of the magnetic recording layer at
a film thickness of 0.5 nm, no difference in crystal grain diameter
and grain isolation was observed between the manufacturing examples
1 and 2 and the comparative example 1. On the other hand, in the
planar TEM images of magnetic recording layer 4 at a film thickness
of 10 nm, the comparative example 1 is remarkably different from
the manufacturing examples 1 and 2. Crystal grain diameter 10 of
the magnetic recording layer at a film thickness of 10 nm, as
compared with magnetic recording layer 4 at a film thickness of 0.5
nm, is larger in the comparative example 1, and is decreased down
to 65 to 70% in the manufacturing examples 1 and 2. In the
manufacturing examples 1 and 2, grain boundary 11 has significantly
expanded corresponding to shrinkage in the diameter of crystal
grains 10.
[0095] Sectional Structure
[0096] To examine a sectional structure in this configuration of
growth, cross-sectional TEM observation was conducted on the
perpendicular magnetic recording media. FIG. 8 is a sectional TEM
image of the perpendicular magnetic recording medium of
manufacturing example 1. Distance between crystal grains of
magnetic recording layer 4 at an initial stage of growth is not
larger than 0.5 .mu.m as shown in region 61. On the other hand, the
distance between crystal grains of magnetic recording layer 4 on
the film surface is about 3 nm as shown in region 62. Thus, crystal
grains 10 in magnetic recording layer 4 in the perpendicular
magnetic recording medium 100 of manufacturing example 1 have been
shown to have a configuration like a truncated cone shape in which
diameter of the crystal grain is smaller on the film surface than
at the initial stage of growth.
[0097] Sectional TEM observation on manufacturing example 2 and
comparative example 1 demonstrated that perpendicular magnetic
recording medium 100 of manufacturing example 2 was confirmed to
show a configuration of growth like a truncated cone shape just as
perpendicular magnetic recording medium 100 of manufacturing
example 1, while the perpendicular magnetic recording medium of
comparative example 1 showed a cylindrical configuration of growth.
These results on sectional TEM observation agree well with the
results on planar TEM observation.
[0098] Although the manufacturing methods are different between
manufacturing examples 1 and 2, configuration of growth of magnetic
recording layer 4 is the same configuration like a truncated cone
shape. Consequently, the configuration of growth can produce
improvements in electromagnetic conversion characteristics
including noise reduction and signal output enhancement of
perpendicular magnetic recording medium 100.
[0099] Thus, a perpendicular magnetic recording medium mounted on
various magnetic recording apparatuses and a method for
manufacturing such a medium has been described according to the
present invention. Many modifications and variations may be made to
the techniques and structures described and illustrated herein
without departing from the spirit and scope of the invention.
Accordingly, it should be understood that the media and methods
described herein are illustrative only and are not limiting upon
the scope of the invention.
* * * * *