U.S. patent application number 10/952735 was filed with the patent office on 2005-02-24 for perpendicular magnetic recording medium and process of production thereof.
This patent application is currently assigned to Hitachi, Ltd.. Invention is credited to Hosoe, Yuzuru, Nakagawa, Hiroyuki, Nemoto, Hiroaki.
Application Number | 20050042367 10/952735 |
Document ID | / |
Family ID | 27750765 |
Filed Date | 2005-02-24 |
United States Patent
Application |
20050042367 |
Kind Code |
A1 |
Nemoto, Hiroaki ; et
al. |
February 24, 2005 |
Perpendicular magnetic recording medium and process of production
thereof
Abstract
A perpendicular magnetic recording medium, which has a low level
of recording noise and sufficiently large perpendicular magnetic
anisotropy energy relative to demagnetizing field energy, includes
a substrate and a multi-layered magnetic film. The multi-layered
magnetic film is composed of ferromagnetic metal layers of Co alloy
containing at least Cr and non-magnetic metal layers of Pd alloy,
each one layer of which are laminated alternately on top of one
layer of the other. The ferromagnetic metal layers and the
non-magnetic metal layers have a thickness of d1 and d2,
respectively, with the ratio of d1/d2 being in the range of 1.5 to
4.0. This specific layer structure reduces the magnetic exchange
interaction between magnetic particles in the multi-layered
magnetic film. Therefore, the perpendicular magnetic recording
medium is stable against thermal disturbance and has a low level of
recording noise.
Inventors: |
Nemoto, Hiroaki; (Kokubunji,
JP) ; Nakagawa, Hiroyuki; (Yokohama, JP) ;
Hosoe, Yuzuru; (Hino, JP) |
Correspondence
Address: |
REED SMITH LLP
Suite 1400
3110 Fairview Park Drive
Falls Church
VA
22042
US
|
Assignee: |
Hitachi, Ltd.
|
Family ID: |
27750765 |
Appl. No.: |
10/952735 |
Filed: |
September 30, 2004 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
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10952735 |
Sep 30, 2004 |
|
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|
10217478 |
Aug 14, 2002 |
|
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6815083 |
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Current U.S.
Class: |
427/130 ;
427/131; G9B/5.241; G9B/5.295 |
Current CPC
Class: |
Y10T 428/12646 20150115;
Y10T 428/265 20150115; G11B 5/656 20130101; Y10T 428/12931
20150115; Y10T 428/24975 20150115; G11B 5/66 20130101; G11B 5/84
20130101; Y10T 428/12875 20150115; G11B 2005/0029 20130101 |
Class at
Publication: |
427/130 ;
427/131 |
International
Class: |
B05D 003/00 |
Foreign Application Data
Date |
Code |
Application Number |
Feb 26, 2002 |
JP |
P2002-048937 |
Claims
What is claimed is:
1. A process for producing a perpendicular magnetic recording
medium, said process comprising a step of forming an underlying
layer on a substrate, a step of forming on said underlying layer an
initial layer of Co alloy containing Cr or Cu, a step of forming on
said initial layer a multi-layered magnetic film composed of
non-magnetic metal layers containing Pd and ferromagnetic metal
layers containing Co which are laminated alternately on top of the
other, a step of heat treatment at a temperature higher than
350.degree. C. which is performed after said multi-layered magnetic
film has been formed, and a step of forming a carbon protective
film on said multi-layered magnetic film.
2. A process for producing a perpendicular magnetic recording
medium according to claim 1, wherein said ferromagnetic metal
layers contain less than 20 at % of Cr.
3. A process for producing a perpendicular magnetic recording
medium according to claim 1, wherein said ferromagnetic metal
layers contain mores than 12 at % of Cr.
4. A process for producing a perpendicular magnetic recording
medium according to claim 1, wherein said initial layer of Co alloy
contain more than 30 at % of Cr or Cu.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The present invention relates to a perpendicular magnetic
recording medium which comprises a substrate and a multi-layered
magnetic film composed of ferromagnetic metal layers and
non-magnetic metal layers which are laminated alternately on top of
the other.
[0003] 2. Description of Related Arts
[0004] The recent advance in information society demands data
recording apparatus, such as hard disk drive ("HDD" hereinafter) ,
with higher performance. Improving the performance of HDD is
synonymous with increasing the recording density. One promising
technology to meet this demand is the perpendicular magnetic
recording system which magnetizes the medium perpendicularly to the
film plane, rather than in the plane. There are several candidates
as the magnetic material for the recording layer of the
perpendicular magnetic recording medium. Among them are CoCr-based
alloys incorporated with Pt, Ta, B, etc. as additives, which have
been investigated most closely.
[0005] A recording medium of CoCr-based alloy is superior in
recording characteristics, but CoCr-based alloy itself has
insufficient perpendicular magnetic anisotropy energy (K.sub.u)
which holds the magnetization in its orientation. For this reason,
CoCr-based alloy gives an M-H curve with a squareness ratio much
smaller than 1 and permits small reversed magnetic domains to
appear in magnetic recording domains, thereby reducing the strength
of reproduced signals. Even though its M-H curve has a squareness
ratio close to 1, CoCr-based alloy is still susceptible to "thermal
signal loss" which is a phenomenon that the state of magnetization
changes after recording due to thermal disturbance.
[0006] To cope with this situation, there has been proposed a
recording medium in which the recording layer is a multi-layered
film composed of thin films (with a thickness of an atomic order)
laminated alternately on top of the other. This recording layer can
be designed such that the magnetic structure formed by the magnetic
head remains unchanged with time owing to its sufficiently large
value of K.sub.u.
[0007] The structure of the multi-layered film mentioned above is
generally referred to as superlattice structure. It takes on a
variety of physical properties owing to the state peculiar to the
interface between atomic layers. It is known that the magnetic thin
film of multi-layered structure has a large perpendicular magnetic
anisotropy energy if it is formed from ferromagnetic metal (such as
Co and Fe) and noble metal (such as Pd, Pt, and Au) laminated
alternately. This multi-layered structure of ferromagnetic metal
and noble metal is used in the perpendicular magnetic recording
medium disclosed in U.S. Pat. Nos. 4,587,176, 4,678,721, and
5,106,703.
[0008] Unfortunately, the magnetic recording medium with the
magnetic thin film mentioned above has a larger recording noise
than the perpendicular recording medium of CoCr-based alloy. In
other words, it is not necessarily superior in recording
properties. The cause of recording noise arises from the reversal
mechanism of the multi-layered magnetic film as has been pointed
out in many studies.
[0009] The fact that CoCr-based alloy thin film is capable of
low-noise recording is because there exist ferromagnetic fine
particles in the film, with each particle being magnetically
isolated by a phase with rich Cr which has spread out on its
periphery. By contrast, the multi-layered magnetic film, which
takes no special effort to form the fine structure in the magnetic
film, permits magnetization reversal to take place in a large area
as a unit. Therefore, the magnetic domain for recording has a
zigzag contour regardless of the distribution of the magnetic field
applied by the recording head. This zigzag contour reflects the
random distribution of the magnetic properties of the magnetic
film, and hence it brings about recording noise. To address the
problem involved with the multi-layered magnetic film,.
comprehensive studies have been made on a variety of additives and
underlying materials.
[0010] Attempts have been made to use a multi-layered magnetic film
of Co and Pd or Pt as a magnetic recording medium. Its composition,
structure, and manufacturing method intended for better recording
characteristics are disclosed in Toku-Hyo-Hei 11-501755 (Japanese
translation of PCT international publication WO96/24927). According
to this disclosure, each magnetic metal layer is made of Co or Co
alloy with a thickness of 0.15-1.0 nm and a noble metal layer with
a thickness of 0.5-1.5 nm. The number of the magnetic metal layers,
each including one noble metal layers, is 10-30. The magnetic
recording medium of this structure has a coercive force larger than
2.5 kOe. In addition, according to the disclosure, the
multi-layered magnetic film and its nuclei-forming layer (such as
Pd) should have a total thickness smaller than 150 nm so as to
avoid an unnecessarily large space between the recording head and
the backing soft magnetic layer (NiFe).
[0011] Moreover, an effective way of reducing the noise of
recording medium is also disclosed in the patent publication just
mentioned above. The disclosure mentions that the recording film
should be made by sputtering with an oxygen-containing sputtering
gas at a reduced degree of vacuum and at a high sputtering gas
pressure, and annealing should be performed before and after the
film forming step. The disclosure further mentions that it is
possible to reduce the recording noise if the Co alloy layer is
formed from CoCr or CoCrTa.
[0012] Another perpendicular magnetic recording medium with a
multi-layered magnetic film of Pd/CoCr is disclosed in Appl. Phys.
Lett. 64(21) pp. 2891-2893 by B. M Lairson et. al. This thesis
deals with how the recording characteristics vary depending on the
thickness of the CoCr layer and the amount of Cr added. With the
thickness of the Pd layer fixed at 0.4 nm, it concludes that the
recording characteristics are satisfactory if the content of Cr is
about 15 at %.
[0013] The present inventors repeated the procedure mentioned in
the foregoing patent publication (11-501755) to reproduce the
sample No. 12 given in page 22. This sample is a multi-layered
magnetic recording film composed of ferromagnetic layers of
CoCr-based alloy and noble metal layers which are laminated
alternately on top of the other. The recording film is of
superlattice structure consisting of CoCr layers (0.35 nm) and Pd
layers (1.0 nm), with a Pd underlying layer (20 nm). The CoCr-based
alloy contains 12 at % Cr. The reproduced sample was actually
tested for recording and reproducing performance. The test results
indicate that the sample is stable to thermal disturbance but is
not particularly superior to the conventional CoCrPt recording
medium (proposed by Takano et al., Digest of Intermag 2000,
AD-06).
[0014] The recording medium reproduced by the present inventors has
a higher level of recording noise than the CoCrPt medium (just
mentioned above) despite incorporation with Cr. A probable reason
for this is that magnetic exchange interaction between particles is
not sufficiently reduced. On the other hand, the magnetic film has
a greatly decreased level of saturation magnetization (.about.100
kA/m) because the CoCr-based alloy contains 12 at % Cr. This
results in a significant decrease in reproduced signal intensity.
It was found that the S/N ratio of the reproduced recording medium
is lower by 10 dB or less than that of the conventional CoCrPt
recording medium, when measured with the same recording/reproducing
head due to the increased noise (N) and the decreased signal
(S).
[0015] The reason why the reproduced recording signal shows the
sufficiently low recording noise is that Cr in the ferromagnetic
layer of CoCr-based alloy does not fully block magnetic exchange
interaction between magnetic fine particles. The laminate structure
disclosed in the above-mentioned patent publication (11-501755) is
characterized in that the ferromagnetic layer of CoCr-based alloy
(of 0.35 nm thick) is relatively thinner than the noble metal (Pd)
layer (of 1.0 nm thick). This thickness ratio is inadequate to
reduce the magnetic exchange interaction between magnetic particles
to such an extent as to suppress recording noise because there
exists the magnetic exchange interaction between particles also in
the Pd alloy layer due to magnetization induced in the Pd alloy
layer by the ferromagnetic layer of CoCr-based alloy. In addition,
the small saturation magnetization is apparently due to the
excessively thin ferromagnetic layer of CoCr-based alloy.
[0016] However, the above-mentioned report by Lairson et al. shows
(in FIG. 5) how the perpendicular magnetic anisotropy energy
(K.sub.u) depends on the thickness of the CoCr-based alloy layer.
It is shown that the K.sub.u value decreases with the increasing
thickness of CoCr layer to such an extent that it is not useful for
the magnetic recording medium. Moreover, in the above-mentioned
report, the K.sub.u value is merely a little over 1.times.10.sup.5
J/m.sup.3 (FIG. 4) in the case of a multi-layered magnetic film
composed of Pd layers (0.4 nm thick) and CoCr layers (0.2 nm thick)
containing 15 at % Cr, which gave comparatively good results in
recording and reproducing experiments. The reason for this small
K.sub.u is the insufficient surface magnetic anisotropy energy
(K.sub.s).
[0017] The foregoing suggests that the magnetic layer for recording
cannot be applied in the practical perpendicular magnetic recording
system simply because its axis of magnetization is perpendicular to
the magnetic recording medium. Tackling this problem motivated the
present invention. Accordingly, it is an object of the present
invention to provide a perpendicular magnetic recording medium
characterized by reduced recording noise such that the magnetic
particles become less susceptible thermal disturbance.
SUMMARY OF THE INVENTION
[0018] In order to achieve the above-mentioned object, the present
inventors studied the magnetic recording medium having a
multi-layered magnetic film composed of ferromagnetic metal layers
containing Co and non-magnetic metal layers containing Pd which are
laminated alternately on top of the other. The result suggests that
it is possible to obtain a perpendicular magnetic recording medium
exhibiting sufficient perpendicular magnetic anisotropy energy with
the recording noise reduced below a certain level, if the
multi-layered magnetic film has a specific composition and layer
structure and is produced under specific conditions.
[0019] The present invention is based on this finding. Thus, the
gist of the present invention resides in a perpendicular magnetic
recording medium having a substrate and a multi-layered magnetic
film formed thereon with or without an underlying layer interposed
between them. The multi-layered magnetic film is composed of
ferromagnetic metal layers containing Co and non-magnetic metal
layers containing Pd which are laminated alternately on top of the
other, characterized in that the ratio of film thickness defined by
d1/d2 ranges from 1.5 to 4.0, where d1 denotes the thickness of
each of said ferromagnetic metal layers and d2 denotes the
thickness of each of said non-magnetic metal layers which is
between 0.6 nm and 2.0 nm (0.6nm.ltoreq.d2.ltoreq.2.0 nm).
[0020] The perpendicular magnetic recording medium constructed as
mentioned above may be modified such that the ferromagnetic metal
layers are formed from CoCr-based alloy. The resulting
multi-layered magnetic film has a low level of recording noise due
to the reduction in magnetic exchange interaction between magnetic
particles and also has good resistance to thermal disturbance due
to the sufficient perpendicular magnetic anisotropy energy to
resist the demagnetizing field energy induced by magnetization of
the magnetic film.
[0021] The gist of the present invention resides also in a process
for producing the perpendicular magnetic recording medium, which
comprises a step of forming a first underlying layer on a substrate
which may contains, ex. Pd, a step of forming a second underlying
layer which may contains, ex. CoCr.sub.40, then a multi-layered
magnetic film composed of alternately laminated ferromagnetic metal
layers containing Co and non-magnetic metal layers containing
Pd.
[0022] The above-mentioned process is preferably modified such that
an initial layer containing a paramagnetic Co alloy interposed
between the underlying layer and the multi-layered magnetic film.
In addition, the step of forming the multi-layered magnetic film is
preferably followed by heat treatment in a vacuum at a temperature
higher than 350.degree. C.
BRIEF DESCRIPTION OF THE DRAWINGS
[0023] A preferred embodiment of the present invention is
illustrated in the accompanying drawings in which:
[0024] FIG. 1 is a cross-sectional view of the magnetic recording
medium to which the present invention is applied;
[0025] FIG. 2 is a diagram of a ternary rotary cathode device;
[0026] FIG. 3 is a diagram of the multi-layered film formed by a
simultaneous discharging or an alternate discharging with the
rotary cathode;
[0027] FIG. 4 shows the relation between the Ar gas pressure and
the perpendicular magnetic anisotropy energy which is observed
during the production of the multi-layered film;
[0028] FIG. 5 shows how the surface magnetic anisotropy depending
on the thickness of Pd film in the magnetic recording medium of the
Example 1;
[0029] FIG. 6 shows how recording noise depending on the ratio of
d1/d2 (where d1 denotes the thickness of Pd layer and d2 denotes
the thickness of Co alloy layer) in the magnetic recording medium
of the Example 1;
[0030] FIG. 7 shows how the thermal stability parameter
K.sub.u/2.pi.M.sub.s.sup.2 depending on d1 and d2 in the magnetic
recording medium of the Example 1;
[0031] FIG. 8 is a diagram in which the reduction of reproducing
signal is plotted against the elapsed time;
[0032] FIG. 9 shows the relation between the thermal stability
parameter K.sub.u/2.pi.M.sub.s.sup.2 and the thermal
demagnetization factor S in the magnetic recording medium of the
Example 1;
[0033] FIG. 10 shows the relation between the thermal stability
parameter K.sub.u/2.pi.M.sub.s.sup.2 and the ratio of d1/d2 in the
magnetic recording medium of the Example 1; and
[0034] FIG. 11 shows how the thermal stability parameter
K.sub.u/2.pi.M.sub.s.sup.2 depending on d1 and d2 in the magnetic
recording medium of the Example 2.
[0035] FIG. 12 shows how the thermal stability parameter
K.sub.u/2.pi.M.sub.s.sup.2 depending on d1 and d2 in the magnetic
recording medium of the Example 3.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0036] As shown in FIG. 1(a), the perpendicular magnetic recording
medium 10 according to the present invention consists of a glass
substrate 11, an underlying layer 12 of NiTa.sub.37Zr.sub.10 alloy,
an underlying layer 13 of Pd, a multi-layered magnetic film 14, a
Pd cap layer 15, and a carbon protective layer 16. (Subscripts
indicate at %.) The multi-layered magnetic film 14 is composed of
ferromagnetic metal layers 14-1 and non-magnetic metal layers 14-2
each one layer of which is alternately laminated on top of one
layer of the other kind. This perpendicular magnetic recording
medium differs from the sample used for evaluation of recording and
reproducing which has a soft magnetic backing layer (400 nm thick)
between the glass substrate 11 and the underlying layer 12.
However, there is no difference between them in the magnetic
characteristics of the multi-layered magnetic film.
[0037] The multi-layered magnetic film 14 shown in FIG. 1(b) is
composed of ferromagnetic metal layers 14-1 and non-magnetic metal
layers 14-2. The former are formed from CoCr-based alloy, and the
latter are formed from Pd alloy. The multi-layered magnetic film 14
is characterized in that the ratio of d1/d2 ranges from 1.5 to 4.0,
where d1 denotes the thickness of each of the ferromagnetic metal
layers and d2 denotes the thickness of each of the non-magnetic
metal layers.
[0038] To incorporate with Cr to reduce magnetic exchange
interaction between particles, the ferromagnetic metal layers of
CoCr-based alloy ("CoCr-based alloy layers" hereinafter) should be
thicker than the non-magnetic metal layers of Pd alloy ("Pd alloy
layers" hereinafter), so that the grain boundary with a high Cr
concentration produces a strong effect. Specifically, the ratio of
d1/d2 should range from 1.5 to 4.0, where d1 denotes the thickness
of each of the ferromagnetic metal layers and d2 denotes the
thickness of each of the non-magnetic metal layers.
[0039] The multi-layered film structure in the perpendicular
magnetic recording medium 10 exhibits the interface magnetic
anisotropy K.sub.s and increases the perpendicular magnetic
anisotropy energy K.sub.u only when the thickness d2 of the Pd
alloy layer is 0.6 nm or larger. If the Pd alloy layer is
excessively thin, the surface magnetic anisotropy becomes small
despite the multi-layered film structure. If the Pd alloy layer is
excessively thick, magnetic exchange interaction between CoCr-based
alloy layers may be insufficient and individual layers may have
different coercive force. Therefore, the thickness of the Pd alloy
layer should be 2.0 nm or less. Thus the thickness of the Pd alloy
layer should be in the range of 0.6 nm to 2.0 nm, preferably 0.8 nm
to 1.2 nm.
[0040] The ratio of d1/d2 should be in the range of 1.5 to 4.0. If
the ratio of d1/d2 is smaller than 1.5, there is large magnetic
exchange interaction between particles and large domain wall
displacement at the time of recording, which lead to large
recording noise. If the ratio of d1/d2 is larger than 4.0, the
thickness of each of the ferromagnetic metal layers is large and
the effect of increasing perpendicular magnetic anisotropy energy
by K.sub.s become negligible. Thus, the multi-layered film
structure will not improve thermal stability.
[0041] In the case of magnetic recording medium with a
multi-layered film composed of CoCr-based alloy layers and noble
metal layers which are laminated alternately, it has been common
practice to reduce the laminating period (.lambda.) so as to
increase the surface magnetic anisotropy energy K.sub.s induced by
the layer interfaces.
[0042] Under the condition specified by the thickness ratio of
d1/d2, the value of .lambda. is inevitably large and hence the
crystal magnetic anisotropy energy K.sub.v of each CoCr-based alloy
layer accounts for a larger portion in the total perpendicular
magnetic anisotropy energy K.sub.u of the multi-layered magnetic
films. Therefore, K.sub.s merely enhances K.sub.u, and it is
essential that each CoCr-based alloy layer has a large value of
K.sub.v. Therefore, it is necessary to produce the multi-layered
structure to obtain a high value of K.sub.v.
[0043] One way to increase the value of K.sub.v is to keep as low
as possible the Cr concentration in the CoCr-based alloy layer.
Unfortunately, reducing the Cr concentration in CoCr-based alloy
generally tends to weaken the effect of reducing the magnetic
exchange interaction between particles at the Cr grain boundary.
(This is also applicable to monolayer CoCr-based alloys.) Up to
now, a Cr concentration of 20% is the minimum amount to reduce the
magnetic exchange interaction between particles and to suppress the
recording noise.
[0044] However, the present inventors found that it is possible to
sufficiently reduce the magnetic exchange interaction between
particles in the multi-layered magnetic film 14 even though the Cr
concentration is reduced below 20% in the CoCr-based alloy layer,
if the following production process is adopted.
[0045] (1) On the underlying layer formed an initial layer (about 3
nm thick) of paramagnetic CoCr-based or CoCu.
[0046] (2) On the initial layer is formed the multi-layered
magnetic film 14.
[0047] (3) After the multi-layered magnetic film 14 has been
formed, heat treatment is performed at 350.degree. C. or above for
about 10 seconds.
[0048] It was found that if the initial layer contains more than 30
at % of Cr or Cu, the magnetic exchange interaction between
particles in the multi-layered magnetic film 14 greatly decreases,
and consequently recording noise decreases. A probable reason for
this phenomenon is that the reheating process causes Cr or Cu atoms
in the initial layer to diffuse through grain boundary (with high
Cr concentration) in the direction perpendicular to the
multi-layered magnetic film 14, and this diffusion enhances the
effect of decreasing the magnetic exchange interaction between
particles at clear grain boundaries.
[0049] A single-layered recording film of CoCr-based alloy has a
large value of K.sub.v, but because of low Cr content, it also-has
a high value of Ms (saturation magnetization) and hence a high
value of demagnetizing energy. This large demagnetic-energy makes
saturation recording difficult, and hence it is not suitable for
use as the perpendicular magnetic recording medium.
[0050] On the other hand, the multi-layered magnetic film 14 which
applies the above high-Kr CoCr (-based) alloy decreases in M.sub.s
due to the lamination with the Pd alloy layer and hence the
demagnetizing field energy decreases to a proper value. Therefore,
the CoCr-based alloy thin film, which has a high value of K.sub.v
(crystal magnetic anisotropy energy) and a high value of M.sub.s
(saturation magnetization), is provided for use as the CoCr-based
alloy layer in the multi-layered magnetic film 14 according to the
present invention.
[0051] Moreover, the magnetic recording medium 10 of the present
invention is preferably modified such that the ferromagnetic metal
layer in the multi-layered magnetic film 14 contains Pt in addition
to Co and Cr. (The Pt-containing metal layer will be simply
referred to as a "CoCrPt alloy layer".)
[0052] The incorporation of Pt causes the conventional CoCr-based
alloy medium to increase in perpendicular magnetic anisotropy
energy has been reported many times. The present inventors found
that the same phenomenon is also applicable to the multi-layered
magnetic film according to the present invention.
[0053] In other words, the CoCrPt alloy layer used as the
ferromagnetic metal layer in the multi-layered magnetic film 14 has
a larger value of K.sub.u than the CoCr-based alloy layer. This
suggests that Pt added to the CoCr-based alloy layer increases the
value of K.sub.v (crystal magnetic anisotropy energy). Thus, the
incorporation of Pt into the CoCr-based alloy layer is
preferred.
[0054] It appears that the perpendicular magnetic recording medium
exhibits better magnetic characteristics if it has the
multi-layered magnetic film 14 composed of CoCrPt alloy layers
(with a low Cr concentration) and Pd alloy layers (non-magnetic
metal layers) which are laminated alternately, and if heat
treatment is performed after the multi-layered magnetic film 14 has
been formed.
[0055] A detailed description is given below of the kind and amount
of elements to be added to the multi-layered magnetic film 14. The
Cr concentration in the ferromagnetic metal layer should be in the
range of 12 at % to 21 at %. As mentioned above, the lower the Cr
concentration in the ferromagnetic metal layer, the greater the
value of K.sub.v. However, if the Cr concentration is lower than 12
at %, the resulting magnetic recording medium has a high level of
recording noise due to magnetic exchange interaction between
particles which cannot be reduced even by heat treatment. By
contrast, if the Cr concentration is 21 at %, the resulting
magnetic recording medium has an adequate level of magnetic
exchange interaction between particles even though no heat
treatment is performed.
[0056] Also, the Pt concentration in the ferromagnetic metal layer
should be in the range of 10 at % to 16 at %. The Pt added to the
ferromagnetic metal layer increases the value of K.sub.v. The
effect of Pt becomes maximum when the Pt concentration is higher
than 10 at %. However, if the Pt concentration exceeds 16 at %, the
magnetic exchange interaction between particles increases, and the
recording noise increases accordingly. A probable reason for this
is that Pt atoms prevent Cr atoms from displacement toward the
grain boundaries.
[0057] In addition, it is desirable to add a small amount of B or
Ta to the multi-layered film. In general, B or Ta added to the
multi-layered film promotes displacement of Cr atoms toward the
grain boundaries. However, an excess amount of B or Ta greatly
lowers the perpendicular magnetic anisotropy; therefore, their
amount is preferably several at %, at most.
[0058] The present inventors also studied the magnetic recording
medium in which a Pt layer is used as the non-magnetic metal layer.
However, it was found that the Pt layer did not contribute to good
characteristics. If a Pt alloy layer is used as the non-magnetic
metal layer, the interface between the Pt alloy layer and the
CoCr-based alloy layer is mixed during sputtering at a low gas
pressure. This leads to an insufficient level of K.sub.s (surface
magnetic anisotropy energy). In order to use Pt non-magnetic metal
layer, the production of the multi-layered magnetic film needs a
high sputtering gas pressure in order to achieve a large value of
K.sub.s. On the other hand, it is necessary to raise the degree of
crystallization of CoCr-based alloy in order to increase the value
of K.sub.v. To this end, it is desirable to perform sputtering at a
low gas pressure in order to prevent the sputtering gas from
entering the alloy. These film-forming conditions are incompatible
with one another for the Pt non-magnetic metal layer.
[0059] The results of the present inventors' investigation revealed
that a large value of K.sub.s can be obtained at a comparatively
low gas pressure if the non-magnetic metal layer is composed mainly
of Pd the multi-layered magnetic film 14 composed of ferromagnetic
metal layers and non-magnetic metal layers each one layer of which
is alternately laminated on top of one layer of the other kind. The
magnetic recording film 14, in which Pd is a major component in the
non-magnetic metal layer, has high surface magnetic anisotropy,
with the CoCr-based alloy layer keeping high crystal magnetic
anisotropy.
[0060] Incidentally, the multi-layered magnetic film 14 is not
specifically restricted in thickness. However, it should have an
adequate thickness corresponding to the perpendicular magnetic
anisotropy of the multi-layered magnetic film 14, because it is
vulnerable to thermal disturbance if it is excessively thin.
[0061] The perpendicular magnetic recording medium according to the
present invention is described more in detail with reference to the
following examples.
EXAMPLE 1
[0062] This example demonstrates the perpendicular magnetic
recording medium according to the present invention, in which the
substrate is a glass disk (76 mm in diameter) with a planarized
surface suitable for HDD. This glass disk has a hole (an inside
diameter of 15 mm) at its center through which it is placed on to a
spindle.
[0063] This glass disk was mounted on a sputtering apparatus and
then heated at about 100.degree. C. to remove any adsorbed water.
On the glass disk was formed an underlying layer (40 nm thick) from
NiTa.sub.37Zr.sub.10 alloy. On the underlying layer was
sequentially formed a Pd film (5 nm thick), a multi-layered
magnetic film, a Pd film (1 nm thick), and a carbon protective
film. The multi-layered magnetic film is composed of noble metal
layers and ferromagnetic metal layers each one layer of which are
laminated alternately on top of one layer of the other kind.
[0064] The multi-layered magnetic film is formed by using a rotary
cathode device as shown in FIG. 2. The rotary cathode has three
target cathodes 22 which rotate along the same circle to form a
film on the substrate 21 placed right above the center of the
circle.
[0065] If two target cathodes (A and B) are mounted on the rotary
cathode device and they are subjected to discharging, it is
possible to form a multi-layered film (composed of layers A and
layers B which are laminated alternately) in a short time
(hereinafter "simultaneous discharging method").
[0066] This rotary cathode device gives rise to a multi-layered
magnetic film 31 shown in FIG. 3(a). This multi-layered magnetic
film 31 is not composed of complete layers. That is, the noble
metal layers 32 and the ferromagnetic metal layers 33 are formed
such that the position of their interface periodically moves.
However, the multi-layered magnetic film 31 may be regarded as
lamellar because the circumference of the glass disk is
sufficiently large (50 mm or above) relative to the thickness of
each layer (about 1 nm).
[0067] It is also possible to form the multi-layered structure as
shown in FIG. 3(b) if the targets A and B on the rotary cathode
device are actuated for discharging alternately (hereinafter
"alternate discharging method").
[0068] In general, the simultaneous discharging method is
advantageous over the alternate discharging method in producing the
multi-layered magnetic film of periodic structure in a short time.
Therefore, the simultaneous discharging method was employed in this
and following examples.
[0069] In this example, a recording medium of multi-layered
structure having a multi-layered magnetic film composed of
ferromagnetic metal layers of CoCr.sub.21 (referred to as
CoCr.sub.21 alloy layers) and noble metal layers of Pd which are
laminated alternately on top of the other was prepared.
("Pd/CoCr.sub.21 medium" hereinafter.)
[0070] In this example, a recording medium of multi-layered
structure having a multi-layered magnetic film composed of
ferromagnetic metal layers of CoCr.sub.21 (referred to as
CoCr.sub.21 alloy layers) and noble metal layers of Pt which are
laminated alternately on top of the other was also prepared for
comparison. ("Pd/CoCr.sub.21 medium" hereinafter.)
[0071] The multi-layered magnetic film was formed such that each
CoCr.sub.21 alloy layer is 2.0 nm thick and each noble metal layer
is 0.2-1.4 nm thick, with the total thickness being 20 nm. The
layer thickness d1 and d2, the thickness ratio d1/d2, and the
laminating period were controlled by regulating the sputtering
power onto the noble metal target and Co alloy target, as well as
the speed of the rotary cathode device. The substrate was preheated
so that its temperature was 250.degree. C. when the multi-layered
magnetic film was formed.
[0072] In a comparative example, a perpendicular magnetic recording
medium which consists of a substrate, an underlying layer (40 nm
thick) of NiTa.sub.37Zr.sub.10 alloy, a Pd film (5 nm thick), a
single-layered magnetic film (20 nm thick) of CoCr.sub.21, a Pd
film (1 nm thick), and a carbon protective film was prepared
("CoCr.sub.21 medium" hereinafter). The single-layered magnetic
film is a CoCr.sub.21 film having the axis of magnetization in the
perpendicular direction. The substrate was heated at 250.degree. C.
immediately before the magnetic film was formed as in the case of
forming the Pd (Pt) /CoCr.sub.21 medium.
[0073] The sputtering gas pressure affects the surface magnetic
anisotropy energy K.sub.s of the multi-layered magnetic film and
the crystal magnetic anisotropy energy K.sub.v of the CoCr.sub.21
film (which is a single-layered magnetic film) as shown in FIG. 4.
The noble metal layer in the multi-layered magnetic film is 0.8 nm
thick.
[0074] The perpendicular magnetic anisotropy energy K.sub.u in the
multi-layered magnetic film medium depends on K.sub.s and K.sub.v,
as approximately represented by equation (1) below. (H. J. G.
Draaisam et al., J. Magn. Magn. Mater. 66 (1987) 351-355)
K.sub.u.multidot..lambda.=2.multidot.K.sub.s+t.multidot.K.sub.v
(1)
[0075] In equation (1), K.sub.u represents the perpendicular
magnetic anisotropy energy of the multi-layered magnetic film as a
whole, K.sub.s represents the surface magnetic anisotropy energy
(per unit area) in the interface between the CoCr.sub.21 alloy
layer and the noble metal layer, and K.sub.v represents the crystal
magnetic anisotropy energy of the CoCr.sub.21 alloy layer. T
denotes the thickness of each CoCr.sub.21 alloy layer, .lambda.
denotes the period of lamination, and 2.multidot.K.sub.s denotes
the surface magnetic anisotropy energy (for two interfaces) due to
one period in the multi-layered film.
[0076] The value of K.sub.v (crystal magnetic anisotropy energy)
includes the shape anisotropy of the film (-2.pi.M.sub.s.sup.2)
which occurs when the recording film is magnetized in one
direction. As equation (1) suggests, the ratio of K.sub.v to
K.sub.u increases in proportion to the thickness (t) of the
CoCr.sub.21 alloy layer.
[0077] In the present invention, the surface magnetic anisotropy
energy was calculated according to equation (1) under on the
assumption that K.sub.v of the CoCr.sub.21 alloy layer in the
multi-layered magnetic film is equal to K.sub.v of the CoCr.sub.21
film (20 nm thick) as a single-layered magnetic film.
[0078] FIG. 4 suggests that the value of K.sub.s increases as the
sputtering gas pressure increases if Pt is used in the noble metal
layer. By contrast, the value of K.sub.s increases as the
sputtering gas pressure decreases if Pd is used in the noble metal
layer. On the other hand, the K.sub.v value of CoCr.sub.21 alloy
film rapidly decreases as the sputtering gas pressure
increases.
[0079] In order to achieve the object of the present invention, it
is necessary to form the multi-layered magnetic film which has high
surface anisotropy energy K.sub.s such that the CoCr.sub.21 alloy
layer keeps high crystal magnetic anisotropy. FIG. 4 suggests that
for the CoCr.sub.21 alloy layer to have high perpendicular magnetic
anisotropy, it is necessary to perform sputtering at a low
sputtering gas pressure. If the noble metal layer and the
CoCr.sub.21 alloy layer are formed in the same sputtering chamber,
it is necessary to obtain high surface magnetic anisotropy energy
under the condition of low gas pressure. This apparently suggests
that it is desirable to use Pd for the noble metal layer
constituting the multi-layered magnetic film.
[0080] The results shown in FIG. 4 are discussed in the following.
In general, sputtering method for forming a film tends to cause
sputtering gas (such as Ar) to enter the film during the
film-forming process. Therefore, the sputtering gas pressure should
be as low as possible to give a high-purity alloy so long as
discharging can be performed under acceptable conditions. The value
of K.sub.v (crystal magnetic anisotropy energy) depends largely on
the degree of crystallization of the CoCr-based alloy. It is a
matter of course that the value of K.sub.v (crystal magnetic
anisotropy energy) increases as the sputtering gas pressure
decreases.
[0081] As mentioned, it is desirable to increase the sputtering gas
pressure in the production of superlattice. In fact, U.S. Pat. No.
5,106,703 (mentioned above) mentions that the coercive force
increases if the sputtering gas pressure is increased, and Ar, as
the sputtering gas, is replaced by Xe or Kr. Usually, the coercive
force is proportional with the perpendicular magnetic anisotropy
energy.
[0082] However, FIG. 4 indicates that the Pt/CoCr.sub.21 medium and
the Pd/CoCr.sub.21 medium behave greatly differently toward the
sputtering gas pressure. It is noted that the noble metal layer
with Pd has a high value of K.sub.s when the pressure of Ar
sputtering gas is low.
[0083] Such a difference between Pd and Pt in the noble metal layer
arises from the difference in their atomic weight (Pd=195.1,
Pt=106.4). The multi-layered structure requires that the layer
interface (as the source of surface magnetic anisotropy) be formed
neatly. To this end, it is necessary to keep sufficiently low the
kinetic energy of sputtering particles sticking to the film such
that sputtering particles impinge upon the film without destroying
the layer interface.
[0084] Sputtering particles are scattered by collision with
sputtering gas at a high pressure in the chamber, so that they lose
their kinetic energy. However, Pt particles do not readily lose
their kinetic energy unless the sputtering gas pressure is
considerably high, because Pt has a much larger atomic weight than
Ar. (Ar=40) By contrast, Pd particles readily lose their kinetic
energy at a comparatively low sputtering gas pressure although the
difference between Pd and Ar in atomic weight is not so large.
[0085] The relation between sputtering gas pressure and sputtering
rate was investigated by using the same chamber as mentioned. It
was found that when the sputtering gas pressure was raised from 3
Pa to 10 Pa, the sputtering rate of Pt remained almost unchanged
but the sputtering rate of Pd decreased by more than half. This
result suggests that there are more Pd particles which do not reach
the substrate due to scattering by the sputtering gas. This
coincides with the above-mentioned prediction.
[0086] Sputtering at a low gas pressure is close to the condition
under which the conventional single-layered CoCr-based alloy
crystalline recording film is formed. In this case, a good layer
interface is readily obtained because there is only a small amount
of impurities in the multi-layered magnetic film. In the case where
Pd is used to form the noble metal layer, the value of K.sub.s is
high when the gas pressure is low because the effect of making a
good layer interface excels the effect of destroying the interface
by the sputtering particles.
[0087] Incidentally, since Co has a smaller atomic weight (58.9)
than noble metal, Co sputtering particles are less likely to damage
the interface than the noble metal sputtering particles. Sputtering
with Xe or Kr (with an atomic weight of 131.2 and 83.8,
respectively) would be favorable to Pt; however, Ar is advantageous
costwise.
[0088] An experiment was carried out to see how the surface
magnetic anisotropy energy of the multi-layered magnetic film
depends on the thickness of the noble metal layer in the case of
CoCr.sub.21 alloy medium in which the thickness of the CoCr.sub.21
alloy layer is fixed at 2.0 nm. The result of the experiment is
shown in FIG. 5. It is noted that the surface perpendicular
magnetic anisotropy energy (K.sub.s) increases with the increasing
thickness of the noble metal layer of Pd. It rapidly increases
until the thickness reaches 0.6 nm, and it becomes saturated when
the thickness further increases from 0.8 nm to 1.0 nm. This
suggests that an excessively thin noble metal layer (Pd) does not
contribute to the surface perpendicular magnetic anisotropy energy
arising from the multi-layered film structure.
[0089] Why the surface perpendicular magnetic anisotropy energy is
small when the thickness of the Pd layer is small may be explained
as follows. (1) If the Pd layer is excessively thin, no good layer
structure is formed. (2) There are no sufficient electrons to
generate the magnetic anisotropy energy if the Pd layer is
excessively thin. (Magnetic anisotropy is caused by electrons in
the Pd layer.)
[0090] It was found that each of the CoCr.sub.21 alloy layers have
individually different coercive force if the Pd noble metal layer
is made thicker than 2.0 nm. This suggests that magnetic exchange
interaction between CoCr.sub.21 alloy layers becomes so small that
domains in each layer can reverse independently. Under this
situation, the multi-layered magnetic films as a whole do not
function as a single magnetic film. That is, it cannot be used as
the recording magnetic film. The Pd noble metal layer in the
multi-layered magnetic film should be thinner than 2.0 nm.
[0091] A probable reason why the surface magnetic anisotropy energy
becomes saturated when the thickness of the Pd noble metal layer is
approximately 1.0 nm is that magnetization induced by the Pd noble
metal layer is limited to about 0.5 nm (about two-atom layer) from
the interface between the CoCr.sub.21 alloy layer and the Pd noble
metal layer caused by Co.
[0092] The present inventors measured the magnitude of
magnetization induced in the Pd noble metal layer by the adjacent
CoCr.sub.21 alloy layer. It was found that average magnetization
remains nearly constant at about 100 kA/m when the Pd noble metal
alloy is thinner than 1.0 nm but it decreases as the thickness of
the Pd noble metal layer exceeds 1.0 nm. It is considered that
magnetization induced in the Pd noble metal layer is responsible
for at least part of surface magnetic anisotropy energy.
[0093] Then, the present inventors prepared several samples of
multi-layered magnetic film medium (Pt/CoCr.sub.21 medium) which
vary in the thicknesses of the Pd noble metal layer and the
CoCr.sub.21 alloy layer. The multi-layered magnetic film was tested
for perpendicular magnetic anisotropy energy and saturation
magnetization. The results are shown in Tables 1 and 2.
[0094] As a reference, the present inventors also prepared a
CoCr.sub.21 medium with a 20-nm thick single-layered magnetic film.
It was found that the single-layered magnetic film has
perpendicular anisotropy energy of about 0.8.times.10.sup.5
J/m.sup.3 and saturation magnetization of 250 kA/m. This
single-layered magnetic film gave an M-H curve with a squareness
ratio of about 0.55.
1TABLE 1 Structure and perpendicular magnetic anisotropy energy of
multi-layered magnetic film in Pd/CoCr.sub.21 medium (unit:
.times.10.sup.5 J/m.sup.3) Thickness of Pd film (nm) 0.2 0.4 0.6
0.8 1.0 1.2 1.5 Thickness of 0.3 0.70 1.54 3.42 3.65 3.15 2.68 2.35
CoCr.sub.21 alloy 0.5 0.75 1.32 2.95 3.30 2.82 2.45 2.15 film (nm)
0.8 0.75 1.23 2.52 2.75 2.38 2.36 1.95 1.0 0.80 1.15 2.15 2.42 2.21
2.20 1.83 1.5 0.83 1.02 1.90 2.12 1.95 1.93 1.67 2.0 -- 1.10 1.73
1.95 1.76 1.68 1.55 2.5 -- 0.93 1.55 1.82 1.65 1.58 1.46 3.0 -- --
1.51 1.66 1.52 1.46 1.42 4.0 -- -- 1.32 1.45 1.40 1.32 1.30 5.0 --
-- -- -- 1.32 1.33 1.20 6.0 -- -- -- -- 1.33 1.25 1.06
[0095]
2TABLE 2 Structure and saturation magnetization (M.sub.s) of
multi-layered magnetic film in Pd/CoCr.sub.21 medium (unit: kA/m)
Thickness of Pd film (nm) 0.2 0.4 0.6 0.8 1.0 1.2 1.5 Thickness of
0.3 190 168 150 140 135 135 130 CoCr.sub.21 alloy 0.5 209 185 167
152 150 142 135 film (nm) 0.8 220 202 186 178 168 163 160 1.0 222
210 192 182 175 163 158 1.5 230 218 205 193 190 179 170 2.0 -- 226
215 202 200 186 188 2.5 -- 230 221 205 205 205 194 3.0 -- -- 226
212 207 205 200 4.0 -- -- 230 225 226 209 210 5.0 -- -- -- -- 224
233 215 6.0 -- -- -- -- 226 226 220
[0096] It is noted that the multi-layered magnetic film has much
larger perpendicular anisotropy energy than the single-layered
magnetic film of CoCr.sub.21 medium. It is noted from Table 2 that
the multi-layered magnetic film decreases in saturation
magnetization due to the Pd layer.
[0097] For the perpendicular magnetic recording medium to have
stable magnetization, it is necessary that its M-H curve has a
squareness ratio close to 1. This can be checked in terms of a
parameter of K.sub.u/2.pi.M.sub.s.sup.2 according to Shimazu et al.
(Nippon Ouyou Jiki Gakkaishi, vol. 25, No. 4-2, pp. 539-542). The
reference pointed out that this parameter should essentially be
greater than 3 for a practical magnetic recording medium (with
magnetic particles with an approximate diameter of 10 nm) in
consideration of effect on it by thermal disturbance and
demagnetizing field. Since K.sub.u/2.pi.M.sub.s.sup.2=H.s-
ub.k/4.pi.M.sub.s, this parameter represents the magnitude ratio of
the anisotropic magnetic field to the demagnetizing field (H.sub.k
is a value of the physical property which indicates how hard it is
for magnetic particles to undergo magnetization reversal). In other
words, it is an index indicating how stable the magnetization is in
the medium against the demagnetization field induced when the
recording bits are uniformly magnetized.
[0098] The CoCr.sub.21 alloy medium referenced in this example has
a value of K.sub.u/2.pi.M.sub.s.sup.2=1.73. This value suggests
that the squareness ratio apparently deviating from 1. This
parameter was calculated also for the multi-layered magnetic film
in this example. The results are shown in Table 3.
3TABLE 3 Structure and K.sub.u/2.pi.M.sub.s.sup.2 value of
multi-layered magnetic film in Pd/CoCr.sub.21 medium (unit: none)
Thickness of Pd film (nm) 0.2 0.4 0.6 0.8 1.0 1.2 1.5 Thickness of
0.3 3.086 8.684 24.19 29.64 27.51 23.40 22.13 CoCr.sub.21 alloy
film 0.5 2.733 6.138 16.83 22.73 19.95 19.34 18.78 (nm) 0.8 2.466
4.798 11.59 13.81 13.42 11.14 12.12 1.0 2.583 4.150 9.282 11.63
11.49 13.18 11.67 1.5 2.497 3.416 7.196 9.058 8.597 9.587 9.197 2.0
-- 3.428 5.956 7.606 7.003 7.729 6.980 2.5 -- 2.798 5.051 6.893
6.249 5.984 6.174 3.0 -- -- 4.705 5.878 5.646 5.529 5.650 4.0 -- --
3.971 4.559 4.362 4.810 4.692 5.0 -- -- -- -- 4.187 4.257 4.132 6.0
-- -- -- -- 4.144 3.895 3.486
[0099] It is noted from Table 3 that the multi-layered magnetic
film has a considerably large value of K.sub.u/2.pi.M.sub.s.sup.2
because the surface magnetic anisotropy enhances the perpendicular
magnetic anisotropy energy and the Pd layer suppresses the average
magnetization of the multi-layered film as a whole.
[0100] For the multi-layered magnetic film to be a suitable
perpendicular magnetic recording medium, it should have
perpendicular magnetic anisotropy energy, saturation magnetization,
and K.sub.u/2.pi.m.sub.s.sup- .2 value as desired, as well as good
recording characteristics.
[0101] Thus, the present inventors tested the perpendicular
magnetic recording medium in this example so as to realize low
recording noise for the structure of the multi-layered magnetic
film.
[0102] The test procedure consists of recording a continuous
pattern with a recording density of 300 kFCI on each sample of the
perpendicular magnetic recording medium and reproducing the
continuous pattern to measure the integral value of recording
noise.
[0103] The perpendicular magnetic recording medium for recording
and reproducing tests has an FeTaC soft magnetic layer (400 nm
thick) on that side of the NiTaZr underlying layer which faces the
substrate. Recording was accomplished by using a single pole type
head, and reproducing was accomplished by using a GMR sensor
mounted on the same head.
[0104] It was found that recording noise depends on the ratio of
d1/d2 (where d1 denotes the thickness of the CoCr.sub.21 alloy
layer and d2 denotes the thickness of the Pd noble metal layer) as
shown in FIG. 6. In FIG. 6, recording noise is given in terms of
relative noise strength compared with the noise of the CoCr.sub.21
medium with the single-layered magnetic film (for reference). The
noise strength of each medium is normalize with respect to the
low-frequency signal strength.
[0105] It is noted that the relative noise strength decreases as
the relative thickness of the CoCr.sub.21 alloy layer increases (or
it is about 1 when d1/d2 is about 1.5). A probable reason why the
recording noise is large when the d1/d2 is small is that there is
large magnetic exchange interaction between particles and there is
large domain wall displacement at the time of recording. This
result suggests that the multi-layered magnetic recording medium
has a low level of recording noise as in the CoCr.sub.21 medium
with the single-layered magnetic film if it is constructed such
that the CoCr.sub.21 alloy layer is more than 1.5 times thick than
the Pd noble metal layer. It was necessary to keep the substrate at
about 250.degree. C. when layers are formed thereon.
[0106] The values of K.sub.u/2.pi.M.sub.s.sup.2 (smaller than 10)
in Table 3 are plotted against the thickness of the Pd noble metal
layer, with the thickness of the CoCr.sub.21 alloy layer varied, as
shown in FIG. 7. All layer thickness conditions plotted in FIG. 7
satisfy a condition that its d1/d2 value is larger than 1.5 such
that the magnetic exchange interaction between particles is
sufficiently reduced. It is noted from FIG. 7 that all of the
multi-layered magnetic recording medium have larger
K.sub.u/2.pi.M.sub.s.sup.2 values than the CoCr.sub.21 medium for
reference. This suggests that the multi-layered magnetic film
medium is more thermally stable than the single-layered magnetic
film medium.
[0107] In FIG. 7, all the plotted points are within the thick
broken line. If the Pd noble metal layer is excessively thin, the
surface magnetic anisotropy is insufficient. If the Pd noble metal
layer is excessively thick, the density in the interface decreases
because it is necessary that the CoCr.sub.21 alloy layer should
also be thick. Consequently, the thickness of the Pd noble metal
layer which gives the maximum value of K.sub.u/2.pi.M.sub.s.sup.2
is 0.8 to 1.0 nm, which is equivalent to the thickness at which the
surface magnetic anisotropy energy becomes saturated.
[0108] The present inventors examined the relation between the
K.sub.u/2.pi.M.sub.s.sup.2 value and the thermal demagnetizing
factor S of the multi-layered magnetic film medium in this example
by recording a continuous pattern (20 kFCI) on each sample of the
medium and then measuring the amount of loss in the reproduced
signal. The thermal demagnetizing factor S is the coefficient of
magnetic viscosity which relates the rate of signal loss to
log.sub.10 (time). It was determined by plotting the amount of
signal loss (measured for a period of 1000 seconds) on a graph in
which the abscissa represents log.sub.10 (time) and the ordinate
represents the rate of signal loss. An example of measurements is
shown in FIG. 8.
[0109] In FIG. 9, the thermal demagnetizing factor S is plotted
against the K.sub.u/2.pi.M.sub.s.sup.2 value of each medium. FIG. 9
indicates that the thermal demagnetizing factor S approaches zero
when K.sub.u/2.pi.M.sub.n.sup.2 increases. In other words, thermal
demagnetization is practically negligible for the
K.sub.u/2.pi.M.sub.s.su- p.2 value exceeding 6. The result of this
investigation suggests that the K.sub.u/2.pi.M.sub.s.sup.2 value
should be greater than 6 so that the perpendicular magnetic
recording medium permits recorded signals to remain thermally
stable.
[0110] It is noted from FIG. 7 that this condition is satisfied
when the thickness (d2) of the Pd noble metal layer is larger than
0.6 nm. If the Pd alloy layer is excessively thick (or thicker than
2.0 nm), magnetic exchange interaction between CoCr-based alloy
layers is insufficient and hence CoCr-based alloy layers would have
different coercive force independently. Therefore, the thickness
(d2) of the Pd noble metal layer should be from 0.6 nm to 2.0 nm,
and preferably, from 0.8 nm to 1.2 nm.
[0111] The data in FIG. 7 is rearranged in FIG. 10. In FIG. 10, the
K.sub.u/2.pi.M.sub.s.sup.2 value is plotted against the ratio of
d1/d2 (where d1 is the thickness of the CoCr-based alloy layer and
D2 is the thickness of the Pd noble metal layer). It is noted that
the K.sub.u/2.pi.M.sub.s.sup.2 value is not large when the
thickness of the Pd layer is 0.2 nm or 0.4 nm. The above-mentioned
condition is not met when the ratio of d1/d2 is smaller than 1.5
(requirement for good signal quality). On the other hand, for the
Pd layer with a thickness of 0.6 nm, the K.sub.u/2.pi.M.sub.s.sup.2
value suddenly increases and the condition that d1/d2 is larger
than 1.5 and K.sub.u/2.pi.M.sub.s.sup.2 is larger than 6 is met.
However, with an excessively large d1/d2, the condition that
K.sub.u/2.pi.M.sub.s.sup.2 is larger than 6 is not met because the
laminate period decreases (and hence the number of interfaces
between the Co layers and Pd layers decreases), and the surface
magnetic anisotropy energy decreases. FIG. 10 shows limited data
for the Pd layer with a thickness up to 0.8 nm. Beyond this
thickness, the value of K.sub.u/2.pi.M.sub.s.sup.2 tends to
decrease. It is concluded from FIG. 10 that the allowable maximum
value of d1/d2 is 4.
EXAMPLE 2
[0112] This example demonstrates a magnetic recording medium in
which the ferromagnetic metal layer is formed from a material
having a large value of Kv in place of the CoCr-based alloy used in
Example 1. In Example 1, the CoCr layer as the ferromagnetic metal
layer is inherently limited in crystal perpendicular magnetic
anisotropy energy K.sub.v and hence the range of d1/d2 for keeping
the K.sub.u/2.pi.M.sub.s.sup.2 value exceeding 6 (with the maximum
being about 10) is rather narrow. The advantage of the recording
medium in this example is that the problem with thermal
demagnetization can be avoided even in the case where the diameter
of magnetic particles is reduced further in order to decrease
recording noise.
[0113] The multi-layered magnetic recording medium prepared in this
example consists of a substrate, an underlying layer of
NiTa.sub.37Zr.sub.10 alloy (40 nm thick), a Pd film (5 nm thick) a
multi-layered magnetic film of Pd/CoCr.sub.21Pt.sub.14, a Pd film
(1 nm thick), and a carbon protective film, which are sequentially
placed on top of the other. The multi-layered magnetic film of
Pd/CoCr.sub.21Pt.sub.14 is referred to as "Pd/CoCr.sub.21Pt.sub.14
film" hereinafter. The multi-layered magnetic recording medium is
referred to as "Pd/CoCr.sub.21Pt.sub.14 medium" hereinafter.
Incidentally, the total thickness of the multi-layered magnetic
film is about 20 nm, and the substrate was heated to 250.degree. C.
immediately before film lamination.
[0114] A comparative sample of perpendicular magnetic recording
medium was also prepared, which consists of a substrate, an
underlying layer of NiTa.sub.37Zr.sub.10 alloy (40 nm thick), a Pd
film (5 nm thick), a single-layered magnetic film of
CoCr.sub.21Pt.sub.14, a Pd film (1 nm thick), and a carbon
protective film, which are sequentially placed on top of the other.
The single-layered magnetic film of CoCr.sub.21Pt.sub.14 is
referred to as "CoCr.sub.21Pt.sub.14 film" hereinafter. The
single-layered magnetic recording medium is referred to as
"CoCr.sub.21Pt.sub.14 medium" hereinafter. As in the
Pd/CoCr.sub.21Pt.sub.14 medium, the substrate was heated to
250.degree. C. immediately before film deposition.
[0115] The CoCr.sub.21Pt.sub.14 film was found to have
perpendicular anisotropy energy of about 1.6.times.10.sup.5
J/m.sup.3 and saturation magnetization of 250 kA/m. The
CoCr.sub.21Pt.sub.14 film is quite similar in saturation
magnetization to the CoCr.sub.21 alloy layer (in Example 1) which
does not contain Pt. However, the CoCr.sub.21Pt.sub.14
single-layered magnetic film increases in crystal magnetic
anisotropy.
[0116] As shown above, the incorporation of Pt into the
ferromagnetic metal layer of CoCr-based alloy is an effective way
for increasing the magnetic anisotropy of the magnetic film. The
comparison of the Pd/CoCr.sub.21Pt.sub.14 medium with the
Pd/CoCr.sub.21 medium revealed that the former has larger
perpendicular magnetic anisotropy energy than the latter but there
is no significant difference between them in the average saturation
magnetization of the entire multi-layered film. The increment of
perpendicular magnetic anisotropy is due mostly to crystal magnetic
anisotropy energy but very little to surface magnetic anisotropy
energy. The CoCr.sub.21Pt.sub.14 alloy is suitable for use as the
ferromagnetic metal layer of the multi-layered magnetic film
medium, because it increases in crystal magnetic anisotropy owing
to the incorporation of Pt even if it is made into multi-layered
thin film.
[0117] The multi-layered magnetic film medium
(Pd/CoCr.sub.21Pt.sub.14 medium) in this example was examined for a
relation between d1/d2 and recording noise. It was found that
recording noise can be suppressed if d1/d2 is larger than 1.5 as in
Example 1.
[0118] Samples of Pd/CoCr.sub.21Pt.sub.14 medium varying in their
layer thickness were measured for K.sub.u/2.pi.M.sub.s.sup.2
values. The results are shown in FIG. 11. In comparison of FIG. 11
with FIG. 7 (in term of the Pd/CoCr.sub.21 medium), it is noted
that the Pd/CoCr.sub.21Pt.sub.14 medium has larger
K.sub.u/2.pi.M.sub.s.sup.2 values than the Pd/CoCr.sub.21 medium,
with the maximum value being about 12 when d2 is in the range of
0.8 to 1.0 nm. It is also noted that the Pd noble metal layer
should have a thickness (d2) larger than 0.6 nm so as to meet the
condition that K.sub.u/2.pi.M.sub.s.sup.2 of the
CoCr.sub.15Pt.sub.14B.sub.2 alloy is greater than 6 (as the index
for thermal stability). If the Pd alloy layer is excessively thick
(or thicker than 2.0 nm), magnetic exchange interaction between
CoCr-based alloy layers is insufficient, and hence CoCr-based alloy
layers would have different coercive force independently.
[0119] Samples of Pd/CoCr.sub.21Pt.sub.x medium were examined for
perpendicular magnetic anisotropy, with the amount of Pt varying in
the ferromagnetic metal layer of CoCr-based alloy. The
CoCr.sub.21Pt.sub.x alloy layer is 1.6 nm thick, and the Pd noble
metal layer is 0.8 nm thick.
[0120] It was found that the perpendicular magnetic anisotropy
energy linearly increases in proportion to the amount of Pt added
(up to 10 at %). However, the perpendicular magnetic anisotropy
remained almost unchanged beyond 10 at %, and recording noise
begins to increase beyond 16 at %. This indicates that
incorporation with Pt increases magnetic exchange interaction
between particles. It is concluded from the foregoing that the
ferromagnetic metal layer of CoCr-based alloy should be
incorporated with Pt in an amount from 10 at % to 16 at %.
EXAMPLE 3
[0121] This example demonstrates a multi-layered magnetic film
recording medium ("Pd/CoCr.sub.15Pt.sub.14B.sub.2 medium"
hereinafter) which was prepared in the following manner. First, a
substrate was coated sequentially with an underlying layer (40 nm
thick) of NiTa.sub.37Zr.sub.10 alloy and a Pd film (5 nm thick).
With the substrate heated to 250.degree. C., the Pd film was coated
with an initial layer (3 nm thick) of CoCr.sub.40 alloy and
subsequently with a multi-layered magnetic film of
Pd/CoCr.sub.15Pt.sub.14B.sub.2 ("Pd/CoCr.sub.15Pt.sub.14- B.sub.2
film" hereinafter) by using the rotary cathode device. After the
multi-layered magnetic film had been formed, a heat treatment was
performed at 350.degree. C. or above for 10 seconds. After cooling,
a Pd film (1 nm thick) and a carbon protective film were formed
sequentially.
[0122] In general, the crystal magnetic anisotropy energy (K.sub.v)
increases but the effect of reducing the magnetic exchange
interaction between particles at Cr grain boundaries is lost as the
Cr concentration decreases in the CoCr-based alloy. According to
the conventional technology, the minimum Cr concentration was 20%
which decreases the magnetic exchange interaction between particles
without increase in recording noise.
[0123] The present inventors found that the magnetic exchange
interaction between particles decreases to a level suitable for
magnetic recording if heat treatment is performed at 350.degree. C.
or above for 10 seconds or so after the multi-layered magnetic film
has been formed.
[0124] The reason why the magnetic exchange interaction between
particles decreases upon a heat treatment may be understood as
follows. The CoCr.sub.40 initial layer just under the multi-layered
magnetic film contains Cr atoms which, during reheating, diffuse
through the grain boundaries of magnetic particles in the direction
perpendicular to the film surface.
[0125] A comparative sample of single-layered magnetic film medium
("CoCr.sub.15Pt.sub.14B.sub.2 medium" hereinafter) was prepared in
the following manner. A substrate was coated sequentially with an
underlying layer (40 nm thick) of NiTa.sub.37Zr.sub.10 alloy and a
Pd film (5 nm thick). With the substrate heated to 250.degree. C.,
the Pd film was coated with an initial layer (3 nm thick) of
CoCr.sub.40 alloy and subsequently with a single-layered magnetic
film of CoCr.sub.15Pt.sub.14B.sub.2 ("CoCr.sub.15Pt.sub.14B.sub.2
film" hereinafter). After the CoCr.sub.15Pt.sub.14B.sub.2 film had
been formed, a heat treatment was performed at 350.degree. C. or
above for 10 seconds. After sufficient cooling, a Pd film (1 nm
thick) and a carbon protective film were formed sequentially.
[0126] The CoCr.sub.15Pt.sub.14B.sub.2 alloy film was found to have
large perpendicular anisotropy energy of about 2.5.times.10.sup.5
J/m.sup.3 and saturation magnetization of 320 kA/m (which is larger
than that of the CoCr.sub.15 alloy film) due to the small Cr
content (15%). Therefore, the recording medium has a large
demagnetizing field, and the single-layered film of
CoCr.sub.15Pt.sub.14B.sub.2 has a K.sub.u/2.pi.M.sub.s.sup.2 value
of 3.9. In other words, the single-layered film of
CoCr.sub.15Pt.sub.14B.sub.2 differs very little from the
CoCr.sub.21Pt.sub.14 film with a large Cr content (without heat
treatment, referred to Example 2) in terms of
K.sub.u/2.pi.M.sub.s.sup.2 (thermal stability), because the
increased perpendicular magnetic anisotropy is cancelled by the
increased demagnetizing field.
[0127] The Pd/CoCr.sub.15Pt.sub.14B.sub.2 medium, which underwent
heat treatment after the multi-layered magnetic film had been
formed, was examined for perpendicular magnetic anisotropy energy
(K.sub.u) and saturation magnetization (M.sub.s), with the
thickness of the multi-layered magnetic film being adequately
varied. The results are shown in Tables 4 and 5.
4TABLE 4 Structure and perpendicular magnetic anisotropy energy
(K.sub.u) of multi-layered magnetic film in
Pd/CoCr.sub.21Pt.sub.14B.sub.4 medium with reheating (unit:
.times.10.sup.5 J/m.sup.3) Thickness of Pd film (nm) 0.2 0.4 0.6
0.8 1.0 1.2 1.5 Thickness of 0.3 2.02 2.92 5.56 6.87 6.45 5.72 4.75
CoCr.sub.21Pt.sub.14B.sub.2 0.5 1.98 2.65 5.43 6.15 5.92 5.30 4.67
alloy film 0.8 2.03 2.55 4.85 5.64 5.26 4.95 4.30 (nm) 1.0 2.15
2.42 4.35 5.45 4.98 4.75 4.05 1.5 2.10 2.30 3.98 4.81 4.45 4.30
3.68 2.0 -- 2.32 3.78 4.33 4.07 3.77 3.30 2.5 -- 2.32 3.66 4.05
3.80 3.85 3.35 3.0 -- -- 3.42 3.75 3.58 3.42 3.32 4.0 -- -- 3.15
3.45 3.20 3.35 3.04 5.0 -- -- -- -- 3.25 3.05 3.12 6.0 -- -- -- --
3.04 3.05 2.95
[0128]
5TABLE 5 Structure and saturation magnetization (M.sub.s) of
multi-layered magnetic film in Pd/CoCr.sub.21Pt.sub.14B.sub.4
medium with reheating (unit: kA/m) Thickness of Pd film (nm) 0.2
0.4 0.6 0.8 1.0 1.2 1.5 Thickness of 0.3 240 222 175 164 150 152
143 CoCr.sub.21Pt.sub.14B.sub.2 0.5 275 248 205 190 175 170 165
alloy film 0.8 284 268 231 215 210 195 167 (nm) 1.0 290 272 244 226
210 208 190 1.5 298 280 270 250 237 226 220 2.0 -- 290 272 270 250
242 230 2.5 -- 300 285 268 265 256 243 3.0 -- -- 285 275 270 263
260 4.0 -- -- 296 295 280 270 262 5.0 -- -- -- -- 293 272 275 6.0
-- -- -- -- 295 292 284
[0129] It is noted from that the multi-layered magnetic film has
higher values of both K.sub.u and K.sub.s as compared with that in
Example 2 for the same layer thickness. On the other hand, the
multi-layered magnetic film has larger surface magnetic anisotropy
energy (K.sub.s) as compared with that of Pd/CoCr.sub.21 medium in
Example 1 and that of Pd/CoCr.sub.21Pt.sub.14 medium in Example 2,
both by more than 50%, for the same structure. This may be
attributable to the increased Co concentration in the Co alloy
layer and the increased area of Pd/Co interface.
[0130] As in Example 1 (FIG. 7) and Example 2 (FIG. 11), the
K.sub.u/2.pi.M.sub.s.sup.2 values are plotted against d2, with d1
varied such that d1/d2 is greater than 1.5 (for sufficiently small
magnetic exchange interaction between particles). The result is
shown in FIG. 12. This result is quite similar to that in Example
2. However, the recording medium in this example has more
preferable properties than that in Example 2. One of such
properties is good thermal stability for high recording
density.
[0131] What K.sub.u/2.pi.M.sub.s.sup.2 values represent is thermal
stability which manifests itself when magnetization in the medium
is oriented in the same direction or when the recording density is
low. With a low recording density, the
Pd/CoCr.sub.15Pt.sub.14B.sub.2 medium in this example is almost
identical in thermal stability to the medium in Example 2, due to
the effect of the demagnetizing field. However, as the recording
density increases, the medium in this example, which has a larger
value of K.sub.u, has the advantage in thermal stability. Another
of the preferable properties is the high strength of reproduced
signals. In other words, the Pd/CoCr.sub.15Pt.sub.14B.sub.2 medium
in this example has larger magnetization than the medium in Example
2, which provides a larger output of reproduced signals.
[0132] The present inventors closely studied heat treatment
performed after the CoCr.sub.15Pt.sub.14B.sub.2 film has been
formed. It was found that reheating does not cut off the magnetic
exchange interaction between particles in the multi-layered
magnetic film if the Cr content is lower than 12 at %. This result
suggests that it is necessary that the Cr content in the Co alloy
should be higher than 12 at %.
[0133] It was found that Cu in place of Cr is also useful as the
additive to the initial layer. The amount of Cu in the initial
layer should be larger than that in the multi-layered magnetic
film. An amount more than 30 at % is necessary for reheating to
produce a remarkable effect.
[0134] In addition, this example is characterized by that the Co
alloy is incorporated with a small amount (2 at %) of B. The
incorporation of B reduces the size of magnetic particles by about
10%. It is known that, in general, such elements as B and Ta added
in small quantities to the CoCr-based alloy promote segregation of
Cr atoms on the grain boundary and reduce the size of crystal
grains. It was confirmed that this is effective also in the
multi-layered magnetic film medium. More details are discussed in
Example 4.
EXAMPLE 4
[0135] This example demonstrates a multi-layered magnetic film
recording medium of the same structure as in Example 1, except that
the multi-layered film is incorporated with boron (B). This
recording medium has a ferromagnetic metal layer of
CoCr.sub.21Pt.sub.14B.sub.x (1.8 nm thick) and a noble metal layer
of PdB.sub.x (0.8 nm thick). The multi-layered magnetic film has a
total thickness of 20 nm.
[0136] The amount of B added is expressed in terms of x at %. The
layers containing B were prepared in the same way as in Example 1.
The substrate was heated to 250.degree. C. after the underlying
layer had been formed. With x varied from 0 at % to 5 at %, the
multi-layered magnetic film was examined for magnetic properties
and crystal grain sizes. The results are shown in Table 6.
Incidentally, the measurements of crystal grain size were carried
out by using a scanning tunnel electron microscope (TEM) and
crystal structure analysis.
6TABLE 6 Magnetic properties and crystal grain size of
multi-layered magnetic thin film (20 nm thick) composed of
PdB.sub.x (0.8 nm) and CoCr.sub.21Pt.sub.14B.sub.x (1.8 nm)
Perpendicular Saturation magnetic Crystal magnetization Coercive
anisotropy grain Amount of B (M.sub.s) force (H.sub.c) energy
(K.sub.u) size (D) added (x at %) (kA/m) (kA/m) (J/m.sup.3) (nm) 0
202 415 2.42 .times. 10.sup.5 11.5 1 200 376 2.33 .times. 10.sup.5
10.5 2 195 352 2.04 .times. 10.sup.5 10.2 3.5 191 310 1.95 .times.
10.sup.5 9.7 5 185 292 1.81 .times. 10.sup.5 9.2
[0137] It is noted from Table 6 that the incorporation of B into
the non-magnetic metal layer (Pd) slightly decreases the crystal
grain size as well as K.sub.u and H.sub.c. Because of the decrease
in K.sub.u and the grain size, the perpendicular magnetic recording
medium is subject to thermal disturbance and hence is poor in
thermal stability. Nevertheless, it has a low level of recording
noise owing to the reduction in grain size.
[0138] In this example, the incorporation of 5 at % B did not cause
appreciable thermal demagnetization. Rather, the incorporation of B
reduces the grain size and adjust K.sub.u and M.sub.s to adequate
values suitable for perpendicular magnetic recording. This in turn
improves resistance to thermal disturbance and contributes to the
multi-layered magnetic film suitable for the perpendicular magnetic
recording medium.
[0139] The same effect as mentioned above was also obtained when B
was replaced by tantalum (Ta).
[0140] As mentioned above, the perpendicular magnetic recording
medium consists of a substrate and a multi-layered magnetic film,
with or without an underlying layer interposed between them. The
multi-layered magnetic film is composed of ferromagnetic metal
layers containing Co and non-magnetic metal layers containing Pd,
each one layer of which are laminated alternately on top of one
layer of the other. The ferromagnetic metal layers and the
non-magnetic metal layers have a thickness of d1 and d2,
respectively, with the ratio of d1/d2 being in the range of 1.5 to
4.0. This specific layer structure reduces the magnetic exchange
interaction between magnetic particles in the magnetic recording
film and increases the perpendicular magnetic anisotropy energy
(K.sub.u) relative to the demagnetizing field of the recording
medium. As a result, the perpendicular magnetic recording medium is
stable against thermal disturbance and has a low level of recording
noise.
[0141] The invention may be embodied in other specific forms
without departing from the spirit or essential characteristics
thereof. The present embodiment is therefore to be considered in
all respects as illustrative and not restrictive, the scope of the
invention being indicated by the appended claims rather than by the
foregoing description and all changes which come within the meaning
and range of equivalency of the claims are therefore intended to be
embraced therein.
* * * * *