Note: Descriptions are shown in the official language in which they were submitted.
~Z~9~72
BACKGROUND OF THE INVENTION
(1) Field of the Invention
This invention relates to a magnetic recording
medium, and more particularly to a so-called anisotropic
magnetic recording medium having a magnetic layer of Co
or Co-Ni alloy and having anisotropic magnetic
characteristics along a surface of the magnetic layer.
(2) Description of the Prior Art
Recently, there has been extensively studied a
thin ~ilm type magnetic recording med~ium, that is, a
magnetic recording medium produced by forming a
ferromagnetic thin film of Co or Co-Ni, etc. on a non-
magnetic substrate by a method such as vacuum deposltion
for purpose of achieving a magnetic recording of high
density. Especially, high coercive force Hc and high
rectangular ratio Mr/Ms have been required in an
anisotropic magnetic recording medium having a
ferromagnetic thin film formed by substantially vertical
vacuum deposition.
OBJECT AND SUMMARY OF THE INVENTION
It is an object of the present invention to
- 2 - ~
~255972
provide an improved magnetic recording medium having a
ferromagnetic metal layer formed by physical vapour
deposition. It is another object of the present
invention to provide a magnetic recording medium having
anisotropic magnetic characteristics along a surface of
a magnetic layer.
It is a further object of the present
invention to provide a magnetic recording medium having
high coercive force and high rectangular ratio.
According to one aspect oE the present
invention, there is provided a magnetic recording me~ium
comprising a non-magnetic substrate and a magnetic layer
having anisotropic magnetic characteristics along a
surface of the magnetic layer formed on the non-magnetic
substrate. The magnetic layer is composed of Co or Co-
Ni alloy having a composition represented as Coloo_xNix,
where x standing for atomic percent of nickel, and the
magnetic layer is composed of f.c.c. crystal phase and
h.c.p. crystal phase. Ratio of the two phases is
defined as y = f.c.c. phase/(f.c.c. phase + h.c.p.
phase) x 100 volume percent, wherein values of (x) and
(y) satisfying to be in an area on Cartesian coordinate
surrounded by lines y = ~0 + 0.8x, y = 10 + x, x = 0 and
~L2559~
Y = 50.
Other objects and advantages of the invention
will become apparent ~rom the following description and
accompanying drawing.
BRIEF DESCRIPTION OF THE DRAWI~G
The drawing is a graph showing a relation
between proportion of Ni and proportion of f.c.c. phase
in composition of Coloo-xNix.
DET~ILED DESCRIPTION OF THE PREFERRED EMBODIMENT
~ ccording to the present invention, a non-
magnetic base metal layer of Bi, etc. is formed on a
non-magnetic substrate, and a ferromagnetic metal layer
of Co or Co-Ni alloy is formed on the non-magnetic base
metal layer by physical vapour deposition to form a
magnetic recording medium. The magnetic recording
medium prepared as is above described has a high
coercive force Hc because a non-magnetic metal in the
base metal layer is diffused in the ferromagnetic metal
layer, and thereby crystalline particles in the
ferromagnetic metal layer are finely fractionized.
Further, the magnetic recording medium has anisotropic
125S9~72
magnetic characteristics along a surface of the magnetic
layer since it is prepared by applying vapour beam of
magnetic metal substantially perpendicularly to a
surface of the substrate.
In a bulky binary alloy of Co-Ni, modification
between hexagonal closed-pack structure, h.c.p. (~ and
face-centered cubic structure, f.c.c. (~) occurs in a
region of 22 - 32 atomic % Ni at room temperature. The
binary alloy shows h.c.p. phase if Ni content is not
more than 22 atom. %, and shows f.c.c. phase if Ni
content i5 not less than 32 abom. ~. Referring to
magnetic anisotropy, the h.c.p. phase is of unaxial
anisotropy, while f.c.c. phase is of cubic anisotropy.
Anisotropy constants (uniaxial anisotropy constant Ku1;
cubic anisotropy constant Kl~ are Kul = 4.3 x 106
erg/cc, and Kl = -1.0 x 106 erg/cc, respectively.
Coercive force Hc which is an important factor as a
property of the magnetic recording medium is
proportional to magnitude of the anisotropy constant.
In the anlsotropic magnetic recording medium where easy
axis of magnetization is randomly two-dimensionally
distributed, rectangular ratio Mr/Ms is theoretically
Mr/Ms = 0 64 in the h.c.p. phase having uniaxial
~2~ 72
anisotropy, while it is Mr/Ms = O.9B in the f.c.c. phase
having cubic anisotropy.
; The inventor has investigated phases of Co and
Co-Ni alloy in the magnetic layer by electron beam
diffraction analysis, and has found that change in the
phases is not necessarily the same as that of the bulky
Co-Ni alloy having the same composition. Although there
occurs some variation in dependence upon conditions of
preparation of the magnetic layer, proportion of the
f.c.c. phase tends to become large with increase in Ni
content. In this manner, when both the h.c.p. phase and
the f.c.c. phas~ exist in ~he magnetic layer, magnetic
characteristics are varied in the following manner,
provided that the proportion of the f.c.c. phase (y) as
a parameter is defined to y = amount of f.c.c. phase
/(amount of h.c.p. phase ~ amount of f.c.c. phase) x 100
(vol. %). The anisotropy constant of the entire
magnetic thin film is decreased with increase in the
value of (y), and accordingly the coercive force Hc is
decreased. The rectangular ratio Mr/Ms becomes high
with increase in the value of (y). A high density
magnetic recording medium requires high coercive force
Hc and high rectangular ratio Mr/Ms, and both the
'lZ5S9~
factors depend on the value of (y), that is, the
proportion of the f.c.c. phase.
Accordingly, in the anisotropic magnetic
recording medium having magnetic layer of Co or Co-Ni
alloy according to the present invention, provided that
composition of the magnetic layer is represented by
C100-xNix, and proportion of the f.c.c~ phase to the
h.c.p. phase is represented by y = amount of the f.c.c.
phase)/(amount of the h.c.p. phase + amount of the
f.c.c. phase) x 100 (vol.%), values of (x) and (y) are
selected so as to exist in a region enclosed by two
s~raight line~ as represented by y ~ a ~ bx, that is to
say, y = ~0 ~ O.~x and y = 10 ~ x in the range o~
0 -~ x -~ 50, thus obtaining an anisotropic magnetic
recording medium having magnetic characteristics where
both the coercive force and the rectangular ratio are
rendered high and well-balanced.
A non-magnetic metal, e.g., Bi which
volumetrically expands on solidifying was deposited on a
non-magnetic substrate, e.g., poiyimide film by vacuum
evaporation, and subsequently a magnetic layer Gf
C100-xNix ( ~ x ~ 50) was deposited on the Bi layer.
Thickness of the Bi layer as a base layer was 100 R, and
~25S~ 2
temperature of the substrate upon deposition was varied
from 100C to 300C. A crystal structure of the
magnetic layer as prepared was investigated by electron
beam diffraction analysis, and as the result,
diffraction peaks of both the h.c.p. phase and the
f.c.c. phase were observed in all of the magnetic layer.
According to this observation, coexistence of both the
phases was confirmed. Proportion of the f.c.c. phase to
the h.c.p. phase (y), that is, y = amount of f.c.c.
phase/(amount of h.c.p. phase + amount of f.c.c. phase)
x 100 (vol. %) was obtained from integral values of
relative stren~ths oE the diEEraction peaks.
As the result, it became clear that when the
proportion of the f.c.c. phase (y) and the proportion of
Ni (x) in the magnetic layer of Coloo_xNx were selected
in a region (I) enclosed by two straight lines as
represented by y = 40 -~ 0.8x and y = 10 + x in the range
of 0 ~ x ' 50 as shown by an oblique line in the
drawing, magnetic characteristics where both the
coercive ~orce Hc and the rectangular ratio Mr/Ms in the
magnetic recording medium are high and well-balanced may
be obtained irrespective of the fact that the non-
magnetic layer is formed of Bi. Concretely, the
12559~7Z
coercive force Hc and the rectangular ratio Mr/Ms in the
magnetic characteristics were Hc = 800 - 1300 Oe and
Mr/Ms = 0.70 - 0.94.
Example 1
A Bi base layer of 100 A thiclcness was
deposited on a non-magnetic substrate of polyimide film
having a thickness of 30 microns under vacuum of 10-4 Pa
at a substrate temperature of 150C, and subsecluently,
Co-Ni alloy (80 atom. ~ of Co; 20 atom. ~ of Ni) was
deposited on the Bi layer to Eorm a macJnetic layer
having a thickne~,s oE 300 R.
The proportion oE the f.c.c. phase to the
h.c.p. phase (y) in the magnetic in the magnetic layer
was analyzed by electron beam diffraction to obtain y =
41 ~. Further, magnetic characteristics of the magnetic
recording me~ium was such that the coercive force Hc was
Hc = 1020 Oe and the rectangular ratio Mr/Ms was Mr/Ms =
0.84. The magnetic characteristics were almost
indentical even when measurement was carried out in any
directions in a surface of the magnetic layer.
Example 2
_ g _
~Z55972
In the same manner as in Example 1, except
that the base layer was formed of Ga and the thickness
thereof was 100 ~, a magnetic recording medium was
prepared. Proportion of the f.c.c. phase to the h.c.p.
phase in the magnetic layer was analyzed to obtain y =
36~. Magnetic characteristics of the magnetic recording
medium were such that the coercive force Hc was Hc = 880
Oe and the rectangular ratio Mr/Ms was Mr/Ms = 0.82.
Comparison 1
In the same manner as in Example 1, except
that the sLIbstrate temperature was set to 250C, and the
thickness oE the Bi base layer was 200 ~, a macJnetic
recording medium was prepared. proportion of the f.c.c.
phase to the h.c.p. phase in the magnetic layer was
analyzed to obtain y = 20 ~. Magnetic characteristics
of the magnetic recording medium were such that the
coercive force Hc was Hc = 1050 Oe and the rectangular
ratio Mr/Ms was Mr/Ms = 0.68.
Comparison 2
In the same manner as in Example 1, except
that the substrate temperature was set to 140C, and the
-- 10 --
. ~ ,
~2~S~72
thickness of the Bi base layer was 40 ~, a magnetic
recording medium was prepared. Proportion of the f.c.c.
phase to the h.c.p. phase in the magnetic layer was
analyzed to obtain y = 50 %. Magnetic characteristics
of the magnetic recording medium were such that the
coercive force Hc was Hc = ~30 Oe and the rectangular
ratio Mr/Ms was Mr/Ms = 0.92.
The non-magnetic metal which volumetrically
expands on solidifying as the base layer may be selected
from Sb, Tl, Sn, Pb, In, Zn and alloy thereof as well as
Bi and Ga.
As will be apparent from ~xamples 1 and 2,
when values oE proportion oE Ni ~x) and the proporti.on
oE f.c.c. phase (y) exist in the region (I) as shown in
the drawing, a magnetic recording medium having high
coercive force and high rectangular ratio may be
obtained. On the contrary, in Comparisons 1 and 2, when
values of (x) and (y) do not exist in the region (I),
either of the coercive force or the rectangular ratio is
low, and an anisotropic magnetic recording medium having
magnetic characteristics where both the coercive force
and the rectangular ratio are high may not be obtained.
Further, when Ni content exceeds 50 atom. %,
....
~25sg72
high coercive force is not obtained, and such a ~agnetic
layer is not suitable for a high density magneti~
recording meaium.
Consequently, according to the present
invention, the substrate temperature on vacuum
evaporation is set to 100 - 300C, and material for the
base layer is selected from Bi, Ga, Sb, Sn, Pb, In, Zn
and alloy thereof. In addition, provided that
conditions in the region (I) in the drawing are
satisfied, it is possible to obtain an anisotropic
magnetic recording medium having satisfactory magnetic
characteristics.
While the invention has been described with
reee~ence to specific embodiments, the description is
illustrative and is not to be construed as limiting the
scope of the invention. Various modifications and
changes may occur to those skilled in the art without
departing from the spirit and scope of the invention as
defined by the appended claims.
