Stock material or miscellaneous articles – Composite – Of inorganic material
Reexamination Certificate
2001-03-13
2004-01-27
Rickman, Holly (Department: 1773)
Stock material or miscellaneous articles
Composite
Of inorganic material
C428S690000, C428S690000, C427S127000, C427S130000, C252S06251C, C252S062550, C252S062560, C252S062580
Reexamination Certificate
active
06682834
ABSTRACT:
CROSS-REFERENCE TO RELATED APPLICATION
The present application is based on Japanese priority application No.2000-291144 filed on Sep. 25, 2000, the entire contents of which are hereby incorporated by reference.
BACKGROUND OF THE INVENTION
The present invention generally relates to magnetic storage of information and more particularly to a magnetic storage medium for use in high-density magnetic information storage devices.
With recent advancement in the field of information processing, magnetic disk devices, particularly those used in computers and other information processing apparatuses as external or auxiliary information storage device, are exposed to a stringent demand of more recording density, more resolution, and higher signal-to-noise ratio.
In a general magnetic recording medium for a longitudinal magnetic recording, a pulse width Pw
50
of a reproduced magnetic signal is defined as
Pw
50
=(2(
a+d
)
2
+(
a/
2)
2
)
½
a
∝(
t×B
r
/H
c
)
½
(1)
wherein H
c
represents a coercive force of a magnetic layer provided in the magnetic recording medium, B
r
represents a remnant magnetic flux density in the magnetic layer, t represents the thickness of the magnetic layer, and d represents a magnetic spacing between the magnetic layer and a magnetic head.
The narrower the width Pw
50
of the magnetic pulse, the better the resolution of the reproduced signal. Thus, in order to increase the recording density and resolution of the magnetic storage medium, it is effective to reduce the thickness t of the magnetic layer and increase the coercive force H
c
thereof.
Meanwhile, there is another demand for a high-density magnetic storage medium, in relation to the requirement of minimizing a medium noise, in that the magnetic layer has a high S/Nm (signal-to-medium noise) ratio. In order to suppress the medium noise, it has been practiced to reduce the grain size of the magnetic particles in the magnetic layer and suppress the magnetic interaction between the magnetic particles as much as possible.
For example, Japanese Laid-Open Patent Publication 3-31638 describes a magnetic storage medium that uses a magnetic layer of a Co-alloy film containing therein Cr and Ta with respective concentration levels of 6-20 at % (atomic percent) and 9 at %, wherein improvement is achieved in the foregoing prior art magnetic storage medium with regard to the S/Nm ratio by incorporating Cu with a concentration level of 0.5-7 at %. By doing so, it is possible to reduce the particle size in the Co alloy film used in the magnetic storage medium as the magnetic layer.
However, the demand for higher density recording has become more stringent these days, and it was discovered by the inventor of the present invention that the magnetic layer of the foregoing prior art composition can no longer meet for the demand of recent, leading-edge magnetic storage devices. Further, no solution has been proposed conventionally for improving anisotropy magnetic field H
k
and for preventing degradation of the coercive force H
c
under the situation that the product (t×B
r
) is set small for improved resolution and for improved S/Nm ratio.
In a magnetic storage media for use in high-density magnetic storage devices, it is noted that there is a serious problem known as thermal fluctuation. When the thickness of the magnetic layer is reduced or the grain size of the magnetic crystals therein is reduced extremely for improved resolution and improved S/Nm ratio, there is a tendency that magnetic relaxation is promoted in the magnetic layer and the remnant magnetization of the magnetic layer is degraded as a result.
Thus, the phenomenon of thermal fluctuation has to be suppressed as much as possible particularly in the case of high-density magnetic storage medium, while this minimization of the thermal fluctuation has to be accompanied with simultaneous minimization of the product (t×B
r
) for minimization of medium noise and also for simultaneous improvement of resolution.
It is known that the relaxation time &tgr; of a magnetic layer is represented, according to the Nee-Arrhenius relationship as
&tgr;
−1
=f
0
exp(−&Dgr;
E/kT
)
&Dgr;
E=K
u
·V
·(1
−H/H
0
)
1
; n=
⅔,
K
u
=H
k
·M
s
/2,
H=H
e
+H
d
, (2)
where f
0
represents a spin precession frequency having an order of 10
9
/s, k represents Boltzmann's constant, T represents a temperature of the magnetic layer, K
u
represents an anisotropy energy constant, V represents an effective volume of a magnetic particle in the magnetic layer, H
0
represents an intrinsic coercive force in the absence of thermal fluctuation, M
s
represents a saturated magnetization, H
e
represents an external magnetic field, H
d
represents a demagnetization field at the bit transition, and H
k
represents an anisotropy magnetic field.
Referring to Eq.(2) above, it is noted that the attempt to reduce the medium noise by reducing the product (t×B
r
) invites a decrease of the relaxation time &tgr; by way of causing a reduction of the particle volume V or causing reduction of the saturation magnetic field Ms. When the relaxation time &tgr; is reduced, the resistance of the magnetic layer against thermal fluctuation is degraded and the strength of the output signal reproduced from the magnetic storage medium may become smaller with time.
In view of the foregoing, a demand has emerged recently for a technology that can maintain a high value for the product (K
u
×V), so that a sufficient resistance is maintained against thermal fluctuation while simultaneously minimizing the product (t×B
r
).
It should be noted that the coercive force H
c
of a magnetic layer is a function of temperature and time. Thus, the coercive force H
c
appears low when the measurement of the coercive force is conducted at a high temperature.
A remnance coercive force H
cr
is given as
H
cr
/H
0
=1
−{C·ln
(
f
0
·t
im
/ln
2)}
n
C
−1
=&Dgr;E/kT
(3)
where t
im
represents the duration in which the external magnetic field H
e
is applied.
From Eq.(3), it can be seen that the coercive force H
c
is degraded in the magnetic recording medium susceptible to thermal fluctuation when the product (t×B
r
) is reduced. With the degradation of the coercive force H
c
, the resistance to thermal fluctuation is degraded and also the S/Nm ratio.
As noted previously, it is desirable to design the magnetic layer of the magnetic storage medium such that a large coercive force H
c
is maintained even when the product (t×B
r
) is set small. However, various magnetic properties of the magnetic layer are interrelated, and thus, it has been extremely difficult to make a magnetic storage medium that maintains a high coercive force H
c
for the magnetic layer therein even when the value of the product (t×B
r
) is reduced.
SUMMARY OF THE INVENTION
Accordingly, it is a general object of the present invention to provide a novel and useful magnetic storage medium wherein the foregoing problems are eliminated.
Another and more specific object of the present invention is to provide a magnetic storage medium for high-density magnetic recording having a large coercive force H
c
for a magnetic layer therein even in such a case the value of the product (t×B
r
) is set small.
Another object of the present invention is to provide a magnetic storage medium, comprising:
a non-magnetic substrate;
an under layer provided on said non-magnetic substrate; and
at least one magnetic layer provided above said under layer,
said magnetic layer comprising at least an alloy layer of a system Co—Cr—Pt—B—Cu,
said alloy layer having a thickness t and a remnant magnetic flux density B
r
satisfying a relationship for a product (t×B
r
) as
2.0
nT·m≦
(
t×B
r
)≦7.0
nT·m,
said alloy layer containing, in addition to Co, Cr with a concentration &bgr; of 20-26 at % (20 at %≦&bgr;≦26 at %), Pt with a concentration &ggr; of 6-20 a
Endo Atsushi
Kikuchi Akira
Murao Reiko
Okuyama Chiaki
Greer Burns & Crain Ltd.
Rickman Holly
Uhlir Nikolas J.
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