Stock material or miscellaneous articles – Composite – Of inorganic material
Reexamination Certificate
2001-11-28
2004-03-23
Rickman, Holly (Department: 1773)
Stock material or miscellaneous articles
Composite
Of inorganic material
C428S611000, C428S667000, C428S900000, C427S128000, C427S129000, C427S131000, C427S307000, C204S192200
Reexamination Certificate
active
06709775
ABSTRACT:
TECHNICAL FIELD
The present invention relates to a magnetic recording medium, a production method thereof, and a magnetic recording device. More specifically, the present invention relates to a magnetic recording medium comprising a ferromagnetic metal layer of magnetic crystal grains which are not susceptible to the effects of thermal agitation, as well as a production method thereof, and a magnetic recording device. The magnetic recording medium of the present invention can be ideally applied to hard disks, floppy disks, and magnetic tapes and the like.
BACKGROUND ART
In recent years, magnetic recording media have been widely used as high density, large capacity recording media in magnetic recording devices such as hard disks, although improvements are now required in recording and playback characteristics in order to progress to even higher recording densities.
FIG. 13
is a schematic cross-sectional view showing a magnetic recording medium
50
used in a conventional magnetic recording device. Furthermore,
FIGS. 14A-D
are diagrams showing go each of the steps in a production method for a conventional magnetic recording medium, and a cross-sectional view of the sample at each of those steps. A base material
51
of a non-magnetic material can utilize, for example, a non-magnetic layer
53
comprising nickel-phosphorus (Ni—P) provided on the surface of a base
52
comprising an aluminum (Al) based alloy. An underlayer
54
comprising a non-magnetic metallic film such as chromium (Cr) for example, a recording layer
55
comprising a magnetic film, and a protective layer
56
are then layered sequentially on top of the base material
51
.
In a medium comprising a Co based magnetic layer
55
laminated on top of a Cr underlayer
54
, the following facts are well known.
(1) The thinner the Cr layer
54
becomes, the smaller the grain diameter of the crystal grains
54
a
which make up the Cr layer
54
will be, and the thicker the Cr layer
54
becomes, the larger the grain diameter of the crystal grains
54
a
will become.
(2) The grain diameter of the crystal grains
55
a
which make up the Co based magnetic layer
55
laminated on top of the Cr underlayer
54
will be of approximately the same size as the grain diameter of the crystal grains
54
a
of the Cr layer
54
[FIG.
14
(
d
)].
In contrast, in order to improve the recording and playback characteristics of a magnetic recording medium
50
of the construction described above, the present inventor and others have already reported [M. Takahashi, A. Kikuchi and S. Kawakita: IEEE Trans. on Magn., 33, 2938 (1997)] that a reduction in the interaction between grains of the magnetic crystal grains
55
a
which make up the magnetic film which functions as the recording layer
55
, and a reduction in the film thickness of the magnetic film, are essential. In particular, in order to achieve a reduction in noise level for the medium, the above reference introduces a fabrication method effective for miniaturizing the crystal grains
55
a
of the magnetic film, by reducing the film thickness of the magnetic film of the recording layer
55
.
However, there are limits to the micro-structure control and volume reduction in the crystal grains
55
a
which are possible by reduction in the thickness of the magnetic film comprising the recording layer
55
. The reason being that as the thickness of the magnetic film comprising the recording layer
55
is reduced, there is an accompanying miniaturization of the crystal grains
55
a
which make up the magnetic film, and a problem arises in that the magnetic characteristics, such as the magnetization (residual magnetization) recorded on the magnetic film, can vary significantly over time, in other words the magnetic film becomes more susceptible to thermal agitation.
The present inventor has keenly pursued the development of a method for reducing the grain diameter of the crystal grains
55
a
of a magnetic film by reducing the film thickness of the underlayer
54
, in other words, a method of reducing the volume of the crystal grains
55
a
of the magnetic film. Specifically, by developing a method of preparing a medium comprising a Cr underlayer and a Co based magnetic layer under ultra clean process conditions, the inventor has succeeded in developing a medium capable of achieving a coercive force exceeding 2000 [Oe] even with an ultra-thin Cr layer with a thickness of 2.5 nm as the underlayer
54
(International Patent Application No. PCT/JP97/01092).
The ultra clean process disclosed in the aforementioned application is a process comprising principally an increase in the ultimate vacuum of the film formation chamber from the 10
−7
Torr level of a typical conventional sputtering device, to the 10
−9
Torr level, as well as a reduction in the concentration of impurities such as water in the ultra pure Ar gas introduced into the film formation chamber by a further two digits beyond the levels found in normal ultra pure Ar gas, down to the 1 ppb level.
According to the aforementioned method, the average grain diameter of the crystal grains
55
a
making up the magnetic film decreased, although there was a tendency for the variation (standard deviation) in size to increase. This effect reflects localized variations in the interactions between grains within the magnetic film, and as the shift to higher recording densities proceeds, and recording magnetization becomes smaller and smaller, the above effect can no longer be ignored.
Furthermore, the effects achieved by reducing the film thickness of the underlayer using the aforementioned medium production method are approaching a limit, and the tendency for the interaction between grains of the magnetic film to increase with reductions in the film thickness of the underlayer has also been confirmed for specific magnetic films [J. Nakai, A. Kikuchi, K. Nakatani, M. Hirasaka, T. Shimatsu and M. Takahashi: J. Magn. Magn. Mater., 155, 234 (1996)].
Accordingly, there has been considerable demand for the development of a magnetic recording medium which enables a reduction in the grain diameter of the magnetic crystal grains
55
a
using a method other than reducing the film thickness of the underlayer
54
or the recording layer
55
, and also enables the suppression of variation in the grain diameter of the magnetic crystal grains
55
a
, as well as a production method for such a medium, and a magnetic recording device.
One object of the present invention is to provide a magnetic recording medium capable of suppressing the effects of thermal agitation by simultaneously reducing the average grain diameter and the standard deviation of the magnetic crystal grains of a ferromagnetic metal layer, without changing the film thickness of a metal underlayer or the film thickness of the ferromagnetic metal layer which forms the recording layer.
Furthermore another object of the present invention is to provide a method of producing a magnetic recording medium capable of suppressing the effects of thermal agitation by simultaneously reducing the average grain diameter and the standard deviation of the magnetic crystal grains of a ferromagnetic metal layer without changing the film thickness of a metal underlayer or the film thickness of the ferromagnetic metal layer which forms the recording layer.
In addition, yet another object of the present invention is to provide a magnetic recording device comprising a magnetic recording medium capable of suppressing the effects of thermal agitation by simultaneously reducing the average grain diameter and the standard deviation of the magnetic crystal grains of a ferromagnetic metal layer without changing the film thickness of a metal underlayer or the film thickness of the ferromagnetic metal layer which forms the recording layer.
DISCLOSURE OF THE INVENTION
A magnetic recording medium according to the present invention is a magnetic recording medium comprising a ferromagnetic metal layer of a cobalt based alloy formed on a base material with a metal underlayer com
Djayaprawira David
Shoji Hiroki
Takahashi Migaku
Yoshimura Satoru
Rickman Holly
Takahashi Migaku
Young & Thompson
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