Stock material or miscellaneous articles – Web or sheet containing structurally defined element or... – Physical dimension specified
Reexamination Certificate
1998-09-15
2001-06-05
Kiliman, Leszek (Department: 1773)
Stock material or miscellaneous articles
Web or sheet containing structurally defined element or...
Physical dimension specified
C428S336000, C428S690000, C428S690000, C428S690000, C428S900000, C204S192220, C204S192230, C204S192150, C204S192200, C204S298120, C204S298130
Reexamination Certificate
active
06242085
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a magnetic recording medium for use in a magnetic recording/reproducing device having large capacity and high recording density, and a method for producing such a magnetic recording medium.
2. Description of the Prior Art
There is a tendency in a magnetic recording/reproducing device toward high density recording in order to achieve large capacity with a compact size. In the field of a hard disk drive, which is a typical magnetic recording device, a device having a recording density per unit of area (surface recording density) more than 1 Gbit/in
2
has been already commercially available. It is expected to see a technological advancement so rapid that a device with 10 Gbit/in
2
will be commercially practical in a few years.
An improvement in medium performance and head-disk interface performance, and an enhancement in a linear recording density resulting from the advent of new signal processing techniques such as partial response have allowed such high density recording. Recently, however, a tendency toward higher track density is greater than a tendency toward higher linear recording density, and this constitutes a main factor for the improvement in the surface recording density. This tendency is caused by the fact that a magneto-resistive type head (MR head) having more excellent reproducing output performance than that of a conventional induction type magnetic head has been put into practical use. At present, the magneto-resistive head allows signals in a track width as small as several gm to be reproduced in a good SIN ratio. In the near future, the head performance will further improve, and a track pitch will be in a sub-micron region.
When an MR head is used to reproduce signals, the signals to be reproduced are more affected by noise stemming from a recording medium, as compared to a conventional induction type magnetic head. For this reason, it is more important to reduce medium noise. In order to reduce noise, it is effective to minimize crystal grains in a magnetic layer and to isolate magnetic grains.
Conventionally, it is known to perform sputtering, for example, a CoCrPt based magnetic layer at a substrate temperature as high as 300° C. or more, in order to facilitate the isolation of magnetic grains. This method allows atoms sputtered onto a substrate to move easily on the substrate. As a result, a magnetic layer is formed comprising CoPt based magnetic crystal grains and Cr-rich non-magnetic substances at grain boundaries between the magnetic crystal grains. The formation of the non-magnetic substances at the grain boundaries facilitates the isolation of the magnetic crystal grains.
However, according to the method of heating the substrate so as to facilitate the movement of Cr to the grain boundaries, the atoms that have reached the substrate can move readily so that the magnetic crystal grains become large. Thus, it is difficult to make the crystal grains smaller. A large size of crystal grains is detrimental to a decrease in medium noise.
SUMMARY OF THE INVENTION
Therefore, with the foregoing in mind, it is the object of the present invention to provide a magnetic recording medium having low medium noise and being suitable for high density recording, and a method for producing the same.
In order to achieve the object, a magnetic recording medium of the present invention comprises a non-magnetic substrate, an underlying layer formed on the non-magnetic substrate and a magnetic layer formed on the underlying layer. The magnetic layer comprises clusters, each of which is formed of aggregated magnetic grains, and a non-magnetic substance between the clusters. Each magnetic grain has a grain diameter of 5 nm to 20 nm (preferably 8 nm to 10 nm). The average gap between the clusters is 1.5 nm to 5 nm.
In this magnetic recording medium, clusters of aggregated magnetic grains having small diameters are dispersed and isolated by the non-magnetic substance, thereby obtaining a magnetic recording medium having low medium noise and a high S/N ratio. In the specification of the present invention, “average gap between clusters” means an average value of the shortest distances between adjacent clusters.
This magnetic recording medium can be obtained by the following method. Namely, a method for producing a magnetic recording medium of the present invention comprises the steps of forming an underlying layer on a non-magnetic substrate and forming a magnetic layer on the underlying layer. The magnetic layer can be formed by simultaneously sputtering a magnetic substance and a non-magnetic substance that does not form a solid solution with the magnetic substance in an atmosphere having a pressure of 30 mTorr to 75 mTorr.
When a magnetic substance and a non-magnetic substance that do not form a solid solution are sputtered simultaneously in an atmosphere having a relatively high pressure, clusters in which fine magnetic grains are aggregated are formed, and the isolation of the clusters from each other is facilitated.
According to the present invention, a magnetic layer formed on a non-magnetic substrate and an underlying layer comprises clusters of aggregated magnetic grains having a grain diameter of 5 nm to 20 nm and a non-magnetic substance between the clusters. Furthermore, an average gap between the clusters is 1.5 nm to 5 nm. Therefore, the present invention can provide a magnetic recording medium having low noise and a high S/N ratio, which is suitable for high density recording. This magnetic recording medium can be formed efficiently by simultaneously sputtering a magnetic substance and a non-magnetic substance that does not form a solid solution with the magnetic substance in an atmosphere having a pressure of 30 mTorr to 75 mTorr, in the step of forming the magnetic layer on an underlying layer formed on a non-magnetic substrate.
These and other advantages of the present invention will become apparent to those skilled in the art upon reading and understanding the following detailed description with reference to the accompanying figures.
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patent: 5-93270 (1993-04-01), None
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Ichihara, K., et al. “Low noise CoPt-SiO2granular media”, 20th Japan Applied Magnetism Society, 1996, p. 195.
Hamada Taizou
Ishida Tatsuaki
Miyata Keizou
Ryonai Hiroshi
Tohma Kiyokazu
Kiliman Leszek
Matsushita Electric - Industrial Co., Ltd.
Merchant & Gould P.C.
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