Stock material or miscellaneous articles – All metal or with adjacent metals – Having magnetic properties – or preformed fiber orientation...
Reexamination Certificate
2001-02-23
2003-07-08
Rickmon, Holly (Department: 1773)
Stock material or miscellaneous articles
All metal or with adjacent metals
Having magnetic properties, or preformed fiber orientation...
C428S336000, C428S667000, C428S690000, C428S690000, C428S900000, C427S128000, C427S131000, C204S192200
Reexamination Certificate
active
06589669
ABSTRACT:
BACKGROUND
The present invention relates to a magnetic recording medium installed in various magnetic recording devices including an external memory of a computer, and to a manufacturing method therefor.
Non-magnetic substrates of magnetic recording media for magnetic recording devices conventionally employ an aluminum alloy substrate with electroless NiP plating or a glass substrate. So-called plastic substrates using plastic resin have been proposed for use as non-magnetic substrates of magnetic recording medium, as disclosed in Japanese Unexamined Patent Application Publication (KOKAI) No. H10-146843, for example. The plastic substrates, which can be manufactured economically and in a great mass by injection molding, are very promising for non-magnetic substrates of magnetic recording media that require further cost reduction.
When the plastic substrate is utilized as a non-magnetic substrate of a magnetic recording medium, since the temperature which the plastic substrate can withstand is low, at most below 200° C., it is not acceptable to heat the plastic substrate to several hundred degrees Centigrade prior to depositing functional layers on the substrate. This temperature limitation is quite different from conventional substrates of aluminum alloy or glass. Accordingly, methods have been proposed to attain desired magnetic properties and electromagnetic conversion characteristics even with low temperature deposition. One example of such a method, as disclosed in Japanese Patent No. 2763165, employs a so-called granular magnetic layer. The granular magnetic layer is composed of a structure in which a grain of magnetic substance is surrounded by non-magnetic and non-metallic substances such as oxides or nitrides. In another example, disclosed in Japanese Unexamined Patent Application Publication (KOKAI) No. H11-154320, elevated argon gas pressure is employed in the process of depositing a granular magnetic layer by sputtering.
A magnetic recording device using a flying magnetic head must maintain a narrow gap between a magnetic recording medium and the head as small as a few tens of nanometers. Therefore, durability of the device is greatly affected by the friction and wear characteristic of the head-medium interface. To improve the friction and wear characteristic with a head, the surface of the medium is generally coated with liquid lubricant having a molecular weight of several thousands. It is known that the liquid lubricant on the medium surface can be dissolved as a result of cobalt atoms occasionally precipitated from the magnetic layer. Loss of the liquid lubricant results in significant deterioration of durability of the medium. A protective layer is conventionally deposited between the magnetic layer and the liquid lubricant for suppressing the precipitation of cobalt atoms. Control of thickness and quality of the protective layer and control of surface roughness of the medium are essential.
OBJECTS AND SUMMARY OF THE INVENTION
An object of the present invention is to provide a magnetic recording medium utilizing a plastic substrate, the medium exhibiting excellent magnetic property and an electromagnetic transformation characteristic, and at the same time, superior durability, and to provide a method for manufacturing such a recording medium.
Studies by inventors of the present invention revealed that the cobalt atoms contained in the magnetic layer of a magnetic recording medium readily precipitate on the medium surface when the medium is manufactured by the same method as is conventionally used with an aluminum alloy substrate or a glass substrate. The quantity of precipitated cobalt atoms is particularly large when the granular magnetic layer is employed as a magnetic layer, or when argon gas of elevated pressure is used in the sputtering process for attaining favorable magnetic property and electromagnetic conversion characteristics. The cobalt atoms, precipitated on the medium surface, decompose the molecules of the liquid lubricant on the medium surface, and thus deteriorate the resistance of the medium to friction and wear.
The inventors of the present invention made numerous studies for solving the above-described problems and have found that the amount of cobalt dissolved out and precipitated onto the medium surface strongly correlates with the material and the thickness of the under-layer of a magnetic recording medium.
More specifically, a magnetic recording medium of the present invention based on the above-described finding comprises a non-magnetic substrate made of plastic resin such as polycarbonate or polyolefin, a non-magnetic under-layer, a magnetic layer including cobalt, a protective film, and a liquid lubricant layer sequentially laminated on the substrate. The non-magnetic under-layer is composed of a chromium alloy that has a crystal structure of a body centered cubic lattice, and contains at least one element selected from the group consisting of Zr, Nb, Mo, Ru and Pd in a total amount of 15 at % or more.
In another aspect of the embodiments of the present invention, the non-magnetic under-layer is composed of a chromium alloy that has a crystal structure of a body centered cubic lattice, and contains at least one element selected from the group consisting of Hf, Ta, W, Re, Pt and Au in a total amount of 10 at % or more.
In a magnetic recording medium of the invention, an amount of cobalt dissolved out and precipitated onto the medium surface (also called simply ‘dissolved-out cobalt’) is 15 &mgr;g/m
2
or less. After exposing the medium to a high temperature and high humidity environment of 85° C. and 80% relative humidity for 96 hours, the amount of dissolved-out cobalt is 20 &mgr;g/m
2
or less.
Thickness of the non-magnetic under-layer of the medium is preferably from 5 nm to 15 nm.
The inventors have further found that controlling argon gas pressure in at least one of the processes for depositing the magnetic layer and depositing the under-layer is effective to prevent the cobalt atoms from dissolving-out and precipitation onto the medium surface.
More specifically, a method for manufacturing a magnetic recording medium according to the present invention comprises a step for forming a non-magnetic under-layer on a non-magnetic substrate of plastic resin, a step for forming a magnetic layer on the under-layer, a step for forming a protective film on the magnetic layer, and a step for forming a liquid lubricant layer on the protective film. In the method, the non-magnetic under-layer is composed of a chromium alloy having a crystal structure of a body centered cubic lattice. The chromium alloy contains at least one element selected from the group consisting of Zr, Nb, Mo, Ru and Pd in a total amount of 15 at % or more, or contains at least one element selected from the group consisting of Hf, Ta, W, Re, Pt and Au in a total amount of 10 at % or more.
The step for forming an under-layer is preferably performed by sputtering under gas pressure of 30 mTorr or less. The step for forming a magnetic layer is preferably performed by sputtering under gas pressure of 15 mTorr or less. Most preferably, both of the gas pressure conditions are employed, that is, the step for forming an under-layer is performed by sputtering under gas pressure of 30 mTorr or less and the step for forming a magnetic layer is performed by sputtering under gas pressure of 15 mTorr or less.
Aspect of Embodiments of the Invention
The present invention will now be described in detail in the following with reference to a drawing.
FIG. 1
is a schematic cross-sectional view of a magnetic recording medium of the invention. Referring to
FIG. 1
, a magnetic recording medium of the invention comprises a plastic non-magnetic substrate
1
, a non-magnetic under-layer
2
, a magnetic layer
3
, a protective film
4
, and a liquid lubricant layer
5
sequentially formed on the substrate
1
in this order.
Meanwhile, the phrase “quantity of cobalt dissolved out and precipitated onto the medium surface” is a value obtained by measuring the quantity of cobalt extracted by oscillat
Oikawa Tadaaki
Uwazumi Hiroyuki
Darby & Darby
Fuji Electric & Co., Ltd.
Rickmon Holly
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