Stock material or miscellaneous articles – Composite – Of inorganic material
Reexamination Certificate
2001-10-15
2004-06-08
Rickman, Holly (Department: 1773)
Stock material or miscellaneous articles
Composite
Of inorganic material
C428S690000, C428S900000, C428S697000, C428S699000, C428S701000
Reexamination Certificate
active
06746786
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a magnetic recording medium and a process for producing the magnetic recording medium, and more particularly, to a magnetic recording medium capable of exhibiting a relatively high coercive force, especially a coercive force of not less than 159 kA/m (2,000 Oe) despite as small a film thickness as not more than 200 nm, and an excellent surface smoothness, and a process for producing the magnetic recording medium at one step.
In recent years, in magnetic recording apparatuses such as hard disk devices, there has been a remarkable tendency that information devices or systems used therefor are miniaturized and required to have a high reliability. With such a recent tendency, in order to deal with a large capacity data, there is an increasing demand for providing magnetic recording media on which information can be stored with a high density.
In order to satisfy such requirements, the magnetic recording media have been strongly required to have not only a high coercive force, but also reduce a distance between a magnetic head and a magnetic recording layer (magnetic spacing).
As magnetic recording media having a high coercive force, there is widely known those comprising a substrate and a magnetic thin film formed on the substrate.
The magnetic thin films which have been already practically used in magnetic recording media, are generally classified into magnetic oxide thin films composed of iron oxides such as magnetite, maghemite, etc. (“Technical Report of the Institute of Electronics, Information and Communication Engineers”, published by the of Electronics, Information and Communication Engineers, (1981) MR81-20, pp. 5 to 12; “Ceramics”, published by Japan Institute of Ceramics, (1986) Vol. 24, No. 1, pp. 21 to 24; and Japanese Patent Publication (KOKOKU) Nos. 51-4086(1976) and 5-63925(1993)), and magnetic alloy thin films composed of Co—Cr alloy or the like.
The magnetic oxide thin films composed of iron oxides are excellent in corrosion resistance due to inherent properties of the oxides. As a result, the magnetic oxide thin films can show an excellent stability independent of change with passage of time, and less change in magnetic properties with passage of time. Further, since oxides exhibit a higher hardness than that of metals, it is not necessary to form an overcoat thereon. As a result, it becomes possible to reduce the magnetic spacing of the obtained magnetic recording medium as compared to those magnetic recording media using the magnetic alloy thin film. Therefore, the magnetic oxide thin film is optimum for the production of high-density magnetic recording media.
It has been attempted to enhance a coercive force of the thin iron oxide film by incorporating cobalt thereinto. However, with the increase in cobalt content, the iron oxide thin film tends to be deteriorated in stability independent of change with passage of time due to adverse influences of heat or the like.
Meanwhile, the present inventors have already proposed a maghemite thin film which can exhibits a high coercive force even with a less cobalt content by controlling the spacing of specific plane of the maghemite thin film (Japanese Patent Application Laid-Open (KOKAI) Nos. 11-110731(1999) and 11-110732(1999)).
On the other hand, the magnetic alloy thin films composed of Co—Cr alloy or the like, have a coercive force as high as not less than about 159 kA/m (2,000 Oe). However, these alloy materials tend to be readily oxidized in themselves and, therefore, tend to be deteriorated instability independent of change with passage of time as well as magnetic properties.
In order to prevent the deterioration of magnetic properties due to the oxidation, a overcoat composed of diamond-like carbon, SiO
2
or the like, having a thickness of usually about 10 to 20 nm, is formed on the surface of the magnetic alloy thin film, resulting in undesired increase in magnetic spacing corresponding to the thickness of the overcoat.
In magnetic recording media, in order to reduce the magnetic spacing, it is necessary to reduce the flying height of a magnetic head as far as possible, and always cause the magnetic head to be flying stably. In conventional hard disk drive, magnetic recording media used therefor have been required to have a certain surface roughness in order to prevent the magnetic head from being absorbed thereonto owing to a meniscus force therebetween upon stopping the magnetic head. At the present time, as a result of current improvement in these hard disk systems, magnetic recording media have been no longer required to show such a surface roughness for preventing the magnetic head from being absorbed thereonto. On the contrary, it has been required that a magnetic thin film used in these magnetic recording media exhibits a more excellent surface smoothness.
Also, it is known that the non-smooth surface of magnetic recording media causes media noise. In order to eliminate such a media noise, it is necessary to reduce a surface roughness of the magnetic thin film.
At present, in magnetic recording media using a magnetic oxide thin film, with the recent tendency toward high recording density, the magnetic oxide thin film is required to have a very small thickness such as not more than 200 nm. For this reason, the surface properties of the magnetic thin film are considerably influenced by surface properties of a substrate. As a result, it has been required not only to use such a substrate having an excellent surface smoothness, but also to develop techniques for further smoothening the surface of the magnetic thin film.
Hitherto, as methods of producing iron oxide thin films, there are known (1) a method of forming a magnetite thin film on a substrate, and then oxidizing the magnetite thin film at a temperature of not less than 300° C.; (2) a method of forming a film composed of &ggr;-Fe
2
O
3
, Fe
3
O
4
or FeO
x
(4/3<x<3/2) using &ggr;-Fe
2
O
3
, Fe
3
O
4
or FeO
x
(4/3<x<3/2) as a target (Japanese Patent Publication (KOKOKU) Nos. 62-49724(1987) and 6-61130(1994)); (3) a method of forming a cobalt-containing spinel-type iron oxide thin film directly on a substrate by sputtering a target while irradiating the substrate with a high-density oxygen plasma (Japanese Patent Application Laid-Open (KOKAI) Nos. 1-298029(1989) and 3-78114 (1991)); or the like.
Presently, it has been strongly demanded to provide magnetic recording media comprising a spinel-type iron oxide thin film capable of showing a high coercive force and an excellent surface smoothness despite its small thickness. However, such magnetic recording media satisfying these requirements have not been obtained until now.
Namely, in the above method (1) for producing the iron oxide thin film, the obtained magnetite thin film is taken out into atmosphere and further subjected to oxidation treatment at a temperature of 300 to 450° C., thereby obtaining the maghemite thin film. In this method, since it is necessary to conduct the heat-treatment at a temperature as high as not less than 300° C., there arises such a problem that the obtained thin film is deteriorated in magnetic properties due to migration from the substrate or the like. Therefore, it is required to select the material of the substrate from those having an excellent heat resistance, so that only limited substrates are usable therefor. In addition, since the magnetite thin film is taken out into atmosphere, there also arise problems such as contamination thereof.
In the method (2), the obtained iron oxide thin film shows a poor coercive force and, therefore, unsatisfactory in magnetic properties.
In the method (3) for producing the iron oxide thin film as described in Japanese Patent Application Laid-Open (KOKAI) No. 1-298029(1989), the spinel-type iron oxide thin film is formed by irradiating the substrate with a high-density oxygen plasma generated from an ECR plasma device while sputtering each metal target. The obtained thin film exhibits a coercive force as high as 191 kA/m (2,400 Oe), but has a very large thi
Kakihara Yasuo
Matsuura Mitsuru
Santoki Teruaki
Yamamoto Setsuo
Nixon & Vanderhye P.C.
Rickman Holly
Toda Kogyo Corporation
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