Stock material or miscellaneous articles – Web or sheet containing structurally defined element or... – Physical dimension specified
Reexamination Certificate
2002-07-01
2004-11-23
Nakarani, D. S. (Department: 1773)
Stock material or miscellaneous articles
Web or sheet containing structurally defined element or...
Physical dimension specified
C428S336000, C428S690000, C428S690000, C204S192100
Reexamination Certificate
active
06821618
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 comprising a spinel-type iron oxide thin film comprising maghemite as a main component that is constituted by grains having a small average grain size and an excellent grain size distribution, and exhibits a high coercive force and an excellent squareness, and a process for producing the magnetic recording medium in an industrially and economically useful manner.
In recent years, in magnetic recording apparatuses, it has been more increasingly demanded to provide magnetic recording media having a high-density recording property and a high reliability in order to deal with a large capacity data such as various back-up data, images and audio data. Especially, in hard disk drives, there has been a remarkable tendency of a miniaturization and a high reliability in accordance with information devices being miniaturized and systems used therefor being required to have a high reliability.
In order to satisfy such properties, the magnetic recording media have been strongly required to have not only a high coercive force, but also exhibit a less noise, an excellent durability and a reduced distance (magnetic spacing) between a magnetic recording layer and a magnetic head.
As magnetic recording media having a high coercive force, there are widely known magnetic recording media comprising a substrate and a magnetic thin film formed on the substrate, in case of the hard disk drives.
The magnetic thin films practically used in magnetic recording media are generally classified into magnetic alloy thin films such as Co—Cr-based alloy thin films and magnetic oxide thin films such as spinel-type iron oxide thin film comprising maghemite as a main component.
The magnetic alloy thin films such as Co—Cr-based alloy thin films, have a coercive force as high as not less than 159 kA/m (2,000 Oe). However, these alloy materials themselves tend to be readily oxidized and, therefore, deteriorated in stability as well as magnetic properties with the passage of time.
In order to prevent the deterioration of magnetic properties due to oxidation and enhance the durability, the surface of the magnetic alloy thin film is coated with a protective film composed of carbon, SiO
2
or the like having a thickness of usually about 5 to 10 nm, resulting in undesired increase in magnetic spacing corresponding to the thickness of the protective layer.
On the other hand, the magnetic oxide thin films such as spinel-type iron oxide thin film comprising maghemite as a main component are excellent in oxidation resistance or corrosion resistance due to inherent properties of oxides. For this reason, the magnetic oxide thin films can show an excellent stability independent of the passage of time, and a less change in magnetic properties with the passage of time. Further, since the oxides exhibit a higher hardness as compared to metals, the magnetic oxide thin films can also show an excellent durability. As a result, in the magnetic oxide thin film, the thickness of a protective layer which is essential to the magnetic alloy thin film can be reduced or the protect layer can be omitted. Therefore, the magnetic oxide thin films can exhibit a small magnetic spacing as compared to the magnetic alloy thin films, so that the magnetic oxide thin films are optimum ones for the production of high-density magnetic recording media.
It has been attempted to enhance a coercive force of the spinel-type iron oxide thin film comprising maghemite as a main component by incorporating cobalt thereinto. However, with the increase in cobalt content, the spinel-type iron oxide thin film comprising maghemite as a main component tend to be deteriorated in stability with the passage of time due to adverse influences of heat or the like.
With the increasing demand for providing magnetic recording media capable of high-density recording, it has been required to enhance a coercive force of the magnetic recording media. In addition, in order to reduce the noise due to the media themselves, the magnetic recording media have been required to have a thin magnetic layer and a smooth surface. Further, in order to obtain magnetic recording media having a high recording resolution such as an excellent overwrite characteristics or the like, the magnetic recording media have been required to exhibit a high squareness. The squareness mentioned in the present invention means a coercive squareness ratio S* (referred in Magnetic Charecterisation of Thin Film Recording Media, published in IEEE TRANSACTIONS ON MAGNETICS, Vol. 29, No. 1, Jan. 1, 1993, pages 286-289) in the case of a longitudinal recording medium, and a squareness ratio S (Mr/Ms) in the case of perpendicular recording medium.
The noises caused from magnetic recording media are mainly classified into transition noises and modulation noises. The transition noises depend upon the size of magnetic grains constituting the magnetic thin film. Therefore, it has been required to reduce an average grain size of grains constituting the magnetic thin film.
Also, in the case where the magnetic recording media have a less smooth surface, modulation noises are caused therefrom. Therefore, in order to reduce the modulation noises, it is necessary to improve a surface smoothness of the magnetic thin film.
On the other hand, in order to reduce the magnetic spacing of magnetic recording media, it is required to minimize flying height of a magnetic head therefrom, and always allow the magnetic head to be flying stably. In conventional hard disk drives, magnetic recording media used therefor have been required to have a certain surface roughness in order to prevent the magnetic head from being absorbed thereon owing to a meniscus force therebetween upon stopping the magnetic head. However, as a result of current improvement in these hard disc systems, magnetic recording media have been no longer required to have such a surface roughness for preventing the magnetic head from being absorbed thereon. Therefore, it has been demanded that a magnetic thin film used in these magnetic recording media exhibits a more excellent surface smoothness.
At present, in the magnetic recording media using a magnetic oxide thin film, since the magnetic thin film has a thickness as small as 50 nm or less, the surface property of the magnetic thin film considerably depends upon that of a substrate. As a result, it has been required not only to use a substrate having an excellent surface smoothness, but also to develop techniques for further smoothening the surface of the magnetic thin film.
As conventional methods for producing spinel-type iron oxide thin film comprising maghemite as a main component, there are known, for example, (1) a method of forming a hematite thin film on a substrate, reducing the hematite thin film at a temperature of 230 to 320° C. to transform the hematite thin film into a spinel-type iron oxide thin film comprising magnetite as a main component, and then oxidizing the spinel-type iron oxide thin film comprising magnetite as a main component at a temperature of 290 to 330° C.; (2) a method of forming a spinel-type iron oxide thin film comprising magnetite as a main component on a substrate, and then oxidizing the spinel-type iron oxide thin film comprising magnetite as a main component at a temperature of 280 to 350° C.; or the like.
Furthermore, as the conventional methods for producing a spinel-type iron oxide thin film comprising maghemite as a main component, there may be exemplified the following methods in addition to the above-mentioned methods (1) and (2):
(3) a method of forming a spinel-type iron oxide thin film on an underlayer composed of Cr, V or the like (Japanese Patent Publication (KOKOKU) No. 55-21451(1980));
(4) a method of conducting a sputtering treatment using a target containing Fe
3
O
4
as a main component in an inert gas atmosphere containing 1.5 to 5% by volume of oxygen to directly form a film
Doi Takanori
Koujima Jun
Maekawa Masaaki
Nakata Kenichi
Santoki Teruaki
Nakarani D. S.
Nixon & Vanderhye P.C.
Toda Kogyo Corporation
Uhlir Nikolas J.
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