Stock material or miscellaneous articles – Composite – Of inorganic material
Reexamination Certificate
2000-12-08
2004-01-20
Rickman, Holly (Department: 2651)
Stock material or miscellaneous articles
Composite
Of inorganic material
C428S690000, C428S900000
Reexamination Certificate
active
06680133
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a magnetic recording medium used in an apparatus such as a magnetic disk apparatus.
BACKGROUND OF THE INVENTION
In recent years, enhancement in the recording density of a magnetic disk apparatus has resulted in demand for a reduction in the distance between a magnetic head and a magnetic recording medium during data readout.
The distance between the aforementioned head and medium is preferably reduced, for example, to 10 nm or less, so as to attain a magnetic recording medium having a recording density of 10 Gbits/inch
2
or more. In order to avoid collision with the head, the magnetic recording medium preferably has a smooth surface.
In addition, a substrate of a magnetic recording medium must have sufficient hardness in order to prevent formation of dimples on a surface thereof caused by collision with a magnetic head in the event of impact-induced damage in a magnetic disk apparatus.
Generally, a substrate which is formed of an aluminum alloy and coated with NiP alloy plating film (hereinafter referred to as an “NiP-plated Al substrate”) is widely used for producing a magnetic recording medium. The NiP alloy plating film is provided to harden the surface of the substrate to enhance durability, and to facilitate polishing and evening of the surface.
Typically, on the surface of NiP alloy plating film, grooves which are collectively called texture are mechanically formed along the circumferential direction by use of a lapping tape or free abrasive grains to reduce contact resistance between the surface of a medium and a magnetic head and to enhance durability.
The texture uniformly aligns crystal axes of Co alloy to the circumferential direction of a substrate, forming a magnetic film, and preventing excessive growth of magnetic particles to make grain size distribution sharp.
Although an NiP-plated Al substrate is surface-polished to enhance surface smoothness for making the distance between a magnetic head and medium fall within the aforementioned range, surface polishing is unsatisfactory in view of reliability and cost.
Furthermore, since an NiP-plated Al substrate comprises NiP alloy plating film having satisfactory hardness and an Al substrate having poor hardness, a magnetic recording medium employing the NiP-plated Al substrate has a drawback. For example, when a magnetic disk apparatus is damaged by impact, a magnetic head of the apparatus collides with the recording medium thereby deforming a collision portion of the Al substrate and forming a dimple, called “head slap,” in the surface of the magnetic recording medium. The dimple may cause errors during reproduction of recorded data.
Thus, in many cases a hard non-metallic substrate formed of material such as glass or ceramic has recently replaced an Al substrate. Particularly, when a glass substrate is employed, sufficient surface smoothness for adjusting the distance between a magnetic head and medium to the aforementioned value can be attained easily at low cost. In addition, such substrates per se have high hardness, thereby advantageously preventing head slap.
In contrast, when a substrate formed of glass or ceramic is employed in a magnetic recording medium, texturing of the substrate is disadvantageously difficult, because the substrate has high hardness.
Texturing includes providing uniform crystalline orientation in an undercoat film so as to orient the crystalline orientation of a magnetic film to a predetermined direction, and is an important step for obtaining excellent magnetic properties during formation of films such as an undercoat film, a magnetic film, and a protective film on a substrate.
Therefore, when the aforementioned non-metallic substrate formed of glass or ceramic, which substrate is difficult to subject to texturing, is employed, the produced magnetic recording medium has unsatisfactory magnetic characteristics.
In order to overcome such drawbacks, there has been proposed formation of a hard film which can be easily textured on a non-metallic substrate formed of material such as glass or ceramic (e.g., Japanese Patent Application Laid-Open (kokai) Nos. 4-295614 and 9-167337). Specifically, a magnetic recording medium employing a non-metallic substrate which is plated with film such as electroless plating film is disclosed therein.
However, the disclosed magnetic recording medium is produced through cumbersome steps, and an electroless plating substrate must contain a heavy metal such as palladium or platinum as a plating catalyst. Thus, there still exist problems, such as post-treatment of wastewater and handling of catalyst material.
In order to solve these problems, there has been proposed a magnetic recording medium having a non-metallic substrate formed of material such as glass or ceramic, and the substrate is coated through sputtering with NiP film serving as a hard film which can be textured (e.g., Japanese Patent Application Laid-Open (kokai) No. 5-197941).
There has also been proposed formation of film comprising NiP alloy and a third element on a non-metallic substrate in order to prevent magnetization of the NiP film caused by heating during production of a magnetic recording medium (e.g., Japanese Patent Application Laid-Open (kokaz) No. 6-267050).
Even though these disclosed magnetic recording media employ a hard non-metallic substrate formed of material such as glass or ceramic, texturing can be carried out.
However, the aforementioned magnetic recording media still have problems; i.e., still exhibit insufficient processability during texturing of a substrate.
Specifically, a NiP film which is formed on a surface of a substrate through sputtering has hardness lower than that of a film formed by electroless plating and is embrittled, thereby resulting in easy formation of fins and deep grooves in the surface of the NiP film during texturing. As a result, surface smoothness sufficient for reducing the distance between a head and a medium cannot be obtained, and errors may occur during reproduction of recorded data.
In production of a magnetic recording medium disclosed in Japanese Patent Application Laid-Open (kokai) No. 6-267050, a third element (Z) is added to NiP to form NiPZ film which is magnetized at higher temperature. However, the NiPZ film has high hardness and exhibits poor processability (e.g., grinding amount, and density of texture lines) during texturing. Thus, magnetic characteristics of the recording medium are disadvantageously and easily deteriorated.
In addition, the aforementioned conventional magnetic recording media have unsatisfactory S/N ratio, error rate, and resistance to thermal fluctuation.
In view of the foregoing, an object of the present invention is provide a magnetic recording medium having excellent surface smoothness and magnetic characteristics such as S/N ratio, error rate, and resistance to thermal fluctuation.
SUMMARY OF THE INVENTION
The present invention provides a magnetic recording medium comprising (a) a non-metallic substrate, (b) a non-magnetic undercoat film, (c) a magnetic film, (d) a protective film, and (e) an orientation-determining film, which determines crystalline orientation of a film provided directly on the orientation-determining film when subjected to surface-texturing, between the non-metallic substrate and the non-magnetic undercoat film. The orientation-determining film comprises NiPX; and X is one or more species selected from the group consisting of Group IIA, IIIA, VIIA, VIII, IB, IIIB, IVB, and VB, other than Ac (actinide)-series elements and X has a melting point of 600-2000° C.
Preferably, the orientation-determining film comprises a material containing P and X in a total amount of 20-50 atomic %. Preferably, the P content is 15-40 atomic % and the X content is 2-25 atomic %.
Preferably, a non-magnetic adhesive film which prevents defoliation of the orientation-determining film from the substrate is formed between the non-metallic substrate and the orientation-determining film, and the non-magnetic adhesive film comprises one or more species selected fro
Kobayashi Masakazu
Sakai Hiroshi
Sakawaki Akira
Rickman Holly
Showa Denko Kabushiki Kaisha
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