Magnetic recording medium in the form of a disc

Details Magnetic recording medium in the form of a disc Magnetic recording medium in the form of a disc

2003-03-11

2004-08-10

Resan, Stevan A. (Department: 1773)

Stock material or miscellaneous articles

Circular sheet or circular blank

Recording medium or carrier

C428S141000, C428S213000, C428S480000, C428S690000

Reexamination Certificate

active

06773783

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to a magnetic recording medium in the form of a disc having a magnetic layer containing ferromagnetic metal fine powder or hexagonal ferrite fine powder for high density recording.
BACKGROUND OF THE INVENTION
In the field of the magnetic disc, a 2 MB MF-2HD floppy disc using Co-modified iron oxide has been generally loaded in a personal computer. However, along with the rapid increase in the amount of data to be dealt with, the capacity of the disc has become insufficient and the increase of the capacity of the floppy disc has been demanded.
Magnetic recording media comprising a magnetic layer containing an iron oxide, a Co-modified iron oxide, CrO
2
, ferromagnetic metal powder, or hexagonal ferrite powder dispersed in a binder coated on a nonmagnetic support have been conventionally widely used. Ferromagnetic metal fine powders and hexagonal ferrite fine powders have been known to have excellent high density recording characteristics among these.
As high capacity discs using ferromagnetic metal fine powders which are excellent in high density recording characteristics, there are 10 MB MF-2TD and 21 MB MF-2SD, and as high capacity discs using hexagonal ferrite, there are 4 MB MF-2ED and 21 MB Floptical, however, any of them is not satisfactory with respect to capacities and properties. As is the circumstance, various attempts have been made to improve high density recording characteristics.
On the other hand, a magnetic recording medium in the form of a disc comprising a thin magnetic layer and a functional nonmagnetic layer has been developed and floppy discs of the class with the capacity of 100 MB are now on the market. As floppy discs showing these characteristics, JP-A-5-109061 (the term “JP-A” as used herein means an “unexamined published Japanese patent application”) proposes the constitution comprising a magnetic layer having Hc of 1,400 Oe or more and a thickness of 0.5 &mgr;m or less and a nonmagnetic layer containing electrically conductive particles, JP-A-5-197946 proposes the constitution containing an abrasive having a particle size larger than the thickness of the magnetic layer, and JP-A-5-290354 proposes the constitution comprising a magnetic layer having a thickness of 0.5 &mgr;m or less with the fluctuation of the thickness of the magnetic layer being within ±15%, in which the surface electric resistance is regulated. Further, JP-A-10-21529 discloses that stable head touch can be obtained by regulating the outside diameter of a disc and the thickness of a support by using aramid as the support, and JP-A-10-21530 by using polyethylene naphthalate as the support.
However, with the rapid trend of the increase of the density of magnetic recording media in the form of a disc, it has become difficult to obtain satisfactory characteristics even with these techniques. In particular, when reproduction is carried out with a high speed magneto-resistance head (an MR head) for high density not with a conventional electromagnetic induction type head, thermal asperities caused by the contact heat by disc and head occur frequently, further, the abrasion of the MR element increases, as a result, the output becomes unstable and sufficient performance cannot be obtained.
SUMMARY OF THE INVENTION
Accordingly, an object of the present invention is to provide a magnetic recording medium in the form of a disc which is markedly improved in electromagnetic characteristics, in particular, high density recording characteristics, and can exhibit the excellent characteristics in every size of a disc when reproduction is performed by an MR head, e.g., AMR, GMR and TMR.
As a result of eager investigation of the above object, the present inventors have found that the problems of conventional techniques can be overcome by satisfying the following constitution and physical properties.
That is, the present invention is as follows.
(1) A magnetic recording medium in the form of a disc comprising a polyethylene naphthalate support having provided thereon a magnetic layer comprising ferromagnetic metal fine powder or ferromagnetic hexagonal ferrite fine powder dispersed in a binder, wherein the central plane average surface roughness of the magnetic layer of the magnetic recording medium is 5 nm or less, the thickness variation rate of the disc is 3% or less, and the total thickness of the disc X &mgr;m and the outside diameter of the disc Y mm satisfy the relationship of 0.3≦Y/X≦0.8.
Further preferred embodiments of the present invention are as follows.
(2) The magnetic recording medium in the form of a disc as described in the above item (1), wherein a substantially nonmagnetic subbing layer is provided between the support and the magnetic layer.
(3) The magnetic recording medium in the form of a disc as described in the above item (1) or (2), wherein the central plane average surface roughness of the magnetic layer is 3 nm or less.
(4) The magnetic recording medium in the form of a disc as described in the above item (1), (2) or (3), wherein reproduction is performed by a magneto-resistance head.
The magnetic recording medium according to the present invention is markedly improved in electromagnetic characteristics, in particular, high density recording characteristics, and can exhibit the excellent characteristics in every size of a disc when reproduction is carried out by an MR head, e.g., AMR, GMR and TMR, and in particular exhibits high density characteristics in reproduction by an MR head.
The reason for this fact is not clearly known but is presumed as follows.
In a conventional magnetic disc medium, a designing method has been taken so as to improve the head touch with a disc by thinning the thickness of the support of a disc to a certain degree to thereby reduce the rigidity of the disc and improve adhesion with the head. However, when the signal recorded on such a magnetic disc is reproduced with an MR head, thermal asperities are caused by heat generated by the contact of the head with minute protrusions on the surface of the disc due to too close adhesion of the head and disc.
Further, it is also thought that a disc which is low in rigidity is liable to wrap around a head part, so that the abrasion of the weak part of head, e.g., an MR element, is accelerated.
Furthermore, it has also been found that when a disc low in rigidity is subjected to high speed rotation (from 2,000 to 10,000 rpm), violent vertical motion is generated and the stable contact of the disc with a head cannot be maintained at all.
It is thought that the magnetic recording medium in the form of a disc (hereinafter also referred to as “the disc”) of the present invention is highly rigid, so that excessive contact of the disc with a head can be obviated, as a result, the generation of thermal asperities and the partial abrasion of the head part can be inhibited. In addition, it is thought that since the disc of the invention is highly rigid, stable head touch can be achieved without increasing vertical motion of the disc at high speed rotation.
Further, it is presumed that by making the central plane average surface roughness of the magnetic layer surface 5 nm or less, the touch of the disc with a head is made uniform on a micro-level, so that stable head touch can be achieved even in such a highly rigid disc.
Furthermore, it is thought that vertical motion of the disc at rotation can be made small by making the thickness variation rate of the disc 3% or less, thus the contact state of the disc and head can be maintained constant even in such a highly rigid magnetic disc.
Still further, the reasons that the present invention exhibits excellent effect are presumably due to the facts that since polyethylene naphthalate is more rigid than polyethylene terephthalate, the vertical motion of the disc at high speed rotation is more stable, and that since polyethylene naphthalate has a high modulus of elasticity, the support itself is deformed with difficulty and more excellent in the flatness of disc.
DETAILED DESCRIPTION OF THE INVENTION
The magnetic r

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