Stock material or miscellaneous articles – Circular sheet or circular blank – Recording medium or carrier
Reexamination Certificate
2002-06-28
2004-03-09
Resan, Steven A. (Department: 1773)
Stock material or miscellaneous articles
Circular sheet or circular blank
Recording medium or carrier
C428S141000, C428S690000
Reexamination Certificate
active
06703101
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a coating type magnetic recording medium for high recording density, and in particular to a magnetic recording particulate medium (i.e., a coated-type magnetic recording medium) for high density recording comprising a magnetic layer and a substantially non-magnetic low layer including fine particles of hexagonal (system) ferrite in an uppermost layer.
BACKGROUND OF THE INVENTION
A magnetic head (induction-type magnetic head) having a working principle of electromagnetic induction has been conventionally used and distributed. But, for using it in a recording/reproducing range of higher density, a limitation seems to arise. That is, for obtaining a large reproduction output, a coiling number of a reproducing head must be increased, but there are problems that inductance increases and resistance in a high frequency becomes much so that the reproduction output goes down consequently. A reproducing head having a working principle of MR (magnetoresistance) has recently been proposed and used in hard disks, etc. MR (i.e., magnetoresistive) head can obtain the reproduction output of several times than an induction-type magnetic head, and besides since it does not use the induction coil, machine noises as impedance noises largely reduce, and a large SN ratio can be obtained by lowering noises of a magnetic recording medium. In other words, if reducing noises of the magnetic recording medium having hidden conventional machine noises, a good recording reproduction can be carried out and the characteristics for high density recording can be extremely improved. The MR head has a particular phenomenon called as a thermal asperity. If a recording medium strongly contacts to the MR head, a thermal energy thereby induces changes in resistance of the MR head, and reproduction signals disorder and occur errors. In particular, a special attention should be paid to flexible media as flexible disks or tapes, because they are ready for making irregular contacts to the MR heads.
SUMMARY OF THE INVENTION
An object of the present invention is to provide a coating-type magnetic recording medium for reproduction of MR head which has conventionally been used, and is excellent in productivity, cheaply available, little in the thermal asperity, and superior in property of high density and low in noises.
The inventors made earnest studies for providing a magnetic recording medium which is good in an electromagnetic (conversion) characteristic, less in the thermal asperity which is particular to reproduction of the MR head, and is by far improved in the high density recording characteristic, and consequently they found provision of the high density recording property, and have come to the present invention.
That is, the present invention relates to a magnetic recording medium for reproduction of MR head comprising a support having thereon a substantially non-magnetic low layer and a magnetic layer comprising ferromagnetic hexagonal system ferrite particles dispersed in a binder, in this order, wherein a power spectrum of density at a spatial frequency of 100/mm having frequency-analyzed a surface roughness of the magnetic layer is 10,000 nm
3
or less, the power spectrum of density at a spatial frequency of 500/mm is 50 to 500 nm
3
, and an average tabular diameter of the ferromagnetic hexagonal system ferrite particles is 15 to 35 nm.
Preferable embodiments of the present invention are as follows.
(1) The magnetic recording medium as described above, wherein the power spectrum of density at a spatial frequency of 100/mm having frequency-analyzed the surface roughness of the support, is 50,000 nm
3
or less, and the power spectrum of density at a spatial frequency of 500/mm is 100 to 5,000 nm
3
.
(2) The magnetic recording medium as described above wherein projections (i.e., protrusions) having a height of 0.1 to 0.5 &mgr;m which are present on the surface of the support, are 0.5 pieces/mm
2
or less.
(3) The magnetic recording medium as described above, wherein the magnetic recording medium is disk- or tape-shaped.
The present invention specifies the power spectrum of density at a spatial frequency of 100/mm having frequency-analyzed the surface roughness of the magnetic layer, the power spectrum of density at a spatial frequency of 500/mm, and the average tabular diameter of the ferromagnetic hexagonal system ferrite particles.
Herein, the power spectrum of density of each spatial frequency means a degree of a specific wavelength component in the surface roughness.
If the power spectrum of density at a spatial frequency of 100/mm exceeds 10,000 nm
3
, the electromagnetic (conversion) characteristic is deteriorated, and the power spectrum of density at a spatial frequency of 100/mm is preferably 7,000 nm
3
or less, and more preferably 5,000 nm
3
or less. A lower limit of the power spectrum of density at a spatial frequency of 100/mm is not especially provided, and the smaller, the better.
If the power spectrum of density at a spatial frequency of 500/mm is less than 50 nm
3
, the thermal asperity is much. If the power spectrum of density exceeds 500 nm
3
, the electromagnetic (conversion) characteristic is worsened. The power spectrum of density at a spatial frequency of 500/mm is preferably 100 to 400 nm
3
. A reason why the power spectrum of density at a spatial frequency of 500/mm gives influences to the thermal asperity is not apparent, but it is assumed to moderate strong impact between the MR head and fine projections.
The average tabular diameter of the hexagonal system ferrite used to the magnetic layer means an average of a hexagonal tabular diameter, and it is 15 to 35 nm, preferably 20 to 35 nm, and more preferably 22 to 30 nm. If the average tabular diameter is larger than 35 nm, noises are made large owing to the ferromagnetic particle itself, and besides it is difficult to obtain distribution in intensity having frequency-analyzed the surface roughness of the present invention. If the average tabular diameter is smaller than 15 nm, a dispersion liquid is not enough obtained by a nowadays' dispersion technique. In the present invention, the average tabular diameter of the hexagonal system ferrite is 15 to 35 nm for bringing about large effects.
There are several methods for bringing the power spectrum of density into the range of the present invention. For example, enumerated are to control the power spectrum of density of the support, to control adjustment of a coating solution for forming the non-magnetic lower layer and the magnetic layer, to select coating methods, to select treating conditions of the calendering, or to polish the surface after the calender. The present invention can be accomplished by combination of these methods.
In the above methods, those which give comparatively large influences for controlling the power spectrum of density of the medium, are to control the power spectrum of density of the support, and to select the average tabular diameters of the hexagonal system ferrite. Projections on the surface of the support appear on the surface of the magnetic recording medium, so that the thermal asperity is easy to occur, and therefore it is desirable that projections of 0.1 to 0.5 &mgr;m in height on the surface of the support for the present invention are 0.5 pieces/mm
2
or less.
DETAILED DESCRIPTION OF THE INVENTION
Magnetic Layer
The magnetic recording medium may have a magnetic layer either on one side only or on each side of the support. When a magnetic layer is to be formed on a lower layer, a coating solution for forming the magnetic layer (also called “upper layer” or “upper magnetic layer”) may be applied either by the wet-on-wet (W/W) method in which it is applied while a coating solution for lower-layer formation which has been applied is still in a wet state or by the wet-on-dry (W/D) method in which the coating solution for magnetic-layer formation is applied after the coating solution for lower-layer formation applied has been dried. From the standpoint of production efficiency, simultaneous or su
Saito Shinji
Yamazaki Nobuo
Fuji Photo Film Co. , Ltd.
Resan Steven A.
Sughrue & Mion, PLLC
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