Stock material or miscellaneous articles – Structurally defined web or sheet – Continuous and nonuniform or irregular surface on layer or...
Reexamination Certificate
2002-09-03
2004-09-14
Resan, Stevan A. (Department: 1773)
Stock material or miscellaneous articles
Structurally defined web or sheet
Continuous and nonuniform or irregular surface on layer or...
C428S329000, C428S409000, C428S690000, C428S690000, C428S690000, C428S690000
Reexamination Certificate
active
06790511
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a coating type magnetic recording medium (i.e., a magnetic recording particulate medium) capable of high density recording. More specifically, the present invention relates to a coating type magnetic recording medium for high density recording which comprises a magnetic layer on a substantially nonmagnetic lower layer wherein the magnetic layer contains a hexagonal ferrite fine powder.
BACKGROUND OF THE INVENTION
Magnetic heads working with electromagnetic induction as the principle of operation (an induction type magnetic head) are conventionally used and spread. However, magnetic heads of this type are approaching their limit for use in the field of higher density recording and reproduction. That is, it is necessary to increase the number of winding of the coil of a reproduction head to obtain larger reproduction output, but when the winding number is increased, the inductance increases and the resistance at high frequency heightens, as a result, the reproduction output lowers. In recent years, reproduction heads which work with MR (magneto-resistance) as the principle of operation are proposed and come to be used in hard discs. As compared with the induction type magnetic disc, several times of reproduction output can be obtained by the MR head. Further, since an induction coil is not used in the MR head, noises generated from instruments, e.g., impedance noises, are largely reduced, therefore, it becomes possible to obtain a great S/N ratio by lowering the noise coming from magnetic recording media. In other words, good recording and reproduction can be done and high density recording characteristics can be drastically improved by lessening the noise of magnetic recording media hiding behind the instruments. However, there is another problem that when the surface of media is smoothed for lessening the noise of magnetic recording media, the running property of magnetic recording media cannot be ensured.
SUMMARY OF THE INVENTION
An object of the present invention is to provide a coating type magnetic recording medium (i.e., a magnetic recording particulate medium) which is excellent in electromagnetic characteristics, markedly improved in a C/N ratio in high density recording region, excellent in productivity, can be produced inexpensively, and excellent in high density characteristics when used in a recording and reproducing system integrated with an MR head while maintaining running property and low in noise.
The above object of the present invention has been attained by a magnetic recording medium comprising a support having thereon a substantially nonmagnetic lower layer and a magnetic layer containing a ferromagnetic hexagonal ferrite powder dispersed in a binder provided in this order, wherein the average tabular diameter of the ferromagnetic hexagonal ferrite powder is from 10 to 40 nm, and the friction coefficient of the magnetic layer against an SUS420J pole and a polyoxymethylene (POM) pole is from 0.15 to 0.50.
The preferred modes of the present invention are as follows.
(1) The magnetic recording medium is a magnetic recording medium for reproduction with a magneto-resistance type magnetic head (an MR head),
(2) The magnetic recording medium wherein the intensity of spatial frequency of 100/mm which is obtained by frequency analysis of the surface roughness of the magnetic layer is 10,000 nm
3
or less, and the intensity of spatial frequency of 500/mm is from 50 to 500 nm
3
, and
(3) The magnetic recording medium wherein the surface of the support has protrusions having a height of from 0.1 to 0.5 &mgr;m in number of from 0.01 to 0.5/mm
2
.
DETAILED DESCRIPTION OF THE INVENTION
Hexagonal ferrite powders which are used in a magnetic layer in the present invention are tabular particles and the tabular diameter of hexagonal ferrite powders should be restrained to 10 to 40 nm. Further, the present invention can provide a magnetic recording medium improved in running durability and excellent in electromagnetic characteristics by restraining the friction coefficient of a magnetic layer against an SUS420J pole and a POM (polyoxymethylene) pole to 0.15 to 0.50.
When the friction coefficient of a magnetic recording medium is higher than 0.50, the reduction of running durability of the magnetic recording medium is actualized.
Further, the average tabular diameter of hexagonal ferrite powders for use in a magnetic layer is preferably from 20 to 35 nm, more preferably from 22 to 30 nm. When the average tabular diameter is higher than 40 nm, noise becomes large due to the hexagonal ferrite powders themselves and, in addition to that, a magnetic layer having excellent surface property which contributes to the reduction of noise is obtained with difficulty. While when the average tabular diameter is smaller than 10 nm, a sufficient dispersion solution cannot be obtained by the techniques of dispersion of the present time. The effect of the present invention becomes great by the average tabular diameter of hexagonal ferrite powders of from 10 to 40 nm.
It is preferred in the present invention that the intensity of spatial frequency of 100/mm which is obtained by frequency analysis of the surface roughness of the magnetic layer is 10,000 nm
3
or less, and the intensity of spatial frequency of 500/mm is from 50 to 500 nm
3
.
When the intensity of spatial frequency of 100/mm is 10,000 nm
3
or more, electromagnetic characteristics are liable to be deteriorated. The intensity of spatial frequency of 100/mm is preferably 7,000 nm
3
or less, and more preferably 5,000 nm
3
or less. The lower limit of the intensity of the spatial frequency of 100/mm is not particularly limited and the smaller the better.
Further, when the intensity of spatial frequency of 500/mm is less than 50 nm
3
, a friction coefficient is liable to become high, as a result, running durability shows a tendency to lower. While when the intensity of spatial frequency of 500/mm is higher than 500 nm
3
, electromagnetic characteristics are liable to be deteriorated. The intensity of spatial frequency of 500/mm is preferably from 100 to 400 nm
3
.
The reason that the intensity of spatial frequency (i.e., the power spectrum of density) or the distribution of the intensity influences electromagnetic characteristics and friction coefficient is not clear, but it is presumed due to the fact that the contact of an MR head and the medium is relaxed by that.
Several methods can be used to bring the friction coefficient into the range of the present invention, e.g., controlling the preparation of the coating solution forming a magnetic layer, controlling the conditions of calendering treatment, and controlling the protrusions on the surface of a support, are exemplified.
The present invention is preferably attained by the combination of the above methods. Of the above methods, it is preferred to select the average tabular diameter of a hexagonal ferrite powder and select the number of the surface protrusions of a support for controlling the intensity of spatial frequency (i.e., the power spectrum of density) of a magnetic layer.
The protrusions of the surface of a support for use in the present invention are preferably controlled so that protrusion density P of a height of from 0.1 to 0.5 &mgr;m comes to 0.01 to 0.5/mm
2
. When P is more than 0.5/mm
2
, electromagnetic characteristics are liable to be deteriorated, while when P is less than 0.01/mm
2
, a friction coefficient is liable to increase due to the increment of a contact area.
Protrusion density P in the present invention is obtained by measuring the number of surface protrusions having a height of from 5 to 10 nm in the area of 225 &mgr;m
2
of a magnetic layer surface by an atomic force microscope (AFM).
The magnetic recording medium according to the present invention is described with every constituent element below.
Magnetic Layer
The magnetic recording medium according to the present invention may be provided with a magnetic layer on either one side of a support or may be provided on both sides. When a magn
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