Stock material or miscellaneous articles – Composite – Of inorganic material
Reexamination Certificate
2001-04-09
2003-02-18
Le, H. Thi (Department: 1773)
Stock material or miscellaneous articles
Composite
Of inorganic material
C428S690000, C428S690000
Reexamination Certificate
active
06521361
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a particulate magnetic recording medium for high-density recording.
2. Description of Related Art
In recent years, recording wavelengths have tended to shorten as recording densities have increased. The problem of self-magnetization losses during recording, where output drops due to the thickness of the magnetic layer, has become significant. Thus, the magnetic layer has been reduced in thickness. However, when a magnetic layer of 2 &mgr;m or less is directly coated onto a support, the nonmagnetic support tends to affect the magnetic surface, and deterioration of electromagnetic characteristics and dropout tend to appear.
One means of solving this problem employs a method of forming a magnetic layer by a simultaneous multilayer coating method in which a nonmagnetic lower layer is provided on a support and a high-density magnetic coating solution is thinly applied (Japanese Patent Unexamined Publication Nos. Sho 63-191315 and Sho 63-187418). Such inventions have made it possible to achieve good electromagnetic characteristics with dramatically improved yields.
Magnetoresistive reproduction heads designed to further increase recording density have been brought into practical use with hard disks, and tests on flexible media such as metal evaporation tapes (ME) have been reported (IEEE. Trans. Mag. Vol. 35. No. 2, p. 729 (1999)).
Although magnetic recording media suited to reproduction with MR heads have been examined with regard to metal evaporation tapes (ME), there has not been adequate examination with regard to particulate magnetic recording media with good production and preservation properties.
Accordingly, an object of the present invention is to provide a particulate magnetic recording medium exhibiting a high C/N ratio in high-density magnetic recording, particularly when employing MR heads for reproduction.
SUMMARY OF THE INVENTION
The present invention relates to a magnetic recording medium comprising on a flexible nonmagnetic support, in order, a lower layer comprising a nonmagnetic powder and a binder, and a magnetic layer comprising a ferromagnetic powder and a binder, wherein an average thickness of said magnetic layer ranges from 0.02 &mgr;m to 0.2 &mgr;m; said ferromagnetic powder contained in said magnetic layer is a magnetic powder of platelike hexagonal ferrite with a mean plate diameter equal to or less than 40 nm; said nonmagnetic powder contained in said lower layer is an inorganic acicular powder with a mean major axis length equal to or less than 0.2 &mgr;m; and a plate ratio of said ferromagnetic powder is equal to or less than an acicular ratio of said inorganic powder of the lower layer.
The operating mechanism of the present invention will be described here.
Adopting a magnetic layer thickness of 0.02-0.2 &mgr;m permits improvement in the C/N resolution of digital recordings with MR heads. This is documented by reports of the results of experiments employing evaporation tapes (IEEE, Trans. Mag. Vol. 35, No. 2, p. 729 (1999) and the Picture Information Media Society Technical Reports, Vol. 23, No. 78, p. 21 (1999)). However, it is difficult to achieve magnetic layer film thicknesses in coated media equivalent to those of thin metal films. Further, in coated media, with magnetic alloy powders yielding high performance with conventional inductance heads, there is significant noise during reproduction with MR heads and saturation tends to occur, precluding high C/N ratios. In the present invention, a magnetic layer with low magnetic nonuniformity can be achieved by providing a magnetic layer comprising a magnetic powder of hexagonal ferrite of small (mean plate diameter=40 nm) particle size over a nonmagnetic lower layer comprising a nonmagnetic powder having a mean major axis length of 0.2 &mgr;m or less and an acicular ratio greater than the plate ratio of the magnetic powder.
In the present invention, incorporating a nonmagnetic powder with a higher acicular ratio than the magnetic particles into the nonmagnetic lower layer permits improvement in the flow orientation properties of the lower layer, improvement in the orientation of magnetic material of micrograms tending not to assume orientations, and reduction in the variation in thickness of the magnetic layer. The relation whereby the upper layer acicular ratio is less than or equal to that of the lower layer acicular ratio is disclosed in Japanese Patent Unexamined Publication Nos Hei 8-102037 and Hei 9-106533. However, the present invention has been found to achieve never before seen results by incorporating a microgranular magnetic powder of hexagonal platelike ferrite. That is, since magnetic hexagonal ferrite materials have lower saturation magnetization than magnetic alloy powders, the thixotropic property of the coating solution is low and orientation reversion tends to occur. Accordingly, although mixing with the lower layer takes place during the coating drying process, resulting in significant orientation reversion, the use of the lower layer in the present invention has been found to yield good upper and lower film boundaries and orientation properties. This effect is marked when the platelike, ferromagnetic powder (magnetic material of hexagonal ferrite) is oriented in a vertical direction. Further, the greater the acicular ratio of the lower layer magnetic powder, the better the molding properties during calendering, so it was found that by compensating for the fact that magnetic layers are difficult to form when using microgranular magnetic materials with low void rates in the film-forming stage, good surface properties could be achieved.
The following are desirable forms of the magnetic recording medium of the present invention:
(1) The mean grain volume of the ferromagnetic powder ranges from 1,500 to 15,000 nm
3
, and the coercivity Hc of the magnetic layer in the longitudinal direction is 167 KA/m (2,100 Oe) or greater;
(2) The squareness of the magnetic layer in the vertical direction SQ⊥ is 0.3 or greater and the coercivity in the vertical direction Hc⊥ is 119 KA/m (1,500 Oe) or greater; and
(3) The mixing in of 10-30 parts of granular particles with a mean primary grain diameter of 50 nm or less and a true specific gravity of 5 or less per 100 parts of acicular material in the nonmagnetic lower layer is desirable to achieve uniform formation of the above-described thin magnetic layer.
The mean grain volume of the ferromagnetic powder is preferably 1,500-15,000 nm
3
from the perspective of noise reduction, and the Hc is preferably 167 KA/m (2,100 Oe) or greater to maintain stable recording magnetization. A grain volume of 1,500 nm
3
or greater and an Hc of 167 KA/m (2,100 Oe) or greater maintains good thermal stability of recording magnetization. When the grain volume is 15,000 nm
3
or less, noise is controlled and an adequate C/N ratio can be achieved. Due to recording principles, a large coercivity Hc is desirable, but when the recording head capacity is considered, 175-279 KA/m (2,200-3,500 Oe) is actually suitable.
An SQ⊥ in the vertical direction of the magnetic layer of 0.3 or greater and an Hc⊥ in the vertical direction of 119 KA/m (1500 Oe) or greater are advantageous to effectively exploit the vertical magnetization component and achieve good resolution.
Further, mixing in 10-30 parts of granular particles with a mean primary grain diameter of 50 nm or less and a true specific gravity of 5 or less per 100 parts of acicular material in the nonmagnetic layer forms voids between the acicular grains in the film, improves forming properties during calendering, and yields good surface properties.
[The Magnetic Layer]
In the magnetic recording medium of the present invention, a suitable average magnetic layer thickness (d) is 0.02-0.2 &mgr;m, preferably 0.03-0.15 &mgr;m.
The object of the present application can be achieved with a single layer or multilayered magnetic layer. When employing multiple magnetic layers, the technique described in Japanese P
Ejiri Kiyomi
Mori Masahiko
Fuji Photo Film Co. , Ltd.
Le H. Thi
Stroock & Stroock & Lavan LLP
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