Magnetic recording medium

Details Magnetic recording medium Magnetic recording medium

2002-08-13

2004-08-10

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, C428S336000, C428S690000, C428S690000, C428S690000

Reexamination Certificate

active

06773788

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to a magnetic recording medium capable of high-density magnetic recording, in particular, suited to reproduction employing an MR head.
BACKGROUND OF THE INVENTION
Recording wavelengths have tended to shorten as recording densities have increased in recent years. The problem of self-demagnetization loss during recording, where a thick recording layer results in low output, has become substantial. For this reason, magnetic layers are made thin. However, when a magnetic layer equal to or less than 2 &mgr;m is directly coated on a support, the nonmagnetic support tends to affect the surface of the magnetic layer, and electromagnetic characteristics and dropout tend to deteriorate.
One way of solving this problem is to employ a simultaneous multilayer coating method to apply a nonmagnetic layer as a lower layer and then apply a thin magnetic coating liquid of high concentration, as described in Japanese Unexamined Patent Publication(KOKAI) Showa Nos. 63-191315 and 63-187418. These methods have permitted marked improvement in yield and made it possible to achieve good electromagnetic characteristics. Magnetoresistive reproduction heads (MR heads) have been practically applied to hard disks to further increase recording density. In flexible media, there are test results for metal evaporation tapes (ME) (IEEE. Trans. Mag. Vol. 35, No. 2, p. 729 (1999)). However, since it is difficult to manufacture magnetic recording media suitable for reproduction with MR heads in particulate magnetic recording with excellent productivity and storage properties, they are not employed in practical.
Taking the above states into consideration, it is an object of the present invention is to provide a magnetic recording medium exhibiting high C/N ratio and suited to high-density magnetic recording, particularly, to reproduction with MR heads.
SUMMARY OF THE INVENTION
The present inventors conducted extensive research focusing on magnetic powders, particularly ferrite magnetic powders, for achieving the above-stated objects. As a result, they discovered a certain relation between the thickness of the magnetic layer and the characteristics of the magnetic powder particles to achieve the present invention.
That is, the object of the present invention is achieved by a magnetic recording medium comprising a nonmagnetic layer comprising a nonmagnetic powder and a binder and a magnetic layer comprising a ferromagnetic powder and a binder in this order on a flexible nonmagnetic support, characterized in that
the magnetic layer has an average thickness ranging from 0.02 to 0.2 &mgr;m,
the ferromagnetic powder contained in the magnetic layer is a plate-shaped hexagonal ferrite ferromagnetic powder having an average plate diameter of 10 to 40 nm,
the ratio of a volume of magnetic reversal to a volume of magnetic material particles is equal to or less than 3,
the magnetic layer has a coercive force Hc equal to or higher than 159 kA/m (2,000 Oe), and
components magnetically reversing in the magnetic field of 80 kA/m (1,000 Oe) or less are less than 1 percent in the magnetization distribution of the magnetic layer.
The preferred modes of the magnetic recording medium of the present invention are as follows;
(1) The magnetic recording medium in which the 10 &mgr;m Power spectrum Density of Roughness abbreviated to “PSD” is equal to or less than 9,000 nm
3
on the magnetic layer surface;
(2) The magnetic recording medium employing an MR head during at least reproduction.
Further, the magnetic recording medium of the present invention is obtained by the following manufacturing method:
A method of manufacturing a magnetic recording medium comprising a step in which a nonmagnetic layer coating liquid comprising a nonmagnetic powder and a binder and, over the nonmagnetic layer, a magnetic layer coating liquid comprising a ferromagnetic powder and a binder are coated on a nonmagnetic flexible support by a wet-on-wet (wet/wet) method, characterized in that the ratio of the viscosity of the magnetic layer coating liquid to the viscosity of the nonmagnetic layer coating liquid is equal to or less than 0.8 at a high shear rate (45,000 s
−1
)
The magnetic recording medium of the present invention is described in detail below.
The magnetic recording medium of the present invention is primarily characterized in that the average thickness of the magnetic layer is 0.02 to 0.2 &mgr;m, the ferromagnetic powder contained in the magnetic layer is plate-shaped hexagonal ferrite ferromagnetic powder with an average plate diameter of 10 to 40 nm, and the ratio of the volume of magnetic reversal to the volume of magnetic particles is equal to or less than 3.
The average thickness of the magnetic layer of the present invention is 0.02 to 0.2 &mgr;m, preferably 0.03 to 0.1 &mgr;m, and more preferably, 0.03 to 0.08 &mgr;m. When the thickness of the magnetic layer is equal to or higher than 0.02 &mgr;m, the magnetic particles in the magnetic layer are able to assume a longitudinal orientation, distributing magnetic material to all areas and yielding good magnetic layer squareness. When the thickness of the magnetic layer is equal to or less than 0.2 &mgr;m, self-demagnetization loss can be inhibited, a low level of magnetization is possible in the magnetic layer, and noise due to saturation can be prevented in MR heads.
Further, the particle size of the ferromagnetic powder contained in the form of the plate-shaped hexagonal ferrite powder in the magnetic layer, defined by the average of the maximum major axis length of the hexagonal plates, that is, the average plate diameter, is 10 to 40 nm, preferably 10 to 35 nm, and more preferably 15 to 35 nm. In reproduction with MR heads, it is necessary to inhibit noise as much as possible. When the average plate diameter ranges from 10 to 40 nm, it is possible to prevent destabilization of magnetization due to thermal fluctuation and noise is also low. Both are suited to high-density magnetic recording.
The ratio of the volume of magnetic reversal to the volume of magnetic particles in the magnetic layer is equal to or less than 3. That is, in the present invention, based on the average particle volume of the plate-shaped hexagonal ferrite ferromagnetic powder, the ferromagnetic powder is dispersed in the binder so that the volume of magnetic reversal is within three times the average particle volume, preferably within 1.7 times, and more preferably, within 1.2 times.
The volume of magnetic reversal V can be obtained from the following relational equation between coercive force Hc due to thermal fluctuation and the volume of magnetic reversal, V.
Hc
=(2
K/Ms
){1−[(
kT/KV
)
ln
(
At/
0.693)]
1/2
}
Here, K denotes the constant of anisotropy, Ms denotes saturation magnetization, k denotes the Boltzmann constant, T denotes absolute temperature, V denotes the volume of magnetic reversal, A denotes the spin precession frequency, and t denotes the magnetic field reversal time.
Since the ratio of the volume of magnetic reversal to the volume of magnetic particles exceeds 3 in the magnetic powder conventionally employed in the magnetic layers of magnetic recording media, noise during reproduction with MR heads is high and a high C/N ratio cannot be achieved. This is thought to occur due to magnetic coupling of multiple plate-shaped hexagonal ferrite particles as the result of stacking between plate-shaped hexagonal ferrite magnetic particles, resulting in a high volume of magnetic reversal in the magnetic material.
The present inventors have extensively examined ways of achieving a ratio of the volume of magnetic reversal equal to or less than 3 to the volume of magnetic particles, that is, reducing the volume of magnetic reversal by inhibiting stacking between plate-shaped hexagonal ferrite magnetic particles as much as possible.
In the particulate magnetic recording medium, as set forth above, a simultaneous multilayer coating method is employed to coat a high-concentration magnetic layer coating liquid on a nonmagnetic

Affiliated with

Also associated with

Rating

Magnetic recording medium does not yet have a rating. At this time, there are no reviews or comments for this patent.

If you have personal experience with Magnetic recording medium, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Magnetic recording medium will most certainly appreciate the feedback.

Rate now

Comments { 0 }

Profile ID: LFUS-PAI-O-3276096