Magnetic recording medium

Details Magnetic recording medium Magnetic recording medium

2002-07-25

2004-07-27

Resan, Stevan A. (Department: 1773)

Stock material or miscellaneous articles

Structurally defined web or sheet

Continuous and nonuniform or irregular surface on layer or...

C428S690000, C428S690000

Reexamination Certificate

active

06767612

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to a particulate magnetic recording medium with high recording density. More specifically, the present invention particularly relates to a magnetic recording medium comprising a nonmagnetic layer and a magnetic layer comprising a hexagonal ferrite ferromagnetic powder in this order, which is suited to a magnetic recording and reproducing system in which an MR head is adopted.
BACKGROUND OF THE INVENTION
In the field of magnetic recording disks, 2 MB MF-2HD floppy disks employing Co-modified iron oxide have been mounted normally in personal computers. However, today, with the rapid increase in the volume of data handled, the capacity of such disks is inadequate and there is a need for the development of high-capacity floppy disks. On the other hand, in the field of magnetic tapes, with the widespread popularity of office computers such as minicomputers, personal computers, and work stations in recent years, a large amount of research has been conducted into magnetic recording tapes (so-called “back-up tapes”) used to record computer data as an external memory medium. In the practical implementation of magnetic recording tapes for such applications, particularly as the size of computers has decreased and information processing capability has increased, there had been strong demand for an increase in recording capacity to achieve high-capacity recording and size reduction.
Further, magnetic heads operating on the principle of electromagnetic induction (inductive magnetic heads) have been widely employed in recording and reproduction systems employing magnetic recording media. However, limits to the use of inductive magnetic heads in the range of even higher density recording are problematic. That is, obtaining high reproduction output requires increasing the number of windings in the coil of the reproduction head. However, when the number of coil windings is increased, inductance increases, which ends up increasing resistance at high frequency. This results in the problem of decreased reproduction output.
In light of this state of affairs, recording and reproduction magnetoresistive (MR) heads operating on the principle of magnetoresistance have been recently developed and put to use on hard disk devices and the like.
In addition, application of MR heads to magnetic tapes has also been studied. Employing MR heads in a magnetic tape recording and reproduction system yields several times the reproduction output of inductive magnetic heads, and since inductive coils are not employed, device noise such as impedance noise can be greatly reduced. Thus, the use of MR heads can reduce the noise of the magnetic recording medium itself, making it possible to achieve a high SN ratio. That is, the magnetic recording medium noise that was previously concealed in device noise is reduced, permitting good recording and reproduction and greatly improved high-density recording characteristics.
Accordingly, magnetic recording media in the form of magnetic tapes have recently been proposed (Japanese Unexamined Patent Publication (KOKAI) Nos. Heisei 8-227517 and 2001-84549).
Japanese Unexamined Patent Publication (KOKAI) No. 2001-84549 discloses a magnetic recording medium manufactured by a method of inhibiting error generation in a linear serpentine type recording and reproduction system by evaluating the pits on the medium surface by optical interference roughness meter and limiting the number of pits to a specified level.
However, since the magnetic head employed in this magnetic recording medium is an inductive head, it cannot adequately resolve the problem of decreasing noise in MR heads designed for use in high-density recording. Accordingly, there is a need for the development of a technique that is capable of resolving the problem of reducing medium noise when employing MR heads from the magnetic recording medium side.
Thus, the present invention was devised in light of the above-described problems, and it is an object of the present invention to provide a particulate magnetic recording medium affording great improvement in medium noise in a recording and reproduction system adopting MR heads.
The present inventors conducted extensive research into the spacing between heads and magnetic tapes in magnetic recording and reproduction systems adopting MR heads, resulting in the discovery that pits of a certain depth on the magnetic layer surface have a marked effect on noise; the present invention was devised on that basis.
SUMMARY OF THE 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 hexagonal ferrite powder and a binder in this order on a nonmagnetic support, characterized in that the number of pits having a depth of ⅓ or more of the minimum recording bit length present on a surface of said magnetic layer is equal to or less than 100/10000 &mgr;m
2
, and the center surface average roughness of said magnetic layer surface SRa is equal to or less than 6.0 nm.
On said magnetic layer surface, the number of pits having a depth as measured by Atomic Force Microscope (AFM) of 50 nm or more is preferably equal to or less than 100/10000 &mgr;m
2
.
The magnetic recording medium of the present invention can be employed for the following magnetic recording and reproducing system;
A magnetic recording and reproducing system which is a magnetic recording and reproducing system employing a magnetic recording medium comprising a nonmagnetic power and a binder and a magnetic layer comprising a hexagonal ferrite powder and a binder in this order on a nonmagnetic support, wherein an MR head is employed for reproducing a signal, and said magnetic recording medium is characterized in that the number of pits having a depth of ⅓ or more of the minimum recording bit length present on a surface of said magnetic layer is equal to or less than 100/10000 &mgr;m
2
, and the center surface average roughness of said magnetic layer surface SRa is equal to or less than 6.0 nm.
By controlling the surface roughness of the magnetic layer within the above-mentioned range, medium noises can be reduced during reproducing by an MR head.
Further, preferred modes of the present invention are as follows;
(1) The magnetic recording medium in which said nonmagnetic layer comprises at least carbon black and a binder composed of a radiation-setting resin and a thermosetting resin.
(2) The magnetic recording medium employed for a magnetically recording and reproducing system in which an MR head is adopted.
Modes of implementing the invention are described in detail below.
The magnetic recording medium of the present invention is characterized in that the number of pits having a depth of ⅓ or more of the minimum recording bit length present on a surface of the magnetic layer is equal to or less than 100/10,000 &mgr;m
2
and the center surface average roughness SRa of the magnetic layer surface is equal to or less than 6.0 nm.
The magnetic recording medium of the present invention is desirably characterized in that the number of pits having a depth of 50 nm or more as measured by atomic force microscope (AFM) on the magnetic layer surface is equal to or less than 100/10,000 &mgr;m
2
.
The depth of pits on the magnetic layer surface is the distance from the average plane (the plane where the volume of protrusions and the volume of pits in the measurement surface are equal) of magnetic layer surface roughness to the deepest portion of the pit when three-dimensional surface roughness is measured with Nanoscope III made by Digital Instruments in U.S.A.
Pits on the magnetic layer surface are presumed to compromise contact conditions between the MR head and the magnetic tape and to be related to noise. That is, it is thought that when there are pitted portions of prescribed depth on the magnetic layer surface, contact conditions between the MR head and the magnetic tape are compromised and output decreases locally in

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