Magnetic recording medium

Details Magnetic recording medium Magnetic recording medium

2003-09-12

2004-12-28

Resan, Stevan A. (Department: 1773)

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Physical dimension specified

C428S690000

Reexamination Certificate

active

06835451

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to a magnetic recording medium capable of high-density recording.
BACKGROUND OF THE INVENTION
Conventionally, magnetic recording media such as video tapes, audio tapes, and magnetic disks having a magnetic layer of ferromagnetic iron oxide, cobalt-modified ferromagnetic iron oxide, CrO
2
, ferromagnetic alloy powders, or the like dispersed in binder that is coated on a nonmagnetic support have been widely employed. In recent years, the trend has been toward high densification accompanied by narrowing of the track width and shortening of the recording wavelength. Thus, the use of high-sensitivity magnetoresistive heads (referred to hereinafter as “MR heads”) in reproduction has been proposed and put into practice.
When employing MR heads, characteristics differing from those of conventional inductive head media are required. That is, since a thick magnetic layer and excessive residual magnetization result in saturation of an MR head, it is necessary to reduce residual magnetization. Further, since MR heads are highly sensitive, it is necessary to employ a microgranular magnetic powder and smoothen the magnetic surface to reduce medium noise. To that end, the magnetic layer thickness is made from 0.01 to 0.3 &mgr;m and residual magnetization is kept to 5 to 50 mA to prevent saturation, while specifying roughness at specified spacial frequency in an attempt to reduce modulation noise (see Japanese Unexamined Patent Publication (KOKAI) No. 2001-256633). Further, the magnetic layer is made thinner than the shortest bit length and nonmagnetic powder is added to the magnetic layer to achieve a volume packing density of 15 to 35 percent in the magnetic layer in an attempt to achieve both saturation and low noise (see Japanese Unexamined Patent Publication (KOKAI) No. 2002-92846). The technique described in Japanese Unexamined Patent Publication (KOKAI) No. 2001-256633 permits a reduction in the noise caused by surface roughness. The technique described in Japanese Unexamined Patent Publication (KOKAI) No. 2002-92846 reduces the volume packing density of the magnetic material, thereby reducing magnetostatic interaction, but nonmagnetic powder and magnetic powder aggregate, resulting in nonuniform distribution. That is, the cited prior art is problematic in that uniform distribution of magnetic particles in the magnetic layer, required to reduce noise, is not achieved.
Considerable analytic research is being conducted into medium noise caused by the continuous aggregation of magnetic particles and loop-shaped aggregation. However, this is a presumption based on mathematic calculation, and specific medium parameters and medium control methods have yet to be proposed (see Jiro Norihashi, “Noise Theory of Microgranular Recording Media and Methods of Isolating and Estimating Noise Sources”,
Bulletin of the Japan Applied Magnetism Society,
1997, Vol. 21, No. 4-1, pp. 149-159, and P. Luo, H. N. Bertram, “Tape Medium Noise Measurements and Analysis”, IEEE Transactions on Magnetics, USA, 2001, Vol.37, No.4, pp.1620-1623). A large number of inventions have been made relating to improvement in dispersibility, but they have not achieved improvement in the microstructure of the magnetic layer.
It is an object of the present invention to provide a magnetic recording medium which has excellent electromagnetic characteristics in an MR head and which is suited to high-density digital recording, especially suited to reproduction in an MR head.
The present inventors conducted extensive research into medium noise, resulting in the discovery of the structural parameters of the magnetic layer that are required to reduce noise from the perspective of micromagnetics. They also discovered that by controlling these structural parameters, it was possible to achieve reduction in noise to an unprecedented level; the present invention was devised on that basis.
That is, the aforementioned object of the present invention is achieved by:
a magnetic recording medium comprising a magnetic layer comprising a ferromagnetic powder and a binder on a nonmagnetic support, wherein
the magnetic layer has a residual magnetization &PHgr;r ranging from 5 to 50 mA, and
the ratio (Sdc/Sac) of the average area Sdc of magnetic clusters under DC magnetized condition to the average area Sac of magnetic clusters under AC erased condition ranges from 0.8 to 2.0.
In the aforementioned magnetic recording medium, it is preferable that the ferromagnetic powder has an average particle size (which denotes a maximum length in a powder having an anisotropic shape) ranging from 5 to 120 nm, as well as, the volume packing density of the ferromagnetic powder in the magnetic layer is equal to or higher than 35 percent.
The mechanism of the present invention is described below.
[Saturation Magnetization &PHgr;r]
MR heads can achieve higher reproduction output than inductive heads. With prior art inductive heads, increased residual magnetization of the magnetic layer has been examined. However, with MR heads, when the residual magnetization becomes excessively high, noise increases because the nonlinear portion of the operating curve is employed. Thus, in MR heads, it is known that high residual magnetization is not necessarily associated with improved performance. Accordingly, in the present invention, the residual magnetization &PHgr;r of the magnetic layer is made 5 to 50 mA based on the element thickness and saturation magnetization of the MR head employed, thereby preventing saturation of the head. The residual magnetization &PHgr;r of the magnetic layer preferably ranges from 7 to 40 mA, more preferably from 10 to 35 mA. When residual magnetization exceeds 50 mA, head saturation occurs, and when residual magnetization is less than 5 mA, magnetization is inadequate and adequate reproduction output is precluded.
There are various methods of controlling residual magnetization. For example, when reproducing with an MR head, it is suitable to set the level of residual magnetization to a low value within the above-stated range while increasing the number of particles. In that case, for example, it is suitable that magnetic powder with a &sgr;s of 50 to 130 A·m
2
/kg is employed and the amount of binder in magnetic and nonmagnetic layers is reduced to yield a fill density as high as possible. Specifically, iron alloy power with a &sgr;s of 80 to 130 A·m
2
/kg and hexagonal ferrite, magnetite, cobalt-ferrite, and the like with a &sgr;s of 40 to 80 A·m
2
/kg may be employed.
[The Magnetic Cluster Area Ratio]
It is also widely theoretically known that a high packing density of magnetic microparticles reduces noise. However, particularly when magnetic microparticles are employed, there are problems in that the magnetic particles aggregate, generating a portion behaving just like a large magnetic material and diminishing the S/N ratio. The present inventors discovered that magnetic blocks (referred to hereinafter as “magnetic clusters”) measured by magnetic force microscopy (MFM) were related to medium noise, changing with aggregation and magnetostatic bonding of magnetic particles. Accordingly, they conducted extensive research into the relation between the S/N ratio and the ratio (Sdc/Sac) of the average area Sdc of magnetic clusters under DC magnetized condition to the average area Sac of magnetic clusters under AC erased condition, resulting in the discovery that a good SIN ratio was achieved when the Sdc/Sac ratio fell within a range of 0.8 to 2.0. Accordingly, in the present invention, the range of Sdc/Sac ratio is set to 0.8 to 2.0, preferably 0.8 to 1.7, and more preferably from 0.8 to 1.5. The Sdc/Sac ratio is essentially never less than 0.8, and when it exceeds 2.0, noise increases and a good S/N ratio cannot be achieved.
In the magnetic recording medium of the present invention, the Sdc and the Sac are each preferably from 3,000 to 50,000 nm
2
, more preferably from 4,000 to 40,000 nm
2
, and further preferably from 5,000 to 35,000 nm
2
. The minimum value of the magnetic

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