Magnetic recording medium and method of manufacturing same

Details Magnetic recording medium and method of manufacturing same Magnetic recording medium and method of manufacturing same

2001-06-20

2003-05-06

Resan, Stevan A. (Department: 1773)

Stock material or miscellaneous articles

Structurally defined web or sheet

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

C428S323000, C428S328000, C428S329000, C428S336000, C428S690000, C428S690000

Reexamination Certificate

active

06558772

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to magnetic recording media, and more particularly, to a magnetic recording medium having a magnetic layer and a nonmagnetic layer that is capable of high-density recording, and a method of manufacturing the same.
RELATED ART
Conventionally, magnetic recording media in which a magnetic layer—comprising a ferromagnetic iron oxide, cobalt-modified ferromagnetic iron oxide, CrO
2
, a ferromagnetic alloy powder, or the like, dispersed in a binder—is coated on a nonmagnetic support, have been widely employed as video tapes, audio tapes, and magnetic discs. In recent years, the trend has been toward high density and shorter recording wavelengths, and the problems of self-demagnetization loss, whereby output decreases as the thickness of the magnetic increases, and loss of thickness during reproduction have become substantial. Thus, the magnetic layer has been made thinner. However, when the magnetic layer is thinned to about 2 &mgr;m or less, the nonmagnetic support member tends to affect the surface of the magnetic layer, with a tendency toward deterioration of electromagnetic characteristics and dropout.
Japanese Patent Unexamined Publication Nos. Sho 63-191315 and Sho 63-187418 describe methods for solving this problem by employing a simultaneous multilayer coating method to provide a nonmagnetic lower layer and to thinly apply a highly concentrated magnetic coating liquid thereon. These inventions markedly improve yield and achieve good electromagnetic characteristics. However, even higher density magnetic recording media are needed.
The principal magnetic head has been the inductive head, which uses conventional magnetic induction to record and reproduce. However, in recent years, MR heads exploiting changes in magnetic resistance based on the level of magnetization of the medium have become widespread in the area of high-density digital recording, mostly on hard disks, and are becoming more widespread for tapes, flexible disks, and the like employed in high-density recording.
A large amount of research has been conducted and numerous papers have been written on the various problems associated with magnetic recording employing MR heads. However, the great majority of these researches have been on thin metal film media and there has been little research on particulate media having a thin magnetic layer.
The present inventors conducted extensive research into improving the suitability of particulate recording media having good production properties for MR heads. As a result, they found that when high-density recording with MR heads was conducted on a particulate recording medium having a magnetic layer of not more than 0.1 &mgr;m in thickness manufactured by a simultaneous multilayer coating method, there were problems in that electromagnetic characteristics (reproduction output and CNR) deteriorated.
Accordingly, the object of the present invention is to provide a magnetic recording medium affording good electromagnetic characteristics with MR heads in the form of a particulate recording medium having good production properties and a magnetic layer not greater than 0.1 &mgr;m, and more particularly, to provide a magnetic recording medium affording good recording and reproduction capabilities at short recording wavelengths, as well as good yields.
SUMMARY OF THE INVENTION
The present inventors conducted extensive research into magnetic layer and nonmagnetic layer structures and manufacturing methods achieving the above-stated objects. As a result, they discovered that these objects could be accomplished by a magnetic recording medium which comprises a nonmagnetic layer comprising a nonmagnetic powder and a binder and a magnetic layer comprising a ferromagnetic powder and a binder provided in this order on a flexible nonmagnetic support, wherein said magnetic layer is 0.01-0.1 &mgr;m in thickness and a ratio of an exposed area of said nonmagnetic powder appeared on said magnetic layer surface is equal to or less than 10 percent.
In the magnetic recording medium of the present invention, recording and reproduction capabilities at short recording wavelengths are desirably further improved by employing an acicular alloy powder with a major axis length of not greater than 0.1 &mgr;m or a hexagonal ferrite magnetic powder with a plate diameter of not greater than 40 nm as the ferromagnetic powder incorporated into the magnetic layer.
The use of an MR head at least during reproduction is a prerequisite of the magnetic recording medium of the present invention.
The present invention also relates to the magnetic recording medium obtained by a method of manufacturing a magnetic recording medium exhibiting a ratio of an exposed area of a nonmagnetic powder to be contained in a nonmagnetic layer on the surface of a magnetic layer is equal to or less than 10 percent, which comprises forming a magnetic layer by coating a coating liquid for a nonmagnetic layer on a flexible nonmagnetic support, followed by coating a coating liquid for a magnetic layer while the coated nonmagnetic layer is in a wet-state, wherein said coating liquid for nonmagnetic layer has a yield stress equal to or higher than 100 &mgr;N/cm
2
(10 dyne/cm
2
) and the solid component concentration of said coating liquid for magnetic layer is greater than the solid component concentration of said coating liquid for nonmagnetic layer.
The present invention will be described next.
MR heads achieve greater reproduction output than inductive heads. With conventional inductive heads, research has been conducted into increasing residual magnetization in the magnetic layer; with MR heads, by contrast, it is known that this leads to increased noise and that high residual magnetization is not necessarily related to enhanced performance. In thin metal film media employed in hard disks, research has been conducted into thinning the magnetic layer to several tens of angstroms to reduce magnetization. However, when applying magnetic particles, thinning of the magnetic layer is limited by the size of the magnetic particles. That is, the size at which magnetization of the magnetic particles is thermally stable is the limit to layer-thinning and is about 0.01 &mgr;m. Due to such particularities of particulate media, saturation of the MR head is prevented in the present invention by limiting the thickness of the magnetic layer to not less than 0.01 &mgr;m and not greater than 0.1 &mgr;m.
Reducing the size of the magnetic particles is known to diminish noise and increase the CNR. However, it was found that, when forming on a nonmagnetic layer a thin magnetic layer as set forth above comprising a magnetic powder of micro-particles, the nonmagnetic powder contained in the nonmagnetic layer tends to become exposed on the magnetic layer surface during coating and drying processes, and this exposure of the nonmagnetic powder on the magnetic layer surface compromises electromagnetic characteristics. Accordingly, in the present invention, the ratio of the exposed area of nonmagnetic powder (powder to be incorporated into the nonmagnetic layer) on the magnetic layer surface is controlled to equal to or less than 10 percent.
Further, the reason why the nonmagnetic powder contained in the nonmagnetic layer tends to be exposed on the magnetic layer surface during coating and drying processes is thought to be that the small particle diameter of the magnetic particles causes them to be pushed aside by nonmagnetic particles moving toward the surface during coating and drying processes. To suppress this phenomenon, in the manufacturing method of the present invention, a coating liquid with a large (that is, with a large force required to bring about flowing of the particles) yield stress of not less than 100 &mgr;N/cm
2
or 10 dyne/cm
2
is employed as the nonmagnetic layer coating liquid, and the solid component concentration of the magnetic layer coating liquid is made higher than that of the nonmagnetic layer, thereby increasing resistance of the magnetic layer to movement of nonmagnetic powder in the no

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