Stock material or miscellaneous articles – Structurally defined web or sheet – Including components having same physical characteristic in...
Reexamination Certificate
2000-08-25
2002-11-19
Resan, Stevan A. (Department: 1773)
Stock material or miscellaneous articles
Structurally defined web or sheet
Including components having same physical characteristic in...
C428S216000, C428S323000, C428S328000, C428S329000, C428S690000, C428S690000, C428S900000
Reexamination Certificate
active
06482506
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a magnetic recording medium, especially, a magnetic recording medium comprising two or more magnetic layers and a nonmagnetic layer which are capable of high-density recording.
2. Description of Related Art
Conventionally, as a magnetic recording medium such as a video tape, an audio tape and a magnetic disc, a nonmagnetic support coated with a magnetic layer in which a ferromagnetic iron oxide, a Co-modified ferromagnetic iron oxide, CrO
2
, a ferromagnetic alloy powder or the like are dispersed in a binder is widely used. Due to recent high densification of recording, a recording wavelength tends to become short. If a recording wavelength becomes short, in a relatively thick magnetic layer, problems such as a self-demagnetization loss during recording and a thickness loss during reproducing appear significant, thereby lowering the output level. Therefore, magnetic recording media comprising a thinner magnetic layer are being developed. However, if a magnetic layer is made thinner to have a thickness equal to or less than 2 &mgr;m, the influence of a nonmagnetic support may easily appear on the surface of the magnetic layer, and a deteriorating tendency in electromagnetic characteristics and dropouts are seen.
To solve such problems, there has been proposed a method for forming a thin thickness magnetic layer and a nonmagnetic layer under the magnetic layer, on a nonmagnetic support, by using a simultaneous multilayer coating technique as well as a concentrated coating liquid for the magnetic layer as disclosed in Japanese Unexamined Patent Publication (KOKAI) Showa No.63-191315 and U.S. Pat. No. 4,863,793. By utilizing these techniques, a magnetic recording medium having improved electromagnetic characteristics could be obtainable and production yields were remarkably improved. However, in order to satisfy the recent demands for further high-density recording, development of a magnetic recording medium capable of further high-density recording is desired.
Specifically, investigations for improving a track density by using a high-sensitive magnetoresistive head (MR head) have been done with respect to a flexible medium. In this case, the medium is required to exhibit further high-densification and a lower noise. As an example of the means for obtaining a high-density medium, use of magnetic particles having a large magnetization is known. However, if a sufficient magnetization amount is tried to secure using magnetic particles presently available, a particle volume becomes large, thereby, disadvantageously, a magnetic recording medium having an expected high output cannot be obtained. In addition, in order to lower a noise, it is preferable to lower a volume of magnetic particles. However, reduction of the magnetic particles volume results in lowering a magnetization amount and an output.
SUMMARY OF THE INVENTION
Therefore, it is an object of the present invention to provide a particulate magnetic recording medium having excellent electromagnetic characteristics, especially, a particulate magnetic recording medium which achieves a high output and a low noise and which is suitable for reproduction with an MR head.
The present inventors have diligently studied for characteristics of a magnetic material used for each magnetic layer and a thickness construction of a magnetic recording medium comprising multiple magnetic layers on a lower nonmagnetic layer mainly containing a nonmagnetic powder and a binder, which is formed on a flexible nonmagnetic support. As a result, it was found that such a magnetic recording medium can solve the above-mentioned problems as characterized in that a sum of a thickness d(L) of a lower magnetic layer approximate to a lower nonmagnetic layer (hereinafter referred to as a lowermost magnetic layer) and a thickness d(u) of an upper magnetic layer on a surface side (hereinafter referred to as an uppermost magnetic layer) is in the range of from 0.02 &mgr;m to 0.5 &mgr;m, and an average particle volume V(u) and a saturation magnetization &sgr;s(u) of magnetic particles contained in the uppermost magnetic layer are, respectively, larger than a volume V(L) and saturation magnetization &sgr;s(L) of magnetic particles contained in the lowermost magnetic layer, thereby the present invention was accomplished.
It is to be noted that a medium comprising multiple magnetic layers on a nonmagnetic layer, in which a size of magnetic particles contained in an uppermost magnetic layer is smaller than a size of magnetic particles contained in a lowermost nonmagnetic layer has been known (see U.S. Pat. No. 5,447,782). However, the medium of the present invention is different from the invention disclosed therein. That is, although the above-mentioned invention is a medium suitable for accomplishing a high output and a low noise in a case a low-sensitive inductive head was used, a noise was not necessarily lowered when this medium was reproduced with an MR head. It can be considered that this is because particles having a large volume are located at a deep part of the magnetic layer, and an influence of a noise of the deep part becomes large in using a high-sensitive MR head while a magnetic noise of a deep part can be ignored in a case a conventional inductive head was used.
In the magnetic recording medium of the present invention, it is further effective that a d(u):d(L) is in the ratio of from 2:8 to 8:2, preferably from 3:7 to 7:3, further preferably from 3:7 to 6:4 from the viewpoint that a high output is compatible with a low noise. A preferable ratio depends on a recording wavelength of a recorded signal and an uppermost magnetic layer can be preferably thinner as the wavelength is shorter.
In addition, in the magnetic recording medium of the present invention, it is further effective that each coercive force Hc of an uppermost magnetic layer and a lowermost magnetic layer is equal to or higher than 2000 Oe from the viewpoint of improving an output in the recording with a short wavelength.
As for a magnetic material contained in the magnetic layer, it is preferable that an acicular alloy magnetic powder containing Fe as a main component is used in the uppermost magnetic layer and an acicular magnetic powder containing Fe as a main component or a magnetoplumbite hexagonal ferrite is used in the lowermost magnetic layer from the viewpoint that a high output can be compatible with a low noise.
Moreover, it is preferable that the total weight of a nonmagnetic material which is a solid at an ordinary temperature and contained in the uppermost magnetic layer is larger than the total weight of a nonmagnetic material which is a solid at an ordinary temperature and contained in the lowermost magnetic layer from the viewpoint that a filling density of the lowermost magnetic layer becomes higher and a noise becomes lower, at the same time, running durability is secured. It is to be noted that the nonmagnetic material which is a solid at an ordinary temperature is, for example, a binder containing a hardening agent, an abrasive, carbon black and the other solid lubricant.
MODE FOR CARRYING OUT OF THE INVENTION
A mechanism of the present invention will be explained hereinafter. The present invention is characterized by forming a thin magnetic layer which comprises two or more layers and has the total thickness equal to or less than 0.5 &mgr;m on a nonmagnetic lower layer. By the thin magnetic layer, a self-demagnetization loss is reduced, a reproduction output in high-density recording becomes higher and a resolution property is improved. A role of the nonmagnetic lower layer is the same as that of a conventional medium, such as coating a thin magnetic layer without coating loss and improving a surface smoothing effect by calendering.
The thin magnetic layer is divided into two or more layers, and then magnetic particles which have a high &sgr;s (but a large volume) and easily obtain an output are used in an uppermost magnetic layer on a surface side and magnetic particles which have a low noise
Ejiri Kiyomi
Mori Masahiko
Resan Stevan A.
Stroock & Stroock & Lavan LLP
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