Magnetic recording medium

Details Magnetic recording medium Magnetic recording medium

2001-03-02

2003-08-05

Kiliman, Leszek (Department: 1773)

Stock material or miscellaneous articles

Structurally defined web or sheet

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

C428S212000, C428S216000, C428S336000, C428S425900, C428S690000, C428S690000, C428S900000

Reexamination Certificate

active

06602576

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a magnetic recording medium, particularly high-density magnetic recording media for recording/reproducing systems having a small bit area, i.e. a short recording wavelength and a narrow track width.
2. Description of Related Art
Particulate magnetic recording media comprising a magnetic layer consisting of a powder of ferromagnetic iron oxide, Co-modified ferromagnetic iron oxide, CrO
2
, ferromagnetic alloy or the like dispersed in a binder applied on a nonmagnetic substrate have widely been used as magnetic recording media such as video tapes, audio tapes, magnetic disks or the like. Recently, there is a tendency toward shorter recording wavelength with higher densification, resulting in the aggravation of the problems of self-demagnetization loss during recording leading to output loss with thick magnetic layers and thickness loss during reproducing. Thus, thinner magnetic layers have been made. However, magnetic layers having a small thickness of about 2 &mgr;m or less are susceptible to the influence of the nonmagnetic substrate on their surfaces to deteriorate electromagnetic characteristics or aggravate dropouts.
As a possible solution to this problem, simultaneous multilayer coating was proposed and commercialized wherein a nonmagnetic layer is applied on a substrate and a thin layer of a concentrated magnetic coating solution is applied thereon to form a magnetic layer (JP-A Nos. 191315/88 and 187418/88). These inventions dramatically improved the yield of particulate magnetic recording media to attain good electromagnetic characteristics. However, there is a demand for magnetic media having a further higher density.
High densification was generally achieved by reducing the bit area, i.e. shortening the recording wavelength and narrowing the track width. However, both means of shortening the recording wavelength and narrowing the track width invite an output loss to make it difficult to ensure C/N. In recent tape systems, attempts were made to ensure C/N in the case of shortening the recording wavelength or narrowing the track width by adopting a laminated head or MR head to achieve a high output.
In order to provide a magnetic recording medium compatible for high densification, attempts were made to further reduce the thickness of the magnetic layer or to enhance the dispersivity of the ferromagnetic powder. However, it was found that as the recording wavelength becomes shorter, the ferromagnetic powder in the form of a rod or loop larger than the bit length aggregates to develop magnetic pinholes (defects of magnetic development).
It was also found that thin magnetic layers of 0.1 &mgr;m or less involve magnetic layer defects, i.e. the powder of the lower layer is partially exposed or magnetic pinholes appear as a result of the presence of nonmagnetic materials (solid lubricants such as carbon blacks or abrasives such as alumina) or spaces occurring in the surroundings.
The presence of such magnetic pinholes invited a fluctuation in bit amplitude due to noise increase or output loss, leading to an increase in error rate.
It is an object of the present invention to provide a magnetic recording medium for digital recording/reproducing systems having a small bit area, i.e. a short recording wavelength and a narrow track width, which is a particulate medium showing good electromagnetic characteristics.
SUMMARY OF THE INVENTION
As a result of careful studies of the structures or magnetic characteristics of magnetic and nonmagnetic layers to solve the above problems, we succeeded in preparing a magnetic recording medium showing good electromagnetic characteristics in MR heads as described below.
Accordingly, the present invention provides 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 provided in this order on a nonmagnetic substrate wherein said magnetic layer has an average thickness of 0.01-0.1 &mgr;m and a ratio of a magnetic recording defects area to a total magnetic recording area is in the range of from 0.2 to 20% in a case recording is made at the shortest wavelength in a recording system.
In the present invention, said ferromagnetic powder is preferably a ferromagnetic metal alloy powder having a major axis length of 0.1 &mgr;m or less and an average particle volume of 25000 nm
3
or less.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
When used with an MR head tested herein, the magnetic layer preferably has a thickness in the range defined above with &phgr;r (residual magnetic flux) being 50-250 G. &mgr;m from the viewpoint of narrowing the half width (PW50) without inducing saturation. However, the lower limit of &phgr;r is defined to ensure output.
In magnetic recording media of the present invention, the magnetic layer has an average thickness of 0.01-0.1 &mgr;m. If the average thickness of the magnetic layer is less than 0.01 &mgr;m, the average number of ferromagnetic powder particles in the thickness direction is one or less. That is, the area of defects in the magnetic layer increases to cause a marked output loss. If the average thickness of the magnetic layer exceeds 0.1 &mgr;m, PW50 (the pulse width at 50% of the peak of an isolated reproducing wave) increases to lower high-density recording characteristics.
When a magnetic recording medium having a magnetic layer based on a ferromagnetic metal alloy powder is used for reproducing in a system using an MR head, the MR head is saturated by various factors of the MR head (saturated magnetic flux density and thickness of the MR head and saturated magnetic flux density and thickness of the SAL film) to lower C/N if the Br (residual magnetic flux density) is 0.25 T or more.
The magnetic layer preferably has an average thickness of 0.02-0.08 &mgr;m.
For example, the following methods can be used to reduce the thickness of the magnetic layer to 0.1 &mgr;m or less.
(1) A method of forming a nonmagnetic layer and a magnetic layer by simultaneous multilayer coating on a nonmagnetic substrate is adopted while decreasing the amount of binders in both magnetic and nonmagnetic layers with the binder amount in the magnetic layer <the binder amount in the nonmagnetic layer. In addition, the solid contents in the magnetic coating solution and the nonmagnetic coating solution are lowered. The reduction of binder amounts and solid contents in the coating solutions suppresses elastic behaviors of the magnetic and nonmagnetic solutions and prevents repulsion of the solutions immediately after coating die slot during high-speed coating to attain a thin magnetic layer.
(2) In addition, the thixotropy of the nonmagnetic solution can be increased to inhibit turbulence at the interface between the magnetic layer and the nonmagnetic layer due to the above reduction of solids. In this case, the amount of carbon blacks on the nonmagnetic layer formulation can be increased or the nature or particle diameter of the main nonmagnetic powder in the nonmagnetic layer can be selected to lower the viscosity under high shearing while increasing the viscosity under low shearing.
If the thickness of the magnetic layer is simply reduced to 0.1 &mgr;m or less, however, the magnetic particle density varies to increase noise and lower C/N because of failures caused by defects in the magnetic layer even by simultaneous multilayer coating. Therefore, the present invention defined that the ratio of the magnetic recording defects area to the total magnetic recording area (hereinafter may be referred to as the area ratio of magnetic development defects) is 0.2-20%. Provided that the area ratio of magnetic development defects to be considered in this invention is that estimated in a case recording is made at the shortest wavelength in a recording system. The magnetic recording defects area is recognizable upon magnetic development as illustrated in Example and from the results, the area ratio can be obtained by comparison with the total magnetic

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