Stock material or miscellaneous articles – Web or sheet containing structurally defined element or... – Including a second component containing structurally defined...
Reexamination Certificate
2001-04-06
2004-12-28
Ahmed, Sheeba (Department: 1773)
Stock material or miscellaneous articles
Web or sheet containing structurally defined element or...
Including a second component containing structurally defined...
C428S336000, C428S690000, C428S690000, C428S690000, C428S690000, C427S541000, C427S544000, C427S547000, C427S548000, C427S550000
Reexamination Certificate
active
06835450
ABSTRACT:
CROSS-REFERENCE TO RELATED APPLICATIONS
The present application claims priority under 35 U.S.C. § 119 of German Patent Application No. P 100 17 489.2, filed Apr. 7, 2000, the disclosure of which is expressly incorporated by reference herein in its entirety.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a magnetic recording medium, in particular a multilayer magnetic recording medium having a thin upper magnetic recording layer which has good electromagnetic recording properties in the entire recording wavelength range, a flat frequency response and good mechanical properties, and a process for its production.
2. Discussion of Background Information
Magnetic recording media, for example magnetic tapes, floppy disks or magnetic cards, have long been known for the recording of analog or digital audio and video signals or for data recording.
There has for some years been an increasing demand for media which are suitable for increasingly high recording densities, i.e., which are capable of storing large quantities of information in a very small space. Increasingly small thicknesses of the recording layer and magnetic pigments having increasingly small dimensions and increasingly high coercive forces are essential for this purpose.
Binder-containing double-layer magnetic media in which production difficulties, arising from the reduction in the thickness of the upper recording layer, and mechanical stability problems can be compensated by a relatively thick lower layer without having to dispense with a high recording density became established some time ago. They have higher stability and better productivity during manufacture compared with the magnetic recording media of the ME (metal evaporated) type, which are also known and are very suitable for high recording densities.
For information storage, they have a very smooth magnetic upper layer which is generally less than 0.5 &mgr;m thick and comprises magnetic pigments having a high coercive force, preferably metal pigments or pigments comprising metal alloys. A thicker, likewise pigmented, binder-containing lower layer is present between the generally nonmagnetic substrate and the magnetic recording layer.
The lower layers used here are layers which contain both magnetic and nonmagnetic inorganic pigments, as well as layers which contain only nonmagnetic pigments in addition to binders and conventional additives. The recent developments of magnetic recording media have concentrated mainly on the refinement of this concept of metal pigment magnetic media in double-layer technology. For example, it was proposed to use novel metal pigments having a higher coercive force, improved uniformity and dispersibility (S. Hisano and K. Saito, Research and Development of Metal Powder for Magnetic Recording, J. Magn. Mat., 190 (1998), 371-381). By means of this technology, it was possible to achieve the quality of ME recording media with a 100 nm thick upper-layer containing metal powder and binder. However, the upper layer containing metal powder is still relatively thick compared with sputtered hard disk layers, in spite of its reduction in layer thickness. As a result of this, the layers containing metal powder have large stray magnetic signal fields at long recording wavelengths.
The magnetoresistive heads used for modern recording media and high recording densities were however originally developed for hard disks and have high sensitivity. Consequently, large stray magnetic signal fields can lead to saturation problems on the heads, with the result that weaker signal levels can no longer be read or signal distortions occur.
As a solution to this problem, it was proposed to use extremely thin upper layers having a thickness of only 55 nm. However, it is not clear whether such thin coatings can be produced at all in the case of recording media containing particulate metal powder. At a typical metal pigment particle thickness of about 20 nm, only three particles at most would fit one on top of the other in such layers, which would lead to considerable technical difficulties in the production process.
The usual way of solving this problem for layer thicknesses which can currently be produced is to write additional pulses for long wavelengths at which head saturation and, associated therewith, signal distortion would occur, i.e., to perform a write equalization.
At a high recording speed, however, the head and the electronics of the recording apparatus are subject to a considerable load as a result of this process, since a very much higher switching frequency of the write field is required. To date, the literature has not described any method of solving the saturation problem in magnetoresistive read heads for particulate recording media in such a way that cheap products can be produced in large quantities thereby.
SUMMARY OF THE INVENTION
The present invention relates to providing a magnetic recording medium which has good electromagnetic properties in the entire recording wavelength range and good mechanical properties, is suitable for recording digital data and can be produced using conventional technologies and in which the problem of head saturation does not occur.
The present invention also relates to providing a process for the production of such a magnetic recording medium.
According to the present invention, there is provided a multilayer magnetic recording medium which comprises, on a nonmagnetic substrate, at least one upper binder-containing magnetic recording layer which has a thickness of less than 0.5 &mgr;m and contains a finely divided magnetic pigment having a coercive force H
c
of 80-250 kA/m, and at least one lower binder-containing layer which contains an isotropic magnetically soft pigment which is selected from &ggr;-Fe
2
O
3
, Fe
3
O
4
and a solid solution of these components, the coercive force H
c
of the lower layer being less than 4 kA/m and the anhysteretic susceptibility of the lower layer at 2 kA/m being greater than 7.
Furthermore, the present invention is directed to a process for the production of a multilayer magnetic recording medium, which comprises:
mixing, kneading and dispersing an isotropic magnetically soft pigment which is selected from &ggr;-Fe
2
O
3
, Fe
3
O
4
and a solid solution of these components, a binder, a solvent and further additives and applying the dispersion to a nonmagnetic substrate, a lower layer forming;
mixing, kneading and dispersing a finely divided magnetic pigment having a coercive force H
c
of 80-250 kA/m with a binder, a solvent and further additives and applying the dispersion to the lower layer, an upper magnetic recording layer forming;
orienting the moist layers in a magnetic field;
drying the moist layers until the upper layer reaches a thickness of less than 0.5 &mgr;m; and
subsequently calendering and separating,
so that the coercive force of the lower layer is less than 4 kA/m and the anhysteretic susceptibility of the lower layer at 2 kA/m is greater than 7.
The coercive force H
c
of the pigment in the upper layer can be from 130 to 220 kA/m.
The magnetic pigment in the upper layer can be a metal pigment or metal alloy pigment.
The magnetic pigment in the upper layer can be a hexagonal ferrite pigment or a Co-modified &ggr;-Fe
2
O
3
, a Co-modified Fe
3
O
4
or a solid solution of these components.
The isotropic magnetically soft pigment in the lower layer can have a mean crystallite size of from 7 to 17 nm.
The amount of the magnetically soft pigment in the lower layer can be more than 45% by weight, or more than 75% by weight, based on the weight of all pigments in the lower layer.
The magnetically soft pigment in the lower layer can be surface-treated with an aluminum compound or with a silicon compound or with a mixture of the two compounds.
The magnetic pigment in the lower layer can be spherical, cubic or amorphous.
The lower layer can contain at least one nonmagnetic pigment in addition to the magnetically soft pigment. The nonmagnetic pigment can be acicular, having a mean longitudinal axis of from 5 to 200 nm, or s
Heilmann Peter
Ilmer Andreas
Jakusch Helmut
Kohl Albert
Kress Ria
Ahmed Sheeba
Imation Corp.
Levinson Eric D.
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