Stock material or miscellaneous articles – Structurally defined web or sheet – Including components having same physical characteristic in...
Reexamination Certificate
2000-07-26
2002-04-16
Thibodeau, Paul (Department: 1773)
Stock material or miscellaneous articles
Structurally defined web or sheet
Including components having same physical characteristic in...
C428S213000, C428S690000, C428S690000, C428S690000
Reexamination Certificate
active
06372330
ABSTRACT:
TECHNICAL FIELD
This invention relates generally to magnetic recording media, and more particularly to thermally stable high density media.
BACKGROUND OF THE INVENTION
Conventional magnetic recording media, such as the magnetic recording disks in hard disk drives, typically use a granular ferromagnetic layer, such as a sputter-deposited cobalt-platinum (CoPt) alloy, as the recording medium. Each magnetized domain in the magnetic layer is comprised of many small magnetic grains. The transitions between magnetized domains represent the “bits” of the recorded data. IBM's U.S. Pat. Nos. 4,789,598 and 5,523,173 describe this type of conventional rigid disk.
As the storage density of magnetic recording disks has increased, the product of the remanent magnetization Mr (the magnetic moment per unit volume of ferromagnetic material) and the magnetic layer thickness t has decreased. Similarly, the coercive field or coercivity (H
c
) of the magnetic layer has increased. This has led to a decrease in the ratio Mrt/H
c
. To achieve the reduction in Mrt, the thickness t of the magnetic layer can be reduced, but only to a limit because the layer will exhibit increasing magnetic decay, which has been attributed to thermal activation of small magnetic grains (the superparamagnetic effect). The thermal stability of a magnetic grain is to a large extent determined by K
u
V, where K
u
is the magnetic anisotropy constant of the layer and V is the volume of the magnetic grain. As the layer thickness is decreased, V decreases. If the layer thickness is too thin, the stored magnetic information will no longer be stable at normal disk drive operating conditions.
One approach to the solution of this problem is to move to a higher anisotropy material (higher K
u
). However, the increase in K
u
is limited by the point where the coercivity H
c
, which is approximately equal to K
u
/Mr, becomes too great to be written by a conventional recording head. A similar approach is to reduce the Mr of the magnetic layer for a fixed layer thickness, but this is also limited by the coercivity that can be written. Another solution is to increase the intergranular exchange, so that the effective magnetic volume V of the magnetic grains is increased. However, this approach has been shown to be deleterious to the intrinsic signal-to-noise ratio (SNR) of the magnetic layer.
Magnetic recording media with high intrinsic SNR (low intrinsic media noise) is desirable because it is well known in metal alloy media, such as CoPt alloys, that the intrinsic media noise increases with increasing linear recording density. Media noise arises from irregularities in the magnetic transitions and results in random shifts of the readback signal peaks. These random shifts are referred to as “peak jitter” or “time jitter”. Thus higher media noise leads to higher bit error rates. It is therefore desirable to develop a thin film metal alloy magnetic media that generates noise below a maximum acceptable level so that data can be recorded at maximum linear density. It is known that substantially improved SNR can be achieved by replacing a single magnetic layer with a laminated magnetic layer of two (or more) separate magnetic layers that are spaced apart by an nonmagnetic spacer layer. This discovery was made by S. E. Lambert, et al., “Reduction of Media Noise in Thin Film Metal Media by Lamination”,
IEEE Transactions on Magnetics,
Vol. 26, No. 5, September 1990, pp. 2706-2709, and subsequently patented in IBM's U.S. Pat. No. 5,051,288. The reduction in media noise by lamination is believed due to a decoupling of the magnetic interaction or exchange coupling between the magnetic layers in the laminate. The use of lamination for noise reduction has been extensively studied to find the favorable spacer layer materials, including Cr, CrV, Mo and Ru, and spacer layer thicknesses, from 5 to 400 Å, that result in the best decoupling of the magnetic layers, and thus the lowest media noise. This work has been reported in papers by E. S. Murdock, et al., “Noise Properties of Multilayered Co-Alloy Magnetic Recording Media”,
IEEE Transactions on Magnetics,
Vol. 26, No. 5, September 1990, pp. 2700-2705; A. Murayama, et al., “Interlayer Exchange Coupling in Co/Cr/Co Double-Layered Recording Films Studied by Spin-Wave Brillouin Scattering”,
IEEE Transactions on Magnetics,
Vol. 27, No. 6, November 1991, pp. 5064-5066; and S. E. Lambert, et al., “Laminated Media Noise for High Density Recording”,
IEEE Transactions on Magnetics,
Vol. 29, No.1, January 1993, pp. 223-229. U.S. Pat. No. 5,462,796 and the related paper by E. Teng et al., “Flash Chromium Interlayer for High Performance Disks with Superior Noise and Coercivity Squareness”,
IEEE Transactions on Magnetics,
Vol. 29, No. 6, November 1993, pp. 3679-3681, describe a laminated low-noise disk that uses a discontinuous Cr film that is thick enough to reduce the exchange coupling between the two magnetic layers in the laminate but is so thin that the two magnetic layers are not physically separated.
What is needed is magnetic recording media that will support very high density recording while retaining good thermal stability and SNR.
SUMMARY OF THE INVENTION
The invention is a magnetic recording medium wherein the magnetic recording layer is at least two ferromagnetic films antiferromagnetically (AF) coupled together across a nonferromagnetic spacer film. Because the magnetic moments of the two antiferromagnetically-coupled films are oriented antiparallel, the net remanent magnetization-thickness product (Mrt) of the recording layer is the difference in the Mrt values of the two ferromagnetic films. This reduction in Mrt is accomplished without a reduction in the thermal stability of the recording medium because the volumes of the grains in the antiferromagnetically-coupled films add constructively. The medium also enables much sharper magnetic transitions to be achieved with reduced demagnetization fields, resulting in a higher linear bit density for the medium. In one embodiment the magnetic recording medium comprises two ferromagnetic films, each a granular film of a sputter deposited CoPtCrB alloy, separated by a Ru spacer film having a thickness to maximize the antiferromagnetic exchange coupling between the two CoPtCrB films. One of the ferromagnetic films is made thicker than the other, but the thicknesses are chosen so that the net moment in zero applied magnetic field is low, but nonzero.
The AF-coupled magnetic recording layer of the present invention, i.e., at least two ferromagnetic films antiferromagnetically coupled together across a nonferromagnetic spacer film, can be used as the individual magnetic layers in the laminated medium described in the above-cited 288 patent to produce a medium with both thermal stability and low intrinsic media noise.
For a fuller understanding of the nature and advantages of the present invention, reference should be made to the following detailed description taken together with the accompanying figures.
REFERENCES:
patent: 4789598 (1988-12-01), Howard et al.
patent: 5051288 (1991-09-01), Ahlert et al.
patent: 5408377 (1995-04-01), Gurney et al.
patent: 5462796 (1995-10-01), Teng et al.
patent: 5465185 (1995-11-01), Heim et al.
patent: 5523173 (1996-06-01), Doerner et al.
patent: 6153320 (2000-11-01), Parkin
S. S. P. Parkin, et al., “Oscillations in Exchange Coupling and Magnetoresistance in Metallic Superlattice Structures: Co/Ruy, Co/Cr, and Fe/Cr” —Cal Review Letters, vol. 64, No. 19, May 7, 1990, pp. 2304-2307.
S.E. Lambert, et al., “Reduction of Media Noise in Thin Film Metal Media by Lamination” —IEEE Transactions on Magnetics, vol. 26, No. 5, Sep. 1990, pp. 2706-2708.
E. S. Murdock, et al., “Noise Properties of Multilayered Co-Alloy Magnetic Recording Media”—IEEE Transactions on Magnetics, vol. 26, No .5, Sep. 1990, pp. 2700-2705.
A. Murayama, et al., “Interlayer Exchange Coupling in Co/Cr/Co Double-Layered Recording Films Studied by Sping-Wave Brillouin Scattering”—IEEE Transactions on Magnetics, vol. 27, No .6, Nov. 1991, pp.
Do Hoa Van
Fullerton Eric Edward
Margulies David Thomas
Rosen Hal Jervis
Berthold Thomas R.
International Business Machines - Corporation
Rickman Holly C.
Thibodeau Paul
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