Stock material or miscellaneous articles – Composite – Of inorganic material
Reexamination Certificate
2001-03-15
2004-08-10
Rickman, Holly (Department: 1773)
Stock material or miscellaneous articles
Composite
Of inorganic material
C428S212000, C428S213000, C428S336000, C428S611000, C428S667000, C428S900000, C360S097010
Reexamination Certificate
active
06773833
ABSTRACT:
CROSS-REFERENCE TO RELATED APPLICATION
The present application is based on japanese priorty application No. 2000-301466 filed Sep. 29, 2000, the entire contents of which are hereby incorporated by reference.
BACKGROUND OF THE INVENTION
The present invention generally relates to magnetic storage of information and more particularly to a magnetic storage medium for high-density magnetic recording and a magnetic storage device that uses such a magnetic storage medium.
With the progress of information processing technology, the demand for magnetic storage devices having higher recording density is increasing. This demand for increased recording density is particularly acute in a magnetic disk device called hard disk drive that uses a rigid magnetic disk for storing information. Thus, intensive efforts are being made for increasing the recording density of magnetic storage media, and various proposals have been made so far.
Meanwhile, there is a requirement in such high-density magnetic storage media in that a magnetic signal can be reproduced therefrom with low medium noise. Further, the high-density magnetic storage media are required to have a high thermal stability.
As noted before, considerable progress have been made with regard to the improvement of recording density as far as the art of horizontal or lengthwise magnetic recording is concerned. These progresses include development of low-noise magnetic media and high-sensitivity magnetic head such as GMR (giant magneto-resistive) head or spin-valve head.
A typical high-density magnetic recording medium includes a foundation layer formed on a substrate and a magnetic layer is provided on the foundation layer as a recording layer, wherein the magnetic layer is generally formed of a Co alloy layer while the foundation layer may be formed of a Cr layer or a Cr-alloy layer.
Various proposals have been made for reducing the medium noise of high-density magnetic storage media. For example, Okamoto, et al., “Rigid Disk Medium for 5 Gbit/in
2
Recording,” AB-3, Intermag. '96 Digest, describes an approach that achieves the reduction of the medium noise by way of using a CrMo alloy for the foundation layer. By using a CrMo alloy in the foundation layer, it becomes possible to reduce the thickness of the magnetic layer, while the use of such a thin magnetic layer enables a reduction of particle size and particle size variation in the magnetic layer.
Further, the U.S. Pat. No. 5,693,426 describes the use of a foundation layer formed of NiAl. Further, Hosoe, et al., “Experimental Study of Thermal Decay in High-Density Magnetic Recording Media,” IEEE Trans. Magn. vol. 33, 1528, 1997, describes the use of a CrTi alloy for the foundation layer of the magnetic storage media for reducing the medium noise.
The composition of the foundation layer noted above is effective for facilitating in-plane alignment of crystal orientation in the magnetic layer, while such an improvement of crystal orientation in the magnetic layer has an effect of increasing the remnant magnetization and thermal stability of a magnetized recording bit. Furthermore, a decrease of the thickness of the magnetic layer contributes to the improvement of resolution at the time of reading.
Further, investigations are made for reducing the width of the transition region between the recording bits, and further for reducing exchange coupling between the magnetic particles in the magnetic layer by causing segregation of Cr to the grain boundary of the CoCr alloy crystals.
On the other hand, there is a tendency that the thermal stability of the magnetic recording dots formed in the magnetic layer is degraded with decreasing particle size of the magnetic crystals in the magnetic layer, as such a decrease of the particle size facilitates mutual isolation of the magnetic crystals in the magnetic layer. In view of the fact that the demagnetization effect, which is caused in association with the formation of magnetic dots in the magnetic layer, increases with increasing linear density of the magnetic recording dots on the magnetic storage medium, the high-density magnetic storage medium having such ultrafine magnetic particles in the magnetic layer becomes extremely susceptible to thermal agitation.
According to Lu, et al., “Thermal Instability at 10 Gbit/in
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Magnetic Recording,” IEEE Trans. Magn. vol. 30, pp. 4230, 1994, it was demonstrated, by way of micro-magnetic simulation, that a magnetic storage medium containing magnetic particles having a diameter of 10 nm in the magnetic layer experiences an extensive thermal decay when a signal is recorded with a linear recording density of 400 kfci (fci: flux-change per inch), provided that the anisotropy constant Ku is set so as to satisfy the relationship Ku•V/kB•T~60 for suppressing the exchange coupling between the magnetic particles. Here, Ku is a constant representing the magnetic anisotropy, V represents an average mass of the magnetic particles, kB represents the Boltzmann's constant, and T represents the temperature. The foregoing quantity Ku•V/kB•T is also called thermal stability coefficient.
Abarra et al., “Thermal Stability of Narrow Track Bits in a 5 Gbit/in
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Medium,” IEEE Trans. Magn. vol. 33, pp. 2995, 1997, on the other hand, reports that the existence of inter-particle exchange interaction improves the thermal stability of the magnetic dots, based on MFM (magnetic force microscopic) analysis of a CoCrPtTa/CrMo medium designed for a recording density of 5 Gbit/in
2
, for the case the magnetic layer is recorded with a linear density of 200 kfci.
When the linear recording density exceeds the foregoing value of 200 kfci, on the other hand, it was indicated that suppressing of the inter-particle magnetic coupling is necessary. One solution to deal with this problem would be to increase the magnetic anisotropy of the magnetic layer. However, this approach raises the problem that the magnetic head is subjected to excessive load at the time of writing of information.
It is known that the coercive force of a thermally unstable magnetic medium increases rapidly with decrease of the switching time. This is reported for a magnetic tape medium by Ho et al., “High Speed Switching in Magnetic Recording Media,” J. Magn. Mang. Mater. vol. 155, pp. 6, 1999, and for a magnetic disk medium by Richter, J. H., “Dynamic Coercivity Effect in Thin Film Media,” IEEE Trans. Magn. vol. 34, pp. 1540, 1999. Such a dynamic change of the coercive force causes an adversary effect at the time of high-speed writing in that the magnitude of the magnetic field induced by the magnetic head for writing information has to be increased with decreasing switching time.
Meanwhile, there is a proposal to improve the thermal stability of magnetic storage medium by applying a suitable texture processing to the substrate located underneath the magnetic layer. By applying such a texture processing, the alignment of magnetic crystals in the magnetic layer is improved. For example, Akimoto, et al., “Magnetic Relaxation in Thin Film Media as a Function of Orientation,” J. Magn. Magn. Mater., 1999, discovered, based on micromagnetic simulation, that the effective value of the term Ku•V/kB increases in response to slight increase of the crystal alignment in the magnetic layer. Based on this discovery, Abarra, et al. could successfully reduce the time-dependence of the coercive force and improve the overwrite performance of the magnetic medium, as is reported in Abarra, et al., “The Effect of Orientation Ratio on the Dynamic Coercivity of Media for >15 Gbit/in
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Recording,” EB-02, Intermag. '99, Korea.
Further, there is a proposal of the use of a keeper layer for improving the thermal stability of a magnetic storage medium further.
A keeper layer comprises a soft magnetic layer provided parallel to the magnetic layer above or below the magnetic layer, wherein the soft magnetic layer functions to reduce the demagnetization field of the magnetic bits recorded in the magnetic layer. Typically, a Cr magnetic insulation layer is interpos
Abarra E. Noel
Acharya B. Ramamurthy
Inomata Akihiro
Fujitsu Limited
Greer Burns & Crain Ltd.
Rickman Holly
LandOfFree
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