Stock material or miscellaneous articles – Composite – Of inorganic material
Reexamination Certificate
2000-06-06
2003-11-11
Rickman, Holly (Department: 1773)
Stock material or miscellaneous articles
Composite
Of inorganic material
C428S690000, C428S213000, C428S336000, C428S611000, C428S668000, C428S900000
Reexamination Certificate
active
06645646
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention generally relates to magnetic recording media and magnetic storage apparatuses, and more particularly to a magnetic recording medium and a magnetic storage apparatus which are suited for high-density recording.
2. Description of the Related Art
Due to the development of the information processing technology, there are increased demands for high-density magnetic recording media. Characteristics required of the magnetic recording media to satisfy such demands include low noise, high coercivity, high remanence magnetization, and high resolution in the case of a hard disk, for example.
The recording density of longitudinal magnetic recording media, such as magnetic disks, has been increased considerably, due to the reduction of medium noise and the development of magnetoresistive and high-sensitivity spin-valve heads. A typical magnetic recording medium is comprised of a substrate, an underlayer, a magnetic layer, and a protection layer which are successively stacked in this order. The underlayer is made of Cr or a Cr-based alloy, and the magnetic layer is made of a Co-based alloy.
Various methods have been proposed to reduce the medium noise. For example, Okamoto et al., “Rigid Disk Medium For 5 Gbit/in
2
Recording”, AB-3, Intermag '96 Digest proposes decreasing the grain size and size distribution of the magnetic layer by reducing the magnetic layer thickness by the proper use of an underlayer made of CrMo, and a U.S. Pat. No. 5,693,426 proposes the use of an underlayer made of NiAl. Further, Hosoe et al., “Experimental Study of Thermal Decay in High-Density Magnetic Recording Media”, IEEE Trans. Magn. Vol. 33, 1528 (1997), for example, proposes the use of an underlayer made of CrTiB. The underlayers described above also promote c-axis orientation of the magnetic layer in a plane which increases the remanence magnetization and the thermal stability of written bits. In addition, proposals have been made to reduce the thickness of the magnetic layer, to increase the resolution or to decrease the width of transition between written bits. Furthermore, proposals have been made to decrease the exchange coupling between grains by promoting more Cr segregation in the magnetic layer which is made of the CoCr-based alloy.
However, as the grains of the magnetic layer become smaller and more magnetically isolated from each other, the written bits become unstable due to thermal activation and to demagnetizing fields which increase with linear density. Lu et al., “Thermal Instability at 10 Gbit/in
2
Magnetic Recording”, IEEE Trans. Magn. Vol. 30, 4230 (1994) demonstrated, by micromagnetic simulation, that exchange-decoupled grains having a diameter of 10 nm and ratio K
u
V/k
B
T~60 in 400 kfci di-bits are susceptible to significant thermal decay, where K
u
denotes the magnetic anisotropy constant, V denotes the average magnetic grain volume, k
B
denotes the Boltzmann constant, and T denotes the temperature. The ratio K
u
V/k
B
T is also referred to as a thermal stability factor.
It has been reported in Abarra et al., “Thermal Stability of Narrow Track Bits in a 5 Gbit/in
2
Medium”, IEEE Trans. Magn. Vol. 33, 2995 (1997) that the presence of intergranular exchange interaction stabilizes written bits, by MFM studies of annealed 200 kfci bits on a 5 Gbit/in
2
CoCrPtTa/CrMo medium. However, more grain decoupling is essential for recording densities of 20 Gbit/in
2
or greater.
The obvious solution has been to increase the magnetic anisotropy of the magnetic layer. But unfortunately, the increased magnetic anisotropy places a great demand on the head write field which degrades the “overwrite” performance which is the ability to write over previously written data.
In addition, the coercivity of thermally unstable magnetic recording medium increases rapidly with decreasing switching time, as reported in He et al., “High Speed Switching in Magnetic Recording Media”, J. Magn. Magn. Mater. Vol. 155, 6 (1996), for magnetic tape media, and in J. H. Richter, “Dynamic Coervicity Effects in Thin Film Media”, IEEE Trans. Magn. Vol. 34, 1540 (1997), for magnetic disk media. Consequently, the adverse effects are introduced in the data rate, that is, how fast data can be written on the magnetic layer and the amount of head field required to reverse the magnetic grains.
On the other hand, another proposed method of improving the thermal stability increases the orientation ratio of the magnetic layer, by appropriately texturing the substrate under the magnetic layer. For example, Akimoto et al., “Relationship Between Magnetic Circumferential Orientation and Magnetic Thermal Stability”, J. Magn. Magn. Mater. (1999), in press, report through micromagnetic simulation, that the effective ratio K
u
V/k
B
T is enhanced by a slight increase in the orientation ratio. This further results in a weaker time dependence for the coercivity which improves the overwrite performance of the magnetic recording medium, as reported in Abarra et al., “The Effect of Orientation Ratio on the Dynamic Coercivity of Media for >15 Gbit/in
2
Recording”, EB-02, Intermag '99, Korea.
Furthermore, keepered magnetic recording media have been proposed for thermal stability improvement. The keeper layer is made up of a magnetically soft layer parallel to the magnetic layer. This soft layer can be disposed above or below the magnetic layer. Oftentimes, a Cr isolation layer is interposed between the soft layer and the magnetic layer. The soft layer reduces the demagnetizing fields in written bits on the magnetic layer. However, coupling the magnetic layer to a continuously-exchanged coupled soft layer defeats the purpose of decoupling the grains of the magnetic layer. As a result, the medium noise increases.
Various methods have been proposed to improve the thermal stability and to reduce the medium noise. However, there was a problem in that the proposed methods do not provide a considerable improvement of the thermal stability of written bits, thereby making it difficult to greatly reduce the medium noise. In addition, there was another problem in that some of the proposed methods introduce adverse effects on the performance of the magnetic recording medium due to the measures taken to reduce the medium noise.
More particularly, in order to obtain a thermally stable performance of the magnetic recording medium, it is conceivable to (i) increase the magnetic anisotropy constant K
u
, (ii) decrease the temperature T or, (iii) increase the grain volume V of the magnetic layer. However, measure (i) increases the coercivity, thereby making it more difficult to write information on the magnetic layer. In addition, measure (ii) is impractical since in magnetic disk drives, for example, the operating temperature may become greater than 60° C. Furthermore, measure (iii) increases the medium noise as described above. As an alternative for measure (iii), it is conceivable to increase the thickness of the magnetic layer, but this would lead to deterioration of the resolution.
SUMMARY OF THE INVENTION
Accordingly, it is a general object of the present invention to provide a novel and useful magnetic recording medium and magnetic storage apparatus, in which the problems described above are eliminated.
Another and more specific object of the present invention is to provide a magnetic recording medium and a magnetic storage apparatus, which can improve the thermal stability of written bits without increasing the medium noise, so as to enable a reliable high-density recording without introducing adverse effects on the performance of the magnetic recording medium, that is, unnecessarily increasing the magnetic anisotropy.
Still another object of the present invention is to provide a magnetic recording medium comprising at least one exchange layer structure and a magnetic layer provided on the exchange layer structure, the exchange layer structure including a ferromagnetic layer and a non-magnetic coupling layer provided on the ferromagnetic layer, and a magnetic bonding
Abarra E. Noel
Mizoshita Yoshifumi
Okamoto Iwao
Umeda Hisashi
Fujitsu Limited
Greer Burns & Crain Ltd.
Rickman Holly
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