Stock material or miscellaneous articles – Composite – Of inorganic material
Reexamination Certificate
2002-02-06
2003-12-30
Rickman, Holly (Department: 1773)
Stock material or miscellaneous articles
Composite
Of inorganic material
C428S900000, C428S212000, C428S611000
Reexamination Certificate
active
06670057
ABSTRACT:
BACKGROUND OF THE INVENTION
This application claims the benefit of a Japanese Patent Application No. 2001-272601 filed Sep. 7, 2001, the disclosure of which is hereby incorporated by reference.
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 which is suited for high-density recording and capable of carrying out high-speed recording and reproduction, and to a magnetic storage apparatus which uses such a magnetic recording medium.
2. Description of the Related Art
Due to the developments in information processing technology, there are increased demands for high-density magnetic recording media. For example, for a hard disk, the magnetic recording media required to satisfy such demands should include such characteristics as low noise and improved thermal stability.
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 alloy, and the magnetic layer is made of a Co 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. 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 the 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 the transition between written bits. Furthermore, proposals have been made to decrease the exchange coupling between grains by promoting more Cr segregation in a magnetic layer which is made of the CoCr 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 the 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, as demonstrated 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, 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 that is parallel to the magnetic layer. This soft layer can be disposed either 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 the 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.
In order to improve the thermal stability and to reduce the medium noise, magnetic recording media and magnetic storage apparatuses have been proposed in U.S. patent application Ser. No. 09/425,788 filed Oct. 22, 1999, which is incorporated herein by reference, and in which the assignee is the same as the assignee of this application. This previously proposed magnetic recording medium is comprised of at least one exchange layer structure, and a magnetic layer formed on the exchange layer structure, wherein the exchange layer structure includes a ferromagnetic layer and a non-magnetic coupling layer provided on the ferromagnetic layer and under the magnetic layer, and the ferromagnetic layer and the magnetic layer have antiparallel magnetizations. According to this previously proposed magnetic recording medium, it is possible to improve the thermal stability of the written bits, reduce the medium noise, and realize a high-density recording having a high reliability without adversely affecting the performance of the magnetic recording medium.
In other words, in this previously proposed magnetic recording medium, the non-magnetic coupling layer (or the non-magnetic exchange layer) is interposed between the ferromagnetic layer that forms a first magnetic layer and the magnetic layer that forms a second magnetic layer. When the structure includes first and second magnetic layers having antiparallel magnetizations, the first and second magnetic layers mutually cancel portions of the magnetizations. Hence, it is possible to increase the effective grain size of the magnetic layer without substantially affecting the resolution. Therefore, from the point of view of the grain volume, it is possible to increase the apparent thickness of the magnetic layer so as to realize a magnetic recording medium having a good thermal stability.
Accordingly, this previously proposed magnetic recording medium employs a basic structure made up of the ferromagnetic layer (the first magnetic layer) and the magnetic layer (the se
Fujitsu Limited
Greer Burns & Crain Ltd.
Rickman Holly
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