Dynamic optical information storage or retrieval – Optical storage medium structure
Reexamination Certificate
2002-04-25
2004-10-12
Klimowicz, William (Department: 2652)
Dynamic optical information storage or retrieval
Optical storage medium structure
C369S013400
Reexamination Certificate
active
06804822
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a magnetic recording medium suitable for use in a magnetic recording apparatus for magnetic recording of data by (a) heating up (applying a heat onto) a recording region of a magnetic layer (a portion of the magnetic layer) by using a light beam (heat source) and (b) by applying a magnetic field on the recording region, and to the magnetic recording apparatus using the same.
BACKGROUND OF THE INVENTION
Recently, optical memories, such as DVDs (Digital Versatile Discs) and magneto-optical discs, and magnetic memories, such as hard discs, have been significantly improved to have high densities. Especially, an optically assisted magnetic recording/reproducing method has been developed as one of high-density magnetic recording/reproducing methods. For example, U.S. Pat. No. 5,656,385 (registered on Aug. 12, 1997: corresponding to Japanese Unexamined Patent Application (Tokukaihei) No. 4-176034) discloses (a) a magnetic recording medium having a recording layer made of an N-type ferrimagnetic material whose compensation point (magnetic compensation temperature) is substantially at room temperatures, and (b) an optically assisted magnetic recording/reproducing method (hereinafter, referred to as a first prior art) using the same.
For recording in this type of optically assisted magnetic recording/reproducing method, a laser beam heats up (laser heating) a recording region of the magnetic recording medium so as to sufficiently reduce a coercive force, then a recording magnetic head applies an exterior magnetic field on the recording region, thereby recording data. A region in which a recording mark (magnetic bit) is formed during the recording is limited within a region in which a region (laser beam radiation region) on which the laser beam is radiated overlaps a region (magnetic field applied region) on which the magnetic field is applied. Their positional relationship is explained referring to
FIG. 17. A
recording region
113
is a region in which a magnetic field applying region
111
formed by the magnetic head overlaps a heating region
112
(corresponding to a light spot) formed by radiating the laser beam. In the recording region
113
, formed is a recording mark
114
. As a result, it is possible to record, on the magnetic recording medium, a track
115
that has a narrow pitch that is equivalent to a diameter of beam spot of the laser beam (a diameter of the heating region
112
; 0.5 m or less) by using a conventional recording magnetic head of a few m width.
Also for reproducing, the laser beam heats up a reproducing region of the magnetic recording medium so as to intensify residual magnetization, then data is read out of the reproducing region by a reproducing magnetic head. A region that is reproduced during the reproduction is also limited within a region where the region on which the laser beam radiation region overlaps a recording head region (where the recording magnetic head applies a magnetic field). This makes it possible to reproduce the track recorded with the narrow track pitch, by using a reproducing magnetic head having a large width, so that crosstalk will be restrained.
In this manner, the optically assisted magnetic recording/reproducing method, that is the first prior art, uses the laser beam as the heat source so as to selectively heat up a region narrower than the magnetic field applied region. This allows the recording track pitch to be narrower and reduces the crosstalk. For this reason, the recording and reproduction of the first prior art are carried out in high density.
Moreover, the optically assisted magnetic recording/reproducing method uses a magnetic recording medium in which an aluminum nitride (AlN) film of 60 nm is formed as an underlayer on a disc substrate, then a recording layer and a protective layer are formed on the AlN film in this order. The AlN underlayer is provided for preventing the light beam from reflecting and improving efficiency of the heating. In other words, the AlN underlayer is used for improving absorption rate of the light beam (a rate of light absorbed by the recording layer) that comes into the magnetic recording medium, and recording sensitivity.
On the other hand, an article in conference reports, K. OZAKI et al., “TbFeCo as a Perpendicular Magnetic Recording Material”, J. Magn. Soc. Japan, 25, p322-327 (Publication Date: Mar. 15, 2001) (hereinafter, referred to as a second prior art) discloses a perpendicular magnetic recording medium including an underlayer having an uneven structure, for use in a conventional perpendicular magnetic recording method in which the recording is carried out only by using a magnetic head, without radiation of light. With the perpendicular magnetic recording medium, it is possible to prevent magnetic domain walls from moving, that is, to perform pinning (holding) of the magnetic domain walls so as not to move. This increases recording density. Note that an NiP layer is used as the underlayer having the uneven structure in the article.
Furthermore, an article in conference reports, Koji MATSUMOTO et al., k“Perpendicular magnetic recording media using Magneto-Optical media”, the 25th Applied Magnetization Association, p 235 (Publication Date: Sep. 25, 2001), discloses an example in which an NiP layer is used as an underlayer having an uneven structure, as the second prior art. However, this article recited that in reality a carbon layer should be provided between the NiP underlayer and a TbFeCo magnetic layer, so that exchange bonding of TbFeCo and Ni will not occur even when Ni, the magnetic body, is precipitated out.
Moreover, an article in conference reports, H. Kawano, et al., “Effect of Air Gap on Write and Readout Characteristics of Magneto-Optical Media with Solid Immersion Lens”, Technical Digest of Joint Moris/APDSC 2000, p188-189 (Publication Date: Oct. 30, 2000) (hereinafter, referred to as a third prior art), discloses a Magneto-optical recording medium in which an aluminum layer is provided between a glass substrate and a TbFeCo recording layer.
In data recording media, high-density recording is facilitated by increasing a recording frequency (a frequency of magnetic field application in case of magnetic modulation) so as to shorten a shortest length of a recording mark (that length of a recording mark that is along a track, where the recording mark is a minimum unit for data of 1 bit: the length represented by a reference sign M in FIG.
17
).
However, in the optically assisted magnetic recording method recited in Tokukaihei No. 4-176034, the magnetic recording medium used has an insufficient capacity that makes it difficult to form a recording mark having the recording mark of the shortest length of 200 nm or less. This restrains improvement of the recording density in the optically assisted magnetic recording method.
This is based on a result of evaluation of recording and reproducing of the magnetic recording medium used in the publication. In the evaluation, it was observed that quality of signals was suddenly deteriorated when the shortest length of the recording mark approached to near 200 nm. Further, observation of the thus formed recording mark by an MFM (Magnetic Force Microscope) showed that a phenomenon in which individual recording marks are disturbed, for example, a phenomenon in which the recording marks overlap each other, or a phenomenon in which the recording mark is disappeared, was occurred when the shortest length of the recording mark approached to about 200 nm. As to a width of the track, it was observed that the width of the track got narrower and narrower, and finally was narrowed to cause a break-off. Therefore, in the conventional magnetic recording medium, the shortest length of the recording mark is practically 250 nm, considering a reliability of the optically assisted magnetic recording medium.
An exchange interaction force is one of factors that cause instability in shape of the recording mark as described above. As smaller the recording mark is, as a ratio of the exchange interactio
Fuji Hiroshi
Katayama Hiroyuki
Kojima Kunio
Ohta Kenji
Sato Jun-ichi
Conlin David G.
Edwards & Angell LLP
Hartnell, III George W.
Klimowicz William
Sharp Kabushiki Kaisha
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