Stock material or miscellaneous articles – Composite – Of inorganic material
Reexamination Certificate
2001-12-05
2004-08-03
Thibodeau, Paul (Department: 1773)
Stock material or miscellaneous articles
Composite
Of inorganic material
C428S690000, C369S013410, C369S013450, C369S013460
Reexamination Certificate
active
06770387
ABSTRACT:
CROSS-REFERENCE TO RELATED APPLICATION
This application is related to Japanese application No. 2000-327155 filed on Oct. 26, 2000, whose priority is claimed under 35 USC §119, the disclosure of which is incorporated by reference in its entirety.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a magneto-optical recording medium to be embodied as a magneto-optical disk, a magneto-optical tape, a magneto-optical card or the like for use with a magneto-optical recording/reproducing apparatus, and method of reproducing the same.
2. Description of the Related Art
In recent years, magneto-optical recording media have come into limelight as external recording devices for computers. Such a magneto-optical recording medium, which is adapted to form submicron recording bits thereon by application of an external magnetic field and a laser beam, has a greater recording capacity than conventional types of external recording media such as floppy disks and hard disks.
A currently available 3.5-inch magneto-optical recording medium, for example, has 1.1-&mgr;m pitch tracks provided on an area thereof defined between 24-mm radius and 40-mm radius concentric circles, and is adapted to circumferentially write marks of a minimum size of 64 &mgr;m to provide a recording capacity of about 640 MB on each side thereof. The magneto-optical recording medium is a rewritable medium having a very high recording density.
However, the recording capacity should further be increased to record a tremendous amount of data and motion pictures for the upcoming multimedia age. For the increase in the recording capacity, a greater amount of recording marks should be formed on the medium. Therefore, marks having a smaller length should be arranged at smaller intervals than the currently employed marks. For higher density recording with this arrangement, a laser beam to be applied to the medium should have a wavelength smaller than 780 nm or 680 nm. In consideration of the practicality, the reduction in the length of the marks is more effective than the reduction in the wavelength of the laser beam.
Various methods have been proposed for data reproduction from marks having a smaller size than the diameter of the laser beam.
Japanese Unexamined Patent Publication No. HEI 1(1989)-143041, for example, proposed a method called “FAD (Front Aperture Detection) method” (first prior art), which is adapted to read a recording mark in a low temperature region within a laser spot while utilizing a high temperature region as a mask region.
Japanese Unexamined Patent Publications No. HEI 3(1991)-93056 and No. HEI 3(1991)-93058 proposed methods called “RAD (Rear Aperture Detection) method” (second prior art), which is adapted to read a recording mark in a high temperature region within a laser spot while utilizing a low temperature region as a mask region.
Japanese Unexamined Patent Publication No. HEI 4(1992)-271039 proposed a method called “RAD double mask method” (third prior art), which is adapted to read a recording mark in an intermediate region between a low temperature region and a high temperature region within a laser spot while utilizing the low temperature region and the high temperature region as mask regions.
Japanese Unexamined Patent Publication No. HEI 5(1993)-12731 proposed a method called “CAD (Center Aperture Detection) method” (fourth prior art).
The prior art methods described above can read a recording mark in a region having a smaller size than the diameter of a spot of a reproduction laser beam, and provide a resolution substantially equivalent to that provided by reproduction with the use of a light spot smaller in diameter than the spot of the reproduction laser beam.
However, the aforesaid prior art methods have the following drawbacks.
The first prior art method, which is adapted for reproduction in the low temperature region, allows for size reduction of the entire system without the need for provision of an initialization magnet, but is not effective for prevention of crosstalk because recording marks in neighboring tracks may be detected to affect the reproduction.
The second prior art method, which is adapted for reproduction in the high temperature region, is effective for prevention of crosstalk, but does not allow for size reduction of the system with the need for provision of an initialization magnet.
The third prior art method is also effective for prevention of crosstalk, and allows for enhancement of reproduction output. However, it is impossible to reduce the size of the system with the need for provision of an initialization magnet as in the second prior art method.
The fourth prior art method requires no initialization magnet, but fails to provide a high reproduction output because there is a larger transition area in which the orientation of magnetization of a reproduction layer is shifted from an in-plane direction to a perpendicular direction.
Since the prior art methods have the drawbacks described above, the inventors of the present invention have proposed, in Japanese Unexamined Patent Publication No. HEI 7(1995)-244877, a magneto-optical recording medium (fifth prior art) which is capable of providing a magnetic super resolution (MSR) and a high reproduction output without the need for provision of an initialization magnet. An explanation will hereinafter be given to the magneto-optical recording medium according to the fifth prior art.
As shown in
FIG. 10
, the magneto-optical recording medium comprises a reproduction layer
4
, an intermediate layer
5
and a recording layer
6
stacked in this order on a substrate (now shown). The reproduction layer
4
is composed of a rare earth-transition metal amorphous alloy such as GdFeCo, and has a direction of easy magnetization extending perpendicularly thereto. The intermediate layer
5
is composed of a rare earth-transition metal amorphous alloy such as GdFeCo, and has a direction of easy magnetization which extends in an in-plane direction at room temperature but is shifted from the in-plane direction to a perpendicular direction when the layer is heated up to a predetermined temperature by application of a reproduction light beam. The recording layer
6
is composed of a rare earth-transition metal amorphous alloy such as TbFeCo, and has a direction of easy magnetization extending perpendicularly thereto. The reproduction layer
4
, the intermediate layer
5
and the recording layer
6
have Curie temperatures Tc1, Tc2 and Tc3, respectively, which satisfy relationships of Tc2<Tc1 and Tc2<Tc3. Further, the reproduction layer
4
and the recording layer
6
have coercive forces Hc1 and Hc3, respectively, which satisfy a relationship of Hc3>Hc1 at room temperature.
The reproduction layer
4
serves as a mask for reading a signal or for providing a magnetic super resolution. The intermediate layer
5
has an in-plane magnetization property at room temperature and, when the layer is heated, is exchange-coupled to the recording layer
6
, whereby the magnetization direction thereof is copied to the reproduction layer
4
. The recording layer
6
is adapted for thermal magnetic recording which is achieved by heating the layer up to a temperature near its Curie temperature with application of a recording magnetic field for inversion of the direction of the magnetization.
For reproduction of data recorded in the recording layer
6
, smaller size marks are accurately read by utilizing a temperature gradation generated within a laser spot on the medium.
Erasing, recording and reproducing operations to be performed on the magneto-optical recording medium will be explained with reference to
FIGS. 10
to
13
. It is herein assumed that an upward bias magnetic field is applied for recording data and a downward bias magnetic field is applied for reproducing and erasing the data. The explanation will be given on the assumption that the reproduction layer
4
and the recording layer
6
are rich in transition metals (TM-rich) and the intermediate layer
5
is rich in rare earth elements (RE-rich).
As
Mihara Motonobu
Tamanoi Ken
Tanaka Tsutomu
Fujitsu Limited
Greer Burns & Crain Ltd.
Thibodeau Paul
Uhlir Nikolas J.
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