Dynamic information storage or retrieval – Storage or retrieval by simultaneous application of diverse... – Magnetic field and light beam
Reexamination Certificate
2000-01-28
2002-10-08
Dinh, Tan (Department: 2653)
Dynamic information storage or retrieval
Storage or retrieval by simultaneous application of diverse...
Magnetic field and light beam
C369S013380, C428S690000
Reexamination Certificate
active
06463016
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a magneto-optical recording medium such as a magneto-optical disk, magneto-optical tape, a magneto-optical card, etc., for use in a magneto-optical recording and reproducing device.
BACKGROUND OF THE INVENTION
One type of re-writable optical memory medium conventionally implemented for actual use is a magneto-optical disk using a magneto-optical memory medium. In such a magneto-optical disk, a light beam projected by a semiconductor laser is condensed and projected onto the magneto-optical recording medium, thereby raising the temperature of a localized area of the magneto-optical recording medium to perform recording and erasure. Accordingly, recorded information is reproduced by condensing and projecting onto the magneto-optical recording medium a light beam of a strength insufficient to perform recording and erasure, and distinguishing a state of polarization of light reflected therefrom.
However, in this type of magneto-optical recording medium, as the diameter of recorded bits of recorded magnetic domains and the interval between recorded bits becomes small in comparison with the diameter of the light beam spot, reproducing characteristics deteriorate. This is because adjacent recording bits fall within the light beam spot condensed on a target recording bit, thus making it impossible to distinguish and reproduce the individual recorded bits.
One magneto-optical recording medium which has been proposed to solve the foregoing problem is a magneto-optical recording medium structured of a reproducing layer having in-plane magnetization at room temperature and shifting to perpendicular magnetization at temperatures at and above a critical temperature, an in-plane magnetized layer having a Curie temperature in the vicinity of the critical temperature, a non-magnetic intermediate layer, and a recording layer made of a perpendicular magnetized film, for recording information (disclosed in Japanese Unexamined Patent Publication No. 9-320134/1997 (Tokukaihei 9-320134), published on Dec. 12, 1997).
In the foregoing conventional magneto-optical recording medium, the reproducing layer has in-plane magnetization at temperatures below the critical temperature. Accordingly, at temperatures below the critical temperature, the in-plane magnetization of the reproducing layer forms a mask, so that recorded magnetic domain information recorded in the recording layer is not copied to the reproducing layer, and thus the recorded magnetic domain information is not reproduced. At temperatures at and above the critical temperature, however, the reproducing layer shifts to perpendicular magnetization. Accordingly, at temperatures at and above the critical temperature, recorded magnetic domain information is copied to the reproducing layer, and recorded magnetic domain information is reproduced.
With the foregoing structure, even if adjacent recorded bits fall within the light beam spot condensed on the reproducing layer, the individual recorded bits can be distinguished and reproduced, as long as reproducing power of the light beam and the critical temperature at which the reproducing layer shifts to perpendicular magnetization are set appropriately. Consequently, it is possible to reproduce information recorded at high density, i.e., to perform ultra-high resolution reproducing.
The following will explain, with reference to
FIG. 13
, an ultra-high resolution magneto-optical recording medium, which is a magneto-optical recording medium capable of reproducing information recorded at high density.
FIG. 13
is an explanatory drawing showing the principle of ultra-high resolution reproducing operations in a conventional magneto-optical recording medium.
The foregoing conventional ultra-high resolution magneto-optical recording medium is structured of a reproducing layer
101
having in-plane magnetization at room temperature and perpendicular magnetization at temperatures at and above a critical temperature, an in-plane magnetized layer
102
having a Curie temperature in the vicinity of the critical temperature, a non-magnetic intermediate layer
103
, and a recording layer
104
made of a perpendicular magnetized film having a compensation temperature in the vicinity of room temperature.
Reproducing is performed by condensing and projecting a light beam
105
onto the reproducing layer
101
side of the ultra-high resolution magneto-optical recording medium. Condensing and projecting the light beam
105
onto the ultra-high resolution magneto-optical recording medium forms therein a temperature distribution having a Gaussian distribution corresponding to an intensity distribution of the light beam
105
. In accordance with this temperature distribution, the reproducing layer
101
shifts from in-plane to perpendicular magnetization, forming a domain
106
having a temperature at or above the critical temperature and having perpendicular magnetization. Within domains of the reproducing layer
101
retaining in-plane magnetization, an in-plane magnetization mask is formed, and thus a reproducing signal is not produced.
In the domain
106
where the reproducing layer
101
has shifted to perpendicular magnetization, on the other hand, total magnetization is directed in the same direction (up or down) as the direction of magnetic flux leaking from the recording layer
104
. Accordingly, the direction of magnetization of the recording layer
104
is copied to the reproducing layer
101
, and ultra-high resolution reproducing can be realized.
Here, the in-plane magnetized layer
102
, whose Curie temperature is in the vicinity of the critical temperature, is exchange-coupled with the reproducing layer
101
, and is provided in order to strengthen the in-plane magnetization mask in domains of the reproducing layer
101
whose temperature is less than the critical temperature.
As discussed above, in the foregoing conventional ultra-high resolution magneto-optical recording medium, it is preferable to reproduce only the information of the domain
106
having perpendicular magnetization, where the reproducing layer
101
has a temperature at or above the critical temperature.
Here, in order to give the reproducing layer
101
characteristics whereby it has in-plane magnetization at room temperature and shifts to perpendicular magnetization with rising temperature, the composition of the reproducing layer
101
, in contrast to a compensation composition in which moment of rare earth (RE) and transition metal (TM) sub-lattices are the same size in a temperature range at which reproducing is performed, must be an RE-rich composition having more RE sub-lattice moment. In the reproducing layer
101
, the orientation of the TM sub-lattice moment and that of total magnetization are parallel, but are directed in opposite directions, i.e., they are anti-parallel.
The recording layer
104
, on the other hand, is made of an RE-TM alloy having a compensation temperature at room temperature, and in the temperature range at which reproducing is performed, the TM sub-lattice moment is larger than the RE sub-lattice moment. Accordingly, in the recording layer
104
, the orientation of the TM sub-lattice moment and that of total magnetization are parallel, and are directed in the same direction (up or down).
However, in the foregoing conventional ultra-high resolution magneto-optical recording medium, the total magnetization of the in-plane magnetized layer
102
gradually decreases as temperature rises, and in the vicinity of the domain
106
, it is difficult for the in-plane magnetized layer
102
to strengthen the in-plane magnetization mask of the reproducing layer
101
. Consequently, in the vicinity of the domain
106
, even at temperatures below the critical temperature, the reproducing layer
101
is influenced by leaking magnetic flux produced by the recording layer
104
, and the orientation of the magnetization of the reproducing layer
101
tilts with respect to the surface of the layer. Accordingly, when reproducing information from the domain
106
, info
Hirokane Junji
Iwata Noboru
Dinh Tan
Sharp Kabushiki Kaisha
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