Dynamic information storage or retrieval – Storage or retrieval by simultaneous application of diverse... – Magnetic field and light beam
Reexamination Certificate
2000-05-05
2002-08-06
Dinh, Tan (Department: 2653)
Dynamic information storage or retrieval
Storage or retrieval by simultaneous application of diverse...
Magnetic field and light beam
C369S013440, C428S690000
Reexamination Certificate
active
06430116
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a magneto-optical recording medium to be used for information recording and a writing and readout method therefor, and a writing and readout apparatus.
2. Discussion of Background
Magneto-optical recording media are media designed to record information in a magnetic layer as a magnetization direction. They are rewritable with high densities and of low costs, and they are used, for example, as information recording media for e.g. external memory apparatus for computers or for apparatus for recording and replaying music. Among them, a magneto-optical recording medium employing a writing layer made of an amorphous alloy of a rare earth metal or a transition metal, exhibits excellent properties. A magneto-optical disk having a large recording capacity is practically available even now, but in view of the ever increasing quantity of information of the society, modification for a larger capacity is desired. The limit in the recording density of a magneto-optical disk is usually determined by the size of the spot of the readout laser beam. The size of the spot can be made smaller, as the wavelength of the laser is shorter. Accordingly, a study has been made for shortening the wavelength of the laser but with difficulty, and use of a short wavelength laser is a factor for a high cost. On the other hand, attempts for so-called super resolution have been made in recent years, in which it is attempted to obtain a resolution higher than the level determined by the wavelength of the laser, by various measures.
As one of such attempts, a system for magnetically induced super resolution (hereinafter referred to also as MSR) has been reported which employs an exchange coupling force among magnetic layers of a multi-layered structure in a magneto-optical disk.
This system consists essentially of a layer for information recording (a writing layer) and a layer for information readout (a readout layer), whereby writing is carried out against the writing layer, and at the time of readout, the magnetization direction of the writing layer is transferred to the readout layer, and the magnetization direction of the readout layer is readout. Usually, the transferability to the readout layer is controlled by the temperature for heating a magnetic layer of the writing layer or the like.
According to this system, the temperature distribution in a readout laser beam spot is utilized to modify the magnetic domain of the readout layer, whereby the waveform interference of readout signals can be reduced, and high density recorded information can be readout with good quality.
The MSR system includes, for example, one utilizing magnet static coupling and one utilizing exchange coupling.
As one of the MSR systems employing magnet static coupling, the present inventors have proposed in JP-A-7-147029 a system so-called “reversal type MSR” which employs a medium comprising mutually exchange-coupled three layers i.e. a readout layer having a small coercivity, a switching layer having a low Curie temperature and a writing layer having a high Curie temperature and a large coercivity.
Here, each of the readout layer and the writing layer is made of an alloy of a transition metal and a rare earth metal. Magnetization of each layer is determined by the sub-lattice magnetization of the rare earth metal and the sub-lattice magnetization of the transition metal.
A composition in which the magnetization of the rare earth metal and the magnetization of the transition metal cancel out each other, is called a compensation composition, and a composition containing the transition metal in a larger amount than the compensation composition is called transition metal magnetization dominant (hereinafter referred to also as transition metal rich or TM rich), whereby the overall magnetization agrees to the transition metal magnetization. A composition containing the rare earth metal in a larger amount than the compensation composition is called rare earth metal magnetization dominant (hereinafter referred to also as rare earth rich or RE rich), whereby the overall magnetization agrees to the rare earth metal magnetization.
In the reversal type MSR system, at a low temperature portion within the spot of readout laser beam, the readout layer and the writing layer are exchange-coupled via the switching layer. In the exchange coupling, the direction of the sub-lattice magnetization of the readout layer agrees to the direction of the sub-lattice magnetization of the writing layer.
On the other hand, at a high temperature portion, the temperature of the switching layer exceeds the Curie temperature of the switching layer, whereby the exchange coupling of the readout layer and the writing layer will be cut off. Accordingly, in the relation between the readout layer and the writing layer, the magnet static coupling tends to be governing, whereby the magnetization direction of the readout layer agrees to the magnetization direction of the writing layer. If both the readout layer and the writing layer are TM rich, the magnetization directions of the readout layer and the writing layer will always be the same.
However, when the dominant magnetizations differ, for example, in a case where the readout layer is RE rich, while the writing layer is TM rich, the magnetization state in the readout layer will be opposite as between a case where the sub-lattice magnetization directions of the readout layer and the writing layer agree to each other and a case wherein the magnetization direction of the readout layer and the writing layer agree to each other.
Namely, when the medium is heated under irradiation with a readout beam, firstly at a low temperature portion, magnetization of the readout layer will appear due to the exchange coupling with the writing layer, and when it is further heated to a high temperature, the exchange coupling is cut off, whereby the magnetization of the readout layer will be reversed.
In high density recording, this reversed magnetization serves to intensify the signal together with the non-reversed adjacent mark, whereby high resolution can be realized.
In a reversal type MSR system, like a magnetic disk, the signal intensity becomes maximum at the edge of the mark (at the boundary of a magnetic domain) rather than the center of the mark (the magnetic domain), whereby the mark edge can easily be detected by detecting the peak position of the signal. Accordingly, there is an economical merit in that readout of a mark length modulation recording signal can be carried out by utilizing an inexpensive signal-detecting circuit which is commonly used in a magnetic disk apparatus.
Further, as another type of a MSR system employing magnet static coupling, a system so-called “static coupled CAD” has been proposed wherein a readout layer which has in-plane magnetization at a low temperature and which becomes a perpendicular magnetization film as the temperature becomes high and as the magnetization becomes small, is employed as the readout layer, a non-magnetic barrier layer is provided between the writing layer and the readout layer, and the magnetization direction of the writing layer is transferred to the readout layer solely by the magnet static coupling force.
In the static coupled CAD system, only at a high temperature, the magnetization of the writing layer will be transferred to the readout layer, and the signal can be readout. This system is excellent in that at a low temperature, no transfer of the magnetization direction of the writing layer will take place, and the layer remains to be an in-plane magnetization film, whereby a so-called low temperature mask will be formed, and the signal interference (cross talk) with the formed adjacent tracks can be minimized.
On the other hand, as a MSR system utilizing only exchange coupling without employing magnet static coupling, a system so-called “exchange-coupled CAD” is available wherein a readout layer which has in-plane magnetization at a low temperature and which becomes a perpendicular magnet
Kawano Toshifumi
Okamuro Akio
Dinh Tan
Mitsubishi Chemical Corporation
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