Dynamic information storage or retrieval – Storage or retrieval by simultaneous application of diverse... – Magnetic field and light beam
Reexamination Certificate
2002-04-18
2004-06-08
Neyzari, Ali (Department: 2655)
Dynamic information storage or retrieval
Storage or retrieval by simultaneous application of diverse...
Magnetic field and light beam
C369S013520, C428S064200, C428S690000
Reexamination Certificate
active
06747919
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a magneto-optical recording medium used for recording or reproducing information, a method for producing the same, and an apparatus for producing the same.
2. Description of the Related Art
As a repeatedly rewritable recording medium having a high density, a magneto-optical recording medium and a recording/reproducing apparatus for recording a minute domain onto a magnetic thin film with thermal energy of laser light, and reproducing a signal using a magneto-optical effect are being developed actively. In such a magneto-optical recording medium, when the diameter and interval of recording bits (domains for recording) become smaller with respect to the diameter of a light beam focused onto the medium, reproduction characteristics are degraded. This is caused as follows: an adjacent recording bit enters the diameter of a light beam focused onto an intended recording bit, which makes it difficult to reproduce information from individual recording bits separately.
In order to solve the above-mentioned problem, attempts have been made to enhance a recording density by modifying the configuration of a recording medium and a reproducing method. For example, a super-resolution system, a domain wall displacement detection (DWDD) reproducing system using the displacement of a domain wall, and the like have been proposed. Herein, a DWDD reproducing system disclosed in JP6(1997)-290496 A will be described with reference to FIG.
9
.
In a magneto-optical recording medium shown in
FIG. 9
, a reproducing layer (domain wall displacement layer)
91
, an intermediate layer (switching layer)
92
, and a recording layer
93
that constitute magnetic layers
90
are exchange-coupled to each other, and a minute recording domain of the recording layer
93
is enlarged in the reproducing layer
91
, whereby an amplitude of a reproducing signal is increased, making it possible to conduct high-density recording. Arrows represent the sublattice magnetization directions of transition metal in each layer. In each layer, a domain wall
94
is formed between domains in which magnetization directions are opposite to each other. A region
95
of the intermediate layer
92
reaches a temperature equal to or higher than a Curie temperature due to the irradiation with laser light for reproduction, whereby a magnetic order is lost.
The conditions desired for the above-mentioned magneto-optical recording medium are summarized by the following four points:
(1) The magneto-optical recording medium has the recording layer
93
that holds minute domains stably in a range from a room temperature to a reproducing temperature.
(2) Even when the magneto-optical recording medium is heated to the vicinity of a Curie temperature of the intermediate layer
92
, the reproducing layer
91
, the intermediate layer
92
, and the recording layer
93
are exchange-coupled to each other.
(3) When the intermediate layer
92
reaches a temperature exceeding its Curie temperature so as to lose its magnetic order, exchange coupling between the recording layer
93
and the reproducing layer
91
is cut off.
(4) The domain wall coercive force of the reproducing layer
91
is small, and a domain wall energy gradient is caused by a temperature gradient. Therefore, in a region of the reproducing layer
91
where exchange coupling is cut off by the intermediate layer
92
, the domain wall
94
is displaced from a position transferred from a domain of the recording layer
93
. As a result, the magnetization in this region is aligned in the same direction, and an interval (recording mark length) between the magnetic walls
94
of the recording layer
93
is enlarged.
In
FIG. 9
, when the magneto-optical recording medium is moved (rotated in the case of a disk) in the right direction on the drawing surface while laser light is radiated thereto, due to the high linear velocity of the medium, the position at which a film temperature becomes maximum is placed on the backward side from the center of a beam spot in a traveling direction (left direction on the drawing surface) thereof. A domain wall energy density &sgr;
1
in the reproducing layer
91
generally decreases with an increase in temperature to become 0 at a temperature equal to or higher than a Curie temperature. Therefore, in the presence of a temperature gradient, the domain wall energy density &sgr;
1
is decreased toward a higher temperature side.
Herein, a force F
1
represented by the following expression acts on a domain wall present at a position “x” in a medium movement direction (circumferential direction of a disk).
F
1
∞−d&sgr;
1
/dx
The force F
1
acts so as to displace a domain wall in a direction of lower domain wall energy. In the reproducing layer
91
, a domain wall coercive force is smaller and a domain wall mobility is larger compared with those of the other magnetic layers. Therefore, when exchange coupling from the intermediate layer
92
is cut off, a domain wall is displaced very rapidly in a direction of lower domain wall energy due to the force F
1
.
Referring to
FIG. 9
, in a region of the medium before being irradiated with laser light (e.g., a region at a room temperature), three magnetic layers are exchange-coupled to each other, and domains recorded in the recording layer
93
are transferred to the reproducing layer
91
. In this state, the domain walls
94
are present between domains having magnetization directions opposite to each other in each layer. In the region
95
that reaches a temperature equal to or higher than the Curie temperature of the intermediate layer
92
due to the irradiation with laser light, magnetization of the intermediate layer
92
is lost, and the exchange coupling between the reproducing layer
91
and the recording layer
93
is cut off. Therefore, a force for holding a domain wall is lost in the reproducing layer
91
, and a domain wall is displaced to a higher temperature side due to the force F
1
applied to the domain wall. At this time, a domain wall displacement speed is sufficiently higher than that of the medium movement speed. Thus, a domain larger than a domain stored in the recording layer
93
is transferred to the reproducing layer
91
.
In a magneto-optical recording medium using the DWDD reproducing system, for the purpose of displacing a domain wall easily, the following is proposed: guide grooves having a rectangular cross-section are formed on a substrate so that domain walls are not generated on the side of the recording tracks, whereby the respective tracks are separated by the grooves. However, even if guide grooves having a rectangular cross-section are formed, films actually are accumulated to some degree in stepped portions, and magnetic layers are connected to each other. As a result, magnetic separation cannot be conducted completely, which inhibits the displacement of a domain wall.
SUMMARY OF THE INVENTION
The magneto-optical recording medium of the present invention includes a substrate and a multi-layer film formed on the substrate, the multi-layer film including a first dielectric layer, a domain wall displacement layer, a switching layer, a recording layer, and a second dielectric layer in this order from the substrate side, a Curie temperature of the switching layer being lower than those of the domain wall displacement layer and the recording layer, a domain wall in the domain wall displacement layer being displaced to a higher temperature side in a region that reaches a temperature equal to or higher than a Curie temperature of the switching layer due to irradiation with a light beam for reproduction. In the magneto-optical recording medium of the present invention, the magnetic anisotropy of at least one layer selected from the group consisting of the domain wall displacement layer and the recording layer formed between recording tracks is made lower than that of said layers on the recording tracks, and magnetization of at least one magnetic layer selected from the group consisting
Birukawa Masahiro
Kawaguchi Yuuko
Miyaoka Yasuyuki
Murakami Motoyoshi
Shiratori Tsutomu
Matsushita Electric - Industrial Co., Ltd.
Merchant & Gould P.C.
Neyzari Ali
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