Dynamic information storage or retrieval – Storage medium structure – Optical track structure
Patent
1991-06-26
1993-11-02
Dzierzynski, Paul M.
Dynamic information storage or retrieval
Storage medium structure
Optical track structure
369 13, 369 59, 369110, 369121, 3692753, 360131, 360135, G11B 1110
Patent
active
052589730
DESCRIPTION:
BRIEF SUMMARY
TECHNICAL FIELD
The present invention relates to a method of pre-processing a magnetooptical recording medium disk capable of performing an over-write operation by only intensity modulation of light without modulating a direction of a bias field, and the pre-processed disk.
BACKGROUND ART
In recent years, many efforts have been made to develop an optical recording/reproduction method, apparatus and medium which can satisfy various requirements including high density, large capacity, high access speed, and high recording/reproduction speed.
Of various optical recording/reproduction methods, the magnetooptical recording/reproduction method is most attractive due to its unique advantages in that information can be erased after it is used, and new information can be recorded.
A recording medium used in the magnetooptical recording/reproduction method has a perpendicular magnetic anisotropy layer or layers as a recording layer. The magnetic layer comprises, for example, amorphous GdFe GdCo, GdFeCo, TbFe, TbCo, TbFeCo, and the like. Concentrical or spiral tracks are normally formed on the recording layer, and information is recorded on the tracks. In this specification, one of the "upward" and "downward" directions of the magnetization with respect to a film surface is defined as an "A direction", and the other one is defined as a "non-A direction". Information to be recorded is binarized in advance, and is recorded by two signals, i.e., a bit (B.sub.1) having an "A-directed" magnetization, and a bit (B.sub.0) having a "non-A-directed" magnetization. These bits B.sub.1 and B.sub.0 correspond to "1" and "0" levels of a digital signal. The direction of magnetization of the recording tracks can be aligned in the "non-A direction" by applying a strong bias field. This processing is called "initialization". Thereafter, a bit (B.sub.1) having an "A-directed" magnetization is formed on the tracks.
The principle of bit formation will be described below with reference to FIG. 1.
In the bit formation, a characteristic feature of laser, i.e., excellent coherence in space and time, is effectively used to focus a beam into a spot as small as the diffraction limit determined by the wavelength of the laser light. The focused light is radiated onto the track surface to write information by producing bits less than 1 .mu.m in diameter on the recording layer. In the optical recording, a recording density up to 10.sup.8 bit/cm.sup.2 can be theoretically attained, since a laser beam can be concentrated into a spot with a size as small as its wavelength.
As shown in FIG. 1, in the magnetooptical recording, a laser beam L is focused onto a recording layer 1 to heat it, while a bias field Hb is externally applied to the heated portion in the direction opposite to the initialized direction. A coercivity (denoted Hc herein) of the locally heated portion is decreased below the bias field Hb. As a result, the direction of magnetization of that portion is aligned in the direction of the bias field Hb. In this way, reversely magnetized bits are formed.
Ferromagnetic and ferrimagnetic materials differ in the temperature dependencies of the magnetization and Hc. Ferromagnetic materials have Hc which decreases around the Curie temperature and allow information recording based on this phenomenon. Thus, information recording in ferromagnetic materials is referred to as Tc recording (Curie temperature recording).
On the other hand, ferrimagnetic materials have a compensation temperature, below the Curie temperature, at which magnetization M becomes zero. The Hc abruptly increases around this temperature and hence abruptly decreases outside this temperature. The decreased Hc is canceled by a relatively weak bias field Hb. Namely, recording is enabled. This process is called T.sub.comp. recording (compensation point recording).
In this case, however, there is no need to adhere to the Curie point or temperatures therearound, and the compensation temperature In other words, if a bias field Hb capable of canceling a decreased Hc is applied to a ma
REFERENCES:
patent: 4910622 (1990-03-01), Saito et al.
patent: 5014254 (1991-05-01), Van Rosmalen et al.
patent: 5106704 (1992-04-01), Matsumoto
patent: 5128910 (1992-07-01), Iida
"The Bell System Technical Journal", vol. 62 (Sep., 1983), pp. 1923-1936.
Chu Kim-Kwok
Dzierzynski Paul M.
Nikon Corporation
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