The present invention relates to a magneto-optical memory apparatus, and more particularly to a magneto-optical disk apparatus which permits overwriting by a magnetic field modulation method.
The prior art overwritable magneto-optical disk apparatus uses two light spots as disclosed in IEEE Transaction on Magnetics, MAG 20, Vo. 5, page 1013 (1984) or a magnetic field modulation system in which a magnetic field applied to a record film is modulated with information to be recorded as disclosed in JP-A-51-107121, JP-A-59-215008 and JP-B-60-48806. The present invention relates to the overwritable magneto-optical memory apparatus of the latter or magnetic field modulation system.
In the prior art overwritable magneto-optical apparatus by the magnetic field modulation system, a magnetic field generating electromagnetic coil is mounted on a disk to apply the modulation magnetic field. In such a prior art apparatus, a spacing between the disk and the magnetic field application means such as electromagnetic coil cannot be reduced in order to prevent the disk from contacting to the magnetic field application means by fluctuation of the disk and the spacing is fixed to a value between 0.1 mm and 0.5 mm. As a result, if the disk fluctuates as the disk rotates, the spacing between the electromagnetic coil and the disk varies and the magnetic field intensity applied to the record film varies as the record film is moved. Since a high magnetic field is required, it is difficult to modulate it at a high speed and hence a recording frequency is lower than 0.5 MHz. As a result, the record density (line record density) is low.
A magneto-optical memory apparatus of a light modulation type in which the magnetic field applied to the record film is fixed and an intensity of an irradiated laser beam is modulated with the record information is known. However, this type of apparatus does not permit overwriting. It is disclosed in U.S. patent application Ser. No. 932,578 filed on Nov. 20, 1986 by some of the inventors of the present invention, now U.S. Pat. No. 4,803,674.