Among optical memories on/from which information is recorded/reproduced using light, magneto-optical disks having a recording film made of a perpendicularly magnetized film as a recording medium have been practically used. Information is recorded on the magneto-optical disks when a direction of magnetization within light spots is arranged to be upward or downward by applying a magnetic field while irradiating laser light onto the recording film.
As illustrated in FIG. 8, a magneto-optical disk has grooves 51. A light spot 55 accurately follows a land 52 between the grooves 51. The address information of a particular track that the light spot 55 follows is obtained because address information is recorded on each of the lands 52 as pits 53.
Information is recorded on tracks as lands 52. The track pitch is almost equal to the diameter of the light spot 55 which is determined by the wavelength of laser light and the numerical aperture of an objective lens. The objective lens converges the laser light into the light spot 55. Usually, the wavelength of the laser light is between 780 nm and 830 nm and the numerical aperture of the objective lens is between 0.45 and 0.6. Thus, the light spot 55 has a diameter of between 1.2 and 1.4 .mu.m and the track pitch is between 1.4 and 1.6 .mu.m. Accordingly, the minimum diameter of an upwardly or downwardly magnetized recording domain 54 is around 0.8 .mu.m.
A magneto-optical disk with flat mirrored sections 62 shown in FIG. 9 is also well known. The mirrored sections 62 do not have grooves 61 but pits 63. The light spot 55 tracks the grooves 61, and the address information of a particular track that the light spot 55 follows is obtained by reproducing the pits 63. Similar to the above-mentioned optical disk of FIG. 8, the minimum diameter of a recording domain 64 on the groove 61 of this magneto-optical disk is around 0.8 .mu.m.
In recent years, magneto-optical disks including a recording film of a multi-layer structure have been produced so as to achieve magnetic super resolution effects. The magneto-optical disk with such a structure produces a recording domain of a size much smaller than the size of the light spot 55, achieving improved recording density. With the magnetic super resolution technique, recording domains of size almost one half of the conventional size are stably formed. It is therefore possible to reduce the track pitch to around 0.8 .mu.m or one half of the conventional track pitch, improving the recording density significantly. For example, there is a detailed report on magnetic super resolution in "Journal of Japanese Applied Magnetism Association", Vol. 15, No. 5, 1991, pp. 838-845.
With the conventional structures, however, when the track pitch is reduced to a half, the size of the pit 53 is also reduced to a half, resulting in weaker signals from the pits 53.
Further, the distance between pits 53 formed on adjacent tracks is also decreased to a half. This causes crosstalk and prevents accurate address information from being obtained.