This invention relates to a magneto-optical recording capable of recording medium, reproducing/erasing with a laser beam, and more particularly to the structure of a magneto-optical disk effective for the suppression of deterioration in the reproduction output due to repetitions of recording/erasing, while maintaining a high S/N and also for the control of shape of a formed recording domain.
With the recent progress toward a highly informational society, needs for a file memory of larger capacity and higher density are now increasing and optical disks have been regarded as a response to these needs. Particularly, magneto-optical recording is now to be used in the form of a practical, rewritable optical disk. Currently known layer structures are classified into two types, i.e. three-layer structure (underlayer/recording layer/protective layer) and four-layer structure (underlayer/recording layer/protective layer/metallic layer). Most importantly, disks of a four-layer structure utilize both the Kerr effect and the Faraday effect of magneto-optical recording layer concurrently and thus a large rotational angle can be obtained and, also the reproduction output can be increased at the driving. Therefore, disks of a four-layer structure are now extensively studied and are under development.
On the other hand, studies on higher density and higher speed are also underway. For example, it has been proposed to utilize a field modulation recording system, a system for using a double layer based on an exchange couple, etc. in order to attain the overwritting. In order to attain a higher density, it has been studied to form micromagnetic domains or to use pit edge recording. In any event, disks having a high S/N and a good controllability of magnetic domains are required. If magneto-optical disks can be used not only in computer file memory, but also for domestic purposes for example as rewritable compact disks, a larger requirement will be posed on the development. The first condition for this purpose is a lower cost. To satisfy the condition, it is necessary to use low cost plastics as substrate materials and to make the disk structure as simple as possible. This can be attained only as a result of of improvements in disk technology such as (1) technology of preparating substrates, (2) technology of making a magneto-optical recording layer of higher corrosion resistance and (3) optimization of disk structure, etc. In particular, the technology of making a magneto-optical recording layer of higher corrosion resistance, as mentioned in (2), is important.
In the above-mentioned disk technology, research and development have been thus far concentrated on an increase in the rotational angle, while no particular consideration has been paid to the decrease in disk characteristics, especially reproduction output, when recording/erasing are repeated, and therefore no higher reliability has been obtained. This decrease in disk performance results from the amorphous state of magneto-optical recording layers being relaxed by a laser beam used for the recording/erasing.
In the case of higher density recording by field modulation recording, recording domains themselves interfere with one another or no better shapes of recording domains are obtained, and thus no satisfactory reproduction output is obtained. This is because parts in a larger ratio of the domain wall to the domain area exist and consequently the energy of the domain wall is so high that the shapes of recording domains become unstable.
Furthermore, in the case of domestic applications, etc. on the other hand, no satisfactory simplification of layer structure has been made yet, resulting in poor mass production and higher cost. In other words the magneto-optical disks have been found to be unsuitable for the domestic applications. Still furthermore, an underlayer made mainly of inorganic compounds must have a thickness of at least 750 .ANG., in view of the optical effect. This increases an overall stress of the magneto-optical recording medium and causes the underlayer to peel off, especially when formed on a plastic substrate. In order to prevent such a peeling, it has been attempted to reduce the stress on the underlayer of inorganic compounds or to apply some treatment to the substrate surface. The thickness of the underlayer of inorganic compound has been restricted to 700-900 .ANG. in view of the optical characteristics, which case the recording layer has been sometimes corroded via pinholes existing in the underlayer.