1. Field of the Invention
This utility model relates to a magnetic disc for magnetic recording in a direction parallel to or perpendicular to its surface.
2. Background Art
One example of a conventional magnetic disc is a rigid magnetic disc. The substrate of the rigid magnetic disc is made of rigid material. In general, the rigid magnetic disc employs an aluminum substrate (JIS A5086 for instance).
The rigid magnetic disc is generally formed as follows. An aluminum plate is machined on a lathe, and its surface is polished so that a data recording operation can be performed at high density because the space between the disc and the head is minimized. Thereafter, a magnetic layer is formed on the aluminum substrate by vacuum deposition or spin coating. For high density recording and reproducing, the surface of the disc should be smooth. However, it has been difficult for the conventional magnetic disc using the aluminum substrate to have a surface with a central line average roughness Ra of not more than 0.1 micrometers. Furthermore, since the substrate is rigid, in forming the magnetic layer, the continuous coating is limited by the web pass. That is, handling of the conventional rigid substrate is rather troublesome. On the other hand, a high density recording operation is greatly affected by the dust on the disc. Therefore, it is essential to manufacture magnetic discs in a dust-proof environment. Accordingly, the magnetic disc manufacturing process is considerably intricate, thus requiring an enormous sum of investment in equipment.
The conventional substrate, such as an aluminum substrate, is rigid. Therefore the head should not contact the magnetic layer in tracing it. However, it is difficult to maintain constant the narrow space between the head and the magnetic layer, and therefore signal errors frequently occur. Further, it is very difficult to further reduce the space between the head and the disc surface to thereby increase the recording density. If the magnetic head is accidentally brought into contact with the disc during the tracing operation with the narrow space maintained, a sliding frictional force is produced because the substrate is rigid. As a result of this frictional engagement, a great impact is applied collectively to the part of the magnetic layer which has contacted the head, so much so as to break the disc surface, thus reducing the service life of the disc.
Furthermore, the aluminum substrate polished as described above is expensive.
On the other hand, a magnetic disc as shown in FIG. 1 has been proposed in the art. In this magnetic disc, recesses are formed in both sides of a disc substrate 1, and floppy discs or flexible discs 2 (hereinafter referred to as "flexible disc sheets") having a magnetic layer on one side are bonded on both sides of the substrate 1 in such a manner that the magnetic layers are exposed on the exterior of the disc and gaps 3 are formed between the substrate 1 and the inner surface of the flexible disc sheets 2.
The magnetic recording surfaces of the magnetic disc thus constructed are flexible. Therefore, even when the head is accidentally brought into contact with the magnetic recording surface, or when a higher density recording operation is carried out with the head held in contact with the magnetic recording layer, the magnetic layer is scarcely broken, unlike that of the rigid disc. Accordingly, the technique of flexible discs can be utilized as it is. Furthermore, a smooth magnetic layer that is highly durable can be used as a magnetic disc's magnetic layer. Thus, this type of magnetic disc is being watched with great interest since it eliminates the difficulties accompanying a conventional rigid magnetic disc.
The inventors have conducted intensive research on the magnetic disc of this type, and found that it still suffers from serious difficulties. That is, in the case where the flexible disc sheets are only bonded on the substrate as shown in FIG. 1, the flexible disc sheets may relax into a curved shape, may not be satisfactorily bonded to the substrate, or may be creased.