Magnetic disks having a flexible substrate generally known as floppy disks are widely used in computers, word processors and the like. Floppy disks are usually fabricated by coating a magnetic layer on at least one surface of a length or section of nonmagnetic substrate film. The coated sheeting is dried, cured, and calendered before a disk is punched out of the sheeting. The disk is finally received in a jacket or cartridge having a cleaning liner mounted inside.
In addition to magnetic layers of the coating type, magnetic layers of the thin film type are also known which are generally formed from metals or alloys, typically Co-Cr alloy by sputtering or other various thin film forming techniques.
Since the manufacture of floppy disks involves punching as described above, burrs and cracks occur on the disks, which would disturb stable disk rotation.
When the magnetic layer is formed by vapor phase deposition, the substrate is inevitably heated during the deposition process. Such heating would induce stresses or some deformation like deflection in the disk, also causing unstable disk rotation.
Japanese Patent Application Kokai No. 240490/1986 proposes to cover the peripheral edge of a magnetic disk with a plastic film. The configuration of the magnetic disk proposed therein is illustrated in FIGS. 11 and 12. A magnetic recording medium in the form of a disk 100 includes an annular polymer film 102 and a magnetic layer in the form of a metal thin film 103 on each surface thereof. A pair of plastic film rings 104 and 105 enclose the periphery of the disk on both sides. These plastic film rings covering the disk edge prevent the disk edge, which is otherwise sharp, from scratching the liner of the jacket.
Disk substrates are required to exhibit greater flatness as the recording density of the associated magnetic disk increases. If the substrate is less flat, the disk will produce more output variations. For example, magnetic disks of the perpendicular recording type having a magnetic layer in the form of a sputtered Co-Cr alloy film should provide a high degree of flatness in order to allow for high density recording. Nevertheless, these disks tend to deform because not only the magnetic layer, but additional layers such as a soft magnetic film between the magnetic layer and the substrate and a solid protective film on the magnetic layer are also formed by vapor phase deposition.
The use of plastic film rings as proposed in the above-cited Japanese Patent Application Kokai is effective in preventing deformation of disks and hence variation of reproduced output. If the plastic film rings used are thicker than necessary, the disk as a whole becomes too stiff to achieve smooth contact with the magnetic head. As a result, variations occur in the reproduced output.