Physics of Magnetism

13

High-Density Recording Materials

13.1. INTRODUCTION

Several magnetic materials find an application as thin magnetic films in high-density

recording devices. The most common methods for high-density recording are schematically

represented in Fig. 13.1.1.

Case A represents conventional optical recording applied, for instance, in compact

disks. A pattern of pits is burned by means of a pulsed laser beam into a non-magnetic

film at the surface of the disk. This pattern is read out by means of a laser of much lower

intensity, the reflected beam being out or in focus when it hits a pit or does not hit a pit,

respectively.

Case B is a modification of case A. Local heating of a suitable surface layer by means of

a pulsed laser beam leads to temporary local melting. After irradiation the melt cools at a suf­

ficiently high rate to produce the amorphous state. The irradiated spot can be distinguished

from the unmodified matrix by means of its lower reflectivity or larger transmittance. The

advantage of case B compared to case A is the erasability of the written information. Instead

of amorphization, one may also use local color changes produced by irradiation in special

materials.

Case C illustrates the principles involved in magneto-optical recording. The thin sur­

face layer of the disk consists, for instance, of an amorphous Gd–Fe or Gd–Co alloy.

The requirements for alloys to be used as magneto-optical recording media include the

following:

(i)  An easy magnetization direction perpendicular to the film plane. The corresponding

anisotropy of the material has to be sufficiently strong to overcompensate the tendency

of the magnetization vector to lie in the film plane because of the lower demagnetizing

factor in this direction.

(ii)     A sufficiently high coercivity at room temperature but decreasing strongly with

temperature.

(iii)    A low thermal conductivity.

These properties offer the possibility of thermomagnetic writing of bits. This is accom­

plished by switching the magnetization direction of a tiny spot by local heating with a pulsed

laser beam. The local heating brings the material in the spot area into a temperature range

where the coercivity is low. The coercivity has to be low enough for the local demagnetizing

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