132 CHAPTER 13. HIGH-DENSITY RECORDING MATERIALS
field to reverse the magnetization of the irradiated material. After the laser beam has moved
away, the temperature of the material returns to room temperature keeping its reversed
magnetization direction. The written bits are stable at room temperature because the high
room-temperature coercivity prevents further changes of the magnetization direction.
Properties (ii) and (iii) guarantee a comparatively small diameter of the reversed domain
and therefore a large bit density. Reading of the written bits is performed by means of the
Kerr effect, as will be discussed later. The advantage of magneto-optical recording compared
to conventional optical recording (case A) is the erasability of the written information. By
means of a special technique, that will not be discussed here, it is possible to overwrite old
information by new one. A further advantage is the higher bit density.
Case D represents vertical recording by means of a recording head on a tape or a
rigid disk covered with a thin magnetic film. In normal (longitudinal) magnetic recording,
the easy magnetization direction is within the film plane. The difference between vertical
recording and longitudinal recording and the requirement for the corresponding materials
will be discussed together with the physical problems associated with their application.
It is good to bear in mind that magnetic and magneto-optical recording media fall
in the class of hard-magnetic materials because of the requirement of sufficiently high
coercivity that keeps the written artificial domain pattern from changing as a function
of time. By contrast, inductive and magnetoresistive recording-head materials fall in the
class of soft-magnetic materials. These materials and their application will be dealt with in
Chapter 14.