SECTION 13.2. MAGNETO-OPTICAL RECORDING MATERIALS 135We mentioned already that the anisotropy constant does not vary strongly at room temper
ature and below. However, varies extremely strongly near the compensation temperature
In fact, since becomes zero at one expects that at the same temperature
will diverge. The coercivity is correlated with so that it is plausible that the coercivity
shows a very strong increase at In practice, one observes a temperature dependence
of the coercivity around the compensation temperature as shown in Fig. 13.2.2.
The strong temperature dependence of the coercivity is of prime importance for the
writing of the domains with reversed magnetization direction. The local heating by means
of a laser beam brings about a local reduction in coercivity so that the demagnetizing field
can reverse the magnetization in the heated area. A strong decrease of the coercivity with
respect to the room temperature value is most desirable because the temperature excursion
needed to reverse the magnetization can be kept low and the same holds for the writing
power of the laser beam. The temperature will again decrease quickly to room temperature
after the laser beam has moved away. The original coercivity is restored and keeps the local
magnetization in the opposite direction. Unlike an intermetallic R-3d compound of fixed
composition, it is possible to vary the composition of an amorphous alloy continuously. This
compositional freedom associated with the amorphous state makes it possible to choose the
appropriate R/3d composition ratio in such a way that the maximum of the coercivity
(occurring at ) is located at a temperature close to room temperature.
Read-out of the written bits is done by means of a laser beam of lower intensity than
the one used for writing the bits. It is essential for the read-out process that the laser beam
be linearly polarized. In that case, the spots of reversed magnetization can be distinguished
from regions of the original magnetization direction by means of the Kerr effect. In 1877,
Kerr discovered that the plane of polarization of linearly polarized light is rotated over