SECTION 13.2. MAGNETO-OPTICAL RECORDING MATERIALS 137the magneto-optical rotation does not originate from the overall magnetization of the film
but is due to one of the two sublattice magnetizations. This can be understood from the
results shown in Fig. 4.5.1, illustrating that both sublattice magnetizations have a smooth
temperature dependence even in a ferrimagnetic material with a compensation temperature.
In the upper part of Fig. 13.2.5, a schematic representation of the magnitude and direc
tion of the two sublattice magnetizations around the compensation temperature is given.
Here, we have assumed that the direction of the total magnetization
follows the direction of the applied field, meaning that both the Fe-sublattice magnetiza
tion and the Gd-sublattice magnetization reverse their direction when passing from above
to below The Fe-sublattice magnetization is dominant in the high-temperature
regime, whereas the Gd-sublattice magnetization dominates below the compensation
temperature.
Hysteresis loops are shown for both temperature regions in the lower part of the figure.
These results were obtained not by measuring the magnetization as a function of field
strength but by measuring the rotation angle versus field strength. The fact that the hysteresis
loop becomes reversed when passing the compensation temperature agrees with the notion
that the optical rotation originates from only one of the two sublattice magnetizations and
not from the total magnetization.
At this stage, it is difficult to decide which of the two sublattice magnetizations is
responsible for the magneto-optical rotation, since both sublattice magnetizations change
their direction when passing the compensation temperature. This dilemma has been solved
by measuring the optical rotation at a fixed temperature on alloys of increasing Fe con
centration. Results of magneto-optical measurements are shown in Fig. 13.2.6. It can be
seen that the Kerr rotation (full curve) does not follow the total magnetization (broken
curve), but increases with Fe concentration. This shows that the magneto-optical rotation