SECTION 4.3. ANTIFERROMAGNETISM 33which givesThis change in moment direction from the easy direction to a direction perpendicular to
it is accompanied by an abrupt increase in the total magnetization, as illustrated by curve
(b) in Fig. 4.3.3. This phenomenon is called spin flop. Of course, owing to the action of
the field applied, the sublattice-moment directions are not strictly perpendicular to the easy
direction. The sublattice moments have bent already into the field direction to some extent
and will continue to do so above for further increasing fields.
It is interesting to note that the magnetization corresponding to curve (b) for applied
fields higher than is slightly larger than that corresponding to curve (a). The reason
for this is the following. The torque experienced by the sublattice moments due to the
applied field that forces the sublattice moments into the field direction is counteracted in
both cases by the intersublattice coupling that tries to keep the two sublattice moments
mutually antiparallel (see previous section). In the case of curve (a), the torque produced by
the applied field additionally has to overcome a restoring torque caused by the anisotropy
energy that tries to keep the sublattice moments in the easy direction. This latter restoring
torque acts in a favorable way in the case of curve (b) because the field is applied in the easy
direction now. Therefore, for a given field strength above a larger degree of bending of
the sublattice moments into the field direction is achieved in the case of curve (b) than in
the case of curve (a).
A special situation is encountered in materials for which the magnetocrystalline
anisotropy is very large. This is illustrated by means of Fig. 4.3.4 where the field depen
dence of the total magnetization is plotted with the field applied in the hard direction
(curve a) and in the easy direction (curve b). In the case of curve (a), the strong anisotropy
prevents any sizable bending of the sublattice moments into the field direction. A forced