26 CHAPTER 4. THE MAGNETICALLY ORDERED STATE
4.3. ANTIFERROMAGNETISM
A simple antiferromagnet can be visualized as consisting of two magnetic sublattices
(A and B). In the magnetically ordered state, the atomic moments are parallel or ferromag
netically coupled within each of the two sublattices. Any two atomic magnetic moments
belonging to different sublattices have an antiparallel orientation. Since the moments of
both sublattices have the same magnitude and since they are oriented in opposite directions,
one finds that the total magnetization of an antiferromagnet is essentially zero (at least at
zero kelvin). As an example, the unit cell of a simple antiferromagnet is shown in Fig. 4.3.1.
In order to describe the magnetic properties of antiferromagnets, we may use the same
concepts as in the previous section. However, it will be clear that the molecular field caused
by the moments of the same sublattice will be different from that caused by the moments of
the other (antiparallel) sublattice. The total field experienced by the moments of sublattices
A and B can then be written as
where H is the external field and where the sublattice moments and have the same
absolute value:
The intrasublattice-molecular-field constant is different in magnitude
and sign from the intersublattice-molecular-field constant