28 CHAPTER 4. THE MAGNETICALLY ORDERED STATEIt follows from Eq. (4.3.12) that the susceptibility of an antiferromagnetic material follows
Curie–Weiss behavior, as in the ferromagnetic case. However, for antiferromagnets is
not equal to the magnetic-ordering temperature
If we compare Eq. (4.3.10) with Eq. (4.3.13), we conclude that is smaller than
bearing in mind that is negative. In many types of antiferromagnetic materials, one
has the situation that the absolute value of the intersublattice-molecular-field constant is
larger than that of the intrasublattice-molecular-field constant. In these cases, one finds
with Eq. (4.3.13) that is negative. The plot displayed in Fig. 4.2.1d corresponds to
this situation.
In a crystalline environment, frequently, one crystallographic direction is found in
which the atomic magnetic moments have a lower energy than in other directions (see
further Chapters 5 and 11). Such a direction is called the easy magnetization direction.
When describing the temperature dependence of the magnetization or susceptibility at tem
peratures below we have to distinguish two separate cases, depending on whether the
measuring field is applied parallel or perpendicular to the easy magnetization direction of
the two sublattice moments. As can be seen from Fig. 4.3.2, the magnetic response in these
two directions is strikingly different.
We will first consider the case where the field is applied parallel to the easy magneti
zation direction in an antiferromagnetic single crystal, with H parallel to the A-sublattice
magnetization and antiparallel to the B-sublattice magnetization. The magnetization of both
sublattices can be obtained by means of