Physics of Magnetism

SECTION 4.4. FERRIMAGNETISM 37

where and are the Brillouin functions corresponding to the quantum

numbers and respectively, and where

It is to be noted that the two expressions for and in Eq. (4.4.15) are coupled

equations since

The applied field H is assumed to be zero in Eqs. (4.4.17) and (4.4.18), since we are

interested in the spontaneous moment The temperature dependence of can

be derived from the expression

Some illustrative examples of magnetization versus temperature curves are given in
Figs. 4.4.2 and 4.4.3, where we have assumed that The situation shown
in Fig. 4.4.2a refers to a compound in which the A-intrasublattice interaction is antiferromag­
netic or only weakly ferromagnetic while the B-intrasublattice interaction is ferromagnetic
and much stronger. As a result, the effective molecular field experienced by the A moments
is smaller than that experienced by the B moments. This has as a consequence that
decreases more rapidly with temperature than Figure 4.4.2b refers to a case where