Biquadratic Exchange and the Temperature Dependence of Sublattice Magnetization in Lithium Ferrite

Neutron diffraction measurements have shown that the sublattice magnetizations in lithium ferrite are significantly higher, in the temperature range from 0.4 Tc to 0.9 Tc, where Tc is the Curie temperature, than predicted by molecular field calculations. An approximate analysis of the effect of adding a biquadratic term to the exchange interaction suggests that the principal effect is to distort the temperature scale in such a way as to make T/Tc = (T′/Tc) (1+β〈SA·SB〉), where T′ is the temperature calculated from a molecular field model and β is of the order of the ratio of the biquadratic to bilinear exchange coefficients. If the A-A B-B interactions are neglected, but a biquadratic term is included in the A-B interaction with β=0.04, the calculated curves are in good agreement with experiment.