Sublattice Magnetizations of Yttrium Iron Garnet as a Function of Temperature

The intensity of the 220 neutron reflection of a single crystal of yttrium iron garnet has been measured as a function of temperature from 4.2° up to 598°K. The information from this, when combined with measurements of the saturation magnetization, yields values for the sublattice magnetizations Ma(T) and Md(T) individually. As was found previously with lithium ferrite, the sublattice magnetizations are significantly higher at temperatures between 0.5 Tc and 0.9 Tc, where Tc is the Curie temperature, than those predicted by molecular field theory. It has been shown that the effect of a biquadratic term in the exchange interaction can be approximated by distorting the temperature scale such that T/Tc = (T′/Tc)[1+βS2Ma (T′)Md(T′)/Ma (0)Md(0)], where Ma(T′) and Md(T′) are the sublattice magnetizations calculated at temperature T′ by molecular field methods, S is the spin of the magnetic ions, and β is an adjustable parameter. If the a-a and d-d interactions are neglected but the biquadratic correction is introduced with β = 0.04, the same value as in lithium ferrite, the fit with the experimental data is greatly improved. A maximum discrepancy of 5% between the observed and calculated magnetizations corresponds to a discrepancy of 1% in the sublattice magnetizations. This might be accounted for either by an improved approximation or by small values for the a-a and d-d interactions.