Sublattice Magnetization and Resonance Frequency of Antiferromagnets with Large Uniaxial Anisotropy

The effect of large anisotropy on the thermal behavior of the sublattice magnetization and antiferromagnetic resonance frequency is discussed. It is shown that the curves for M(T)/M(0) and ω(T)ω(0) vs TTc are shifted upward from those predicted by a Brillouin function magnetization curve. The shifts are due to a combination of the effect of the large anisotropy energy with that of electron exchange correlation. Corrections to the molecular field approximation arising from pair correlations are taken into account by an extension of the Oguchi method. These corrections are expected to be roughly 1/Z as large as the more accurate corrections which would arise from correlation with all Z neighbors. This expectation is supported by a comparison of the values of kTcJ and S̄(0) for the Oguchi method with those obtained by the Bethe-Peierls-Weiss and spin-wave methods, respectively. The results are related to the experiments on FeF₂ where the anisotropy is comparable to the exchange energy.