Antiferromagnetic Resonance in Twinned Crystals of FeF3

Resonance studies at 70 GHz have been performed in crystals of FeF₃ from liquid‐helium temperature to 400°K. Below 367°K the field for resonance is anisotropic and temperature dependent. Because of a high‐temperature cubic‐to‐rhombohedral phase transition, the observed spectra of the twinned samples consist of superpositions of the resonance lines resulting from single crystals with c axes parallel to each of the four $\text{〈111〉}$ directions of the cubic unit cell. The angular dependence is well described by a two‐sublattice model applied to each of the single‐crystal orientations. The model contains symmetric and antisymmetric exchange interactions and a weak anisotropy field parallel to the c axis. Both high‐ and low‐frequency modes have been observed. The effective fields used in the two‐sublattice model have been obtained as functions of temperature from the resonance data at special applied field orientations. It is found that the c axis is a hard direction of magnetization between the Néel point and 251°K and changes to an easy axis at low temperatures. This effect is interpreted as a result of competing sources of anisotropy. Good agreement is found between the AFMR results and temperature dependences calculated using the molecular‐field approximation.