Antiferromagnetic Resonance in Twinned Crystals of FeF3

Abstract

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 〈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.