Antiferromagnetic Resonance in NaMnF3

Abstract

The angular dependence of the AFMR in oriented single crystals of NaMnF3 has been investigated at 70 GHz and 4.2°K. In addition to the usual AFMR which is found at all applied field orientations, spin-flop resonance modes which are double valued in applied field are observed over a narrow angular interval centered about the [100] axis of the orthorhombic unit cell. A two-sublattice model has been used to derive the equilibrium positions of the sublattice moments and the angular dependence of the field for resonance. The model contains anisotropy fields consistent with the orthorhombic crystal structure and canting fields resulting from both single-ion anisotropy, and antisymmetric-exchange interactions, which produce a weak ferromagnetic moment parallel to the c axis. The effects of both types of canting mechanisms are described, and the constants used in the model have been determined from the resonance data. It was found that resonance absorptions in several crystals displayed structure which depended upon the sizes and shapes of the samples. In particular, nonspherical samples of the order of 1 mm or larger contained many lines with complex angular dependences. The splittings in smaller, spherical-shaped samples are interpreted as resulting from magnetostatic modes.