Magnetic Resonances and Susceptibility in Orthoferrites

Abstract

Magnetic resonances and susceptibilities are calculated for various allowed magnetic structures in orthoferrites and similar magnetic perovskites. The calculation is based on a general form for the free energy which includes canting contributions from both single-ion anisotropy and antisymmetric exchange, and which assumes four distinct interacting magnetic sublattices. The results are compared with those obtained from a simplified 2-sublattice model, and the effect of hidden canting on overt behavior is evaluated. When antisymmetric exchange is the predominant canting mechanism, a 2-sublattice model presents a correct formal (but not necessarily physical) picture for the antiferromagnetic resonance modes and for low-frequency magnetic behavior, provided that exchange effects associated with hidden canting are properly incorporated within an effective anisotropy energy. This effective anisotropy will, in general, not have the physical properties, e.g., temperature dependence, typical of single-ion anisotropy. A 4-sublattice model is required for the analysis of the high-frequency exchange modes. It is found that these modes are coupled by the spin canting mechanism to the antiferromagnetic modes and as a result become optically active. In general, one finds that, of the many interaction coefficients possible in a 4-sublattice system, only relatively few can be determined by direct macroscopic measurement.