Theory of spin-wave interactions in s=1 two-sublattice uniaxial magnets

Abstract

A semi-invariant diagrammatic perturbation formalism is employed to study spin-wave interaction effects in S=1 Heisenberg magnets with single-ion uniaxial anisotropy and in an applied magnetic field. A general two-sublattice model is assumed with both intra-sublattice and inter-sublattice exchange interactions. The theory takes account of the optical branches to the spin-wave spectrum (arising due to the single-ion anisotropy) as well as the usual spin-wave excitations. Expressions are derived for the renormalised energy and damping of all these modes as a function of temperature and applied field in the ordered regime, but excluding the spin-flop phase. Numerical application is given to the metamagnetic system FeCl2 in its zero-applied-field antiferromagnetic phase. The results are found to be in good agreement with recent one-magnon Raman scattering measurements for this material.