Two-Magnon Raman Scattering and Infrared Absorption in MnF2

Abstract

Recent work has shown that it is essential to include magnon‐magnon interaction effects in order to calculate the two‐magnon line shape for optical processes in antiferromagnets. These experiments are very sensitive to any structure in the density of states, and so it is necessary to include both J₁ and J₂ (obtained from inelastic neutron scattering measurements) even though J₁ is much smaller than J₂. We have calculated line shapes for Raman scattering and infrared absorption in MnF₂ and obtained much improved agreement with the experimental results. We show that the sharp structure in the density of states and magnon‐magnon interactions are equally important in determining the line shape. Simple arguments based on the Ising model to get the shift in peak position that are valid for RbMnF₃, where there is a single sharp peak in the density of states, are no longer correct in MnF₂. We conclude that a small J₃ exchange and a possible extended range of interaction between light and the magnetic system can only have a small effect on the optical spectra.