Two-magnon raman scattering and infrared absorption in MnF₂

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.