Magnetic Effects in the Near Infrared Spectrum of FeF2

We have investigated the near infrared spin-allowed transitions in antiferromagnetic Fe${\mathrm{F}}_2$. At low temperatures we have observed a sharp transition accompanied by magnetic dipole structure; from the behavior of this structure with respect to temperature and magnetic field changes, it is possible to conclude that magnetic dipole magnon-assisted sideband processes are present in the spectrum. Using general symmetry considerations, selection rules are developed for magnetic dipole sidebands, and it is shown that the exciton-magnon transitions couple only at the $R$ point of the Brillouin zone. From phenomenological considerations, it is also possible to deduce that the magnon-exciton pair are in fact created in opposite sublattices. These conclusions are then used to predict and identify the sideband structure. The strength and the nature of the exciton-magnon coupling mechanism are discussed as well as the mechanism for temperature broadening of the central or no-magnon, no-phonon transition.