Magnon Sidebands in the Optical Absorption Spectrum of MnF2

The magnon sidebands and other structure in the optical spectrum of antiferromagnetic Mn${\mathrm{F}}_2$ have been studied under various conditions of magnetic fields, uniaxial stress, and temperature. We present the results of such experiments for the ${}_{}{}^4{}_{}{}^{}T_1^{}{}_{}{}^{}\left(\mathrm{I}\right)$, ${}_{}{}^4{}_{}{}^{}A_1^{}{}_{}{}^{}$, ${}_{}{}^4{}_{}{}^{}E\left(\mathrm{I}\right)$, ${}_{}{}^4{}_{}{}^{}T_2^{}{}_{}{}^{}\left(\mathrm{II}\right)$, ${}_{}{}^4{}_{}{}^{}T_1^{}{}_{}{}^{}\left(\mathrm{II}\right)$, and ${}_{}{}^4{}_{}{}^{}E\left(\mathrm{II}\right)$ states of ${\mathrm{Mn}}^{++}$ in Mn${\mathrm{F}}_2$. The magnon sideband shapes have been fitted as well as possible by density-of-states calculations in which the normal magnon dispersion and an adjustable exciton dispersion were used. It has been possible in this way to identify and give the magnitude of the pair moment for the principal exchange mechanism responsible for each sideband absorption, and to give the exciton parameters for the best fit. The discrepancies in this fitting process are probably ascribable to exciton-magnon binding, and indeed one line appears to represent a bound state of an exciton and a magnon.