Magneto-optic absorption and emission experiments on MnF2

We report the magnetic circular dichroism (MCD) and magnetically induced circular emission (MCE) spectra of the ${}_{}{}^6{}_{}{}^{}A_{1g}^{}{}_{}{}^{}\leftrightarrow {}_{}{}^4{}_{}{}^{}T_{1g}^{}{}_{}{}^{}\mathrm{}\left(G\right)\mathrm{}$ region of antiferromagnetic Mn${\mathrm{F}}_2$. Particular emphasis is placed on the magneto-optic properties of the magnon sidebands both in absorption and emission. A theoretical model is developed to treat the magneto-optic properties, which relates the MCD to a single-ion model and successfully predicts the form of the MCD of both sidebands. Expressions are obtained for the moments of magnon sideband line shapes in terms of mixing parameters of the single-ion states and the parameters evaluated from the ratio of the first and zeroth MCD moments. These compare well with values from other experiments. We note the extreme sensitivity of the MCD to small variations in these parameters, which effectively limits the value of this technique in studying the exciton-magnon interaction. The emission spectra are dominated by impurity-induced traps and the MCE of sidebands, $\sigma$, second and third nearest neighbor to an ${\mathrm{Mg}}^{2+}$ trap, $\mathrm{Mg}\sigma \left(\mathrm{II}\right)$ and $\mathrm{Mg}\sigma \left(\mathrm{III}\right)$, have been obtained. An unusual line shape is observed in the MCE of the $\mathrm{Mg}\sigma \left(\mathrm{III}\right)$ which mirrors the relative populations of the related exciton [$\mathrm{Mg}E\left(\mathrm{III}\right)$] components in a magnetic field. The field dependence of the polarization ratio indicates that energy transfer between the two sublattices occurs via the intrinsic exciton and that magnons are involved in the equilibration process. An analysis of the MCE data yields mixing parameters for the $\mathrm{Mg}E\left(\mathrm{III}\right)$ and $\mathrm{Mg}E\left(\mathrm{II}\right)$ states, those for the latter state showing significant deviations from the results obtained in absorption.