Multimagnon Transitions in NiBr2and NiCl2

Abstract

We have made a detailed study of the temperature dependence of the optical absorption of the parity- and spin-forbidden transitions ${}_{}{}^1{}_{}{}^{}T_{2g}^{}{}_{}{}^{}\left(t_{2g}{e}_g\right)$ in Ni${\mathrm{Br}}_2$ and ${}_{}{}^1{}_{}{}^{}T_{2g}^{}{}_{}{}^{}$, ${}_{}{}^1{}_{}{}^{}E_g^{}{}_{}{}^{}\left({t_{2g}}^2\right)$ in Ni${\mathrm{Cl}}_2$. It is proposed that the oscillator strength magnitude, its temperature dependence, and the band shape of both transitions are due to a strong electron-magnon interaction.