Temperature Dependent Oscillator Strengths of Optical Absorptions in MnF₂ and RbMnF₃

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Temperature dependent absorption strength of several absorption bands of MnF₂ and RbMnF₃ in visible and ultra-violet region is investigated experimentally and theoretically. The oscillator strength for the exciton-magnon transitions increases as temperature increases up to Néel temperature T_N or even above T_N, and saturates or varies only slightly at higher temperatures. It is found, however, that the temperature dependence of the cold band intensity below T_N is quite different from this as expected from the theory. The temperature variation of the intensity of the two-exciton transitions is somewhat similar of that of the cold band, namely, the intensity starts to decrease with the increase of T below T_N and is nearly constant or increases slightly above T_N.
Theoretical explanations for these experimental results are presented. The calculation which assumes inter- and intrasublattice pair excitation leads to the results in qualitative agreement with the experiment. Causes of quantitative discrepancy are also discussed.