Magneto-optical activity at the structural phase transition in paramagnetic KMnF3

The magneto-optical Verdet constant ø has been measured in the paramagnetic phase of KMnF₃ and particularly in the neighbourhood of the cubic-tetragonal phase transition driven by the softening of zone boundary modes. ø is positive and displays a sizeable decrease on cooling, thus indicating the presence of a temperature dependent paramagnetic contribution that, in view of the ⁶S ground state of the Mn²⁺ ion, is attributed to the role of spin-orbit interaction. In a temperature range of about 20K around T_c = 186K, a cusp-shaped anomaly of ø is observed. From the dependence on the wavelength, and by taking into account also EPR linewidth and g measurements and optical absorption spectra, an analysis of the possible mechanisms for the cusp-shaped anomaly is given. It is argued that the enhancement of the critical rotational fluctuations of the MnF₆ octahedra can be responsible for this effect.