Optical phenomena in BaMnF4near its phase-transition temperatures

Abstract

Measurements of indices of refraction, birefringence, and rotation of the optical ellipsoid in BaMn${\mathrm{F}}_{4}$ near its incommensurate structural phase-transition temperature and near its two- and three-dimensional magnetic-ordering temperatures have been made. The results are compatible with a single structural phase transition at ${T}_{I}=251$ K for the sample used. Small jumps of all measured quantities have been found at ${T}_{I}$ that proves the transition to be of first order. The point-group symmetry below ${T}_{I}$ is shown to be 2 (${C}_{2}$) or lower, a result assumed before but never confirmed directly. The optical ellipsoid rotates \ensuremath{\sim}4\ensuremath{'} as temperature is lowered below ${T}_{I}$. Optical activity tensor component ${G}_{33}=5\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}5}$ was measured below ${T}_{I}$. Quite unexpectedly the rotation of the optical ellipsoid and the optical activity were found to be temperature independent. Contrary to that, the anomalous part of the index of refraction close to ${T}_{I}$ varies with temperature as $〈({\ensuremath{\xi}}^{2}+{\ensuremath{\eta}}^{2})〉$ in both phases, where $\ensuremath{\xi}$ and $\ensuremath{\eta}$ are components of the order parameter. In the high-temperature phase $\mathrm{Ama}{2}_{1} ({C}_{2v}^{12})$ close to ${T}_{I}$ this varies as ${(T\ensuremath{-}{T}_{I})}^{\frac{1}{2}}$ in agreement with theoretical estimations. Fluctuational contributions to the indices of refraction and to the birefringence persist up to \ensuremath{\sim}350 K. In the low-temperature phase the order-parameter contribution to the index of refraction is found to vary with the critical exponent $\ensuremath{\beta}=0.28$. The magnetic contribution to the refractive index at low temperatures is obtained, the highest value being $\ensuremath{\delta}{n}_{c}\ensuremath{\simeq}1\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}3}$.