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 $\ensuremath{\delta}{n}_{c}\ensuremath{\simeq}1\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}3}$.
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Disclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.