Evidence of soft modes in magnetoelectric CuFeO2: Ultrasonic velocity measurements and Landau theory
G. Quirion, M. J. Tagore, M. L. Plumer, and O. A. Petrenko

Ultrasonic velocity measurements are used to examine the elastic properties of the magnetoelectric compound CuFeO$_2$ in the vicinity of the magnetic and structural phase transition near 14~K. Strong softening of modes related to $C_{66}$ occurs at the transition from the high-temperature rhombohedral $R\overline{3}m$ to lower-temperature monoclinic $C2/m$ symmetry. A Landau free energy is proposed and analyzed in order to understand the elastic properties of CuFeO$_2$. The principal features of sound velocity modes vs temperature are reproduced by the model. Our analysis suggests that the transition at 14~K is primary {\it pseudoproper ferroelastic} with the spin acting as a secondary order parameter.

© 2008 The American Physical Society.