Evidence of soft modes in magnetoelectric CuFeO2:: Ultrasonic velocity measurements and Landau theory

Ultrasonic velocity measurements are used to examine the elastic properties of the magnetoelectric compound CuFeO2 in the vicinity of the magnetic and structural phase transitions near 14 K. Strong softening of modes related to C-66 occurs at the transition from the high- temperature rhombohedral R (3) over barm to lower-temperature monoclinic C2/ m symmetry. A Landau free energy is proposed and analyzed in order to understand the elastic properties of CuFeO2. 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 pseudoproper ferroelastic, with the spin acting as a secondary order parameter.