Landau theory of the structural phase transition in antiferromagnetic CuFe02

Abstract

A Landau-type free energy model is developed to explain experimental ultrasonic velocity measurements on the magnetoelectric compound CuFeO₂ (provided by Dr. G. Quirion at Memorial University) characterizing its low-temperature structural and magnetic phase transitions. -- In the first part of this thesis, we investigate the elastic properties of this compound in the neighbourhood of the magnetic and structural phase transitions at 11 K and 14 K. The goal is to understand the measured temperature dependence of the elastic constants of CuFeO₂. In the high-temperature rhombohedral R3m phase, we observe that the elastic constant C₆₆ shows a strong softening. This softening behavior is also non-linear. Our Landau model reproduces key features of the data and we therefore can conclude from our studies that the structural transition at 14 K is pseudoproper ferroe- lastic. The crystal structural symmetry changes from the high-temperature rhombohedral R3m to lower-temperature monoclinic C2/m at this structural transition. This work has recently been published ([1] G. Quirion, M.J. Tagore, M.L. Plumer and O.A. Petrenko, Phys. Rev. B 77, 094111 (2008)). -- In the last part of this thesis, we examine the impact of the magnetoelastic coupling. First, we develop a Landau model free energy which depends only on magnetic degrees of freedom (the spin density). Analysis shows that there are two magnetic phase transitions at Tn₁ and Tn₂, coincident with anomalies in the ultrasound data. Second, we analyze a Landau model free energy which contains spin, elastic, and magnetoelastic coupling energy. We again obtain the temperature dependence of three of the six independent elastic constants of CuFeO₂ and demonstrate strong effects due to spin-lattice coupling.