Elastic anomalies associated with structural and magnetic phase transitions in single crystal hexagonal YMnO₃

Resonant ultrasound spectroscopy has been used to measure the elastic and anelastic behaviour through known structural and magnetic phase transitions in single crystal hexagonal YMnO₃. Anomalous elastic behaviour is observed at the high temperature structural transition at ~1260 K, with a discontinuity in the elastic constants and nonlinear recovery below T_c, consistent with λeQ²/_s coupling. There is no change in dissipation associated with this high temperature transition, and no evidence in the elastic or anelastic behaviour for any secondary transition at ~920 K, thus supporting the thesis of a single high temperature transformation. Elastic stiffening is observed on cooling through T_N, in accordance with previous studies, and the excess elastic constant appears to scale with the square of the magnetic order parameter. The strains incurred at T_N are a factor of ~20 smaller than those at the structural transition, implying very weak λeQ²/_m coupling and a dominant contribution to the variation in the elastic constants from λeQ²/_m. The increased acoustic dissipation above T_N is consistent with an order–disorder process.