Magnetoelastic properties of TmFeo3 at the spin reorientation region

Experimental results of sound propagation in the spin reorientation region of TmFeO3 are presented. The velocity of a shear wave propagating along the c axis with a polarization in the a direction exhibits pronounced dips at the transition temperatures, Tl=84°K and Tu=93.6°K. A longitudinal wave in the c direction exhibits steplike velocity changes at the transition temperatures. The experimental results are explained by a relatively simple free-energy model, which includes in addition to the magnetic and elastic energy terms, a magnetoelastic coupling linear in the strain. An applied magnetic field in the a direction results in a shift of the lower transition at Tl and a suppression of the higher one. These effects are attributed to a Zeeman term and field-dependent coupling between the rare-earth and iron ions.