Sound-wave-soft-mode interaction near displacive phase transitions: Spin reorientation in ErFeO3

We present experimental and theoretical results of the sound-wave-spin-wave interaction in the spin reorientation region for the orthoferrite ErFeO3. Near the transition temperatures T-l = 87 degrees K and T-u = 96.6 degrees K, a longitudinal sound wave propagating along the c axis exhibits a sound-wave-order-parameter interaction which is linear in the strain and quadratic in the order parameter. This leads to steplike discontinuities in the sound velocity. The experimentally observed velocity change at T-l and T-u is 0.8%. This gives a magnetoelastic coupling constant of vertical bar B-33-B-31 vertical bar similar to 18 x 10(6) erg/cm(3). Attenuation peaks at T-l and T-u arise from the same resonant interaction. Shear waves, with polarization vector along the a axis, exhibit velocity dips at T-l and T-u indicating an interaction linear in the strain and the order parameter. A theoretical fit to the velocity curve yields the magnetoelastic coupling constant vertical bar B-55 vertical bar = 2.2 x 10(6) erg/cm(3). Again, spin-wave damping leads to attenuation peaks at T-l and T-u. Finally, shear waves, with polarization vector along the b axis, do not give any coupling to the order parameter, but only a coupling to the optical branch of the spin-wave spectrum. This leads to a small noticeable sound-wave velocity change in the spin reorientation region. All these effects can be quantitatively described by a linearized set of coupled spin-wave and sound-wave equations of motion.