Ultrasonic Propagation in RbMnF3. II. Magnetoelastic Properties

A study of the propagation of rf ultrasonic waves in the antiferromagnetically ordered state of RbMn${\mathrm{F}}_3$ is presented. Consideration of the effect of the sublattice magnetization orientation on the coupling between elastic and magnetic modes of the system results in the prediction of a strong magnetic field dependence of the measured elastic constants. Quantitative comparison with experiment is given, and the results are used to obtain values of the magnetoelastic coupling constants $b_1$ and $b_2$ and the anisotropy constant $K$ at 4.2 K. The temperature dependence of the measured elastic constants, which show sharp anomalies at $T\simeq 60$ K, is readily explained with the present model.