Phonon-Induced Nuclear Spin-Wave Instabilities in RbMnF3

We report the first observations of nonlinear phonon-induced nuclear spin-wave instabilities in an antiferromagnet possessing a nuclear magnetic moment. The observed instabilities are associated with the transverse nuclear mode of cubic RbMn${\mathrm{F}}_3$. The nuclear relaxation rates are obtained as a function of the magnon frequency and wave number. This observation verifies a recently published prediction by Platzker and Morgenthaler that a sufficiently intense elastic wave of the proper frequency and polarization can indeed drive antiferromagnetic spin waves unstable.