We show that parallel static and rf magnetic fields applied in the basal plane of single-crystal gadolinium samples excite phonons having a frequency equal to that of the rf field. These results, including measurements of the phonon signal as a function of angle between the rf and dc fields and as a function of the power input to the cavity, are shown to be inconsistent with current magnetoelastic theory.

• Received 24 November 1975

DOI:https://doi.org/10.1103/PhysRevLett.36.606