Generation of Phonons in High-Power Ferromagnetic Resonance Experiments

The magnetoelastic energy of ferromagnets leads to a coupling between spin waves and elastic vibrations. Because of this coupling the normal modes are not purely magnetic or purely elastic but contain admixtures of both kinds of excitation, the mixing being strongest when the unperturbed waves have the same frequency and wavelength. It is shown theoretically that under suitable conditions (of frequency, dc field, and sample shape) coupled magnetoelastic waves with wavelengths of the order of 1 μ or less can be generated in high-power ferromagnetic resonance experiments. The magnetoelastic interaction increases the threshold precession angle of the uniform mode, the fractional increase being proportional to the group velocity times the relaxation time for spin waves divided by the same quantity for transverse phonons.