Parametric Excitation of Phonons

The nonlinear relaxation of the ferromagnetic resonance into short-wavelength phonons is investigated theoretically for weak ferromagnets. A generalized theory of first- order Suhl instabilities is elaborated showing that the decisive nonlinear term results from the magnetoelastic interaction. Nonlinearities of the dipole-dipole-interaction, generally predominating parametric effects in ferro- and ferrimagnetic materials, can be neglected. The results fit in satisfactorily with the experimental data obtained for FeBO 3.