Light scattering study of phonons parametrically excited in the weak ferromagnet FeBO3

We report on a new type of ferromagnetic resonance (FMR) instability which is due to the parametric excitation of phonons in FeBO3, and its observation by inelastic light scattering. The new and most remarkable feature of the process is its low power threshold, since it appears even prior to the competing perpendicular pumping Suhl I magnon instability in the coincidence region. The measurements were performed at room temperature and X-band ⊥ pumping configuration. The FMR absorption saturates above a certain power threshold, simultaneously a strong generation of phonons at half the microwave frequency is observed by light scattering. Using a high contrast Fabry-Perot interferometer, the onset of the phonon generation could be measured starting from the thermal level. At higher microwave power the scattered light is so strong that it can be observed visually. It is thereby revealed that the phonons propagate in sharply defined directions as determined by the energy and momentum conservation conditions of the parametric process.