Phonon generation by magnetic films

Abstract

The generation of microwave acoustic waves by the resonance modes in a magnetic film is discussed. An initial orientation is provided by means of a review of previous research, and this is followed by a description of the experimental technique. Next, in a section on theory, equations of motion for the elastic waves with magnetostrictive driving terms are derived and solved for the case of a magnetic film which is isotropic, both elastically and magnetostrictively, and which is evaporated onto a substrate rod of arbitrary acoustic impedance. The resulting phonon power expression shows that space variations in the magnetoelastic constant, as well as in the microwave magnetization, provide sources for the phonon generation. Also, the expression illustrates that an interaction can easily occur for unequal spin wave and phonon k values. The phonon power expression is applied to the cases of ferromagnetic resonance (FMR) and spin-wave resonance (SWR). The observed polarizations of the acoustic waves as a function of magnetization angle check with the theory in the FMR case. Qualitative agreement with theory also appears likely in the case of the phonon generation on certain SWR peaks. Uncertainties concerning the actual precessional mode in the film, however, contribute in making a quantitative comparison of experiment and theory rather difficult. It is shown that transducer frequency-response measurements offer promise of shedding light on the nature of the precessional modes. Finally, the assumed precessional mode and the phonon generation from this mode are described for the case of a microwave interaction with a strip-domain structure in a magnetic film. The strip-domain resonance (SDR) generates longitudinal acoustic waves and occurs at very low dc magnetic field values. By utilizing SDR and the dielectric cavity technique, efficient means for generating coherent acoustic power in the upper microwave-frequency region may be obtained.