Phonon Pumping by Magnonic Spin Currents: Experiments and Theory

Abstract

We report the experimental observation of an excitation in a film of the ferrimagnet yttrium iron garnet (YIG), driven by the spin current generated by the giant spin Hall effect in a platinum strip with nanoscopic silver particles adjacent to the YIG film. The excitation, detected by Brillouin light scattering (BLS), has a frequency different from the thermal magnons and phonons in YIG, and does not vary with the applied magnetic field, but changes with film thickness. We interpret the BLS signal as due to phonons excited by the magnonic spin current injected into the YIG film, in a process that is the Onsager reciprocal of the spin pumping by coherent elastic waves. The observations are supported by a theory based on a process where one magnon in the magnonic spin current creates one phonon and another magnon, with conservation of energy and momentum. The theoretical value of the critical current in the metallic strip necessary to drive phonons is in order of magnitude agreement with the value measured experimentally.The experiments were performed with two samples, sample A made is with a 154 nm thick single-crystal YIG film grown by liquid-phase epitaxy onto a 0.5 mm thick [111]-oriented Gd3Ga5O12 (GGG) substrate, cut in rectangular shape with dimensions 8.0 x 1.0 mm2, and sample B made in the same way with a 93 nm thick YIG film. The bilayer samples were prepared by deposition on the YIG film of a metallic layer strip, made of Pt (3 nm)/Ag (9 nm)/Pt (3 nm) of width 150 µm, by means of DC magnetron sputtering. As shown in Ref. [1], with the appropriate deposition conditions, the strip forms nanoscopic particles of Ag in the midplane of a 6 nm thick Pt layer, that exhibits a giant spin Hall effect (SHA). A charge current in the metallic strip is converted into a spin current that flows into the YIG film as a magnonic spin current. The excitations in the YIG films are detected by back-scattering BLS using a (2x3)-pass tandem Fabry-Perot interferometer.Figure 1a shows the BLS spectrum obtained with the YIG (154 nm)/[Pt-Ag] sample, with no current in the NM strip. The spectrum was collected with 6 000 interferometer scans, with the sample under an in-plane magnetic field of 2.0 kOe. The peak denoted by M, with frequency shift of 12.6 GHz, corresponds to the Stokes scattering by thermal magnons. The inset shows the measured frequency shifts for some values of the magnetic field and the solid line represents the calculations with the magnon dispersion relation. The peak at 26 GHz, denoted by TP-GGG, is due to transverse phonons in the GGG substrate. For positive currents in the NM strip with intensity below 23 mA, there is no change in the BLS spectrum of Fig. 1(a). However, for currents above 23 mA, a new peak in the BLS spectrum shows up with frequency 7.55 GHz, with much larger intensity than the thermal peaks. Figure 1b shows the BLS spectra obtained with only 1 000 interferometer scans. The peak intensity increases with increasing current, as in Fig. 1c, and the frequency increases slightly to 7.8 GHz as the current approaches mA. For currents above this value, the BLS peak broadens and its intensity decreases due to heating effects. The thermal magnon peak cannot be seen in Fig. 1b because with 1 000 scans its amplitude is only 5 counts. The nonmagnetic character of the current driven excitation in is demonstrated in Fig. 1e showing that the frequency of the excitation does not depend on the magnetic field.A clue to the origin of the observed current-driven excitation is given by measurements in a thinner YIG film. Figure 1e shows data obtained with sample YIG/[Pt-Ag]-B, that has a YIG film with thickness 93 nm and a metallic layer as in the previous sample. By passing a current in the metallic strip of the sample, a peak emerges in the BLS spectrum for currents above a critical value smaller than in YIG/[Pt-Ag]-A, and with larger frequency, 13.5 GHz. Figure 1f shows that the frequency also does not vary with the magnetic field. The facts that the frequency of the BLS peaks increase with decreasing YIG film thickness and do not depend on the magnetic field, strongly suggest that the current driven oscillations originate from phonons.The origin of the phonon pumping observed here lies in the magnon-phonon interaction. Calculations show that the mechanism of the phenomenon reported is the generation of phonons by a magnon-phonon-magnon splitting process, driven by magnons of the magnonic spin current in the YIG film generated by the spin Hall effect in the metallic layer [2]. **