Evidence of phonon pumping by magnonic spin currents

Abstract

Spin current phenomena are at the heart of the active research field of spintronics that aims to develop new perspectives for emerging information technologies. In recent years, several groups reported experiments in which spin currents are used to excite coherent magnetization dynamics in magnetic nanostructures. Here, we show experimentally two effects of the large spin current generated by the giant spin Hall effect in a platinum strip with nanoscopic silver particles adjacent to a film of the insulating ferrimagnet yttrium iron garnet (YIG). The first, demonstrated by ferromagnetic resonance (FMR) experiments, is the dramatic reduction of the magnon damping measured by the FMR linewidth due to the spin torque produced by the spin current. The second, observed by Brillouin light scattering (BLS), is the excitation of quasi-particles in the YIG film with frequencies that do not vary with the applied magnetic field. 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.

Thermal Control of the Intrinsic Magnetic Damping in a Ferromagnetic Metal

“…Notably, the absolute values of the line-width changes in Py shown in Fig. 4(a) is 1 order of magnitude larger than in the ferrimagnetic insulator yttrium iron garnet [3,6,36,50]. The proposal of control of magnetic damping is a most intriguing phenomenon associated with thermal effects and, in the experiments described here, a decrease in damping with increasing temperature gradient is visible in several ways.…”

“…In a metallic film such as Py, the application of a thermal gradient generates a voltage due to the ANE, but the spin current due to the SSE accumulates at the top of the Py film and below the natural oxide layer [9,11]. Therefore, with the application of microwave signals, the spin current due to the SSE is influenced by SR and MCP also being converted into charge current due to the Onsager reciprocity relationship in metallic ferromagnetic materials [4][5][6][7]9,11,43,44]. Figures 3(b)-3(f) show measurements of the field dependencies of the voltage, V, in the Py samples, produced by currents generated simultaneously by SR, MCP, SSE, and ANE, at frequencies of 5 and 7 GHz and a temperature difference of T = +6 K, with the Si substrate cooler than the Py layer.…”

“…The continuing discoveries of processes to generate, transport, and detect spin currents are driving the field of spintronics to previously unforeseen possibilities [1][2][3][4][5][6][7]. The root of several spintronic phenomena in magnetic bilayers lies in the flow of spin angular momentum, with or without the flow of charge carriers, generated in a ferromagnetic material (FM) into an adjacent material or vice versa.…”

“…In a bilayer made of a metallic FM film and a normal metal layer with a thermal gradient applied perpendicularly to the plane, one also expects to have a dc voltage due to the SSE [1,2]. Furthermore, in the single metallic FM film, there is also an electric field created by the ANE, E ANE = −α N σ × ∇T, where α N is the anomalous Nernst coefficient [1][2][3][4][5][6][22][23][24][25][26][27][28][29][30][31][32]. In fact, in a single metallic FM film, the currents due to the SSE and ANE coexist, generating a superposition of both phenomena.…”

Magnetization dynamics affected by phonon pumping

“…They are coupled by weak magnetoelastic and magnetorotational interactions, which can often simply be disregarded. However, recent experimental and theoretical research reveals that the magnonphonon interaction may cause spectacular effects in (i) ferromagnets close to a structural phases transition such as Galfenol [1,2] or (ii) magnets with exceptionally high magnetic and acoustic quality such as yttrium iron garnet [3][4][5][6][7][8][9].…”