Radio-frequency spin-orbit torque exciation of coherent spin waves

Spin waves, collective excitations of magnetic moments, are both essential probes of magnetic phenomena and candidates for low-power signal processing. Traditionally, coherent propagating spin waves have been inductively excited through Oersted fields generated by radio-frequency (RF) currents. An alternative mechanism, spin-orbit torque (SOT), provides highly localized excitation via interfacial spin accumulation along a current-carrying wire. To date, most studies of SOT have utilized sub-MHz currents. In this study, we theoretically and computationally investigate the generation of coherent spin waves in thin-film yttrium iron garnet (YIG) using SOT at GHz frequencies, exploring forward volume, backward volume, and surface modes in both linear and nonlinear regimes with single and interdigitated electrode configurations. We compare SOT- and inductively-generated waves, and uncover distinct features of RF-SOT including increased efficiency, unique mode selectivity and directional symmetry, a π/4 phase offset, reduced anharmonic distortion in nonlinear response, and the absence of second harmonic generation. Our findings identify RF-SOT as a potent new mechanism for magnonic and spintronic applications, as well as offering means of experimentally decoupling the effects of SOT spin currents and inductive exciation.