  The discovery of spin-orbit torques (SOTs) generated through the spin Hall or
Rashba effects provides an alternative write approach for magnetic
random-access memory (MRAM), igniting the development of spin-orbitronics in
recent years. Quantitative characterization of SOTs highly relies on the
SOT-driven ferromagnetic resonance (ST-FMR), where a modulated microwave
current is used to generate ac SOTs and the modulation-frequency is usually
less than 100 kHz (the limit of conventional lock-in amplifiers). Here we have
investigated the SOT of typical SOT material/ferromagnet bilayers in an
extended modulation-frequency range, up to MHz, by developing the ST-FMR
measurement. Remarkably, we found that the measured SOTs are enhanced about
three times in the MHz range, which cannot be explained according to present
SOT theory. We attribute the enhancement of SOT to additional magnon
excitations due to phonon-magnon coupling, which is also reflected in the
slight changes of resonant field and linewidth in the acquired ST-FMR spectra,
corresponding to the modifications of effective magnetization and damping
constant, respectively. Our results indicate that the write current of SOT-MRAM
may be reduced with the assistant of phonon-magnon coupling.
