Role of Coercivity in Surface Acoustic Wave Driven Ferromagnetic Resonance

We report the observation of surface acoustic wave (SAW)-driven ferromagnetic resonance (FMR) on polycrystalline FeRh in its ferromagnetic phase. A strong hysteresis of the magnetic fields at resonance is observed and is found to correlate with the static coercivity of the sample, as confirmed by temperature-dependent measurements. The angular dependence of SAW-FMR is furthermore measured and found to exhibit a wide variety of shapes that differ from commonly observed resonance curves. By modeling the hysteresis of the sample using a simple macrospin approach, we show that the observed features result from the softening of the magnetic eigenfrequency and of the magnetoelastic field allowed by hysteresis. This observation opens up the possibility of coupling resonantly SAWs to magnetization dynamics for both low magnetic fields and low frequencies, which is normally possible only for samples presenting magnetic anisotropy. Long considered a problem to be reckoned with, hysteresis may now instead offer an appealing alternative for on-chip integration of magnetic SAW sensors.