November 07, 2022
Minneapolis, United States
Spurious symmetric voltage signals on the spin torque ferromagnetic resonance measurement induced by oxide substrates
Recently, transition metal oxide thin films with strong bulk or interfacial spin-orbit coupling have demonstrated to exhibit exceptionally large spin Hall angle, which can potentially enable energy-efficient spin-orbit torque devices. To characterize the spin Hall efficiency, spin-torque ferromagnetic resonance (ST-FMR) or spin pumping measurements that operate at microwave frequencies have been widely used in oxide epitaxial thin films on various oxide substrates. At high frequency, the dielectric loss of those oxide substrates could significantly influence the resonance spin torque signals. However it has not been well considered when evaluating the spin Hall efficiency. Here we report that some insulating oxide substrates could generate significant spurious symmetric voltage signals on ST-FMR measurements, leading to serious overestimation of spin-Hall angle. We investigate the spurious signals in five widely used insulating oxide substrates SrTiO3, KTiO3, DySrO3, LSAT, and Al2O3 by performing ST-FMR measurements in oxide substrate/ferromagnetic bars. We find that all the devices present considerable resonance voltage signals even with the absence of spin source layer, see Fig.1. Among which, STO, KTO, DSO show significant symmetric voltage signals, while Al2O3 presents S/A smaller than other oxides due to its large A signal. In contrast, the low frequency second harmonic Hall measurements give no signals for all the devices, indicating that the extra resonance signals on ST-FMR are not originated from spin Hall effect. Our work reveals significant influence of oxide insulator substrates on ST-FMR measurement that should be taken into account on study of spin Hall angle.