Measuring spin torque efficiencies in Pt/ferromagnetic insulator bilayers with spin-torque ferromagnetic resonance

Low damping ferromagnetic insulators (FMI) when interfaced with heavy metals (HM) like Pt can be manipulated using spin-orbit torques (SOT), hence making them a key building block for efficient spin-based electronics due to their minimization of dissipative charge currents. However, most studies on HM/FMI bilayers report low spin torque efficiencies of θ = 0.01-0.1, which makes the future of FMI-based devices unclear. Recently, we demonstrated highly efficient SOT switching in a novel spinel FMI Li_0.5Al_1.0Fe_1.5O₄ (LAFO) when interfaced with Pt, with switching current densities ~10⁶ A/cm². This was a result of the carefully engineered Pt/LAFO interface that yielded a large θ = 0.57 from second harmonic measurements. In this talk, we present complementary results by studying the charge-to-spin interconversion in the Pt/LAFO system using spin-torque ferromagnetic resonance (ST-FMR). By analyzing the evolution of the ST-FMR lineshapes over a series of Pt/LAFO samples, we extract the spin torque efficiency and compare them with values determined previously through second harmonic measurements and other equivalent techniques.