Spin-orbit torque efficiencies in Li_0.5(Al,Fe)_2.5O₄/Pt bilayers

Spin-orbit torque (SOT) efficiency quantifies how effectively an electrical current in a conducting layer can control the magnetization of an adjacent ferromagnetic layer. Higher SOT efficiencies translate to lower critical currents required for SOT-driven magnetization switching—promising for next-generation, efficient spintronic memory and logic devices. We study lithium aluminum ferrite (Li_0.5Al_xFe_2.5-xO₄, LAFO) films grown on magnesium gallate (MgGa₂O₄, MGO) substrates. For x = 0.8 – 1.0, we have stabilized a ferromagnetic insulating ground state with perpendicular magnetic anisotropy, enabling faster SOT switching and making these materials ideal for spintronic applications. However, the SOT efficiencies obtained through harmonic Hall (HH) and spin-torque ferromagnetic resonance (ST-FMR) measurements do not agree and range from 0.057 to 0.464. More specifically, HH measurements on LAFO/Pt bilayers with x = 0.8 – 1.0 indicate SOT efficiencies of approximately 0.52 while SOT efficiencies deduced from ST-FMR measurements are significantly lower, on the order of 0.08. The inconsistency suggests that the two techniques do not account for all magnon generation and annihilation processes. A refined analysis of HH data that accounts for magnon creation-annihilation processes has been applied to CoFeB/W and YIG/Pt bilayers with in-plane magnetic anisotropy by Gambardella et al. We apply a similar refined analysis to HH measurements of LAFO with perpendicular magnetic anisotropy.