Quantitative separation of planar Nernst effects in harmonic Hall measurements of spin–orbit torques in Pt/Co/HfO/Ta multilayers

Abstract

Spin–orbit torques (SOTs) provide a promising pathway for efficient magnetization switching in magnetic random-access memory (MRAM) devices, but their quantitative evaluation is complicated by thermal artifacts. While increasing the resistivity of heavy metals can enhance the spin Hall angle through scattering mechanisms, high-resistivity systems also exhibit sizable thermoelectric signals that obscure reliable SOT extraction. In this study, we investigate SOTs in Pt/Co/HfO/Ta multilayers with in-plane magnetic anisotropy, comparing samples with low and high-resistivity Pt layers prepared under different sputtering pressures. The high-resistivity device exhibits both enhanced damping-like torque and an additional unconventional second harmonic signal. By applying symmetry-based analysis to measurements under reversed magnetic field polarities, we separate and identify this component as originating from the planar Nernst effect (PNE), based on its sin2φ angular dependence, quadratic current scaling, and sign reversal across opposite Hall crosses. With thermal effects removed, we find a ~ 39% increase in the damping-like torque for the high-resistivity sample. Our separation technique provides a reliable route to disentangle SOT and thermal contributions in metallic heterostructures with in-plane anisotropy, aiding the precise evaluation and engineering of SOT-based spintronic devices.