Emergent Antisymmetric Magnetoresistance in Ferromagnetic Ni/Pt Bilayers with Enhanced Spin–Orbit Torque Efficiency: Implications for Low-Temperature Nano-oscillators

We report an unconventional out-of-plane magnetoresistance with an antisymmetric component in Ni/Pt bilayers with in-plane magnetization anisotropy emerging at low temperatures below 100 K, dramatically distinct from the observed conventional symmetric magnetoresistance at elevated temperatures or when the field is rotated to in-plane. The observed magnetoresistance is antisymmetric to the magnetic field and changes sign with opposite applied current but symmetric when the magnetic field and current both have a reverse sign. Ferromagnetic resonance (FMR) signals show an enhanced spin–orbit torque efficiency when the temperature is reduced from 300 to 100 K, and an extra resonance peak appears under an out-of-plane magnetic field at around 50 mT, under similar conditions as the emergent antisymmetric magnetoresistance peak. We attribute the observed antisymmetric magnetoresistance to the enhanced spin–orbit-torque efficiency in Ni/Pt bilayers with thicknesses of tens of nanometers at low temperatures.