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The spin coupling between spins of conduction electron in heavy metal (HM) and localized electrons in ferromagnetic (FM) material influences the magnetization of the FM layer via the spin-orbit torque (SOT). The two main phenomena contributing to SOT are bulk spin Hall effect arising from spin scattering in the heavy metal layer and Rashba effect, which is an interfacial spin orbit coupling at the FM/HM interface. The intensity of the SOT is generally characterized by two effective fields: field-like term and damping-like term. Various techniques have been used to quantify the two effective fields, such as current-induced domain wall motion, ferromagnetic resonance techniques, and SOT-assisted magnetization switching. However, the reported amplitude of the fields is highly dependent on the experimental technique.
We present our recent results on the effective fields measurements in multilayered ferromagnetic films and introduce a self-validating method, which simultaneously quantify both the field-like and damping-like terms in structures with in-plane magnetic anisotropy. An analytical expression is derived and experimentally verified using the harmonic Hall resistance measurement. The second harmonic Hall resistance is fitted with our derived equation, to extract both the field-like and damping-like effective fields. We show that in structures with in-plane magnetic anisotropy, the field-like term comprises of a constant component and an azimuth-angular-dependent term. For structures with perpendicular magnetic anisotropy, our analytical expression and experimental results reveals an angular dependence of the damping-like term. Finally, a novel approach to achieving field free bipolar SOT induced magnetization is presented.
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