Evaluation of Spin-Orbit Torque Efficiency in Ta/CoFeB Bilayers with Out-of-Plane and In-Plane Magnetizations Using Harmonic Hall Measurements

Spin-orbit torque (SOT) offers efficient magnetization control using spin currents generated by the spin Hall effect and/or the Rashba-Edelstein effect through charge-to-spin conversion. Quantitative evaluation of the SOT efficiency is crucial for understanding charge-to-spin conversion and optimizing energy-efficient spintronic devices. We evaluate the SOT efficiency in Ta/CoFeB bilayers with different magnetic anisotropies using harmonic Hall measurements, which allow the extraction of damping-like and field-like effective magnetic fields. From these measurements, the effective spin Hall angle, corresponding to the charge-to-spin conversion ratio, is determined to quantify the SOT efficiency. The effective spin Hall angles are found to be -0.052±0.002 and -0.052±0.004 for the up and down magnetization states of the perpendicularly magnetized sample, respectively, and -0.051±0.001 for the in-plane magnetized sample. These results demonstrate that the SOT efficiency remains nearly identical, irrespective of magnetic anisotropy, when the same spin-current source (a 4-nm-thick Ta layer) is employed.