Origin of spin-orbit torque in CoFeB single-layer CoFeB investigated via in-plane harmonic Hall measurements

The current-induced spin-orbit torque in single-layer ferromagnetic CoFeB thin films is quantitatively investigated by using in-plane harmonic Hall measurements. After the subtraction of thermal contributions such as the anomalous and ordinary Nernst effects, the obtained overall spin-orbit torque is successfully decomposed into damping-like (DL) and field-like (FL) terms. The DL and FL torques exhibit opposite trends of ferromagnetic layer thickness dependence before saturation, giving rise to distinctively different spin torque efficiencies: The DL torque efficiency shows a strong thickness dependence, while the FL torque efficiency is almost independent of the thickness. Such a result shows strong evidence that the DL torque originates from a spin-Hall-like charge-spin conversion in the ferromagnet, while the FL torque stems from interfacial effects such as the Rashba-Edelstein effect. With both DL and FL torques quantified in the single-layer CoFeB, our results exhibit an important step toward the understanding of nontrivial spin-orbit torques in single-layer ferromagnetic thin films.