Spin-orbit torques and their angular dependence in ferromagnet/normal metal heterostructures

Spin-orbit torques (SOTs) are investigated for various elements of X (Pt, Ru, Pd, Cu, Mo, W, and Ta) in Pt/Co/X heterostructures. We find that both the angle-independent and angle-dependent torque components are strongly dependent on the choice of X. Large negative anisotropic SOTs are observed for elements exhibiting high SOT efficiencies (such as X = Ta or W), the behavior of which is detrimental to device applications, thus requiring us to understand the underlying physics behind the anisotropic torques. Our careful analysis shows that angle-dependent torques are closely correlated with the work function difference between Co and X, indicating that the interfacial Rashba spin-orbit coupling plays a role in the anisotropy of SOTs.