Spin-orbit torques originating from bulk and interface in Pt-based structures

We investigated spin-orbit torques in prototypical Pt-based spintronic devices. We found that, in Pt/Ni and Pt/Fe bilayers, the damping-like torque efficiency depends on the thickness of the Pt layer. We also found that the damping-like torque efficiency is almost identical in the Pt/Ni and Pt/Fe bilayers despite the stronger spin memory loss at the Pt/Fe interface. These results suggest that although the dominant source of the damping-like torque is the bulk spin Hall effect in the Pt layer, a sizable damping-like torque is generated by the interface in the Pt/Fe bilayer due to the stronger interfacial spin-orbit coupling. In contrast to the damping-like torque, whose magnitude and sign are almost identical in the Pt/Ni and Pt/Fe bilayers, the field-like torque strongly depends on the choice of the ferromagnetic layer. The sign of the field-like torque originating from the bulk spin Hall effect in the Pt layer is opposite between the Pt/Ni and Pt/Fe bilayers, which can be attributed to the opposite sign of the imaginary part of the spin-mixing conductance. These results demonstrate that the spin-orbit torques are quite sensitive to the electronic structure of the FM layer.