Dependence of the efficiency of spin Hall torque on the transparency of Pt/ferromagnetic layer interfaces

We report that spin current transport across Pt/ferromagnet (FM) interfaces as measured by the spin torques exerted on the FM is strongly dependent on the type and the thickness of the FM layer and on post-deposition processing protocols. By employing both harmonic voltage measurements and spin-torque ferromagnetic resonance measurements, we find that the efficiency of the Pt spin Hall effect in exerting a dampinglike spin torque on the FM corresponds to an effective spin Hall ratio ranging from <0.05 to >0.10 under different interfacial conditions. The "internal" spin Hall ratio of the Pt thin films used in this study, after taking the interfacial spin transmission factor into account, is estimated to be similar to 0.30. This suggests that a careful engineering of Pt/FM interfaces can improve the spin Hall torque efficiency of Pt-based spintronic devices. We also note that the dependence on temperature for both vector components of the spin Hall torque is strongly dependent on the details of the Pt/FM interface, and that measurements of magnetic damping as a function of FM layer thickness are not generally reliable for determining the true effective spin mixing conductance for the interface.