Spin Torque Study of the Spin Hall Conductivity and Spin Diffusion Length in Platinum Thin Films with Varying Resistivity.

We report measurements of the spin torque efficiencies in perpendicularly magnetized Pt/Co bilayers where the Pt resistivity ρ_{Pt} is strongly dependent on thickness t_{Pt}. The dampinglike spin Hall torque efficiency per unit current density ξ_{DL}^{j} varies significantly with t_{Pt}, exhibiting a peak value ξ_{DL}^{j}=0.12 at t_{Pt}=2.8-3.9  nm. In contrast, ξ_{DL}^{j}/ρ_{Pt} increases monotonically with t_{Pt} and saturates for t_{Pt}>5  nm, consistent with an intrinsic spin Hall effect mechanism, in which ξ_{DL}^{j} is enhanced by an increase in ρ_{Pt}. Assuming the Elliott-Yafet spin scattering mechanism dominates, we estimate that the spin diffusion length λ_{s}=(0.77±0.08)×10^{-15}  Ω·m^{2}/ρ_{Pt}.