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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 damping-like spin Hall torque efficiency per unit current density, $ξ^j_{DL}$ , varies significantly with $t_{Pt}$, exhibiting a peak value $ξ^j_{DL}=0.12$ at $t_{Pt} = 2.8 - 3.9$ nm. In contrast, $ξ^j_{DL}/ρ_{Pt}$ increases monotonically with $t_{Pt}$ and saturates for $t_{Pt} > 5$ nm, consistent with an intrinsic spin Hall effect mechanism, in which $ξ^j_{DL}$ is enhanced by an increase in $ρ_{Pt}$ . Assuming the Elliott-Yafet spin scattering mechanism dominates we estimate that the spin diffusion length $λ_s = (0.77 \pm 0.08) \times 10^{-15} Ωm^2 /ρ_{Pt}$.