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 ${\rho }_{Pt}$ is strongly dependent on thickness $t_{Pt}$ . The damping-like spin Hall torque efficiency per unit current density, ${\xi }_{DL}^j$ , varies significantly with $t_{Pt}$, exhibiting a peak value ${\xi }_{DL}^j=0.12$ at $t_{Pt}=2.8-3.9$ nm. In contrast, ${\xi }_{DL}^j\mathrm{/}{\rho }_{Pt}$ increases monotonically with $t_{Pt}$ and saturates for $t_{Pt}>5$ nm, consistent with an intrinsic spin Hall effect mechanism, in which ${\xi }_{DL}^j$ is enhanced by an increase in ${\rho }_{Pt}$ . Assuming the Elliott-Yafet spin scattering mechanism dominates we estimate that the spin diffusion length $\lambda_s = (0.77 \pm 0.08) \times 10^{-15} \Omega m^2 /\rho_{Pt}$.