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

Mesoscale and Nanoscale Physics