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 $mathrm{Pt}/mathrm{Co}$ bilayers where the Pt resistivity ${ensuremath{rho}}_{mathrm{Pt}}$ is strongly dependent on thickness ${t}_{mathrm{Pt}}$. The dampinglike spin Hall torque efficiency per unit current density ${ensuremath{xi}}_{mathrm{DL}}^{j}$ varies significantly with ${t}_{mathrm{Pt}}$, exhibiting a peak value ${ensuremath{xi}}_{mathrm{DL}}^{j}=0.12$ at ${t}_{mathrm{Pt}}=2.8–3.9text{ }text{ }mathrm{nm}$. In contrast, ${ensuremath{xi}}_{mathrm{DL}}^{j}/{ensuremath{rho}}_{mathrm{Pt}}$ increases monotonically with ${t}_{mathrm{Pt}}$ and saturates for ${t}_{mathrm{Pt}}>5text{ }text{ }mathrm{nm}$, consistent with an intrinsic spin Hall effect mechanism, in which ${ensuremath{xi}}_{mathrm{DL}}^{j}$ is enhanced by an increase in ${ensuremath{rho}}_{mathrm{Pt}}$. Assuming the Elliott-Yafet spin scattering mechanism dominates, we estimate that the spin diffusion length ${ensuremath{lambda}}_{s}=(0.77ifmmodepmelsetextpmfi{}0.08)ifmmodetimeselsetexttimesfi{}{10}^{ensuremath{-}15}text{ }text{ }mathrm{ensuremath{Omega}}ifmmodecdotelsetextperiodcenteredfi{}{mathrm{m}}^{2}/{ensuremath{rho}}_{mathrm{Pt}}$.