Temperature dependence of the non-local spin signal in Cu-based lateral spin-valves

We report on measurements of the $T$ dependence of the non-local spin signal in lateral metallic spin valves, focusing on the limit of transparent ferromagnet (FM) / normal metal (Cu) interfaces. Devices with channel width 250 nm and contact widths $\approx $100 nm (Ni$_{80}$Fe$_{20}$ or Co) were fabricated using in-situ shadow masking. We employed high purity sources in UHV, enabling one-shot deposition with no air exposure of the interface. Multiple contact separations (250 -- 800 nm) were fabricated on a single substrate to facilitate measurement of the spin diffusion length ($\lambda _{s})$. NiFe/Cu devices with 250 nm contact separation show a maximum non-local transresistance of 420 $\mu \Omega $. Analysis of Hanle effect measurements yields spin lifetimes $\approx $ 8 ps at low $T$ which compare well to those extracted from the measured $\lambda _{s}$ (300 nm) and resistivity (1.5 $\mu \Omega $cm), demonstrating consistency of our analysis. We observe a qualitatively different $T$ dependence of the non-local signal depending on the relative sizes of the contact separation and $\lambda _{s}$. When the separation becomes comparable to $\lambda _{s}$ we observe a maximum in the non-local spin signal at 35 -- 85 K, with strongly thickness dependent magnitude. These measurements of spin lifetime, resistivity, and $\lambda _{s}$ vs $T$ allow a quantitative comparison with the conductivity mismatch model. Work supported by the NSF MRSEC program.