Nonlocal transport in Py/Cu lateral spin valves shows that the Cu spin diffusion length and the apparent Py spin polarization increase with Cu thickness. A proper quantitative analysis shows that the Cu spin diffusion length is dominated by surface spin-flip scattering and that the Py and Cu thickness dependence of spin polarization is due to strong spin-flip back-scattering at the Py/Cu interface. This solves a long-standing puzzle regarding the discrepancy in Py spin polarizations obtained from different measurements. Interestingly, the Cu surface oxidation causes enhanced spin diffusion, contrary to expectations. These surface effects substantially affect the performance of lateral spin valves.
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