Non-local spin valves (NLSVs) generate pure spin currents, providing unique insight into spin injection and relaxation at the nanoscale. Recently it was shown that the puzzling low temperature non-monotonicity of the spin accumulation in all-metal NLSVs occurs due to a manifestation of the Kondo effect arising from dilute local-moment-forming impurities in the nonmagnetic material. Here it is demonstrated that precise control over interdiffusion in Fe/Cu NLSVs via thermal annealing can induce dramatic increases in this Kondo suppression of injection efficiency, observation of injector/detector separation-dependent Kondo effects in both charge and spin channels simultaneously, and, in the limit of large interdiffusion, complete breakdown of standard Valet-Fert-based models. The Kondo effect in the charge channel enables extraction of the exact interdiffusion profile, quantifying the influence of local moment density on the injection efficiency, and presenting a well-posed challenge to theory.Corresponding author: leighton@umn.edu PACS No's: 72.25.Ba, 72.15.Qm, 72.10.Fk 2 Improved understanding of spin transport in metals is important for the development of low resistance alternatives to the tunnel magnetoresistance 1-4 field sensors used in hard disk drive read heads and will require a substantial increase in the current knowledge of spin injection and relaxation in metals. 5 Because they enable separation of charge and spin currents, non-local spin valves (NLSVs) 6 provide critical insight into these issues, particularly at the nanoscale. 7 In the nonlocal geometry (inset, Fig. 1 Contrary to simple expectations based on Elliott-Yafet spin relaxation 8-10 , Δ is observed to be non-monotonic in many all-metal NLSVs, first increasing on cooling, but then decreasing at low T. [11][12][13][14][15][16][17] This T dependence has recently been explained as a manifestation of the Fig. 1(b)). An aberration-corrected FEI Titan G2 60-300STEM equipped with a Super-X EDX system was used, operating at 300 kV. The measured TEM specimen thicknesses, using low-loss EELS, were approximately 72 and 94 nm respectively for T A = 80 and 450 °C. Considering the effects of convergent beam broadening, channelling, and beamspecimen interaction gives an estimate of intrinsic interface broadening of < 1 nm, significantly smaller than the observed interface widths in this study. Methods for transport measurements were reported in more detail in Ref. 20 . They involve AC excitation at 13 Hz with bias currents of 100 μA.Δ was independent of excitation current in the regime investigated. is the highest T experienced during processing). Other values were measured, and will be summarized later, but the focus for now is placed on the illustrative values 80, 450 and 500 °C. The Δ in unannealed devices ( Fig. 1(a)) decreases with increasing d, as expected, with the non-
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