The non-local spin-valve is pivotal in spintronics, enabling separation of charge and spin currents, disruptive potential applications and the study of pressing problems in the physics of spin injection and relaxation. Primary among these problems is the perplexing nonmonotonicity in the temperature-dependent spin accumulation in non-local ferromagnetic/ non-magnetic metal structures, where the spin signal decreases at low temperatures. Here we show that this effect is strongly correlated with the ability of the ferromagnetic to form dilute local magnetic moments in the NM. This we achieve by studying a significantly expanded range of ferromagnetic/non-magnetic combinations. We argue that local moments, formed by ferromagnetic/non-magnetic interdiffusion, suppress the injected spin polarization and diffusion length via a manifestation of the Kondo effect, thus explaining all observations. We further show that this suppression can be completely quenched, even at interfaces that are highly susceptible to the effect, by insertion of a thin non-moment-supporting interlayer.

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-

In the non-local spin valve (NLSV) geometry, four-terminal electrical Hanle effect measurements have the potential to provide a particularly simple determination of the lifetime ( ) and diffusion length ( ) of spins injected into non-magnetic materials. Recent work, however, has demonstrated that traditional models typically used to fit such data provide an inaccurate measurement of in ferromagnet/nonmagnetic metal (FM/N) devices with low interface resistance, particularly when the separation of the source and detector contacts is small. In the transparent limit, this shortcoming is due to the backdiffusion and subsequent relaxation of spins within the FM contacts, which is not properly accounted for in standard models of the Hanle effect. Here we have used the separation dependence of the spin accumulation signal in NLSVs with multiple FM/N combinations, and interfaces in the diffusive limit, to determine in traditional spin valve measurements. We then compare these results to Hanle measurements as analyzed using models that either include or exclude spin sinking. We demonstrate that differences between the spin valve and Hanle measurements of can be quantitatively modelled, provided that both the FM contact-induced isotropic spin-sinking and the full three-dimensional geometry of the devices, which is particularly important at small contact separations, are accounted for.We find, however, that considerable difficulties persist, in particular due to the sensitivity of fitting to the contact interface resistance and the FM contact magnetization rotation, in precisely determining with the Hanle technique alone, particularly at small contact separations. † Corresponding author: lao24@cam.ac.uk No's: 72.25.Ba, 72.25.Mk, 72.25.Rb PACS