Interdiffusion-controlled Kondo suppression of injection efficiency in metallic nonlocal spin valves

Abstract

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-

Room temperature spin Kondo effect and intermixing in Co/Cu non-local spin valves

“…This depolarization increases logarithmically on cooling through the Kondo temperature T K 22 of the FM/N pair. This manifestation of the Kondo effect has now been observed in NLSVs in which the Kondo impurities diffuse into the N from the FM contacts, 11,13 as well as those in which the magnetic impurities are introduced throughout the N channel. 12,16 In the former case, local moments near the interface reduce the polarization a of the injected current.…”

“…A long-standing puzzle in metallic NLSVs has been the widely observed suppression of the spin accumulation signal DR NL ¼ DV NL =I at low temperatures (T) for common FM/N pairings, i.e., non-monotonic DR NL ðTÞ. [6][7][8][9][10][11][12][13][14][15][16][17][18] This is in apparent contrast to predictions based on Elliott-Yafet (EY) spin relaxation in pure N metals with low spin-orbit coupling, [19][20][21] where the spin relaxation time, and hence DR NL , should increase monotonically on cooling.…”

“…Through complementary Scanning Transmission Electron Microscopy/Energy Dispersive X-ray Spectroscopy (STEM/EDX), we also describe and interpret a non-monotonic T A dependence of the charge and spin Kondo effects. The suppression of spin accumulation at high T and the non-monotonic T A dependence are in contrast to Fe/Cu NLSVs, 13 due to the larger T K for Co in Cu, as well as the very different miscibilities of the two pairings. 30 NLSVs [see Fig.…”

“…30 NLSVs [see Fig. 1(a) for device geometry] were fabricated on Si/Si-N substrates by ultra-high vacuum electron beam evaporation of high purity Co and Cu, multi-angle shadow evaporation 11,13,31,32 enabling single-shot deposition of low resistance (transparent) interfaces. The thickness t N of the Cu channels ranged from 50 to 200 nm (with a width of 150 nm), while the Co thickness t F was 16 nm (with widths of 100-150 nm).…”

“…We thus use a eff in place of a FM to account for potential Kondo suppression at the interface. 13 Second, in the EY mechanism, the spin relaxation rate scales with the T-dependent momentum relaxation rate, leading to T-dependent k N . We thus separate k N ðTÞ and a eff ðTÞ by fitting DR NL ðd; TÞ to a 1-D solution of the Valet-Fert model for NLSVs in the transparent limit 33,34…”