Unusual scaling of Kondo spin relaxation

Abstract

It has been demonstrated that the Kondo effect from iron impurities substantially enhances the spin relaxation rate at low temperature in submicron copper channels of nonlocal spin valves (NLSV) [1,2]. In this work, we explore a long standing question about the physical existence of Kondo screening clouds [3] by examining the relationship between Kondo spin relaxation rate and Kondo resistivity. A systematic method is utilized to extract spin and charge transport parameters from each of the 20 NLSV devices investigated. Figure 1 (a) shows that the spin relaxation length of a NLSV reaches its maximum of 3.0 microns at 30K. Low temperature upturns are clearly observed in the spin relaxation rate versus temperature (Figure 1 (b)) and the Cu resistivity versus temperature (inset of Figure 1 (b)) plots. Both upturns are attributed to the Kondo effect from Fe impurities of several ppm. The Kondo spin relaxation rate and Kondo resistivity are extracted by fitting and plotted in Figure 1 (c) for all 20 NLSVs. Surprisingly, the Kondo spin relaxation rate remains almost constant, while the Kondo resistivity varies by nearly a factor of 10. This is significantly different from the linear relationship predicted by the Elliott-Yafet model. The effective Kondo spin flip probability, shown in Figure 1 (d), is greatly suppressed as the Kondo resistivity (proportional to impurity density) increases. The data can be explained by considering the spin dephasing processes through the Kondo screening clouds around Fe impurities, and support the physical existence of Kondo screening clouds.