Low spin flip probability of the Kondo scattering in Cu channels with dilute Fe impurities

The Kondo effect originates from the antiferromagnetic exchange interaction between itinerant electrons and localized spins of magnetic impurities. [1] It is generally accepted that the Kondo scattering process is associated with a high probability of spin flip, which was originally calculated to be 2/3 by Kondo. [1] The measurement of the Kondo spin flip probability is fundamentally intriguing. In nonlocal spin valves (NLSVs) with Cu channels that contain dilute Fe impurities, the Kondo spin-flip probability has been studied. However, different works have reported markedly different values. [3] In this study, the Fe impurity concentration in NLSVs is varied by both intentional and unintentional doping. Six sets of NLSVs are studied, each containing 8 to 15 devices. The Kondo spin-flip probability, determined for each set, decreases from 0.14 to 0.03 as the impurity level increases from 1 ppm to 9 ppm. Contrary to the conventional wisdom, the Kondo spin-flip probability is not a constant and is much lower than unity. We hypothesize a cancellation effect and construct an intuitive physical picture using the Kondo screening cloud to account for the findings.

[1] J. Kondo, Prog. Theor. Phys. 32, 37 (1964).

[2] K. W. Kim et al, Phys. Rev. B 95, 104404 (2017).

[3] J. T. Batley et al, Phys. Rev. B 92, 220420(R) (2015).

[4] X. Y. Shen and Y. Ji, Phys. Rev. B 104, 085101 (2021).