Origin of the Magnetic Field Enhancement of the Spin Signal in Metallic Non-Local Spin Transport Devices

Abstract

The non-local spin valve (NLSV) is an important tool in spintronics, primarily due to its ability to isolate pure spin currents1. As such, NLSVs have been instrumental in spin transport measurements of a wide range of materials2. Yet, even in relatively simple systems, questions remain on the impact of interfaces, defects and impurities3–5. Of particular interest is an increase in the “spin-signal”, ΔRNL, under large magnetic fields (up to 9 T) in all-metallic NLSVs6, in stark contrast to the key tenet that ΔRNL is invariant under a magnetic field. This effect, which appears only for certain combinations of FM and normal metals, has been a long-standing mystery and, thus far, has been a challenge to describe with existing models. Our earlier works on similar materials have shown that a low temperature upturn in spin-relaxation rate, 1/τs, originates from magnetic impurity (MI) scattering near the NLSV interfaces5,7, underlining the importance of the Kondo effect in determining spin-relaxation, particularly in Cu devices. In this study we demonstrate MI scattering as the origin of the field enhancement of ΔRNL, and we provide a theoretical framework. By correlating the magnitude of the field enhancement, δRNL, with the NLSV material pairings, we ascertain that δRNL is only present in those materials that can host MI moments (Figure 1). Next, focusing on Cu/Fe NLSVs, we extract spin-transport parameters from the field-enhanced ΔRNL. Under zero-field, we observe the same increase in 1/τs at low temperatures (Figure 2), due to Kondo scattering from MIs, in agreement with our previous work5. Importantly, we show that this mechanism is quenched by a magnetic field and we successfully model the data using a magnetic-field-modified Kondo expression8. This work not only highlights a systematic overestimation of 1/τs in materials containing MIs, including a clear impact on Hanle spin relaxation measurements, but also provides a simple means to quantify and suppress this scattering mechanism.