Low temperature anomalies in partially disordered lateral spin valves

Two sets of lateral spin valves (LSVs) were fabricated – Permalloy (Py)/ Silver (Ag) and Py/ Copper (Cu). Both sets of devices were made with 99.9999 % (six 9s) pure metal, evaporated at a rate that would optimise their crystal structure. However, the deposition of the non-magnetic material in both sets was paused midway, for 25 minutes, before the growth was completed. This was done in order to purposefully introduce ambient impurities from the deposition chamber (such as N2) into the central channel of the devices and, as such, act to disrupt the quality of the crystal structure. Both ‘local’ and ‘non-local’ direct current (DC) reversal sweeps (± 500 µA) were performed at a range of temperatures (3 K up to 290 K) and applied magnetic field strengths (± 100 mT), for devices with a variety of injector-detector separations (550 nm to 2650 nm) in order to extract their thermal, electrical and spintronic properties. A model recently presented by Stefanou et al. [1] was applied to the thermal voltages measured in both sets of devices resulting in a detailed insight to the thermal differences between Cu and Ag nanowires. It was found that Cu’s relatively larger thermal conductivity resulted in a completely different temperature dependence for thermal voltages measured at temperatures below 50 K. The length dependence of the spin signals in both sets of devices were extracted and fitted with a Valet-Fert model for spin diffusion. Careful analysis showed no correlation between the presence of a characteristic Kondo ‘upturn’ in the resistivity measurements and the magnitude of a low temperature ‘downturn’ in the spin diffusion lengths for both the Ag and Cu. The introduction of non-magnetic impurities during deposition and hence the disruption of the crystal structure, appears to have significantly reduced the applicability of the Elliott-Yafet (EY) model for spin-flip scattering when compared to similar devices [2–4]. Additionally, a low temperature deviation from the usual quadratic, nonlocal, current-voltage relationship (NLIV), was shown to be very strongly correlated to the magnitude of thermal voltages, caused by Joule heating in the devices’ injectors, measured at their detectors [4–6].