Spin transport through nanopillar superconducting spin valves

Abstract

This thesis details an investigation into the interaction of non-equilibrium spin currents with superconductivity using spin valves, where the difference in resistance between the antiparallel (AP) and parallel (P) alignments of the magnetic layers within the device [magnetoresistance, ∆R = RAP − RP ] has been used to quantify the spin decay occurring as the current passes through the central spacer layer. Py (15 nm)/Cu (10 nm)/Nb (x nm)/Cu (10 nm)/Py (15 nm)/FeMn (10 nm) spin valves with 200 nm Cu contact layers were deposited using dc magnetron sputtering, and fabricated into current-perpendicular-to-plane (CPP) nanopillars using optical lithography and Ar ion milling followed by focused ion beam milling. Magnetic and electrical characterisation of these devices at temperatures between 0.3-10 K demonstrate a decrease of magnetoresistance (increasing spin decay), with increasing x, the thickness of the central Nb. This trend occurs in the normal state, and also for devices in the superconducting state, which demonstrate a shorter spin decay length. This is supported by measurement of ∆Tc = TcAP − TcP , where Tc is the superconducting transition temperature of the device, and ∆Tc is negative for these CPP devices demonstrating positive ∆R. However, devices in the superconducting state with x < 26 nm demonstrate negative magnetoresistance and positive ∆Tc, behaviour that is typically seen for superconducting spin valves in the current-in-plane regime, where this behaviour is a result of the dominant effect of the exchange fields of the ferromagnets on the superconducting order parameter of the central layer. A crossover between these two parameters (∆R and ∆Tc) is observed with increasing thickness of the central Nb. A toy model is developed and fit to these data which suggests this crossover occurs when the thickness of the central Nb exceeds two coherence lengths.