Evidence for competition between the superconductor proximity effect and quasiparticle spin-decay in superconducting spin-valves

The difference in the density of states for up- and down-spin electrons in a ferromagnet (F) results in spin-dependent scattering of electrons at a ferromagnet / nonmagnetic (F/N) interface. In a F/N/F spin-valve, this causes a current-independent difference in resistance ($ΔR$) between antiparallel (AP) and parallel (P) magnetization states. Giant magnetoresistance (GMR), $ΔR = R(AP) - R(P)$, is positive due to increased scattering of majority and minority spin-electrons in the AP-state. If N is substituted for a superconductor (S), there exists a competition between GMR and the superconducting spin-valve effect: in the AP-state the net magnetic exchange field acting on S is lowered and the superconductivity is reinforced meaning $R(AP)$ decreases. For current-perpendicular-to-plane (CPP) spin-valves, existing experimental studies show that GMR dominates ($ΔR>0$) over the superconducting spin valve effect ($ΔR<0$) [J. Y. Gu et al., Phys. Rev. B 66, 140507(R) (2002)]. Here, however, we report a crossover from GMR ($ΔR > 0$) to the superconducting spin valve effect ($ΔR < 0$) in CPP F/S/F spin-valves as the superconductor thickness decreases below a critical value.

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