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 ($\Delta R$) between antiparallel (AP) and parallel (P) magnetization states. Giant magnetoresistance (GMR), $\Delta R = R(AP) - R(P)$, 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 ($\Delta R>0$) over the superconducting spin valve effect ($\Delta R<0$) [J. Y. Gu et al., Phys. Rev. B 66, 140507(R) (2002)]. Here, however, we report a crossover from GMR ($\Delta R > 0$) to the superconducting spin valve effect ($\Delta R < 0$) in CPP F/S/F spin-valves as the superconductor thickness decreases below a critical value.