Superconductors as ideal spin sources for spintronics

Abstract

Spin-polarized transport is investigated in normal metal-superconductor (NS) junctions as a function of interface transmissivity as well as temperature when the density of states of a superconductor is Zeeman-split in response to an exchange field (h_exc). Similarly to the "absolute spin-valve effect" predicted by D. Huertas-Hernando et al. [Phys. Rev. Lett. 88, 047003 (2002)] in superconducting proximity structures, we show that NS junctions can be used to generate highly spin-polarized currents, in alternative to half-metallic ferromagnets. In particular, the spin-polarized current obtained is largely tunable in magnitude and sign by acting on bias voltage and h_exc. While for tunnel contacts the current polarization can be as high as 100%, for transparent junctions it is dominated by the minority spin species. The effect can be enhanced by electron "cooling" provided by the superconducting gap.