Spin polarized conductance in ferromagnet/insulator/conventional superconductor junctions

A theory of differential conductance in ferromagnet / insulator / conventional superconductor (F/I/CS) junctions is presented and an experimental conductance is analyzed by the theory. With regard to line shapes of calculated differential conductance, as the magnitude of the exchange interaction in ferromagnets is increased, the differential conductance for the half-metallic state below the energy gap is always reduced in contrast to that for the normal metallic state. This is due to the fact that the retro-reflectivity of the Andreev reflection in the case of CS's is broken by the influence from the exchange interaction. The experimental conductance of ferromagnetic Ru2-xFexCrSi Heusler alloy / superconducting Pb junctions was in good agreement with calculational results of the theory for the F/I/CS junctions.