Theory of the spin-filtering effect in ferromagnet/ferromagnetic insulator/superconductor junctions

Abstract

Although it is well known that the ferromagnetic insulator at the interface of the superconducting junctions works as a spin-filter, its detailed mechanism has not yet been clarified. Here we present a simple factor inducing the spin-filtering effect in ferromagnet/ferromagnetic insulator/superconductor junctions by applying the extended Blonder-Thinkham-Klapwijk theory. From the factor, one can see easily how the spin-filtering effect is induced by the ferromagnetic insulator. Furthermore, it is also shown that the spin-filtering effect can be caused only by ferromagnet without ferromagnetic insulator. As an interesting application, we study the spin-filtering effect for a chiral p-wave superconductor. In ferromagnet/insulator/chiral p-wave superconductor junction, we find that the zero-bias conductance peak is shifted in opposite bias-voltage directions for Stoner model and spin bandwidth asymmetry ferromagents. It is also shown that the additional shift by ferromagnetic insulator may be useful for identifying the dominant mechanism of the Stoner and spin bandwidth asymmetry mixed ferromagnet.