Andreev Reflection in Ferromagnet-Superconductor Junctions

The transport properties of a ferromagnet-superconductor (FS) junction are studied in a scattering formulation. Andreev reflection at the FS interface is strongly affected by the exchange interaction in the ferromagnet. The conductance $G_{\mathrm{FS}}$ of a ballistic point contact between F and S can be either larger or smaller than the value $G_{\mathrm{FN}}$ with the superconductor in the normal state, depending on the ratio of the exchange and Fermi energies. If the ferromagnet contains a tunnel barrier (I), the conductance $G_{\mathrm{FIFS}}$ exhibits resonances which do not vanish in linear response—in contrast to the Tomasch oscillations for nonferromagnetic materials.