Enhancement of Andreev reflection by spin polarization in out-of-equilibrium ferromagnet-superconductor junctions
We report on transport measurements on Nb/Al/Gd/Al/Nb junctions. Bulk Gadolinium is a weakly polarized ferromagnet (5-7%), and is present in the junction in granular form (superparamagnet). We show that Andreev reflection is strongly enhanced by a weak polarization, obtained by applying an external magnetic field. A new model is proposed that accounts for this effect. The transport is described in terms of “hot” carriers that experience Zeeman splitting due to the spin polarized background. The Landauer formula is G = 4e 2 h A(hb), hb being the exchange field at the grain boundary, and A(E) the Andreev current coefficient,that increases with E for resistive junctions.