Enhancement of Andreev reflection by spin polarization in out-of-equilibrium ferromagnet-superconductor junctions
Abstract
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=(4e2)/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.
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