Theory of Andreev Spectroscopy of Ferromagnets

Abstract

On the basis of microscopic approach we derive the Eilenberger-type equations of superconductivity for metals with exchange-split conduction band. The equations are valid for arbitrary band splitting and arbitrary spin-dependent mean free paths within the quasiclassic approximation. Next, we deduce general boundary conditions for the above equations. These boundary conditions take into account explicitly the spin-dependence of F/S interface transparency. We apply our theory for the Andreev reflection at F/S interface and derive an original expression for the Andreev conductance. Based on experimental data and our calculations we give estimations of the conduction band spin polarization for series of ferromagnets in contact with superconductors. We consider the superconducting proximity effect for a contact of a strong and clean enough ferromagnet with a dirty superconductor. We show that superconducting T c of an F/S bilayer oscillates as a function of the F-layer thickness. At small enough superconducting layer thickness the reentrant behavior of superconductivity is possible. The theory gives also non-monotonic dependence of the superconducting layer critical thickness on the spin-polarization of the ferromagnetic layer. These unconventional and distinctive features of the F/S proximity effect fit well experimental observations.