Manifestation of coherent and spin-dependent effects in the conductance of ferromagnets adjoining a superconductor

Abstract

Electrotransport was investigated in the macroscopic ferromagnets (F) Fe and Ni in the presence of F∕In interfaces with different area. The measurements were performed in two configurations—with a constant current flowing parallel and perpendicular to the interfaces. It was found that the transition of In into the superconducting (S) state is accompanied by an increase of the resistance of the ferromagnets. In the first configuration the increase concerned the change of the resistance of the region between the boundaries of the potential probes (superconducting “mirrors”) and was of the order of the typical, weakly localized, contribution to the conductivity of ferromagnets from subgap singlet excitations, arising with Andreev reflection, for macroscopic distances between the “mirrors” at the coherence length characteristic for metals with a strong difference of the energy dispersion of the spin subbands. In the second configuration, where the conductivity of the F/S interface is also included in the measurements, the nonequilibrium resistive contribution of the latter, associated with the spin polarization of the region of the ferromagnet under the interface, was studied. The observed increase of the resistance corresponded to the theoretically predicted magnitude of the change occurring in the resistance of a single-domain region with spin-polarized electrons as a result of spin accumulation on the F/S interface under the conditions of limitations of Andreev reflections. The coefficients of current polarization and the coherence length in the exchange field were found for Fe and Ni from the experiemental data and the lower limit of the spin relaxation length was found for the samples investigated.