Coupling superconducting-ferromagnetic point contacts by Andreev reflections

The properties of a device made of two point contacts between normal (N) or ferromagnetic (F) tips, and a superconductor (S), are discussed as a function of the spin polarization and the distance L between the contacts. When L is smaller than the superconductor coherence length xi, nonlocal Andreev reflections occur: for opposite spin polarizations of the contacts, mixed Cooper pairs made of electrons coming one from each tip can be injected into the superconductor. This leads to unusual properties, for instance, the parallel resistance of two S/F contacts goes from infinity for full and equal polarizations, to a finite Andreev value for opposite ones. (C) 2000 American Institute of Physics. [S0003-6951(00)01504-7].