Andreev reflection in ferromagnet/superconductor/ferromagnet double junction systems

We present a theory of Andreev reflection in a ferromagnet/superconductor/ferromagnet double junction system. In the Andreev reflection process, the injected spin polarized quasiparticles convert into Cooper pairs in the superconductor within the range of the penetration depth from the interface. When the thickness of the superconductor is smaller than or comparable to the penetration depth, the spin polarized quasiparticles pass through the superconductor and therefore the electric current depends on the relative orientation of magnetizations of the ferromagnets. The dependences of the magnetoresistance on the thickness of the superconductor, temperature, the exchange field of the ferromagnets, and the height of the interfacial barriers are analyzed. Our theory explains recent experimental results well.