Specular Andreev reflection and magnetoresistance in graphene-based ferromagnet-superconductor double junctions

Spin-polarized transports of relativistic electrons through graphene-based ferromagnet-superconductor double junctions have been investigated theoretically. In the presence of specular Andreev reflection, some unusual properties are found. The oscillating negative magnetoresistance (MR) is not only observed, but the resonance peak of the MR also appears at a certain bias voltage due to the retroreflection crossing over to the specular Andreev reflection. This means that the MR can be tuned by external bias voltage, which benefits the spin-polarized electron device based on the graphene materials.