▎ 摘　　要

Spin-polarized transport of relativistic electrons through graphene-based ferromagnet/insulator/superconductor single and double junctions has been investigated on the basis of the Dirac-Bogoliubov-de Gennes equation. We have presented a comparative study on two kinds of cases: in the presence and in the absence of specular Andreev reflection. Although both the magnetoresistance (MR) and conductance are oscillating functions of the effective barrier potential and the thickness of the superconductive graphene (SG) layer for the two kinds of cases, some differences in features have also been found. In the presence of specular Andreev reflection, the MR decreases quickly with an increase of the thickness of the SG layer, and negative MR can be observed, which is in contrast to the case for the absence of specular Andreev reflection. It is interesting that the resonance peak of the MR can appear at a certain bias voltage due to the retroreflection crossing over to specular Andreev reflection. This means that the MR can be tuned by an external bias voltage, which benefits spin-polarized electron devices based on graphene materials.