Magnetoresistance in graphene-based ferromagnetic/ferromagnetic barrier/superconductor junction

We study the magnetoresistance (MR) and spin transport in a graphene-based ferromagnetic/ferromagnetic barrier/s-wave superconductor (FFBS) junction for two regimes including retro-Andreev reflection (AR) (E-F = 100 Delta(0)) and retroreflection crossing over to specular Andreev reflection (E-F = Delta(0)). We observed that the variation of exchange energy in the ferromagnetic region h(1) has a stronger effect on the amplitude of the MR oscillations rather than length, exchange energy, and effective gate potential in the F-B region in both regimes. In the E-F = 100 Delta(0) regime, the higher values of exchange energy h(1) approach E-F, which decrease the amplitude of the MR oscillations with respect to FB length. By increasing the exchange energy up to h(1) > E-F, a phase shift occurs for the peaks of the MR curves and the amplitude of the MR oscillations reaches its maximum value at the exchange energy h(c) = 2E(F) + U-0. Furthermore, in the E-F = Delta(0) regime, at the exchange energy h(1) = eV, the amplitude of the MR oscillations approaches its maximum value. At this value of energy of carriers eV + h(1) = E-F (Dirac point), the sign of the energy will be reversed and a transition from the retroreflection to the specular Andreev reflection occurs. Thus, in this regime (E-F = Delta(0)), by observing the maximum of the MR at specified values of eV and h(1), we can measure the Fermi energy of the graphene. While in the E-F = 100 Delta(0) regime, the MR versus bias energy can be tuned from the positive to negative values by varying h(1), but in the E-F = Delta(0) regime, the MR shows only the positive values. In addition, we have investigated the spin conductance in both regimes. (C) 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.4729302]