Graphene based superconducting junctions as spin sources for spintronics

Highlights: • A graphene-based FS junction can be used in spintronic devices to generate high spin-polarized currents and it can be treated as an alternative for half-metallic ferromagnets or non-graphene junctions. • We have found that spin polarization shows a clear difference between the specular Andreev reflection and the conventional Andreev reflection processes. Therefore, spin polarization can be used as a tool to discriminate between the specular and conventional Andreev reflections, which is very important from the physical point of view. • The highly spin-polarized current does not depend on potential barrier which is in opposite to normaljunctions. We investigate spin-polarized transport in graphene-based ferromagnet-superconductor junctions within the Blonder-Tinkham-Klapwijk formalism by using spin-polarized Dirac-Bogoliubov-de-Gennes equations. We consider superconductor in spin-singlet s-wave pairing state and ferromagnet is modeled by an exchange field with energy of $E_x$. We have found that graphene-based junctions can be used to produce highly spin-polarized current in different situations. For example, if we design a junction with high $E_x$ and $E_F$ compared to order parameter of superconductor, then one can have a large spin-polarized current which is tunable in magnitude and sign by bias voltage and $E_x$. Therefore graphene-based superconducting junction can be used in spintronic devices in alternative to conventional junctions or half-metallic ferromagnets. Also, we have found that the calculated spin polarization can be used as a tool to distinguish specular Andreev reflection (SAR ) from the conventional Andreev reflection (CAR ) such that in the case of CAR, spin polarization in sub-gap region is completely negative which means that spin-down current is greater than spin-up current. When the SAR is dominated, the spin polarization is positive at all bias-voltages, which itself shows that spin-up current is greater than spin-down current.