▎ 摘　　要

First-principles investigations combined with the nonequilibrium Green's function method are conducted to understand the spin-polarized transport property in graphene (Gr) based magnetic tunnel junctions (MTJs). Our systematic and comparative investigation is not only restricted to junctions with Ni electrodes on both sides (symmetric junctions) but also the junctions where one side is Ni and the other is Co (non-symmetric junctions). In spite of low magnetic exchange couplings between the electrodes, spin filtering is quite sensitive to electrode material. Distinct hybridization of the Gr pi states with symmetric and non-symmetric junctions is observed. Gr based junctions display higher transmission spectra indicating low contact resistance irrespective of the electrode material. Furthermore, the influence of different stacking of electrode layers on transport properties is also checked. Tunneling magnetoresistance ratios of Or-based MTJs are drastically affected by the Fermi surfaces of electrodes. Both symmetric and non-symmetric junctions exhibit distinctive selection rules due to the incompatibility of wave functions of electrodes on both sides of the interface.