Magnetotransport and current-induced spin transfer torque in a ferromagneticallycontacted graphene

Abstract

We theoretically investigate the spin-dependent transport through a graphene sheetbetween two ferromagnetic (FM) leads with arbitrary polarization directions at lowtemperatures, where a magnetic insulator is deposited on the graphene to induce anexchange splitting between spin-up and spin-down carriers. By using standardnonequilibrium Green’s function (NGF) techniques, it is demonstrated that thedensity of states (DOS) decreases for spin-up and increases for spin-down when thepolarization strength of the two leads in parallel alignment increases. For theelectron energy around the exchange splitting, the DOS for both spin-up andspin-down channels is independent of the polarization. In contrast, the conductanceincreases for spin-up but decreases for spin-down with an increase of the polarization.Interestingly, the magnitude of tunneling magnetoresistance (TMR) can be dramaticallysuppressed with the increase of the exchange splitting in graphene. Furthermore, thecurrent-induced spin transfer torque (STT) dependence on the relative angleθ between the magnetic moments of the two leads shows a sine-like behavior and is enhancedwith an increase of the polarization and/or the bias voltage. We attribute thesespin-resolved effects to the breaking of the insulator-type properties of graphene with anexchange splitting between spin-up and spin-down carriers.