Spin-dependent transport for armchair-edge graphene nanoribbons between ferromagneticleads

Abstract

We theoretically investigate the spin-dependent transport for the system of anarmchair-edge graphene nanoribbon (AGNR) between two ferromagnetic (FM) leads witharbitrary polarization directions at low temperatures, where a magnetic insulator isdeposited on the AGNR to induce an exchange splitting between spin-up and -downcarriers. By using the standard nonequilibrium Green’s function (NGF) technique, it isdemonstrated that the spin-resolved transport property for the system depends sensitivelyon both the width of AGNR and the polarization strength of FM leads. The tunnelingmagnetoresistance (TMR) around zero bias voltage possesses a pronounced plateaustructure for a system with semiconducting 7-AGNR or metallic 8-AGNR in the absence ofexchange splitting, but this plateau structure for the 8-AGNR system is remarkablybroader than that for the 7-AGNR one. Interestingly, an increase of the exchange splittingΔ suppresses the amplitude of the structure for the 7-AGNR system. However, the TMR ismuch enhanced for the 8-AGNR system under a bias amplitude comparable to the splittingstrength. Further, the current-induced spin-transfer torque (STT) for the 7-AGNR systemis systematically larger than that for the 8-AGNR one. The findings here suggest the designof GNR-based spintronic devices by using a metallic AGNR, but it is more favorable tofabricate a current-controlled magnetic memory element by using a semiconducting AGNR.