Half-Metallic Devices from Armchair Graphene Nanoribbons with Transition Metal Guest Atoms

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The spin-dependent transmission properties of (0,8) graphene nanoribbons (GNRs) with two substitutional Fe atom impurities (2Fe-aGNRs) have been studied by the non-equilibrium Green's function (NEGF) method in conjunction with density functional theory (DFT). Emphasis is placed on the spin-filtering activity of current transmission elements derived from these structures. In particular, it is shown that devices based on 2Fe-aGNR approach the limit of half-metallicity, where the magnitude and the sign of the current spin polarization is controlled by the bias across the device as well as the spin state of the 2Fe subsystem. This effect is rationalized by electronic structure and partial-density-of-states (PDOS) analysis of the transmission element. An occupied spin minority state, induced by the Fe-atom moiety and close to the Fermi energy of 2Fe-aGNR, accounts for the predominance of minority spin polarization. Comparison with nanosystems obtained from 2Fe-aGNR, involving vacancies rather than impurities, or both types of defects, reveals that substantial degrees of current spin polarization prevail across a wide variety of device types.