The coherent spin-polarized electron transport through a zigzag-edge graphene flake (ZGF), sandwiched between two semi-infinite armchair graphene nanoribbons, is investigated by means of Landauer-Buttiker formalism. To study the edge magnetism of the ZGF, we use the half-filled Hubbard model within the Hartree-Fock approximation. The results show that the junction acts as a spin filter with high degree of spin polarization in the absence of magnetic electrodes and external fields. By applying a gate voltage the spin-filtering efficiency of this device can be effectively controlled and the spin polarization can reach values as high as 90%.Graphene nanoribbons (GNRs) and graphene junctions are good candidates for electronic and spintronic devices due to high carrier mobility, long spin-relaxation times and lengths, and spin-filtering effect [1][2][3]. In fact, the conduction electrons in carbon-based materials can move very long distances without scattering due to their small spin-orbit coupling and low hyperfine interaction. For instance, in GNRs with zigzag edges, electronic transport is dominated by edge states which have been observed in scanning tunneling microscopy [4]. These states are expected to be spin-polarized and make zigzag-edge GNR (ZGNR) junctions attractive for nanoscale spintronic applications such as spin filters [5][6][7][8][9][10][11][12]. In order to achieve a spintronic device, it is very important to find nonmagnetic materials where a spin-polarized current can be injected and flowed without becoming depolarized. The ground state of ZGNRs has an antiferromagnetic spin configuration where the total spin (S) is zero. However, when the system has different number of A-and B-sublattice sites, the total spin of the ground state is 2S = N A − N B [13] and by appropriate designing, one can form a ferromagnetic spin configuration at the zigzag edges. Most of the previous studies on spinfiltering effects have been focused on the junctions, which consist of ZGNR electrodes.In this letter, a GNR junction which operates as a spin-filter in the absence of an external electric field is presented and the influence of edge atoms on spin transport through the junction will be examined. We also investigate the sensitivity of the spin polarization to the gate voltage to obtain a maximum value for this polarization. An interesting feature of our system is that, in spite of the previous studies, the type of electrodes in this junction is armchair-edge GNR and the central part of the system is a zigzag-edge graphene nanodisk [6,14], as shown in Fig. 1(a). In this junction, the right GNR electrode with a ribbon index n=8 has a width W R =0.86 nm, while the left electrode with a ribbon index n=33 * Electronic address: a-saffar@tpnu.ac.ir has a width W L =3.93 nm. Furthermore, the central region (nanodisk) with a trapezoidal shape consists of N C = 240 carbon atoms and produces a ferromagnetic spin configuration at its edges.We simulate the system depicted in Fig. 1(a) by use of a single-band tight-binding model an...

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Spin-caloritronic effects in armchair graphene nanoribbons (AGNRs) with various ribbon widths and periodic structural defects in the form of triangular antidots were systematically studied. Our results showed that by engineering defects in AGNRs, one could not only reduce the phononic thermal conductance for enhancing the thermoelectric efficiency, but also induce a ferromagnetic ground state. Interestingly, AGNRs with triangular antidots exhibit spin-semiconducting behavior with a tunable spin gap and a narrow spin-polarized band around the Fermi level. Therefore, AGNRs with antidots exhibit spin-up and spin-down currents with opposite flow directions under a temperature gradient, and they also exhibit a giant spin Seebeck coefficient () and spin figure of merit () that are much larger than those of zigzag GNRs. Finally, these results pave the way towards the application of defective AGNRs in spin-caloritronic devices operating at room temperature with a giant spin thermoelectric efficiency.

The effect of electron-electron interaction on the electronic structure of Aharonov-Bohm (AB) graphene quantum rings (GQRs) is explored theoretically using the single-band tight-binding Hamiltonian and the meanfield Hubbard model. The electronic states and magnetic properties of hexagonal, triangular and circular GQRs with different sizes and zigzag edge terminations are studied. The results show that, although the AB oscillations in the all types of nanoring are affected by the interaction, the spin splitting in the AB oscillations strongly depends on the geometry and the size of graphene nanorings. We found that the total spin of hexagonal and circular rings is zero and therefore, no spin splitting can be observed in the AB oscillations. However, the non-zero magnetization of the triangular rings breaks the degeneracy between spin-up and spin-down electrons, which produces spin-polarized AB oscillations.