  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%.
