Bipolar-unipolar transition in thermospin transport through a graphene-based transistor

Based on the mean-field Hubbard model, we study the thermally driven spin-polarized transport through a local-gated magnetic zigzag graphene nanoribbon by using the nonequilibrium Green's function method. The spin currents are tuned by the source temperature, the temperature bias, and the gate voltage. We find this transistor exhibits a transition from the bipolar to unipolar spin transport under associated modulations of thermal bias and gate voltage. It is argued that the result originates from the band selective rule related to parity conservation of wave functions in quantum tunneling. We also find the thermal magnetoresistance of the ribbon between the ferromagnetic excited state and antiferromagnetic ground state could reach up to 10(5)% under a small local gate voltage. This proposed device provides possibility for bettering control of the spin freedom of electrons in graphene materials. (C) 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.4748110]