Seebeck effects in a graphene nanoribbon coupled to two ferromagnetic leads.

We theoretically investigate the Seebeck effects for the system of a narrow graphene nanoribbon between two ferromagnetic (FM) electrodes with noncollinear magnetic moments. Both zigzagedge graphene nanoribbons (ZGNRs) and armchair-edge graphene nanoribbons (AGNRs) have been considered. By using the nonequilibrium Green's function method combining with the tightbinding Hamiltonian, it is demonstrated that, the Seebeck coefficients are sensitive to the chirality and width of the nanoribbon in the absence of magnetic field. Compared with 22-ZGNR and metallic 17-AGNR systems, semiconducting 15-AGNR system is found to posses superior thermoelectric performance, its Seebeck coefficients can be improved by two orders of magnitude. Meanwhile, the Seebeck coefficients for both 22-ZGNR and metallic 17-AGNR systems are the same order as that of graphene system. Furthermore, the Seebeck coefficients are strongly dependent on the magnetization M as well as magnetic configuration of the two FM leads. Particularly, the Seebeck coefficient drastically enhances when the magnetization of the two FM leads is in antiparallel configuration. Interestingly, the Seebeck coefficient for both 22-ZGNR and metallic 17-AGNR systems increases with increasing temperature T, while it decreases with increasing T for semiconducting 15-AGNR system. Moreover, the dependence Seebeck coefficients on magnetic flux φ show an oscillation behavior. The results obtained here may provide a valuable theoretical guidance to experimentally design heat spintronic devices. [ABSTRACT FROM AUTHOR]