▎ 摘　　要

Employing first-principles calculations, we investigate the efficiency of spin injection from a ferromagnetic electrode (Ni) into graphene and a possible enhancement by using a barrier between the electrode and graphene. Three types of barriers, h-BN, Cu(111), and graphite, of various thickness (0-3 layers) are considered, and the electrically biased conductance of the Ni/barrier/graphene junction is calculated. It is found that the minority-spin-transport channel of graphene can be strongly suppressed by the insulating h-BN barrier, resulting in a high spin-injection efficiency. On the other hand, the calculated spin-injection efficiencies of Ni/Cu/graphene and Ni/graphite/graphene junctions are low, due to the spin-conductance mismatch. Further examination of the electronic structure of the system reveals that the high spin-injection efficiency in the presence of a tunnel barrier is due to its asymmetric effects on the two spin states of graphene.