Manipulation of magnetic and electronic structures of graphene nanoflakes is of great technological importance. Here, we systematically study the magnetic and electronic phases of graphene nanoflakes within first-principles calculations. We illustrate the intricate shape and size dependence on the magnetic and electronic properties and further investigate the effects of carrier doping, which could be tuned by gate voltage. A crossover from the nonmagnetic to magnetic phase is observed at a critical flake size for the flakes without sublattice imbalance. We identify this as originating from the armchair defects at the junctions of two sublattices on the edge. Electron or hole doping simultaneously influences the magnetic and electronic structures and triggers phase crossover. Beyond a critical doping, antiferromagnetic to ferromagnetic phase crossover is observed for the flakes without sublattice imbalance. In contrast, suppression of magnetism and a possible crossover from the magnetic to nonmagnetic phase is observed for flakes with sublattice imbalance. Simultaneous with magnetic phase changes, a semiconductor to (half) metal transition is observed upon carrier doping. Our findings should have important implications in graphene-based spintronics.

• Received 21 December 2016

DOI:https://doi.org/10.1103/PhysRevB.95.174419