▎ 摘　　要

Magnetic properties of graphenic carbon nanostructures, which are relevant for future spintronic applications, depend crucially on doping and on the presence of defects. In this paper we study the magnetism of the recently detected substitutional Ni (Ni-sub) impurities. Ni-sub defects are nonmagnetic in flat graphene and develop a nonzero-spin moment only in metallic nanotubes. This surprising behavior stems from the peculiar curvature dependence of the electronic structure of Ni-sub. A similar magnetic-nonmagnetic transition of Ni-sub can be expected by applying anisotropic strain to a flat graphene layer.