  Magnetic properties of graphenic carbon nanostructures, 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 non-magnetic in
flat graphene and develop a non-zero magnetic moment only in metallic
nanotubes. This surprising behavior stems from the peculiar curvature
dependence of the electronic structure of Ni(sub). A similar
magnetic/non-magnetic transition of Ni(sub) can be expected by applying
anisotropic strain to a flat graphene layer.
