Based on our theoretical prediction of magnetic nanographite, we examined magnetism in various graphitic structures in nanometer scale using the first-principles calculation. When di-hydrogenated carbon atoms are created at a zigzag edge, high-spin ground states are found in carbon nanorings, which are short zigzag nanotubes, as well as graphene structures. The maximum total spin is proportional to the length of graphene ribbon or the diameter of the nanoring. The magnetism is interpreted as a realization of flat-band ferromagnetism known in the Hubbard model on bipartite lattices.
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