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Engineering spin exchange in non-bipartite graphene zigzag edges

The rules that govern spin exchange interaction in pristine graphene nanostructures are constrained by the bipartite character of the lattice, so that the sign of the exchange is determined by whether magnetic moments are on the same sublattice or else. The synthesis of graphene ribbons with perfect zigzag edges and a fluoranthene group with a pentagon ring, a defect that breaks the bipartite nature of the honeycomb lattice, has been recently demonstrated. Here we address how the electronic and spin properties of these structures are modified by such defects, both for indirect exchange interactions as well as the emergent edge magnetism, studied both with DFT and mean field Hubbard model calculations. In all instances we find that the local breakdown of the bipartite nature at the defect reverts the sign of the otherwise ferromagnetic correlations along the edge, introducing a locally antiferromagnetic intra-edge coupling and, for narrow ribbons, also revert the antiferromagnetic inter-edge interactions that are normally found in pristine ribbons. Our findings show that these pentagon defects are a resource that permits to engineer the spin exchange interactions in graphene based nano-structures.