Tunable Magnetic Coupling between Atomically-Precise Spin Centers in Bottom-up Fabricated Graphene Nanoribbons

Carbon nanostructures provide a new platform for exploring quantum magnetism in the absence of transition metal atoms. In sp²-bonded nanographene systems spin can be induced through mechanisms such as sublattice imbalance, topology, and dangling bonds. This creates opportunities for creating complex spin systems at the molecular scale, but requires atomically-precise fabrication techniques to control such spin behavior. Here we report the bottom-up synthesis of local spin centers in graphene nanoribbons (GNRs) that can exhibit either ferromagnetic or antiferromagnetic coupling depending on their local geometry. Our bottom-up GNRs were fabricated by covalently assembling molecular precursors on a Au(111) surface. To induce local magnetic moments we designed and synthesized molecular precursors that have a sublattice imbalance of carbon atoms and we incorporated them as dilute dopant impurities into otherwise nonmagnetic GNRs. Using a combination of scanning tunneling microscopy (STM) and atomic force microscopy (AFM) we are able to identify the resulting local spin centers in our GNRs and to probe their electronic and magnetic properties. Isolated spin centers exhibit the Kondo effect whereas pairs of spin centers in close proximity exhibit magnetic coupling as evidenced by spin-flip signatures in STM inelastic tunneling spectroscopy. These signatures reveal both ferromagnetic coupling and antiferromagnetic coupling depending on the local structural arrangement of the spin dimers. We have characterized the structure/function relationship of this new magnetic GNR system.