  Predictions state that graphene can spontaneously develop magnetism from the
Coulomb repulsion of its $\pi$-electrons, but its experimental verification has
been a challenge. Here, we report on the observation and manipulation of
individual magnetic moments localized in graphene nanostructures on a Au(111)
surface. Using scanning tunneling spectroscopy, we detected the presence of
single electron spins localized around certain zigzag sites of the carbon
backbone via the Kondo effect. Two near-by spins were found coupled into a
singlet ground state, and the strength of their exchange interaction was
measured via singlet-triplet inelastic tunnel electron excitations. Theoretical
simulations demonstrate that electron correlations result in spin-polarized
radical states with the experimentally observed spatial distributions. Hydrogen
atoms bound to these radical sites quench their magnetic moment, permitting us
to switch the spin of the nanostructure using the tip of the microscope.
