Spin and Charge Control of Topological End States in Chiral Graphene Nanoribbons on a 2D Ferromagnet

Tailor-made graphene nanostructures can exhibit symmetry-protected topological boundary states that host localized spin-1/2 magnetic moments at half filling. However, one frequently observes charge transfer on coinage metal substrates, which results in closed-shell configurations. Using low-temperature scanning tunneling spectroscopy, it is demonstrated here that pristine topologically nontrivial chiral graphene nanoribbons synthesized directly on the ferromagnet GdAu₂ can either maintain a charge-neutral diradical state, or convert to a singly anionic doublet. As an underlying mechanism, both a work function and an exchange field modulated by the moiré-induced superstructure are identified, as corroborated by Kelvin probe force microscopy and spin-flip spectroscopy. The joint electrostatic and magnetic interactions allow reversibly switching between the three spin multiplicities by atomic manipulation. An effective Hubbard dimer model is introduced that unifies the effects of local electrostatic gating, electron-electron correlation, hybridization and an exchange field to outline the phase diagram of accessible spin states. These results establish a platform for the local control of π-radicals adsorbed on metallic substrates.