  Nanographenes with zigzag edges are predicted to manifest non-trivial
pi-magnetism resulting from the interplay of hybridization of localized
frontier states and Coulomb repulsion between valence electrons. This provides
a chemically tunable platform to explore quantum magnetism at the nanoscale and
opens avenues toward organic spintronics. The magnetic stability in
nanographenes is thus far limited by the weak magnetic exchange coupling which
remains below the room temperature thermal energy. Here, we report the
synthesis of large rhombus-shaped nanographenes with zigzag periphery on gold
and copper surfaces. Single-molecule scanning probe measurements unveil an
emergent magnetic spin-singlet ground state with increasing nanographene size.
The magnetic exchange coupling in the largest nanographene, determined by
inelastic electron tunneling spectroscopy, exceeds 100 meV or 1160 K, which
outclasses most inorganic nanomaterials and remarkably survives on a metal
electrode.
