  The magnetic properties of carbon materials are at present the focus of an
intense research effort in physics, chemistry and materials science due to
their potential applications in spintronics and quantum computations. Although
the presence of spins in open-shell nanographenes has been recently confirmed,
the ability to control magnetic coupling sign has remained elusive, but the
most desirable. Here, we demonstrate an effective approach of engineering
magnetic ground states in atomically precise open-shell bipartite/nonbipartite
nanographenes using combined scanning probe techniques and mean-field Hubbard
model calculations. The magnetic coupling sign between two spins has been
controlled via breaking bipartite lattice symmetry of nanographenes. In
addition, the exchange-interaction strength between two spins has been widely
tuned by finely tailoring their spin density overlap, realizing a large
exchange-interaction strength of 42 meV. Our demonstrated method provides ample
opportunities for designer above-room-temperature magnetic phases and
functionalities in graphene nanomaterials.
