  Magnetic order emerging in otherwise non-magnetic materials as carbon is a
paradigmatic example of a novel type of s-p electron magnetism predicted to be
of exceptional high-temperature stability. It has been demonstrated that atomic
scale structural defects of graphene can host unpaired spins. However, it is
still unclear under which conditions long-range magnetic order can emerge from
such defect-bound magnetic moments. Here we propose that in contrast to random
defect distributions, atomic scale engineering of graphene edges with specific
crystallographic orientation, comprising edge atoms only from one sub-lattice
of the bipartite graphene lattice, can give rise to a robust magnetic order. We
employ a nanofabrication technique based on Scanning Tunneling Microscopy to
define graphene nanoribbons with nanometer precision and well-defined
crystallographic edge orientations. While armchair ribbons display quantum
confinement gap, zigzag ribbons narrower than 7 nm reveal a bandgap of about
0.2 - 0.3 eV, which can be identified as a signature of interaction induced
spin ordering along their edges. Moreover, a semiconductor to metal transition
is revealed upon increasing the ribbon width, indicating the switching of the
magnetic coupling between opposite ribbon edges from antiferromagnetic to
ferromagnetic configuration. We found that the magnetic order on graphene edges
of controlled zigzag orientation can be stable even at room temperature,
raising hope for graphene-based spintronic devices operating under ambient
conditions.
