  The adsorption behavior and the magnetic states of cobalt atoms and dimers on
single vacancies in a graphene sheet are investigated by means of relativistic
density functional calculations. It is found that local magnetic moments are
formed in both cases, despite strong chemical binding. Of particular interest
are kinetically stable isomers with two cobalt atoms attached to the same side
of the graphene sheet. Magnetic bi-stability with an anisotropy barrier of
about 50 meV is possible in this geometry. The feasibility of its preparation
is discussed.
