  The effective spin exchange coupling between impurities (adatoms) on graphene
mediated by conduction electrons is studied as a function of the strength of
the potential part of the on-site energy $U$ of the electron-adatom
interaction. With increasing $U$, the exchange coupling becomes long-range,
determined largely by the impurity levels with energies close to the Dirac
points. When adatoms reside on opposite sublattices, their exchange coupling,
normally antiferromagnetic, becomes ferromagnetic and resonantly enhanced at a
specific distance where an impurity level crosses the Dirac point.
