  The magnetic coupling between single Co atoms adsorbed on a copper surface is
determined by probing the Kondo resonance using low-temperature scanning
tunneling spectroscopy. The Kondo resonance, which is due to magnetic
correlation effects between the spin of a magnetic adatom and the conduction
electrons of the substrate, is modified in a characteristic way by the coupling
of the neighbouring adatom spins. Increasing the interatomic distance of a
Cobalt dimer from 2.56A to 8.1A we follow the oscillatory transition from
ferromagnetic to antiferromagnetic coupling. Adding a third atom to the
antiferromagnetically coupled dimer results in the formation of a collective
correlated state.
