Disappearance of the Kondo resonance for atomically fabricated cobalt dimers

We have used scanning tunneling spectroscopy and atomic manipulation to study the interaction between a single pair of magnetic atoms at the surface of a nonmagnetic metal. The local electronic structure of cobalt adatoms on Au(111) was measured for different cobalt-cobalt interatomic spacings at $T=6$ K. Artificially fabricated cobalt dimers are found to show an abrupt disappearance of the Kondo resonance for cobalt-cobalt separations less than 6 {}. This behavior is explained as the result of reduced exchange coupling between gold conduction electrons and ferromagnetic cobalt dimers.