Disappearance of the Kondo resonance for atomically fabricated cobalt dimers

We have used scanning tunneling spectroscopy and atomic manipulation to stu dy the interaction between a single pair of magnetic atoms at the surface o f 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 disa ppearance of the Kondo resonance for cobalt-cobalt separations less than 6 Angstrom. This behavior is explained as the result of reduced exchange coup ling between gold conduction electrons and ferromagnetic cobalt dimers. [S0 163-1829(99)50436-03].