  A simple double-decker molecule with magnetic anisotropy, nickelocene, is
attached to the metallic tip of a low-temperature scanning tunneling
microscope. In the presence of a Cu(100) surface, the conductance around the
Fermi energy is governed by spin-flip scattering, the nature of which is
determined by the tunneling barrier thickness. The molecular tip exhibits
inelastic spin-flip scattering in the tunneling regime, while in the contact
regime a Kondo ground state is stabilized causing an order of magnitude change
in the zero-bias conductance. First principle calculations show that
nickelocene reversibly switches from a spin 1 to 1/2 between the two transport
regimes.
