Spin-flip scattering selection in a controlled molecular junction

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.