  Recent advances in scanning probe techniques rely on the chemical
functionalization of the probe-tip termination by a single molecule. The
success of this approach opens the tantalizing prospect of introducing spin
sensitivity through the functionalization by a magnetic molecule. Here, we use
a nickelocene-terminated tip (Nc-tip), which offers the possibility of
producing spin excitations on the tip apex of a scanning tunneling microscope
(STM). We show that when the Nc-tip is a hundred pm away from point contact
with a surface-supported object, magnetic effects may be probed through changes
in the spin excitation spectrum of nickelocene. We use this detection scheme to
simultaneously determine the exchange field and the spin polarization of the
sample with atomic-scale resolution. Our findings demonstrate that the Nc-tip
is a powerful probe for investigating surface magnetism with STM, from single
magnetic atoms to surfaces.
