  We reversibly switch the polar environment of an individual magnetic atom
with an electric field to control the energy barrier for reversal of
magnetization. By applying an electric field in the gap between the tip and
sample of a scanning tunneling microscope, we induce bistable changes in the
polarization of the region surrounding a chlorine vacancy in a monolayer of
sodium chloride on copper terminated by a monolayer of copper nitride. The
displacement of the sodium chloride ions alters the local electric polarization
and modifies the magnetocrystalline anisotropy experienced by a single cobalt
atom. When a cobalt atom is near a chlorine vacancy, spin-sensitive inelastic
electron tunneling spectroscopy measurements can reveal the change in
anisotropy. The demonstration of atomic-scale control of magnetic properties
with electric fields opens new possibilities for probing the origins of
magnetoelectric coupling and will stimulate the development of model artificial
mutliferroic systems.
