  The orbital angular moment of magnetic atoms adsorbed on surfaces is often
quenched as a result of an anisotropic crystal field. Due to spin-orbit
coupling, what remains of the orbital moment typically delineates the
orientation of the electron spin. These two effects limit the scope of
information processing based on these atoms to essentially only one magnetic
degree of freedom: the spin. In this work, we gain independent access to both
the spin and orbital degrees of freedom of a single atom, inciting and probing
excitations of each moment. By coordinating a single Fe atom atop the nitrogen
site of the Cu$_2$N lattice, we realize a single-atom system with a large
zero-field splitting--the largest reported for Fe atoms on surfaces--and an
unquenched uniaxial orbital moment that closely approaches the free-atom value.
We demonstrate a full reversal of the orbital moment through a single-electron
tunneling event between the tip and Fe atom, a process that is mediated by a
charged virtual state and leaves the spin unchanged. These results, which we
corroborate using density functional theory and first-principles multiplet
calculations, demonstrate independent control over the spin and orbital degrees
of freedom in a single-atom system.
