  Shrinking spintronic devices to the nanoscale ultimately requires localized
control of individual atomic magnetic moments. At these length scales, the
exchange interaction plays important roles, such as in the stabilization of
spin-quantization axes, the production of spin frustration, and creation of
magnetic ordering. Here, we demonstrate the precise control of the exchange
bias experienced by a single atom on a surface, covering an energy range of
four orders of magnitude. The exchange interaction is continuously tunable from
milli-eV to micro-eV by adjusting the separation between a spin-1/2 atom on a
surface and the magnetic tip of a scanning tunneling microscope (STM). We
seamlessly combine inelastic electron tunneling spectroscopy (IETS) and
electron spin resonance (ESR) to map out the different energy scales. This
control of exchange bias over a wide span of energies provides versatile
control of spin states, with applications ranging from precise tuning of
quantum state properties, to strong exchange bias for local spin doping. In
addition we show that a time-varying exchange interaction generates a localized
AC magnetic field that resonantly drives the surface spin. The static and
dynamic control of the exchange interaction at the atomic-scale provides a new
tool to tune the quantum states of coupled-spin systems.
