  The inelastic scanning tunneling microscopy (STM) has been shown recently
(Loth et al. Science 329, 1628 (2010)) to be extendable as to access the
nanosecond, spin-resolved dynamics of magnetic adatoms and molecules. Here we
analyze theoretically this novel tool by considering the time-resolved spin
dynamics of a single adsorbed Fe atom excited by a tunneling current pulse from
a spin-polarized STM tip. The adatom spin-configuration can be controlled and
probed by applying voltage pulses between the substrate and the spin-polarized
STM tip. We demonstrate how, in a pump-probe manner, the relaxation dynamics of
the sample spin is manifested in the spin-dependent tunneling current. Our
model calculations are based on the scattering theory in a wave-packet
formulation. The scheme is nonpertubative and hence, is valid for all voltages.
The numerical results for the tunneling probability and the conductance are
contrasted with the prediction of simple analytical models and compared with
experiments.
