  We present a theoretical framework to describe experiments directed to
controlling single-atom spin dynamics by electrical means using a scanning
tunneling microscope. We propose a simple model consisting of a quantum
impurity connected to electrodes where an electrical time-dependent bias is
applied. We solve the problem in the limit of weak coupling between the
impurity and the electrodes by means of a quantum master equation that is
derived by the non-equilibrium Green's function formalism. We show results in
two cases. The first case is just a single atomic orbital subjected to a
time-dependent electric field, and the second case consists of a single atomic
orbital coupled to a second spin-1/2. The first case reproduces the main
experimental features Ti atoms on MgO/Ag (100) while the second one directly
addresses the experiments on two Ti atoms. These calculations permit us to
explore the effect of different parameters on the driving of the atomic spins
as well as to reproduce experimental fingerprints.
