  We show how the spin-transfer torque generated by an ac voltage may be used
to excite a paramagnetic resonance of an atomic spin deposited on a metallic
surface. This mechanism is independent of the environment of the atom and may
explain the ubiquity of the paramagnetic resonance reported by Baumann
$\textit{et al.}$ [$\href{http://dx.doi.org/10.1126/science.aac8703}{Science
\textbf{350}, 417 (2015)}$]. The current and spin dynamics are modeled by a
time-dependent Redfield master equation generalized to account for the periodic
driven voltage. Our approach shows that the resonance effect is a consequence
of the nonlinearity of the coupling between the magnetic moment and the
spin-polarized current which generates a large second-harmonic amplitude that
can be measured in the current signal.
