  Electron paramagnetic resonance (EPR) spectroscopy is widely employed to
characterize paramagnetic complexes. Recently, EPR combined with scanning
tunneling microscopy (STM) achieved single-spin sensitivity with sub-angstrom
spatial resolution. The excitation mechanism of EPR in STM, however, is broadly
debated, raising concerns about widespread application of this technique. Here,
we present an extensive experimental study and modelling of EPR-STM of Fe and
hydrogenated Ti atoms on an MgO surface. Our results support a piezoelectric
coupling mechanism, in which the EPR species oscillate adiabatically in the
inhomogeneous magnetic field of the STM tip. An analysis based on Bloch
equations combined with atomic-multiplet calculations identifies different EPR
driving forces. Specifically, transverse magnetic-field gradients drive the
spin-1/2 hydrogenated Ti, whereas longitudinal magnetic-field gradients drive
the spin-2 Fe. Additionally, our results highlight the potential of
piezoelectric coupling to induce electric dipole moments, thereby broadening
the scope of EPR-STM to nonpolar species and nonlinear excitation schemes.
