  ESR-STM is an emerging technique which is capable of detecting the precession
of a single spin. We discuss the mechanism of ESR-STM based on a direct
exchange coupling between the tunneling electrons and the local precessing spin
S. We claim that since the number of tunneling electrons in a single precessing
period is small (~20) one may expect a net temporary polarization within this
period that will couple via exchange interaction to the localized spin. This
coupling will randomly modulate the tunneling barrier and create a dispersion
in the tunneling current which is a product of a Larmor frequency component due
to the precession of the single spin and the dispersion of the spin of the
tunneling electrons. This noise component is spread over the whole frequency
range for random white noise spin polarization of electrons. In opposite case
the power spectrum of the spins of the tunneling electrons has a peak at zero
frequency an elevated noise in the current at omega_L will appear. We discuss
the possible source of this spin polarization. We find that for relevant values
of parameters signal to noise ratio in the spectral characteristic is 2-4 and
is comparable to the reported signal to noise ratio. The magnitude of the
current fluctuation is a relatively weak increaing function of the DC current
and the magnetic field. The linewidth produced by the back action effect of
tunneling electrons on the precessing spin is also discussed.
