  A simple expression for the conductance steps in the inelastic electron
tunneling from spin excitations in a single magnetic atom adsorbed on a
non-magnetic metal surfaces is derived. The inelastic coupling between the
tunneling electron and the spin is via the exchange coupling and is treated in
an impulsive approximation using the Tersoff-Hamann approximation for the
tunneling between the tip and the sample. Our results for conductance steps
justify the analysis carried out by Hirjebedin et al. [Science 317, 1199
(2007)] of observed step-like conductances by inelastic electron tunneling from
spin excitations in a single magnetic adatom using a simple spin matrix
element. In addition, our result gives a simple expression for the magnitudes
of conductance steps and their lateral spatial variation with respect to the
tip position, which can be calculated directly from spin-polarized wave
functions at the Fermi level of the sample.
