  We investigate the dependence of the inelastic spin excitation contribution
to scanning tunneling spectroscopy recorded above Fe adsorbates on the
InSb(110) surface at subkelvin temperatures, on bulk doping and tunnel junction
resistance. To explain our observations, we show how the inelastic contribution
depends on the parameters describing the excitation mechanism and tunnel
conditions, in the framework of a recently developed model. We conclude that in
this particular system of an adsorbate which is strongly relaxed into the
substrate, the tip-sample distance dependent by-tunneling has to be taken into
account in order to explain the observed variations in the inelastic tunnel
spectra.
