  The conductance profiles of magnetic transition metal atoms, such as Fe, Co
and Mn, deposited on surfaces and probed by a scanning tunneling microscope
(STM), provide detailed information on the magnetic excitations of such
nano-magnets. In general the profiles are symmetric with respect to the applied
bias. However a set of recent experiments has shown evidence for inherent
asymmetries when either a normal or a spin-polarized STM tip is used. In order
to explain such asymmetries here we expand our previously developed
perturbative approach to electron-spin scattering to the spin- polarized case
and to the inclusion of out of equilibrium spin populations. In the case of a
magnetic STM tip we demonstrate that the asymmetries are driven by the
non-equilibrium occupation of the various atomic spin-levels, an effect that
reminds closely that electron spin-transfer. In contrast when the tip is not
spin-polarized such non-equilibrium population cannot be build up. In this
circumstance we propose that the asymmetry simply originates from the
transition metal ion density of state, which is included here as a
non-vanishing real component to the spin-scattering self-energy.
