  We have performed single-atom magnetization curve (SAMC) measurements and
inelastic scanning tunneling spectroscopy (ISTS) on individual Fe atoms on a
Cu(111) surface. The SAMCs show a broad distribution of magnetic moments with
$\unit[3.5]{\mu_{\rm B}}$ being the mean value. ISTS reveals a magnetization
excitation with a lifetime of $\unit[200]{fsec}$ which decreases by a factor of
two upon application of a magnetic field of $\unit[12]{T}$. The experimental
observations are quantitatively explained by the decay of the magnetization
excitation into Stoner modes of the itinerant electron system as shown by newly
developed theoretical modeling.
