  The recently discovered giant magnetic anisotropy of single magnetic Co atoms
raises the hope of magnetic storage in small clusters. We present a joint
experimental and theoretical study of the magnetic anisotropy and the spin
dynamics of Fe and Co atoms, dimers, and trimers on Pt(111). Giant anisotropies
of individual atoms and clusters as well as lifetimes of the excited states
were determined with inelastic scanning tunneling spectroscopy. The short
lifetimes due to hybridization-induced electron-electron scattering oppose the
magnetic stability provided by the magnetic anisotropies.
