  In this work the magnetization dynamics of clusters supported on non-magnetic
substrates is shown to exhibit an unprecedented complex response when subjected
to external magnetic fields. The field-driven magnetization reversal of small
Co clusters deposited on a Cu(111) surface has been studied by means of
first-principles calculations and atomistic spin dynamics simulations. For
applied fields ranging from 1 Tesla to 10 Tesla, we observe a coherent
magnetization reversal with switching times in the range of several tenths of
picoseconds to several nanoseconds, depending on the field strength. We find a
non-monotonous dependence of the switching times with respect to the strength
of the applied field, which we prove to have its origin in the complex magnetic
anisotropy landscape of these low dimensional systems. This effect is shown to
be stable for temperatures around 10 K, and is possible to realize over a range
of exchange interactions and anisotropy landscapes. Possible experimental
routes to achieve this unique switching behaviour are discussed.
