  We demonstrate that the magnetization reversal in a ferromagnetic Pt/Co/Pt
stack can be induced by a single femtosecond laser pulse. We find that the size
of the switched spot is comparable to the size of the intrinsic magnetic
domains. It requires an absorbed energy density of $\sim$4 mJ.cm$^{-2}$, beyond
which the excited spot fragments into a multidomain structure. The switching
process occurs back and forth with subsequent laser pulses and it is
helicity-independent. Furthermore, the dynamics of the magnetization reversal
occurs in a timescale less than one microsecond. These results suggest that all
optical switching in ferromagnetic films requires to match the laser spot with
the specific domain sizes.
