We have developed an experimental set-up based on time-resolved Magneto-Optical Kerr Effect (MOKE) that allows to retrieve the vectorial magnetization dynamics in thin films with sub-picosecond resolution. This method has been exploited to measure the variations of the magnetization (modulus and orientation) induced by an ultrashort laser pulse. The initial demagnetization is established at the electronic level within a few hundreds of femtoseconds through electron-magnon excitations. The subsequent dynamics is characterized by a precessional motion on the 100 picosecond time-scale, around an effective, time-dependent field. Following the full dynamics of the magnetization, we have unambiguously determined the temporal evolution of the magneto-crystalline anisotropy, providing the clear experimental evidence that the precession is triggered by the rapid, optically-induced misalignment between the magnetization vector and the effective field. This method provides a simple and widely applicable way to study both magnetization and anisotropy in the sub-picosecond regime and therefore to unravel the mechanisms underlying the ultrafast evolution of the spin order in magnetic media.
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