Magnetization dynamics on the femtosecond time scale in ferromagnetic metals

Summary form only given. Recent experiments have revealed ultrafast optically-induced dynamics in the time-resolved magneto-optical (MO) signals obtained in metallic ferromagnetic samples. The very short time scales that have been reported seem to exclude the role of spin-lattice interactions in the dynamics and open several fundamental questions about the physical mechanism responsible for fast demagnetization. However, the usual interpretation of such polarimetric versions of a pump-probe experiment relies on the strong assumption that the polarization state of the probe is only affected by the magnetization of the material (static MO experiments). This assumption becomes particularly questionable in the very-short delay limit because the coherent effects can completely mask the MO component of the signal that would probe the sample magnetization. In particular, two recent papers suggested that an electronic spin-population previously induced by a circularly polarized pump (in analogy with the inverse Faraday effect) is at the origin of transient MO effects that have been detected with a time-delayed probe beam. The ultrashort characteristic times that have been observed in these experiments open some questions, because at that time-scale the coherent effects, which are not necessarily related to a magnetization change, are expected to play a prominent role. In order to clarify these aspects of the ultrafast magnetization dynamics measurements, we studied the transient pump-induced anisotropy in a non-ferromagnetic material with both linear and circular pump polarizations. We measured such an anisotropy both by time-resolved polarimetry (linear probe azimuth rotation) and by transient transmissivity and reflectivity.