Experimental access to femtosecond spin dynamics

In this paper we discuss various contributions to the transient magneto-optical (MO) response of ferromagnetic metals after laser-heating by a femtosecond pump pulse. It is demonstrated that a simple relation between MO response and the fast magnetization dynamics does not hold. A more general concept is introduced, on the basis of which artifacts are experimentally identified. Special focus is on problems associated with conservation of angular momentum in the ultrafast regime. A model describing dichroic state filling effects, fully including the coherent interactions between pump and probe pulses, is worked out. One of the potential methods to achieve access to the genuine magnetization dynamics, based on a thermal difference scheme, is analysed in detail. Finally, it is discussed how the different dynamical processes triggered by the laser pulse appear in the transient MO signal.