Ultrafast demagnetization of metals: Collapsed exchange versus collective excitations

We have investigated the magneto-optical response of Fe epitaxial films by femtosecond pump-probe polarimetry in a broad probe spectral region (1.8-2.6 eV). From the extrapolated photoinduced variation of the conductivity tensor, spin and charge dynamics have been disentangled. In particular, the analysis of the off-diagonal tensor element rules out any appreciable modification of the electronic band structure upon laser excitation and suggests that ultrafast demagnetization is determined by collective excitations, i.e., spin fluctuations. Our experimental evidence provides significant insight into the microscopic mechanisms governing the complex spin dynamics of metals in the subpicosecond time scale.