On the microscopic origin of femtosecond magnetization dynamics

Over the past few years a wealth of new data on laser-induced ultrafast magnetization dynamics has become available. By now, it is generally agreed on that within a few hundred femtoseconds after laser excitation of ferromagnetic transition metals such as nickel the spin system equilibrates at a reduced magnetization. As to the underlying physics that enables such an ultrafast response little is known. The first part of this presentation is devoted to a careful analysis of proposed mechanisms in view of the restriction of conservation of total angular momentum. The potential role of electrons, photons and the lattice to compensate for the quenched magnetic moment will be discussed. Moreover, I will formulate some specific constraints to be fulfilled to extract reliably information on the electron and magnetization dynamics from time-resolved experiments. In the second part, mechanisms involving the lattice degree of freedom will be discussed in more detail. I will present first results of a fully microscopic approach that we implemented recently. A simple model Hamiltonian describes the interactions between a Fermi sea of (spinless) electrons, an ensemble of two level oscillators representing the phonon system, and the magnetic degree of freedom represented by a mean-field Weiss model. In particular we include an Elliot-Yafet type of spin-orbit induced spin scattering by assigning a spin-flip probability aEY to each electron-phonon scattering event. In principle, the electron- phonon (e-p) scattering time, and the demagnetization time can be fitted from transient reflectivity and time- resolved MOKE experiments. When doing so, comparison with the model would provide an estimate of the required aEY. Among the most exciting observations found, we were able to show that even though the demagnetization in our model is mediated by phonon scattering, it is possible to derive a demagnetization time that is shorter than the e-p equilibration- (and even thermalization) time for reasonable values of aEY below unity. Furthermore, predictions will be shown for demagnetization times while laser-heating the ferromagnet above the Curie temperature. Finally, the model will be discussed in a more general context of magnetization