Prior to the development of pulsed lasers, one assigned a single local temperature to the lattice, the electron gas and the spins. With the availability of ultrafast laser sources, these reservoirs can be driven out of equilibrium. The internal degrees of freedom of the spin system are largely unexplored. To study the basic processes relevant for ultrafast spin dynamics we developed a spin and time resolved photoelectron spectrometer. A high harmonic generation source provides femtosecond pulses of 21eV photon energy. The photo-emitted electrons are analyzed in energy by a hemispheric energy analyzer and subsequently spin-analyzed by an imaging SPLEED detector.
Using this instrument we observe the spin polarization of separate regions within the valence band of the ferromagnet: at the Fermi energy, the polarization is reduced faster than at deeper in the valence band. Therefore, on the femtosecond time scale, the magnetization separates into different parts similar to how the single temperature paradigm changed with the development of ultrafast lasers. Our measurements show the full evolution of the spin polarization as a function of time. We can observe the initial generation of a non-thermal electron distribution followed by the thermalization and de-polarization at the Fermi energy. Only later, the band structure changes, leading to a reduction of the polarization in the valence band.
This talk will give an overview over the different aspect of magneto-dynamics spanning from the geological time scales to the femtosecond demagnetization effect..