Two-color snapshots of ultrafast charge and spin dynamics

Ultrafast phenomena on a femtosecond timescale have to be examined in complex pump-probe experiments. Scientists at the Laboratory of Micro- and Nanotechnology have developed a novel type of diffractive X-ray optical element to disclose the dynamic behavior of two magnetic materials in a single snapshot. The X-ray optics can focus two-color pulses of radiation from a free-electron laser and exploits its time dispersive properties to create time traces on a detector downstream of the sample. In an experiment at the DiProI beamline at FERMI, an international scientific team performed such an experiment on an iron-nickel composite magnetic film. The measurements allowed for the first time to follow the response separately for both magnetic elements with perfect relative timing. It unambiguously revealed a delay of approximately 100 fs between the magnetic response with respect to the electronic excitation. This delay shows that the electronic and spin degrees of freedom are decoupled during the demagnetization process. Furthermore, the electronic dynamics of Ni and Fe show pronounced differences when probed at their respective resonance while the demagnetization dynamics are similar. These observations underline that complex materials may have completely different electronic and magnetic behavior than the pure elements they consist of.