Photo-induced spin dynamics revealed by time-resolved measurements with X-ray free electron laser

We developed a system for measuring time-resolved X-ray magnetic circular dichroism (trXMCD) that uses a pump-probe method at an X-ray free electron laser (XFEL), SACLA BL3, and succeeded in capturing the ultrafast demagneization process of 5d elements using the 5d L edge with a time resolution of about 50 fs. Circularly polarized XFEL pulses were generated by using an X-ray phase retarder, and the jitter of the pulses was compensated for with a timing monitor system. We used FePt thin films, which exhibit perpendicular magneto anisotropy, as our samples and determined the demagnetization time scale of Pt to be τ_pt=610±40 fs. We also measured the time-resolved magneto optical Kerr effect (trMOKE) with visible light, the result of which mainly reflects the Fe magnetic moment. TrMOKE suggested that τ_Fe=100 fs or less. We found that demagnetization time scales have a clear dependence on elements in FePt. These different time scales may be related to the all-optical switching of granular FePt. Our newly developed technique has the potential to elucidate the photo-induced dynamics of 5d element magnetism. (author)