Femtosecond charge and spin dynamics in a CoPt alloy.

The use of advanced x-ray sources plays a key role in the study of dynamic processes in magnetically ordered materials. The progress in x-ray free-electron lasers enables the direct and simultaneous observation of the femtosecond evolution of electron and spin systems through transient x-ray absorption spectroscopy and x-ray magnetic circular dichroism, respectively. Such experiments allow us to resolve the response seen in the population of the spin-split valence states upon optical excitation. Here, we utilize circularly polarized ultrashort soft x-ray pulses from the new helical afterburner undulator at the free-electron laser FLASH in Hamburg to study the femtosecond dynamics of a laser-excited CoPt alloy at the Co L-edge absorption. Despite employing a weaker electronic excitation level, we find a comparable demagnetization for the Co 3-states in CoPt compared to previous measurements on CoPd. This is attributed to the distinctly different spin-orbit coupling between 3 and 4 vs 3 and 5 elements in the corresponding alloys and multilayers.