Ultrafast laser-induced magneto-optical changes in resonant magnetic x-ray reflectivity

We investigate the magneto-optical response of Co to an ultrashort laser excitation by x-ray resonant magnetic reflectivity (XRMR) employing circular polarization. The time-resolved reflectivities detected for opposite sample magnetization are separated into magnetic and nonmagnetic contributions, which contain information about the structural, electronic, and magnetic properties of the sample. Different response times of the different contributions are observed. The experimental results are reproduced numerically by two different simulation approaches. On the one hand, we use a purely thermal model, a time-dependent heat-induced loss of macroscopic magnetization, and an inhomogeneous laser-induced strain profile. On the other hand, we employ time-dependent density-functional theory to calculate the transient optical response to the laser-induced excitation and from that the reflected intensities. While both methods are able to reproduce the time dependence of the magnetic signal, the ultrafast nonmagnetic change in reflectivity is captured satisfactorily only in simulations of the transient optical response function and has thus to be assigned to electronic effects. The energy dependence of the magnetic circular dichroism is investigated in the simulations, highlighting a dependence of the observable on the probing energy. Finally, a phenomenological explanation of the dynamics measured in dichroic x-ray reflectivity in the different channels is offered.