Time: Friday, October 25th, 11:00
Speaker: Felix WILLEMS, Berlin
With the advent of new free-electron and laboratory-based high harmonic radiation sources ultrafast magnetic spectroscopy at 3p to 3d transitions (M-edges) has started to develop into a widely used experimental technique. The advantages of this technique are manifold: (a) it leads to the element-specific response of complex multi-component magnetic systems simultaneously in a single measurement [1, 2] (b) it enables ultrafast distinction between the physics of various energy scales; exchange and spin-orbit interaction as well as collective spin excitations and (c) the wavelength of XUV radiation allows to directly probe nanoscale length scales via small angle X-ray scattering  and coherent imaging methods [2, 3]. However, to quantitatively interpret increasingly complex experimental data from this technique, the scientific community relies on fundamental experimental and theoretical groundwork. The lack of such a systematic and thorough study has resulted in a number of controversies in literature stemming mostly from difficulties to separate possibly overlapping resonances and from a non-zero off-resonant magnetic signal at lower photon energies. In our work we address all these important questions by presenting a complete measurement of the complex dichroic index of refraction for Co, Fe and Ni (i.e. both the dispersive and absorptive contributions) and comparing it with state-of-the-art ab-initio density functional theory (DFT) calculations .
Furthermore, in a time resolved high harmonic experiment, we measure the transient changes of the absorptive refractive index displaying distinct femtosecond dynamics depending on the polarization state of the probing XUV light. Doing so we can distinguish the between excitation of the pure electronic and the spin system. A comparison with time dependent DFT simulations suggests an explanation based on ultrafast, spin-orbit mediated spin-flips .
 F. Willems et al., Physical Review B 92, 220405(R) (2015)
 F. Willems et al., Structural Dynamics 4, 014301 (2017)
 C. von Korff Schmising et al., Phys. Rev. Lett., 112, 217203 (2014)
 F. Willems et al., in preparation