Enabling time-resolved 2D spatial-coherence measurements using the Fourier-analysis method with an integrated curved-grating beam monitor

Direct 2D spatial-coherence measurements are increasingly gaining importance at synchrotron beamlines, especially due to present and future upgrades of synchrotron facilities to diffraction-limited storage rings. We present a method to determine the 2D spatial coherence of synchrotron radiation in a direct and particularly simple way by using the Fourier-analysis method in conjunction with curved gratings. Direct photon-beam monitoring provided by a curved grating circumvents the otherwise necessary separate determination of the illuminating intensity distribution required for the Fourier-analysis method. Hence, combining these two methods allows for time-resolved spatial-coherence measurements. As a consequence, spatial-coherence degradation effects caused by beamline optics vibrations, which is one of the key issues of state-of-the-art X-ray imaging and scattering beamlines, can be identified and analyzed. © 2020 Optical Society of America

Multi-Scale Study of All-Optically Induced Magnetization Dynamics in Co/Pt Multilayers utilizing TR-mSAXS at FEL Sources

This thesis deals with the impact of ultrashort near-infrared~(nIR) and extreme ultraviolet~(XUV) laser pulses on the magnetic multi-domain states of particularly thin Co/Pt multilayers. The laser induced magnetization dynamics are investigated with femtosecond time and nanometer spatial resolution utilizing time-resolved magnetic small-angle X-ray scattering~(TR-mSAXS) at the free-electron lasers~(FEL) FLASH in Hamburg and FERMI@Elettra in Trieste.One part of the thesis deals with ultrafast demagnetization in three different Co/Pt-multilayer samples with total film thicknesses in the range of the attenuation length of nIR radiation in Co and Pt. For excitation of the magnetic states, nIR-laser pulses of different fluence, pulse duration and polarization are used, addressing important aspects of ultrafast demagnetization in such optically thin Co/Pt multilayers, for the first time, by resonant magnetic scattering. In particular, a model that accounts for both the low-temperature behavior of the remagnetization dynamics and its drastic slowing down at high temperatures is proposed, taking into account recent theoretical predictions. Within this model, the remagnetization dynamics are described via energy exchange between a strongly coupled electron-spin system and the phonon system.Another part of the thesis addresses the influence of nIR and XUV-laser~(FEL) pulses on the lateral configuration of nanoscopic multi-domain states. Different multi-domain states are generated in a selected Co/Pt multilayer by using out-of-plane~(OOP) magnetic fields. Aside from ultrafast demagnetization, that behaves similar for the different multi-domain states, permanent lateral domain modifications forming on longer time scales are observed, that do depend on the underlying multi-domain state. Moreover, the permanent modifications only occur if nIR and XUV-laser pulses temporally overlap. Since the action of the combined~(pump/probe) peak intensity alone cannot explain the observed effects, it is concluded that the permanent modifications also depend on the photon energies of the laser pulses. In particular, the permanent modifications in the close-to single-domain state point at laser induced nucleation processes and thus a novel all-optical switching~(AOS) like mechanism that is based on the interplay of two different laser excitations

Single and multi-pulse based X-ray Photon Correlation Spectroscopy

The ability of pulsed nature of synchrotron radiation opens up the possibility of studying microsecond dynamics in complex materials via speckle-based techniques. Here, we present the study of measuring the dynamics of a colloidal system by combining single and multiple X-ray pulses of a storage ring. In addition, we apply speckle correlation techniques at various pulse patterns to collect correlation functions from nanoseconds to milliseconds. The obtained sample dynamics from all correlation techniques at different pulse patterns are in very good agreement with the expected dynamics of Brownian motions of silica nanoparticles in water. Our study will pave the way for future pulsed X-ray investigations at various synchrotron X-ray sources using individual X-ray pulse patterns

Ultrafast Magnetisation Dynamics at the Low-Fluence Limit Supported by External Magnetic Fields

We report on ultrafast magnetisation dynamics in ferromagnetic cobalt/platinum multilayers upon pumping by near and mid to far infrared radiation, utilizing sub-100 femtosecond free-electron laser pulses. The evolution of the excited magnetic state is studied on femtosecond timescales with nanometre spatial resolution and element selectivity, employing time-resolved magnetic small-angle X-ray scattering. The obtained results contribute to the ongoing discussion to what extent either coupling of the electromagnetic field or rather quasi-instantaneous heating of the electron-system is the driving force for phenomena like ultrafast demagnetization or all-optical helicity-dependent switching

Ultrafast Demagnetization Excited by Extreme Ultraviolet Light From a Free-Electron Laser

Ultrashort and intense extreme ultraviolet (XUV) and X-ray pulses readily available at free-electron lasers (FELs) enable studying non-linear light−matter interactions on femtosecond timescales. Here, we report on the non-linear fluence dependence of magnetic scattering of Co/Pt multilayers, using FERMI FEL’s 70-fs-long single and double XUV pulses, the latter with a temporal separation of 200 fs, with a photon energy slightly detuned to the Co M2,3 absorption edge. We observe a quenching in magnetic scattering that sets-in already in the non-destructive fluence regime of a few mJ/cm² typically used for FEL-probe experiments on magnetic materials. Calculations of the transient electronic structure in tandem with a phenomenological modeling of the experimental data by means of ultrafast demagnetization unambiguously show that XUV-radiation-induced demagnetization is the dominant mechanism for the quenching in the investigated fluence regime of <50 mJ/cm², while light-induced changes of the electronic core levels are predicted to additionally occur at higher fluences. The modeling of the data further indicates that the demagnetization proceeds on the sub-20-fs timescale. This ultrashort timescale is consistent with non-coherent models for ultrafast demagnetization, considering the sub-femtosecond lifetime of hot electrons with energies of a few 10 eV generated by the XUV radiation

Ultrafast dynamics of spatial magnetic fluctuations in Co/Pt multilayers studied at European XFEL

One of the intriguing problems of modern magnetism is unravelling the non-equilibrium spin dynamics following laser excitation on the nanometer length scale. In this work the ultrafast magnetic behavior of thin Co/Pt multilayers is studied by resonant magnetic SAXS in transmission geometry. We have for the first time observed a very bright transient scattering from nanometer scale magnetic fluctuations

Megahertz-rate Ultrafast X-ray Scattering and Holographic Imaging at the European XFEL

13 pages, 5 figures. Supplementary Information as ancillary fileThe advent of X-ray free-electron lasers (XFELs) has revolutionized fundamental science, from atomic to condensed matter physics, from chemistry to biology, giving researchers access to X-rays with unprecedented brightness, coherence, and pulse duration. All XFEL facilities built until recently provided X-ray pulses at a relatively low repetition rate, with limited data statistics. Here, we present the results from the first megahertz repetition rate X-ray scattering experiments at the Spectroscopy and Coherent Scattering (SCS) instrument of the European XFEL. We illustrate the experimental capabilities that the SCS instrument offers, resulting from the operation at MHz repetition rates and the availability of the novel DSSC 2D imaging detector. Time-resolved magnetic X-ray scattering and holographic imaging experiments in solid state samples were chosen as representative, providing an ideal test-bed for operation at megahertz rates. Our results are relevant and applicable to any other non-destructive XFEL experiments in the soft X-ray range