Faster chiral versus collinear magnetic order recovery after optical excitation revealed by femtosecond XUV scattering

While chiral spin structures stabilized by Dzyaloshinskii-Moriya interaction (DMI) are candidates as novel information carriers, their dynamics on the fs-ps timescale is little known. Since with the bulk Heisenberg exchange and the interfacial DMI two distinct exchange mechanisms are at play, the ultra-fast dynamics of the chiral order needs to be ascertained and compared to the dynamics of the conventional collinear order. Using an XUV free-electron laser we determine the fs-ps temporal evolution of the chiral order in domain walls in a magnetic thin film sample by an IR pump - X-ray magnetic scattering probe experiment. Upon demagnetisation we observe that the dichroic (CL-CR) signal connected with the chiral order correlator $m_z m_x$ in the domain walls recovers significantly faster than the (CL+CR) sum signal representing the average collinear domain magnetisation $m_z^2 + m_x^2$. We explore possible explanations based on spin structure dynamics and reduced transversal magnetisation fluctuations inside the domain walls and find that the latter can explain the experimental data leading to different dynamics for collinear magnetic order and chiral magnetic order.Comment: 28 pages, 14 figure

Interaction of femtosecond X-ray pulses with periodical multilayer structures

The VUV Free Electron Laser FLASH operates in soft X-ray range and produces high-intensive pulse trains with few tens femtoseconds duration. The transversely fully coherent beam will open new experiments in solid state physics which can not be studied with present radiation sources. The study of the time dependent response of the multilayer to the X-ray pulse can provide insights into the process of interaction of highly intense FEL radiation with matter. To test the influence of electron excitation on the optical properties of boron carbide, the refractive index of B4C was measured near B K-edge by energy-resolved photon-in-photon-out method probing a Bragg reflection from periodical multilayers. The measured data clearly show that the variation of the fine structure of the K-absorption edges due to the chemical nature of the absorber element. The knowledge obtained from experiments with continuous radiation was used to design the respective experiments with pulse from the FEL. In my thesis, it is proposed that the geometrical setup, where the incident pulse arrives from the FEL under the angle close to the 1st order ML Bragg peak, provides the most valuable information. Preliminary simulation considering form factors of neutral and ionized boron showed that due to ionization, pronounced changes in the reflectivity curve are expected. The proposed scheme can be the powerful tool to study the various processes within the electronic sub-system of the FEL pulse interaction with matter. This type of investigations gives a deep understanding of the nature of the electronic excitation and the recombination at the femtosecond scale.Der VUV Freie-Elektronenlaser FLASH arbeitet im weichen Röntgenbereich und produziert Wellenpulse mit zum Teil weniger als 10 Femtosekunden Dauer. Der transversal vollständig kohärente Strahl eröffnet neue Experimente in der Festkörperphysik, die mit bis dato verfügbaren Strahlungsquellen nicht durchgeführt werden konnten. Die Untersuchung der zeitabhängigen Antwort eines Multilayers auf einen Röntgenpuls gibt Aufschluss über die Interaktion der sehr intensiven FEL Strahlung mit der Materie. Dazu wurde der Brechungsindex von B4C in der Nähe der K-Absorptionskante von Bor mit Hilfe einer energieaufgelösten photon-in-photon-out Methode gemessen. Dabei wurde die Braggbeugung eines periodischen Multilayers ausnutzt, um den Einfluss der Elektronenanregung auf die optischen Eigenschaften von B4C zu untersuchen. Die gemessenen Daten zeigen eine klare Variation der Feinstruktur der K-Absorptionskante, die die unterschiedlichen chemischen Eigenschaften des Absorberelements Bor in verschiedenen Multlayern wiederspigelt. Das Wissen aus dem Experiment mit kontinuierlicher Strahlung wurde genutzt, um ein entsprechendes Experiment mit dem FEL-Pulsen zu konzipieren. In meiner Arbeit schlage ich einen geometrischen Setup vor, bei welchem der einfallende Puls des FEL unter einem festen Winkel in der Nähe des Braggpeaks erster Ordnung des gekrümmten Multilayers einfällt, und die gestreute Intensität des gesamten Braggpeaks gleichzeitig gemessen werden kann. Vorläufige Simulationen, die Formfaktoren von neutralem und ionisiertem Bor berücksichtigen, zeigen, dass durch Ionisation klare Änderungen in der Reflektivitätskurve erwartet werden können. Das vorgeschlagene Schema kann ein kraftvolles Instrument sein, um verschiedene Prozesse innerhalb des elektronischen sub-Systems der Interaktion des FEL-Pulses mit Materie zu studieren. Diese Art von Untersuchung liefert ein tiefes Verständnis der Natur der elektronischen Anregung und der Rekombination auf der Femtosekundenskala

Measurements of ultrafast spin-profiles and spin-diffusion properties in the domain wall area at a metal/ferromagnetic film interface

Abstract Exciting a ferromagnetic material with an ultrashort IR laser pulse is known to induce spin dynamics by heating the spin system and by ultrafast spin diffusion processes. Here, we report on measurements of spin-profiles and spin diffusion properties in the vicinity of domain walls in the interface region between a metallic Al layer and a ferromagnetic Co/Pd thin film upon IR excitation. We followed the ultrafast temporal evolution by means of an ultrafast resonant magnetic scattering experiment in surface scattering geometry, which enables us to exploit the evolution of the domain network within a 1/e distance of 3 nm to 5 nm from the Al/FM film interface. We observe a magnetization-reversal close to the domain wall boundaries that becomes more pronounced closer to the Al/FM film interface. This magnetization-reversal is driven by the different transport properties of majority and minority carriers through a magnetically disordered domain network. Its finite lateral extension has allowed us to measure the ultrafast spin-diffusion coefficients and ultrafast spin velocities for majority and minority carriers upon IR excitation

Direct Observations of the Structural Properties of Semiconducting Polymer: Fullerene Blends under Tensile Stretching

International audienceWe describe the impact of tensile strains on the structural properties of thin films composed of PffBT4T-2OD π-conjugated polymer and PC 71 BM fullerenes coated on a stretchable substrate, based on a novel approach using in situ studies of flexible organic thin films. In situ grazing incidence X-ray diffraction (GIXD) measurements were carried out to probe the ordering of polymers and to measure the strain of the polymer chains under uniaxial tensile tests. A maximum 10% tensile stretching was applied (i.e., beyond the relaxation threshold). Interestingly we found different behaviors upon stretching the polymer: fullerene blends with the modified polymer; fullerene blends with the 1,8-Diiodooctane (DIO) additive. Overall, the strain in the system was almost twice as low in the presence of additive. The inclusion of additive was found to help in stabilizing the system and, in particular, the π-π packing of the donor polymer chains

In situ measurements of the structure and strain of a π-conjugated semiconducting polymer under mechanical load

International audienceIn this work, in-situ studies of organic thin films under stretching are developed. A high efficiency PffBT4T-2OD π-conjugated polymer (PCE11) was coated directly on a stretchable substrate in order to examine the impact of tensile strains on the structural properties. For that purpose, in-situ grazing incidence X-ray diffraction (GIXD) coupled with optical microscopic observations have been carried out to measure the structural parameters of PCE11 and to probe the mechanical behavior of polymer chains under uniaxial tensile load. It is observed that in the range between 0 and 15%-20% of stretching, the polymer chains become more oriented. Meanwhile an increase of negative values of deformation i.e. compression of the polymer chains along the film normal was measured. Beyond this range of stretching, the polymer order declined and the stress was relaxed. This relaxation is explained by the increased number of cracks spreading over the entire film as observed by optical microscopy

Probing ultrafast changes of spin and charge density profiles with resonant XUV magnetic reflectivity at the free-electron laser FERMI

We report the results of resonant magnetic XUV reflectivity experiments performed at the XUV free-electron laser FERMI. Circularly polarized XUV light with the photon energy tuned to the Fe M-2,M-3 edge is used to measure resonant magnetic reflectivities and the corresponding Q-resolved asymmetry of a Permalloy/Ta/Permalloy trilayer film. The asymmetry exhibits ultrafast changes on 240 fs time scales upon pumping with ultrashort IR laser pulses. Depending on the value of the wavevector transfer Q(z), we observe both decreasing and increasing values of the asymmetry parameter, which is attributed to ultrafast changes in the vertical spin and charge density profiles of the trilayer film. (C) 2017 Author(s)

Nanoscale Transient Magnetization Gratings Created and Probed by Femtosecond Extreme Ultraviolet Pulses

We utilize coherent femtosecond extreme ultraviolet (EUV) pulses from a free electron laser (FEL) to generate transient periodic magnetization patterns with periods as short as 44 nm. Combining spatially periodic excitation with resonant probing at the M-edge of cobalt allows us to create and probe transient gratings of electronic and magnetic excitations in a CoGd alloy. In a demagnetized sample, we observe an electronic excitation with a rise time close to the FEL pulse duration and ∼0.5 ps decay time indicative of electron-phonon relaxation. When the sample is magnetized to saturation in an external field, we observe a magnetization grating, which appears on a subpicosecond time scale as the sample is demagnetized at the maxima of the EUV intensity and then decays on the time scale of tens of picoseconds via thermal diffusion. The described approach opens multiple avenues for studying dynamics of ultrafast magnetic phenomena on nanometer length scales

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