| 1. |
- Suturin, Sergey M., et al.
(författare)
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Short nanometer range optically induced magnetic fluctuations accompanying ultrafast demagnetization of nanoscale ferromagnetic domains
- 2023
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Ingår i: Physical Review B. - : American Physical Society. - 2469-9950 .- 2469-9969. ; 108:17
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Tidskriftsartikel (refereegranskat)abstract
- We have studied the nature of optically induced short-range magnetic fluctuations occurring at 10-nm length scale during ultrafast demagnetization in ferromagnetic Co/Pt multilayers. The time resolved probing of magnetization dynamics was performed with femtosecond soft x-ray pulses at the European x-ray free-electron laser. A transient high-q magnetic scattering accompanying and directly correlated to the destruction of the maze domain network has been observed at picosecond time scale in the wave vector region of 0.2–0.8nm−1. This high-q scattering has a purely magnetic nature and is ascribed to the optically induced short-range magnetic fluctuations developing in the disturbed but not fully destroyed magnetic domain network. Finally, we have simulated the optically induced response of the domain system using a two-temperature atomistic spin model and have concluded that the reason behind the high-q scattering is the laser-driven heating inducing thermal fluctuations of the domain magnetic structure.
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| 2. |
- Engel, Robin Y., et al.
(författare)
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Electron population dynamics in resonant non-linear x-ray absorption in nickel at a free-electron laser
- 2023
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Ingår i: Structural Dynamics. - : American Institute of Physics (AIP). - 2329-7778. ; 10:5
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Tidskriftsartikel (refereegranskat)abstract
- Free-electron lasers provide bright, ultrashort, and monochromatic x-ray pulses, enabling novel spectroscopic measurements not only with femtosecond temporal resolution: The high fluence of their x-ray pulses can also easily enter the regime of the non-linear x-ray-matter interaction. Entering this regime necessitates a rigorous analysis and reliable prediction of the relevant non-linear processes for future experiment designs. Here, we show non-linear changes in the L-3-edge absorption of metallic nickel thin films, measured with fluences up to 60 J/cm(2). We present a simple but predictive rate model that quantitatively describes spectral changes based on the evolution of electronic populations within the pulse duration. Despite its simplicity, the model reaches good agreement with experimental results over more than three orders of magnitude in fluence, while providing a straightforward understanding of the interplay of physical processes driving the non-linear changes. Our findings provide important insights for the design and evaluation of future high-fluence free-electron laser experiments and contribute to the understanding of non-linear electron dynamics in x-ray absorption processes in solids at the femtosecond timescale.
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| 3. |
- Turenne, Diego, et al.
(författare)
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Nonequilibrium sub–10 nm spin-wave soliton formation in FePt nanoparticles
- 2022
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Ingår i: Science Advances. - : American Association for the Advancement of Science (AAAS). - 2375-2548. ; 8:13
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Tidskriftsartikel (refereegranskat)abstract
- Magnetic nanoparticles such as FePt in the L10 phase are the bedrock of our current data storage technology. As the grains become smaller to keep up with technological demands, the superparamagnetic limit calls for materials with higher magnetocrystalline anisotropy. This, in turn, reduces the magnetic exchange length to just a few nanometers, enabling magnetic structures to be induced within the nanoparticles. Here, we describe the existence of spin-wave solitons, dynamic localized bound states of spin-wave excitations, in FePt nanoparticles. We show with time-resolved x-ray diffraction and micromagnetic modeling that spin-wave solitons of sub–10 nm sizes form out of the demagnetized state following femtosecond laser excitation. The measured soliton spin precession frequency of 0.1 THz positions this system as a platform to develop novel miniature devices.
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| 4. |
- Zhou Hagström, Nanna, 1993-, et al.
(författare)
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Megahertz-rate Ultrafast X-ray Scattering and Holographic Imaging at the European XFEL
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Annan publikation (övrigt vetenskapligt/konstnärligt)abstract
- The 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 examples, providing an ideal test-bed for operation at megahertz rates. Nevertheless, our results are relevant and applicable to any other non-destructive XFEL experiments in the soft X-ray range.
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| 5. |
- Zhou Hagström, Nanna, 1993-, et al.
(författare)
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Megahertz-rate ultrafast X-ray scattering and holographic imaging at the European XFEL
- 2022
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Ingår i: Journal of Synchrotron Radiation. - : International Union of Crystallography (IUCr). - 0909-0495 .- 1600-5775. ; 29, s. 1454-1464
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Tidskriftsartikel (refereegranskat)abstract
- The 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, results from the first megahertz-repetition-rate X-ray scattering experiments at the Spectroscopy and Coherent Scattering (SCS) instrument of the European XFEL are presented. The experimental capabilities that the SCS instrument offers, resulting from the operation at megahertz repetition rates and the availability of the novel DSSC 2D imaging detector, are illustrated. 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.
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| 6. |
- Gamage, Eranga H., et al.
(författare)
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As-Se Pentagonal Linkers to Induce Chirality and Polarity in Mixed-Valent Fe-Se Tetrahedral Chains Resulting in Hidden Magnetic Ordering
- 2022
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Ingår i: Journal of the American Chemical Society. - : American Chemical Society (ACS). - 0002-7863 .- 1520-5126. ; 144:25, s. 11283-11295
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Tidskriftsartikel (refereegranskat)abstract
- A novel mixed-valent hybrid chiral and polar compound, Fe7As3Se12(en)(6)(H2O), has been synthesized by a single-step solvothermal method. The crystal structure consists of 1D [Fe5Se9] chains connected via [As3Se2]-Se pentagonal linkers and charge-balancing interstitial [Fe(en)(3)](2+) complexes (en = ethylenediamine). Neutron powder diffraction verified that interstitial water molecules participate in the crystal packing. Magnetic polarizability of the produced compound was confirmed by X-ray magnetic circular dichroism (XMCD) spectroscopy. X-ray absorption spectroscopy (XAS) and Fe-57 Mossbauer spectroscopy showed the presence of mixed-valent Fe2+/Fe3+ in the Fe-Se chains. Magnetic susceptibility measurements reveal strong antiferromagnetic nearest neighbor interactions within the chains with no apparent magnetic ordering down to 2 K. Hidden short-range magnetic ordering below 70 K was found by Fe-57 Mossbauer spectroscopy, showing that a fraction of the Fe3+/Fe2+ in the chains are magnetically ordered. Nevertheless, complete magnetic ordering is not achieved even at 6 K. Analysis of XAS spectra demonstrates that the fraction of Fe3+ in the chain increases with decreasing temperature. Computational analysis points out several competing ferrimagnetic ordered models within a single chain. This competition, together with variation in the Fe oxidation state and additional weak intrachain interactions, is hypothesized to prevent long-range magnetic ordering.
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| 7. |
- Ilyakov, I., et al.
(författare)
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Terahertz-wave decoding of femtosecond extreme-ultraviolet light pulses
- 2022
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Ingår i: Optica. - : Optica Publishing Group. - 2334-2536. ; 9:5, s. 545-550
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Tidskriftsartikel (refereegranskat)abstract
- In recent years, femtosecond extreme-ultraviolet (XUV) and x-ray pulses from free-electron lasers have developed into important probes to monitor processes and dynamics in matter on femtosecond-time and angstrom-length scales. With the rapid progress of versatile ultrafast x-ray spectroscopy techniques and more sophisticated data analysis tools, accurate single-pulse information on the arrival time, duration, and shape of the probing x-ray and XUV pulses becomes essential. Here, we demonstrate that XUV pulses can be converted into terahertz electromagnetic pulses using a spintronic terahertz emitter. We observe that the duration, arrival time, and energy of each individual XUV pulse is encoded in the waveform of the associated terahertz pulses, and thus can be readily deduced from single-shot terahertz time-domain detection.
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| 8. |
- Khubbutdinov, Ruslan, et al.
(författare)
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High spatial coherence and short pulse duration revealed by the Hanbury Brown and Twiss interferometry at the European XFEL
- 2021
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Ingår i: Structural Dynamics. - : American Institute of Physics (AIP). - 2329-7778. ; 8:4
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Tidskriftsartikel (refereegranskat)abstract
- Second-order intensity interferometry was employed to study the spatial and temporal properties of the European X-ray Free-Electron Laser (EuXFEL). Measurements were performed at the soft x-ray Self-Amplified Spontaneous Emission (SASE3) undulator beamline at a photon energy of 1.2 keV in the Self-Amplified Spontaneous Emission (SASE) mode. Two high-power regimes of the SASE3 undulator settings, i.e., linear and quadratic undulator tapering at saturation, were studied in detail and compared with the linear gain regime. The statistical analysis showed an exceptionally high degree of spatial coherence up to 90% for the linear undulator tapering. Analysis of the measured data in spectral and spatial domains provided an average pulse duration of about 10 fs in our measurements. The obtained results will be valuable for the experiments requiring and exploiting short pulse duration and utilizing high coherence properties of the EuXFEL.
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| 9. |
- Lojewski, Tobias, et al.
(författare)
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The interplay of local electron correlations and ultrafast spin dynamics in fcc Ni
- 2023
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Ingår i: Materials Research Letters. - : Taylor & Francis. - 2166-3831. ; 11:8, s. 655-661
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Tidskriftsartikel (refereegranskat)abstract
- The complex electronic structure of metallic ferromagnets is determined by a balance between exchange interaction, electron hopping leading to band formation, and local Coulomb repulsion. By combining high energy and temporal resolution in femtosecond time-resolved X-ray absorption spectroscopy with ab initio time-dependent density functional theory we analyze the electronic structure in fcc Ni on the time scale of these interactions in a pump-probe experiment. We distinguish transient broadening and energy shifts in the absorption spectra, which we demonstrate to be captured by electron repopulation respectively correlation-induced modifications of the electronic structure, requiring to take the local Coulomb interaction into account.
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| 10. |
- Menushenkov, A. P., et al.
(författare)
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Direct evidence of real-space pairing in BaBiO3
- 2024
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Ingår i: Physical Review Research. - : American Physical Society. - 2643-1564. ; 6:2
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Tidskriftsartikel (refereegranskat)abstract
- The parent compound BaBiO3 of bismuthate high-temperature superconductors (HTSCs) BaBi(Pb)O3 and Ba(K)BiO3 with perovskitelike structure exhibits unusual electronic and structural properties, which can be satisfactorily explained if we assume that all charge carriers are in the paired state. However, the prior experiments and the first-principle calculations only indirectly indicate the existence of paired charge carriers in BaBiO3. In this work, we report the direct evidence of initially paired electrons and holes in the upper antibonding Bi 6s-O 2p sigma* orbital of the neighboring octahedral complexes in the ground state of BaBiO3 using the time-resolved x-ray absorption spectroscopy (XAS) to monitor the electron dynamics after the femtosecond resonant 633 nm laser excitation. We observe strong changes in the oxygen K-edge XAS preedge region, defined by the Bi 6s-O 2p sigma* orbitals. We interpret them as a fast (<= 0.3 ps) breaking of charge carrier pairs and slower (0.3-0.8 ps) lattice rearrangement from the distorted monoclinic structure into the new metastable state with a cubic lattice, which persists at least up to 60 ps after the excitation. Analysis of the intermediate state at the fast excitation shows that the bond disproportionation and monoclinic distortion of BaBiO3 structure are energetically favorable due to the charge carrier pairing. Thus the compound BaBiO3 forms a new quantum state that we define as a local pair density wave. Taking into account a large number of similarities between bismuthate and cuprate high-temperature superconductors, we believe that our work will give a new impetus to understanding the nature of superconductivity in perovskite HTSCs.
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| 11. |
- Shokeen, Vishal, et al.
(författare)
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Real-time observation of non-equilibrium phonon-electron energy and angular momentum flow in laser-heated nickel
- 2024
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Ingår i: Science Advances. - : American Association For Cancer Research (AACR). - 2375-2548. ; 10:5
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Tidskriftsartikel (refereegranskat)abstract
- Identifying the microscopic nature of non-equilibrium energy transfer mechanisms among electronic, spin, and lattice degrees of freedom is central to understanding ultrafast phenomena such as manipulating magnetism on the femtosecond timescale. Here, we use time- and angle-resolved photoemission spectroscopy to go beyond the often-used ensemble-averaged view of non-equilibrium dynamics in terms of quasiparticle temperature evolutions. We show for ferromagnetic Ni that the non-equilibrium electron and spin dynamics display pronounced variations with electron momentum, whereas the magnetic exchange interaction remains isotropic. This highlights the influence of lattice-mediated scattering processes and opens a pathway toward unraveling the still elusive microscopic mechanism of spin-lattice angular momentum transfer.
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| 12. |
- Thielemann-Kühn, Nele, et al.
(författare)
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Optical control of 4f orbital state in rare-earth metals
- 2024
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Ingår i: Science Advances. - : American Association for the Advancement of Science (AAAS). - 2375-2548. ; 10:16
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Tidskriftsartikel (refereegranskat)abstract
- A change of orbital state alters the coupling between ions and their surroundings drastically. Orbital excitations are hence key to understand and control interaction of ions. Rare-earth elements with strong magneto-crystalline anisotropy (MCA) are important ingredients for magnetic devices. Thus, control of their localized 4f magnetic moments and anisotropy is one major challenge in ultrafast spin physics. With time-resolved x-ray absorption and resonant inelastic scattering experiments, we show for Tb metal that 4f-electronic excitations out of the ground-state multiplet occur after optical pumping. These excitations are driven by inelastic 5d-4f-electron scattering, altering the 4f-orbital state and consequently the MCA with important implications for magnetization dynamics in 4f-metals and more general for the excitation of localized electronic states in correlated materials.
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| 13. |
- Wang, Xiaocui, et al.
(författare)
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Ultrafast manipulation of the NiO antiferromagnetic order via sub-gap optical excitation
- 2022
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Ingår i: Faraday discussions. - : Royal Society of Chemistry (RSC). - 1359-6640 .- 1364-5498. ; 237:0, s. 300-316
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Tidskriftsartikel (refereegranskat)abstract
- Wide-band-gap insulators such as NiO offer the exciting prospect of coherently manipulating electronic correlations with strong optical fields. Contrary to metals where rapid dephasing of optical excitation via electronic processes occurs, the sub-gap excitation in charge-transfer insulators has been shown to couple to low-energy bosonic excitations. However, it is currently unknown if the bosonic dressing field is composed of phonons or magnons. Here we use the prototypical charge-transfer insulator NiO to demonstrate that 1.5 eV sub-gap optical excitation leads to a renormalised NiO band-gap in combination with a significant reduction of the antiferromagnetic order. We employ element-specific X-ray reflectivity at the FLASH free-electron laser to demonstrate the reduction of the upper band-edge at the O 1s-2p core-valence resonance (K-edge) whereas the antiferromagnetic order is probed via X-ray magnetic linear dichroism (XMLD) at the Ni 2p-3d resonance (L-2-edge). Comparing the transient XMLD spectral line shape to ground-state measurements allows us to extract a spin temperature rise of 65 +/- 5 K for time delays longer than 400 fs while at earlier times a non-equilibrium spin state is formed. We identify transient mid-gap states being formed during the first 200 fs accompanied by a band-gap reduction lasting at least up to the maximum measured time delay of 2.4 ps. Electronic structure calculations indicate that magnon excitations significantly contribute to the reduction of the NiO band gap.
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