The MagneDyn beamline: Investigating Ultrafast Magnetic Phenomena

The manipulation of magnetic order using light is a critical area of research in contemporary magnetism. In particular, the non-equilibrium conditions that arise in magnetic materials following excitation by intense femtosecond laser pulses have garnered significant attention over the past two decades. These highly non-equilibrium states evolve rapidly, within the ultrafast temporal regime (<1 ps), rendering the traditional framework of classical thermodynamics inadequate for describing the magnetic phenomena. Consequently, the ultrafast mechanisms responsible for transferring energy and angular momentum between photons, electrons, spins, and phonons remain elusive and a subject of ongoing debate. The MagneDyn beamline serves as an innovative research tool in the realm of ultrafast magneto-dynamical studies, offering a combination of pump-probe optical and X-ray spectroscopie Researchers can perform time-resolved extreme ultraviolet (EUV) magneto-optical Kerr/Faraday effect studies (tr-EUV MOKE) and resonant x-ray emission scattering (tr-RXES), while varying sample temperature (from 300K down to 10K) and applying high magnetic fields (1.6 Tesla). This unique combination of capabilities enables a more comprehensive understanding of ultrafast magnetic phenomena and their underlying mechanisms.

  • Ultrafast magnetodynamics with free-electron lasers Marco Malvestuto, Roberta Ciprian, Antonio Caretta, Barbara Casarin, Fulvio Parmigiani . Journal of Physics: Condensed Matter 2018
  • Science Frontiers with X-Ray Free Electron Laser Sources. Synchrotron Radiation. Springer Berlin Heidelberg, 2015 pp. 761-785.
  • Time resolved X-ray absorption spectroscopy in condensed matter: A road map to the future. Martina Dell'Angela et al. Journal of Electron Spectroscopy and Related, 2015 vol. 200 pp. 22-30
  • Magneto dynamical studies at Fermi@Elettra. A white paper.Parmigiani F., Malvestuto M.;(2011)

Time resolved core resonant magneto-optical polarization spectroscopy

A relatively recent innovation in the field of magneto-X-ray spectroscopies is core resonant magneto-optical polarization spectroscopy, which offers comprehensive information on the full polarization state of light after interacting with a sample. Techniques such as X-ray Faraday effect, X-ray L-MOKE (Longitudinal Magneto-Optical Kerr Effect), and X-ray Voigt effect fall within this category. Unlike conventional absorption or intensity measurements, polarization analysis provides additional insights into the phase of monochromatic light, complementing intensity data.
The Kerr/Faraday effect is observed when linearly polarized light is decomposed into two circularly polarized waves with opposite helicities. As these waves transmit through a magnetized sample, a phase shift occurscausing the polarization plane to rotate (
Faraday rotation) and the polarization to become elliptical. A complete polarization analysis determines the polarization ellipse (illustrated at the top), offering a more comprehensive understanding of the light-sample interaction and the underlying material properties.

Yamamoto et al. Phys. Rev, B 89, 064423 (2014)

  1. (a) Element-resolved relative change in sample reflectivity as a function of time delay between the optical pump pulse and the FEL probe pulse, tuned to the Ni M2,3 (67 eV) and Fe M2,3 (55 eV) edges, respectively.
  2. (b) Element-resolved Kerr rotation of light polarization as a function of photon energy (solid lines serve as visual guides).
  3. (c) Demagnetization signals observed at the Ni and Fe M2,3 edges.
  4. (d) Element-resolved unpumped magnetic hysteresis data.

Time resolved resonant x-ray emission spectroscopy

Time-resolved valence-to-core X-ray emission spectroscopy (tr-RXES) is a powerful and versatile technique that enables the investigation of transient electronic structures and their dynamics in various materials, such as solids, solutions, and membranes.  This method provides insights into the occupied electronic states and ligand environment surrounding a metal of interest, enabling the study of orbital splittings, spin-, and oxidation-states.
In comparison, tr-XAS focuses on the unoccupied electronic levels and is more sensitive to the local symmetry and coordination.
In tr-RXES, a sample is excited by a pump pulse (usually a laser), followed by a time-delayed probe pulse consisting of monochromatic X-rays tuned to an appropriate absorption edge. The emitted X-rays are then collected and analyzed, revealing the valence-to-core transitions occurring within the system. These transitions involve the valence electrons of the ligands and provide crucial information about the ligand environment, orbital splittings, and spin- and oxidation-states of the metal center. Additionally, the technique is sensitive to low-energy excitations arising from local, nearest-neighbor, and collective interactions.

One of the main advantages of tr-RXES is its time-resolved capability, which allows for the study of ultrafast processes, such as the formation and decay of reactive intermediates in photoreactions. By varying the time delay between the pump and probe pulses, it is possible to capture transient phenomena occurring on femtosecond to picosecond timescales, providing a detailed picture of the dynamic processes taking place within the sample.
In summary, time-resolved valence-to-core X-ray emission spectroscopy is a cutting-edge technique that offers a unique perspective on the electronic structure and dynamics of various materials. Its ability to probe the ligand environment, capture ultrafast processes, and provide complementary information to other X-ray spectroscopic techniques makes it an invaluable tool for researchers aiming to unravel the intricate processes governing the behavior of matter at the atomic and molecular level.

The pumped (red dotted curve) and unpumped (black dotted curve) RXES spectra at positive (+200 fs) and negative time delays (−100 fs) taken at the carbon K edge (incident photon energy hν = 295 eV) of an HOPG sample are shown. The normalized intensity of the RXES spectra is plotted vs the photon energy loss.

Last Updated on Thursday, 04 May 2023 09:27