Research

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 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.