Soft X-Ray Magneto-Optics: Probing Magnetism by Resonant Scattering Experiments

摘要

The most advanced X-ray sources (third generation synchrotrons, linear free-electron lasers and high-harmonic generation sources) widen the range of application of X-ray scattering techniques considerably. Beyond flux and brilliance, improvements in polarization tuneability, degree of coherence and selectable time-structure promoted new methods for investigating the electronic and magnetic properties of solids. The soft X-ray range (50–2000 eV) is well suited for studying magneto-optical effects in laterally confined submicron sized objects, either artificially built or self-assembled. First, by tuning the photon energy at a core resonance, one provides the X-ray scattering technique with element selectivity. Second, resonant excitations make the optical constants sensitive to the local magnetization by introducing large off-diagonal elements in the dielectric tensor; since magnetic effects are stronger when the core excitation produces a dipolar transition to final states involving the magnetic orbitals ($3d$ for the first row TM; 4 $f$ for RE), the most interesting resonances for X-ray magneto-optics [(2, 3)$p to 3 d$ and (3, 4)$d to 4 f$] are all located in the soft X-ray region. Finally, the wavelengths corresponding to soft X-rays are very well suited for scattering studies of nanometer- to micrometer-sized magnetic structures. We will present the results of recent soft X-ray resonant scattering experiments, showing that the combination of element selectivity, magnetic sensitivity and structural analysis can help disentangling and understanding the magnetic properties of complex self-assembled periodic systems. Lastly, recent applications of coherent scattering to the X-ray holographic imagi- g of magnetic domains will be presented.