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Synchrotron Radiation

Synchrotron X-ray characterisation

Synchrotron radiation is bright x-ray light that allows us to resolve the structure of matter down to the level of atoms and molecules. It is produced inside a storage ring, which is not circular but consists of dipole bending magnets separated by straight sections. Electrons are initially accelerated to a few tens of MeV in a linear accelerator, and are then fully accelerated to an energy of 2-10 GeV by a booster synchrotron in combination with the acceleration in the storage ring itself. Beamlines are constructed to extract the beam from tangent points of the bending magnets or from insertion devices within the straight sections, as is shown in figure 1.

Diagram showing the ESRF-Layout
Fig.1: Schematic representation of the European Synchrotron Research Facility (ESRF) in Grenoble, France showing the storage ring and the beamlines.

Synchrotron X-ray diffraction

   Synchrotron diffraction permits the acquisition of high-resolution data for solving crystal structures and for quantitative phase analysis. It is broadly used in material science and in metallurgy including studies of crystal perfection and phase transitions, residual stress and strain in alloys, magnetic ordering and spin densities. X-ray synchrotron diffraction is a reliable method to determine the structure of foreign phases, due to the use of high energies, yielding more Bragg maxima. The possibility of tuning the energy allows carrying out experiments at resonance energies of particular elements.

Synchrotron XRD-Spectra of GaN:Fe
Fig.2: Synchrotron diffraction spectra of (Ga,Fe)N samples grown at different temperatures, acquired at the Rossendorf Beamline (BM20) at the ESRF.

X-ray absorption spectroscopy

Introduction

    X-ray absorption spectroscopy (XAS) is a technique that permits to probe the local structure (geometric and/or electronic) of matter by measuring the absorption coefficient, μ(E), around a selected x-ray absorption edge of a given element. This technique is usually performed at synchrotron radiation sources and on a wide range of materials (from amorphous to crystalline, from dilute to concentrate). When referred to the oscillations observed on μ(E) above the absorption edge, XAS is called x-ray absorption fine structure (XAFS) spectroscopy. The mechanism leading to the appearance of the fine structure is described pictorially in Fig.3: a monochromatic incoming x-ray is absorbed via the photoelectric effect; a photoelectron is emitted, it scatters with the surrounding atoms and interfere with itself; finally it is re-absorbed and a fluorescence x-ray is emitted. A typical XAFS spectrum is visible in Fig.4 where two energy regions are distinguished: the near-edge region (XANES) and the extended one (EXAFS). The XANES is more sensitive to the electronic structure and the symmetry, while the EXAFS gives more information on bond distances, coordination numbers and local disorder.

The electron wave emitted by the absorbing atom is scattered at surrounding atoms and interferes with itself at the point of absorption, leading to a modified absorption coefficient
Fig.3: Pictorial view of the photoelectron interference effect.
XAFS spectrum showing the near edge (XANES) and extended (EXAFS) regions.
Fig.4: XAFS spectrum of a Fe2O3 model compound (crystalline powder) at the Fe K-edge (E0=7112 eV).

XAS applied to the study of magnetic semiconductors

    We apply this technique in the study of magnetic semiconductors by measuring at the K-edge of the magnetic impurities. In order to extract quantitative information from the experimental data, XANES and EXAFS require a theoretical model that is then adjusted (fitted) to experimental spectra. We achieve these tasks by employing ab initio codes as FEFF or FDMNES for the calculation of theoretical signals while the iFEFFIT package for fitting the experimental data. In Fig.5 is given an example of combined XANES and EXAFS analysis applied to (Ga,Fe)N as a function of the growth temperature. By employing a linear combination fit of theoretical XANES it is possible to evaluate the percentage of different phases present in our samples, while the fit of the EXAFS permits the refinement of the local structure (inset of Fig.5).

XANES spectra of (Ga,Fe)N grown at different temperatures and the corresponding Fourier transforms of the EXAFS.
Fig.5: XANES spectra of (Ga,Fe)N samples with different growth temperature. Fourier transform of the relative EXAFS signals (can be seen as a radial distribution function around the absorbing Fe atoms).

    In addition, the analysis of the position of the absorption edge and the pre-edge peaks in the XANES permits to obtain the charge state of the probed element and the relative electronic configuration as shown for (Ga,Mn)N in Fig.6.

XANES spectra of (Ga,Mn)N grown with different Mn concentration compared to oxides. The simulated XANES is also displayed.
Fig.6: a) The position of the edge position of (Ga,Mn)N for two Mn concentrations compared with Mn-Oxides compounds. b) Simulation of the XANES spectra for Mn in 3d4 and 3d5 electronic configuration compared with experimental data. c) Fit of the pre-edge features.

Further Reading


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