E. Gerdau

Pure nuclear reflexes and combined hyperfine interactions in YIG

Mössbauer spectra of oriented YIG single crystals were taken and the numerical analysis using the transmission integral yielded a consistent set of hyperfine interaction parameters. They are in good agreement with theoretical values obtained by MO-calculations which included clusters up to 62 ions. Finally pure nuclear reflexes are predicted for single crystals and two theoretical spectra are given.

Backscattering mirrors for X-rays and Mössbauer radiation

Observation of exact backscattering of X-rays and studies of its energy and angular dependences; test of the validity of the dynamical theory of diffraction in the extreme case of exact backscattering; backscattering high-energy-resolution monochromators; backscattering interferometers, in particular of the Fabry–Pérot interferometer type; and precise, up to 5·10—9 Å, measurements of crystal lattice parameters: these are central topics of the paper. Special attention is paid to the selection of crystals to be used as backscattering mirrors. Noncubic crystals like Al2O3, SiC, etc., allow backscattering for X-rays with practically any energy above 10 keV. Feasibility of backscattering mirrors for Mössbauer radiation of 57Fe (14.4 keV), 151Eu (21.5 keV), 119Sn (23.9 keV), and 161Dy (25.6 keV) nuclei is demonstrated by Al2O3 crystals. A concrete design of a sapphire Fabry–Pérot–Bragg étalon is presented.

On the theory of an X-ray Fabry-Perot interferometer

The theory of an X-ray Fabry-Perot interferometer (FPI) is developed as a particular case of the dynamical theory of X-ray Bragg diffraction in layered crystalline systems. Mathematical expressions are derived for the transmissivity and reflectivity of the X-ray FPI built as a system of two perfect crystal plates parallel to each other. The performance of the X-ray FPI is similar to that of the optical FPI. Both show fine interference structure in the transmission and reflection dependences. However, for the X-ray FPI this occurs only inside the region of the Bragg back diffraction peak. The influence of possible imperfections, such as the roughness of the crystal plate surfaces and the error in the parallelism of the atomic planes are discussed. It is shown that both factors may significantly deteriorate the performance of the X-ray FPI. Numerical estimations are given.

Nuclear diffraction experiments with grazing incidence antireflection films

The energy dependent reflectivity of grazing incidence antireflection (GIAR) films containing the Mößbauer isotope57Fe has been measured with recoilless 14.4 keV radiation from a radioactive source. The reflectivity is influenced by oxide layers on the surface. Assisted by Mößbauer spectroscopy in transmission geometry and X-ray reflectivity measurements the hyperfine interaction parameters in the layers could be determined. In a nuclear diffraction experiment with synchrotron radiation a GIAR-film has been used as a filter. The diffracted beam was analysed by the (002) reflection of Yttrium Iron Garnet (YIG). The evaluation of the observed quantum beat pattern yielded the energy differential reflectivity of the GIAR-film in agreement with the results from the Mößbauer measurements.

Relaxation experiments with synchrotron radiation

Relaxation phenomena show up in standard energy domain Mössbauer spectra via line broadening. The evaluation of such spectra is in most cases done by adopting the stochastic theory mainly developed in the 60s and 70s. Due to the time structure and the polarization of the synchrotron radiation nuclear resonance forward scattering in the time domain gives valuable information on relaxation mechanisms. We report here mainly on Nuclear Forward Scattering (NFS) experiments investigating the paramagnetic relaxation of the Fe3+ ion in (NH)4Al0.9557Fe0.05(SO4)2·12H2O and briefly on recent investigations on charge fluctuations in Eu3S4.

Techniques for inelastic X-ray spectroscopy with μeV-resolution

Abstract We introduce a novel type of spectrometer that provides a μeV bandpass together with a tunability over a few meV. The technique relies on nuclear resonant scattering (Mossbauer effect) of synchrotron radiation at the 14.4-keV resonance of 57 Fe. Energy tuning is achieved by the Doppler effect in high-speed rotary motion. The resonantly scattered monochromatic radiation is extracted by a polarization filtering technique or by spatial separation due to the “nuclear lighthouse effect”.

The temperature dependence of the hyperfine parameters of YIG below the Curie point

The temperature dependence of the hyperfine parameters of the 57Fe nuclei in yttrium iron garnet Y3Fe5O12 (YIG) between room temperature and the Curie point was studied by means of conventional Mößbauer transmission spectroscopy. The critical exponent β, which describes the temperature dependence of the magnetic hyperfine field of 57Fe in YIG, was found to be 0.29(1) and 0.33(1) in the [a]- and (d)-sublattice, respectively. In the temperature region just below the Curie point relaxation of the internal hyperfine fields could be observed. The phase transition at the Curie point shows a distinct dependence on the grain size in the sample.

Nuclear forward scattering of synchrotron radiation from unmagnetized α-57Fe

Nuclear foward scattering of synchrotron radiation from magnetically polycrystalline and partly magnetized α-57Fe foils has been observed. The forward scattering amplitude in both cases is calculated. The results allow the interpretation of the time spectra from samples which are magnetically polycrystalline or exhibit an unidirectional magnetic texture.

Hyperfine spectroscopy in diffraction geometry

With the advent of third generation synchrotron radiation sources nuclear Bragg diffraction became a powerful technique for the determination of hyperfine parameters and the electronic and magnetic structure of single crystals. Basic features are discussed theoretically and experimentally and are illustrated by examples such as YIG, FeBO3, α-Fe2O3, and Fe3BO6.

Inversion of Target Sequence in Nuclear Forward Scattering of Synchrotron Radiation

The radiation fields reemitted in nuclear forward scattering of synchrotron radiation by a system of two single-line targets of highly different linewidths mounted downstream behind each other were analyzed by applying a stepwise energy shift to one of the targets. The results reveal a pronounced dependence of the fields on target sequence, which demonstrates the important role of radiative coupling.