Session V32 - Scattering and Diffraction.
ORAL session, Thursday morning, March 15
Room 620, Washington State Convention Center

[V32.001] Soft X-ray Speckle in Magnetic Scattering from Rough Surfaces with Antiferromagnetic Domains

Speckle patterns in coherent soft x-ray scattering from LaFeO_3 antiferromagnetic surfaces have been measured. In these experiments x-ray magnetic linear dichroism causes diffuse x-ray scattering with in-plane q related to the size of the antiferromagnetic domains. The sample surface is relatively smooth (0.25 nm rms), nevertheless the diffuse scatter from the surface roughness is of the same order as the magnetic scatter. The 5 \mum pinhole used to aperture the incident radiation creates Fraunhofer fringes which interfere with the diffuse scattering. It is shown that the magnetic effect in the speckle pattern results from mixing of the diffuse magnetic scattering with the Fraunhofer fringes and the surface roughness scattering. A theoretical treatment for the scattering of x-rays from rough surfaces with randomly distributed antiferromagnetic domains is presented. Simulations reproduce the main features of the experimental data. In particular the q-range of the scattering, the relative intensity of the specular and diffuse scattering and the size of the magnetic contribution are in good agreement with observations.

[V32.002] Anisotropic XANES and anomalous X-ray dispersion in ferroelectric potassium niobate

In the last decade major progress in theory of ferroelectricity has led to better understanding of electronic polarization in the ferroelectric state, while experimental verification has lagged behind. We demonstrate the potential of X-ray anomalous dispersion for studying the electronic structure of ferroelectrics. Potassium niobate is a strong ferroelectric possessing an orthorhombic structure with ferroelectric polarization along the c-axis. XANES measurements through the Nb K-edge performed with the direction of X-ray polarization parallel to the orthorhombic axes show a clear difference in the imaginary term of anomalous dispersion between non-bonding b-axis and bonding a- and c-axes, and a smaller difference between a- and c-axes. Through application of the Kramers-Kroenig transformation, the real term of anomalous dispersion is calculated. In agreement with the energy dependence of the real term, the intensity of the thermal diffuse scattering near the (500) and (560) Bragg peaks exhibits an anomalous, resonant-type behavior as the X-ray energy is scanned through the Nb K-edge.

[V32.003] Modeling of Interface Roughness Propagation in Sputtered X-ray Multilayers Studied by Diffuse X-ray Scattering

Scattering data out to a lateral q value of 2 nm^-1 were taken at a bending magnet beamline at the Advanced Photon Source (APS) . The 20 period W/C multilayer was grown on float glass at the APS and had a period of 2.8 nm. Modeling of roughness propagation (D.G. Stearns, J. Appl.Phys.71,4286(1992); S.K. Sinha, J.de Phys.4,1543(1994); V.M. Kaganer, S.A.Stepanov, R.Kohler, Physica B, 221,34(1996).) at each interface was done with mathematical rigor. Diffuse scattering corresponding to Kiessig fringes was studied in detail. Modeling with and without the conventional small roughness approximation showed the appropriateness of this approximation to the case at hand. Although the full power spectrum at the interfaces needed to fit the data is not unusual, the correlation function of the roughness propagated between interfaces is found to have a range of only a few Angstroms. The float glass substrate itself was also studied and found to have a liquid-like correlation function.

[V32.004] A Nuclear Resonant X-Ray Waveguide

We have prepared a nuclear resonant x-ray waveguide, consisting ^57Fe/C/^57Fe layers magnetized along the beam direction. In this waveguide, the standing-wave electric field can be resonantly excited at the guided mode inside the carbon layer (350 Angstroms) between thin ^57Fe layers (50 Angstroms) and can travel through the weakly-absorbing carbon layer without severe attenuation. With the incident angle set to excite each discrete guided mode below the critical angle of the iron and the beam energy tuned to the nuclear resonance of ^57Fe at 14.4125 keV, time-delayed spectra were measured for the guided beam emitted directly from the end face of the waveguide and the beam reflected externally from the surface, respectively. The unusually fast coherent decay was observed in the time spectrum from the guided beam, in contrast to that observed for the total external reflection.

*Work at the Advanced Photon Source was supproted by the U.S. Department of Energy under contract no. W-311-109-Eng-38.

[V32.005] Resonant Scattering in Germanium

The 600 reflection is a forbidden reflection, like the 222 (h + k + \ell = 4n +2). Unlike the 222, it cannot be produced by an anisotropic charge density. F_600 is always zero, even when interstitial charges are placed along the bonds between atoms. At resonant energy (E = 11.11 keV) the 600 is excited and can be easily observed. Using 3-beam diffraction^1, we have demonstrated the feasibility of determining phase and magnitude of F_600. With the 3 \bar3\bar5 as simultaneous reflection, we found a value of F_600 = 0.475 electrons, with a phase of - 90^\circ, as expected from an approximate theory, corresponding to a non-centrosymmetric valence charge density. A similar experiment was done for the 222, which can be excited even out of resonance at E = 11.44 keV. It was found that, at resonance, the 222 intensity was decreased by almost 40%, indicating that the 222 charge scattering and resonant scattering are out of phase. Some change in phase was also observed, from 28^\circ out of resonance, to 40^\circ near resonance. Work is in progress in order to understand these results from a theoretical viewpoint. *[12pt]

1) Q. Shen and R. Colella, Nature 329, 232-233 (1987).

[V32.006] Measurement of the Extended X-ray Raman Fine Structure (XRFS) for Diamond Using a Filter Based Analyzer

Non-resonant x-ray Raman scattering is the inelastic scattering of hard x-rays from the core electrons of low-Z elements. Within certain assumptions, the x-ray Raman fine structure (XRFS) and x-ray absorption fine structure (XAFS) are characterized by the same photoelectron transition matrix elements. XRFS and soft x-ray XAFS therefore provide similar local structural information. However, XRFS has bulk rather than surface sensitivity, and has fewer experimental constraints on the local atomic and overall sample environments. Whereas all previous measurements of XRFS have used bent crystal analyzers, we report the first measurement of XRFS with a filter based analyzer. Our analyzer has an 0.3 sr collection angle, a 7 percent detection efficiency, and a 3.5 eV energy resolution, yielding performance comparable to that of bent crystal analyzers. We will present XRFS results for diamond and other low-Z materials, and discuss applications of XRFS measurement ranging from organic materials to transition metal oxides.

[V32.007] X-ray fluorescence holography--some recent developments

X-ray fluorescence holography (XFH) is a relatively new experimental tool for directly determining the local atomic structure in three dimensions and in an element-specific way. For example, recent studies have demonstrated the ability to image a first-row element in the presence of a transition metal [Tegze et al, Nature 407, 38 (2000)], and to study the local atomic environment in a quasicrystal [Marchesini et al, Phys. Rev. Lett. 85, 4723 (2000)]. A new experimental system for XFH at the Advanced Light Source has been developed, optimized for the energy range of this source. We use fast acquisition electronics, synchronization of the rapid motor movement and data acquisition to suppress beam instabilities, a high speed (MHz-level) germanium solid state detector to enable high count rates and discriminate against both elastically scattered photons and other fluorescent x-rays. By placing the sample and its goniometer inside a vacuum chamber, we have the possibility of measuring low energy fluorescent photons(<6 keV). In particular, by tuning the photon energy on and off core resonances [see abstract by Zhao et al.], it should be possible to perform "true color" imaging of the atomic species around a given type of emitter. Preliminary results obtained with this system will be presented. This work was supported by DOE contract No. DE-AC03-76SF00098.

[V32.008] Anomalous x-ray dynamical scattering and related phenomena

A formalism for calculating x-ray diffraction in crystals is introduced, with inclusion of dynamical multiple scattering and near-field effects. The latter are shown to be important in the vicinity of the atomic absorption edges when the radiation wavelength is of the order of the distance between nearest-neighbor resonating atoms, in which case a macroscopic theory results in strong disagreement with respect to the present microscopic theory. In the case of planar surfaces, significant deviations from standard plane-by-plane methods of x-ray dynamical scattering are also observed. Resonance absorption, transmission, and reflection profiles are computed and compared with available experiments. The new method can be also applied to non-planar geometries, involving nanostructured materials. In that case, the near field can play a dramatic role, which is discussed in connection with multi-atom resonant photoemission (MARPE).

[V32.009] Multiple-Wave X-ray Interference at Absorption Edges: Determination of Phase Shift at Resonance

The effects of anomalous dispersion on multi-wave x-ray interference in crystals at atomic absorption edges are studied. Highly phase-sensitive profiles of the diffraction intensity ratios of two inversion-symmetry-related multiple diffractions at absorption edges exhibiting strong asymmetric characteristics, compared with those far from the edges, are experimentally and theoretically observed for the first time. The changing of asymmetry of these ratios results from the anomalous dispersion corrections in the structure factors due to the significant change of the resonance phase at the absorption edge. The excellent agreement between the experimental and theoretical results is obtained. The proposed approach allows for the determination of the changes of x-ray reflection phases under resonance conditions. This provides a highly sensitive way for experimental investigation of the spectral distribution of reflection phase shift due to the resonance.

[V32.010] Development Of 3d X-Ray Structural Microscopy Using White X-Ray Microbeams

We have developed white x-ray microbeam techniques that provide the ability to measure the orientation, phase, size, and full stress/strain tensor of crystalline materials with micron resolution in three-dimensions. White and monochromatic (interchangeable) synchrotron x-ray beams are focused to ~0.5 micron diameter using elliptically figured K-B mirrors on the MHATT-CAT beamline at the Advanced Photon Source. 3D structural microscopy has been achieved through the development of new techniques for collecting diffraction patterns with micron resolution along the direction of the microbeam in the sample. The experimental techniques developed and analysis methods used will be described and the 3D microstructural capabilities will be demonstrated through micron resolution measurements of strain gradients in thin Si single crystals bent cylindrically to ~5 mm radii. Applications of this technique to the investigation of the structure and evolution of materials on mesoscopic length scales will be discussed.

[V32.011] X-Ray Microbeam Mapping of Local Lattice Orientations and Strains in Oxide Films

We have developed a polychromatic, x-ray microbeam technique for measuring the local lattice structure, crystallographic orientation (texture), and the stress/strain tensor with submicron spatial resolution. We have applied this new synchrotron technique to the study of the epitaxial growth of oxide films on highly-textured, rolled nickel substrates (for superconducting wire applications). Microbeam x-ray Laue patterns from within individual grains were collected and analyzed to obtain orientation and strain maps of each layer of multilayer structures. Crystallographic tilting of the epitaxial oxide films has been measured as a function of the miscut of individual substrate grains and as a function of the growth temperature. Quantitative results for CeO_2 films on Ni are consistent with a ledge-growth model at elevated temperatures (600°-800°C) and island growth at lower temperatures. Temperature-dependent micro-strain measurements will also be presented and compared with macro-strain results from conventional x-ray diffraction

[V32.012] Light Scattering from Magnetic Dipoles

We are studying the optical properties of super-paramagnetic colloids. We scatter light at 90 deg. With an incident polarization perpendicular to the plane of scattering, we observe the common electric dipole scattering. However, with an incident polarization parallel to the scattering plane, we observe a significant scattering component with a polarization parallel to the plane. This we interpret as due to magnetic dipole scattering similar to that observed by Du and Tang.("Light Scattering Properties of Paramagnetic Particles," Y.-B. Du, P. Tong J. Chem. Phys. 107), 355 (1997). Due to the temperature dependent nature of super-paramagnetism, we are now focusing on the behavior of the scattering in our colloids at and near their blocking temperatures, which is a time scale dependent quantity. In the same spirit, we are also attempting to observe magnetic scattering from colloids of highly paramagnetic salts.

[V32.013] Scattering of sound by tilted elastic disks: holographic identification of enhancements

When sound is scattered by elastic objects in water, at certain tilt angles of the object, strong enhancements can be observed in the magnitude of the backscattered signal. Even for many objects as simple as a circular elastic disk, however, exact solutions for the scattering are unavailable. The causes of such enhancements may often be identified by combining ray theory for acousto-elastic coupling mechanisms with displays of the response of the target to acoustic tone bursts. In the example considered here the spatial and temporal response of an elastic disk in water was displayed by using acoustic holography [B. T. Hefner and P. L. Marston, ARLO (accepted for publication)]. The nearfield scattering was sampled in a plane and was numerically backpropagated to the plane of the disk. Compressional waves (corresponding to symmetric leaky Lamb waves) were excited on the disk by the incident sound wave. Mode conversion (at the edge of the disk) of compressional waves to shear waves aids in the production of a significant backscattering enhancement.

[V32.014] Neutron Scattering Data Access, Visualization and Analysis Tool, ISAW

Development of advanced neutron sources and instruments has necessitated corresponding advances in software to manage, visualize, and reduce neutron scattering data. The Integrated Spectral Analysis Workbench, ISAW, is being developed at IPNS with the goal of providing a flexible and powerful tool to do neutron scattering data analysis and visualization. The software, written in Java, includes components for automatic transfer of data to a central server, creation and maintenance of a database of metadata, database search and retrieval, and a portable analysis and visualization package. Presently ISAW supports IPNS run files but it is being expanded to support NeXus file format to enable it to be used widely in the analysis and visualization of neutron scattering data. The NeXus file support in ISAW provides remote access to NeXus data files using a NeXus Data Server developed at PSI. Work performed at ANL is supported by the U.S. DOE-BES under contract number W-31-109-ENG-38.

[V32.015] What can we learn about phonons from neutron powder diffraction?

We use the pair distribution function (PDF), obtained from the spherically averaged structure function S(|q|) in pulsed neutron powder diffraction experiments, to extract information about the phonon dispersion. We discuss the applicability, as well as the advantages and disadvantages, of this method for the determination of phonon dispersion curves as a function of external fields, e.g., temperature and pressure. Finally, we present results of the successful implementation of this method and lay out the road map for future applications.

Part V of program listing