IFF
Scientific Report 1998/1999

Modern solid state physics goes far beyond a phenomenological description and bases the understanding of solid state properties and phenomena on atomistic theories. To obtain information about the atomic structure of solids, probes with sub-nanometer spatial resolution are needed. To study the excitation spectra, an appropriate energy resolution is necessary in addition. All these requirements can be met by scattering methods. In this sense, scattering methods provide the basis of our present understanding of the structure, excitations and phase transitions of condensed matter on a microscopic level.

At the Institute for Scattering Methods (ISM), synchrotron x-ray scattering and neutron scattering are employed for the investigation of condensed matter on an atomistic microscopic level. The emphasis lies on exploiting fully the complementarity of the two probes. Besides the application of scattering methods to solid state problems, major activities are concentrated on the methodology. This includes the further development of experimental techniques by improving instrument components and data treatment algorithms, the development of new experimental methods and the corresponding instruments and the development, construction and operation of scattering instruments at large-scale facilities. At present, ISM operates five instruments at the research reactor DIDO of the Research Center Jülich and two instruments at the Hamburger Synchrotronstrahlungslabor HASYLAB. These instruments are open for the use by external groups from universities, research centers and industry. The instrument responsibles from ISM provide scientific and technical support during the experiment and the data processing. ISM is open for all research areas in condensed matter science, where scattering methods can be applied. At present, the research activities are concentrated in three fields: solid state magnetism, structural disorder in alloys, quasi-crystals and high T_c superconductors and electrocatalytic processes. For the purpose of this research, ISM is also engaged in sample preparation (e.g. by molecular beam epitaxy) and characterisation (e.g. AC and DC susceptibility and magnetisation measurements).

In the field of instrument development, the main activities in 1998 were:

1. Design and construction of a high energy side station at an undulator line of the Advanced Photon Source (APS) at Argonne National Laboratory, USA. This work is done in close collaboration with the Midwest Universities Collaborative Access Team (µCAT) at APS.
2. Development of a nuclear spin polarised ³He-filter for the purpose of neutron polarisation analysis. In 1998, the activities were focussed on the design of the ³He polarisation station.
3. Design and construction of a new type of small-angle scattering instrument, which employs focussing mirror optics.
4. Equipment of the diffuse neutron scattering machine DNS at the DIDO-reactor with supermirrors for neutron polarisation analysis. A progress report on this project can be found in the following pages.
5. Optimisation of the reflectometer HADAS at the DIDO-reactor for experiments on small magnetic thin-film samples and studies for the implementation of a polarisation analysis option.
6. Development of optimised target and moderators for the European Spallation Neutron Source.

Method development was done for high energy magnetic x-ray scattering in the energy range above 100 keV, for resonant exchange scattering from ferro- and antiferromagnets for inelastic x-ray scattering, anomalous small-angle x-ray scattering, characterisation of thin-film devices by means of combined neutron-, x-ray reflectivity- and diffuse scattering measurements and neutron polarisation analysis.

In the field of solid state magnetism, we addressed the following topics:

1. Determination of the frustration mechanism in disordered system with competing interaction by determining elementspecific spin pair correlations.
2. Investigation of the critical behaviour at magnetic phase transitions for frustrated and disordered systems and systems with higher order exchange interactions.
3. Investigation of the effect of fourth order exchange interaction on magnetic structure, phase transitions and low temperature magnetisation.
4. Optimisation of the growth conditions for magnetic multilayer systems by determining the interface morphologies with combined neutron and x-ray scattering experiments.

Examples for the work done at ISM this year are given by the progress reports on the following pages.

Thomas Brückel