IFF
Scientific Report 2001/2002

Modern solid state physics goes far beyond a phenomenological description and bases the understanding of solid state properties and phenomena on atomic 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 atomic 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 largescale 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. In addition, we participate in the operation of a sector at the Advanced Photon Source APS in Argonne, USA. 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 the fields: "solid state magnetism", "structural disorder", "novel materials" and "ultra thin liquid films". For the purpose of this research, ISM is also engaged in sample preparation (e.g, by molecular beam epitaxy and single crystal growth) and characterisation (e.g. AC and DC susceptibility and magnetisation measurements).

Within the framework of the so-called ,HGF Strategiefond Magnetoelektronik", a larger collaboration within the research center has been established. Magnetic nanostructures in the form of magnetic thin films and laterally structured thin films are the subject of investigation. Research topics extend from basic research on quantum phenomena of nanostructured magnetic systems up to application oriented questions. In the Institute for Scattering Methods, we are aiming at a full structural and magnetic characterisation of magnetic nanostructures on a microscopic atomic up to mesoscopic level. For the structure characterisation, we have developed techniques of grazing incidence x-ray reflection employing anomalous scattering for contrast variation. The combination of reflectivity and diffuse scattering data allows us to perform a full statistical description of the interface morphology in layered systems. Polarised neutron scattering under grazing incidence allows us to determine the magnetic coupling, domain structure and magnetic roughness at the interfaces. In collaboration with other groups (e. g. from the Institute for Electronic Properties), we are working on a correlation of the obtained microscopic structure and magnetic parameters with macroscopic properties such as interlayer coupling or magneto-resistance-effects. Many of the systems we study are produced in our own MBE chamber. But we also characterise epitaxial or sputtered layer-systems from other groups within the research centre. A program-package has been developed which allows us to simulate and refine x-ray data as well as polarised neutron data from grazing incidence scattering experiments. In 2001 we have studied transition metal multi-layers such as the FeCo/Mn/FeCo, Fe/Cr/Fe, Co/Cu/Co-systems, rare-earth multilayers of Er/Tb as well as TMR and GMR-systems produced by other groups. For all these systems a systematic study of the interface-morphology as function of preparation condition was performed and the preparation conditions could be optimised. To give an example for a systematic structural study of these multilayers, we mentioned the system Co/Cu/Co where a square root dependence of the roughness on layer thickness was found. Another important result was achieved in collaboration with the IEE: Two samples of Fe/Cr/Fe with the same layer-structure but different interface roughness have been prepared and fully characterised. It was found that the magneto- resistance effect was significantly enhanced for the sample with rougher interfaces. These experiments will be continued to identify the influence of the buffer roughness. In addition to thin films, we are now studying laterally structured magnetic layers. In a first experiment on an optical grating covered with a Ni layer, the magnetic shape anisotropy could be observed in a polarised neutron scattering experiment. Fe/Cr/Fe multi-layer stripes with a period of 400 nanometers could be produced by electrolithography and are now being studied with grazing incidence neutron scattering. The application of resonance exchange scattering of synchrotron x-rays from rare-earth multilayers allows us to obtain new inside into the mechanism of interlayer coupling. This has been shown by studies of the Er/Tb multilayer system. For a certain interface morphology and layer thickness, a coherent cone structure develops in the Er throughout the entire stack of Er-layers. How the coupling is mediated through the Tb layers could be observed with element specific magnetic xray scattering tuning the x-ray energy to the Tb LII and LIII edges. In this way we could prove that a spin density wave corresponding to the magnetic order of the Er layers develops in the 5d-conduction band of the ferromagnetic Tb layers.

Due to their often simple Hamiltonian, magnetic systems are ideally suited to study quantum phenomena in many body systems. We employ advanced methods of neutron or magnetic x-ray scattering to study fundamental questions such as the nature of elementary excitations, magnetic phase-transitions or spin and orbital ordering phenomena. By applying neutron polarisation analysis we could for the first time determine the spectrum of longitudinal excitations in a Heisenberg antiferro-magnet with a small spin-wave gap, the model system MnF2. An excitation-gap appears close to the position of the transverse (spin-wave) excitation separating a quasielastic from a truely inelastic part of the spectrum. Our results are in good agreement with recent theories on multi-magnon-processes. The behaviour of the spin-dynamics and magnetisation in the low- and intermediate temperature range has been studied on a wide range of materials. Deviations from the low temperature Bloch-law have been found and a classification into simple power laws is proposed. A major research effort is concentrated on the investigation of the electronically highly correlated manganites. These are not only interesting from a fundamental point of view but also for possible applications as magnetoelectronics devices. Resonant x-ray scattering experiments reveal the orbital ordering in Lal-,Sr,,Mn03 single crystals. The interplay between spin ordering, orbital ordering, lattice distortions and charge ordering is being investigated by neutron diffraction and diffuse neutron scattering, resonant and non-resonant (highenergy) x-ray scattering and magnetisation and resistivity measurements.

While many research efforts within the Institute for Scattering Methods are dealing with magnetic systems we apply scattering methods for the microscopic investigation of a wide range of novel or interesting materials. With small angle x-ray scattering such varied systems have been investigated as amorphous silicon-germanium alloys relevant for solarcell-technology, CdTe quantum-dots, proton conducting polymer-films or three-dimensional networks of platin nanoparticles separated by organic spacer on molecules. First studies have been made for a structure determination of biological macromolecules in solution from x-ray small angle scattering data. Another research focus is the research on thin liquid or polymeric films, where the film thickness is in the range of only a few molecular diameters. These two-dimensional systems usually have completely different behaviour compared to the bulk and also are of importance for applications as lubricants, coatings or insulating layers. A report on recent results obtained with synchrotron radiation is attached. The research on structural disorder in alloys has shifted away from bulk materials towards the investigation of order-disorder phenomena near surfaces, see the report on a synchrotron radiation study of CuAu.

The Forschungszentrum Jülich has taken a leading role in the research and development for a future new generation neutron source, the European Spallation Source ESS. The Institute for Scattering Method is engaged in this research, wherever the expertise of the neutron scatterers and the material experts are concerned. The research group for "Materials under heavy irradiation loads" has been incorporated in the institute and performs research mainly for the ESS project. Besides the development of ideas for innovative neutron instrumentation, most activities concentrate on the target-moderator- and reflector-module of a future high intensity spallation source. This comprises the investigation of changes of the mechanical and microstructure properties of candidate materials for the spallation target. Studies are made on spend targets from existing spallation sources. Microstructure changes of candidate structure-materials by helium-implantation are being investigated at the cyclotron. The effect of shock-waves within the Hg target are being studied both experimentally and with finite element calculations. For the first time a rather good agreement could be achieved. On the full-size target-moderator-reflector test-stand JESSICA at the COSY-synchrotron the time-structure of neutron-pulses could be measured and gave the expected resolution.

A qualified use of the large facilities for neutron- and synchrotron-radiation research requires the continuous development of the instrument park, largely triggered by the requirements for the scientific problems and by the available state-of the art mechanical, optical and electronical devices.

Neutron instrumentation: A continuous program of instrument development and instrument up-grade is being pursued for the neutron scattering instruments at the DIDO reactor in Jülich. The new high resolution small angle scattering machine KWS-3, based on a technique employing focusing mirrors, is completely erected in the neutron guide-hall ELLA of the research reactor DIDO. A first test of the imaging properties with neutrons did not show any influence of gravity on the focus. A momentum space resolution of up to 2x104A-1 can be reached with an entranceaperture of 1 x 1 mmz. The neutron-reflectometer HADAS has been commissioned and permits the measurement of diffuse scattering with polarisation analysis from 1 cm~ big magnetic layer systems. The new diffractometer/spectrometer for thermal neutrons with full polarisation analysis SV30 is close to completion. The beam-tube insert has been installed during the long reactor shut-down in summer. Major development for this machine include research on neutron spin-filter cells with polarised 3He and on a large solid angle area-detector based on a neutron image-plate. Highly polarised 3He at mbar pressure could be produced with a degree of polarisation exceeding 60 %. Important progress has been achieved with the development of the neutron image-plate with low sensitivity for g-radiation by employing the szintillators KBr and KCl in connection with LiF as neutron converter. While the neutron sensitivity is comparable with commercial image plates, the g-sensitivity could be reduced by at least one order of magnitude.

Synchrotron instrumentation: Forefront experiments with synchrotron radiation can only be performed if a continuous method development program is being pursued. Examples are the monochromator development for the highenergy undulator station at the Advanced Photon Source APS. At this station an image plate detector for the measurement of diffuse scattering of high-energy synchrotron radiation in transmission geometry has been successfully commissioned. For the study of confined liquids with synchrotron radiation a sample cell has been developed with which a pressure of up to 300 kN can be applied perpendicular to the film-surface. The cell has been successfully commissioned in a first experiment on tetrachlormethan which could be confined to 160 A gap-distance between two flat A1203-substrates. A new method to measure resonant exchange scattering from ferromagnetic layersystems has been developed and tested on the model-systems EuS, Tb and Er/Tb multilayers. The method is based on the polarisation properties of synchrotron radiation.

Besides the MBE-techniques for the growth of metallic films, the institute operates a small chemistry lab for sample preparation and facilities for single crystal growth. The existing facilities for the growth of metallic single crystals by the Czochralski and Bridgman-techniques will in the future be complemented by a mirror furnace employing the floating zone technique which is currently under construction at the institute. Manganite single crystals have been grown successfully in the existing mirror furnace of the RWTH Aachen (collaboration with the institute for crystallography, Prof. Heger). Besides the manganites, single crystals of metallic alloys such as CuPd, rare-earth-sulfides (Eul_xGdxS) and carbonites have been prepared. A new x-ray laboratory with a high intensity low temperature four circle diffractometer, a small-angle-scattering machine and reflectometer is currently being set up.

I would like to close this report by thanking all members of the institute for their dedicated work during the past year.

Thomas Brückel

Publications in refereed journals