IFF
Scientific Report 1999/2000

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. 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 three fields: solid state magnetism, structural disorder 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).

• SV 30 (Th. Zeiske, Th. Reif, G. Kluck / E. Küssel, B. Schmitz)                                                                                   
  A new instrument for neutron polarisation analysis in the thermal energy range is under construction. This instrument will replace the old SV 4 triple axis spectrometer at the DIDO reactor. The instrument will employ ³He filters for the production and analysis of polarised neutrons and will work at energies up to 110 meV. For inelastic measurements, a triple axis spectrometer module is foreseen, for diffraction measurements a neutron image plate detector will be developed. The design of the primary spectrometer, which comprises filter, monochromator and biological shielding, is completed. All essential parts are ordered or are built in the workshop of the Forschungszentrum. The design of the secondary spectrometer is in progress.
• Polarised ³He (R. Mueller, Ch. Horriar-Esser)                                                                                                          
  ³He is a very effective neutron absorber (³He +n ® ⁴He* ® T+p). However, the absorption cross section is strongly dependent on the neutron spin direction (6000 barns for antiparallel n and ³He spins, 5 barns for parallel spins). Therefore, ³He can be used to build very efficient filter cells for neutron polarisation allowing polarisation analysis over a wide energy and angular range. We will produce polarised ³He gas by optical pumping at low pressure and subsequent compression to several bar. The apparatus for polarising ³He nuclei is well under way. A major breakthrough has been achieved in collaboration with a laser company by replacing the delicate LNA laser used elsewhere for optical pumping by a new 10 W fibre laser. Assembly of the optical pumping station will be finished in 1999 and the design of the compressor unit is well under way. We plan to use the ³He filter cells for the new SV 30 spectrometer, but possibly also at other instruments. We want to emphasise that ³He filters are an unique possibility to perform polarisation analysis experiments at pulsed sources such as the planed European Spallation Source (ESS).

• Reflectometer HADAS (U. Rücker, B. Alefeld, W. Bergs)                                                                                          
  To determine the magnitude and direction of magnetisation in thin film samples and multilayers, a neutron reflectometer with polarisation analysis is required. The reflectometer HADAS is currently being equipped with polarisation analysis. The supermirrors used to produce and analyse the polarised neutron beam have been delivered and tested. They are being assembled into benders. A concept for the instrument control (electronics hardware and software) has been established and will be installed next year.
• Focusing small angle scattering instrument KWS 3 (B. Alefeld, L. Dohmen)                                                            
  This new instrument, which will work as a small angle camera and a reflectometer, follows a completely new concept by focusing an entrance pinhole onto the detector. The focusing elements are a toroidal mirror and several concentric so-called replica mirrors. We expect an improvement in resolution compared to a standard pinhole camera by a factor of 10. The design and construction of all major mechanical components such as velocity selector, mirror and sample chamber are completed. A new neutron guide section has been installed. The toroidal mirror has been delivered and a process to produce the replica mirrors from a NiCu-alloy has been established.
• Instrument for diffuse neutron scattering DNS (W. Schweika, A. Broch)                                                                   
  This compact time-of-flight instrument optimised for diffuse neutron scattering has been equipped with supermirror benders to allow polarisation analysis experiments. Taking full advantage of neutron focusing, this instrument reaches higher countrates as compared to the so far world leading instrument D 7 at the high flux reactor of the ILL in Grenoble. Additional detectors are being equipped with supermirror benders in order to enlarge the solid angle, for which polarisation analysis is available.
• Developments for the ESS target station (H. Conrad)                                                                                                
  Due to the high power deposition in the target of the planed European Spallation Source (ESS), shock waves represent a serious design problem. The newly developed laser interferrometric method for the measurement of theses shock waves could be validated. The comparison of the experiments at the AGS proton accelerator in Brookhaven and numerical simulations show a quantitative agreement only for the first shock pulse, i. e. up to a time of about 60 µs after the proton pulse. The numerical calculations suggest a strong dependence on the proton beam profile. In a larger international collaboration, an experiment has been set up on the proton accelerator COSY in Jülich with the aim to optimise the ESS target and moderator geometry. One of the aims of this so-called JESSICA experiment is to validate Monte Carlo codes for the simulation of the spallation process and the neutron transport. It is planned that the JESSICA experiment becomes operational during the next year.
• Undulator station at APS (D. Hupfeld, P. Hiller)                                                                                                       
  In collaboration with American partners we are building the instrumentation for a sector (bending magnet plus insertion device line) at the Advanced Photon Source (APS) in Argonne, USA. The undulator line has just become operational and delivers photons in the energy range 4 - 20 keV into an experimental hutch equipped with a diffractometer for magnetic x-ray diffraction. FZ Jülich is currently building a high energy side station for the energy range 30 - 120 keV. The experimental enclosures have been erected, the monochromator unit is under construction and the diffractometer has been ordered. A major breakthrough was the development of water-cooled silicon monochromator crystals that can stand the high load of about 100 W/mm².
• Wiggler station W 1 at HASYLAB (W. Caliebe)                                                                                                         
  A new cryomagnet with x-ray transparent windows designed for fields up to 5 T in the temperature range 1.5 up to 300 K has been designed. It has been tested in a modified setup of the diffractometer, which allows horizontal scattering geometry. The operating system of the beamline has been upgraded to a modern unix standard.
• The JUSIFA instrument at HASYLAB for anomalous small angle x-ray scattering (ASAXS) (G. Goerigk, H.-G. Haubold)
  The beam stability could be drastically improved by a new beam position regulation. For measurements on magnetic materials, a polarisation monitor is now available.

All these developments have only been possible with massive technical support by the central technical divisions at the Forschungszentrum Jülich, HASYLAB and APS. We especially want to acknowledge the support by ZAT, ZEL, BD and the construction, mechanical workshop and electronics group at the IFF.

The large number of activities related to instrumental developments described in the section above is characteristic for a young institute. However, the instrumentation is only a means to an end. The research areas, which are being pursuit in our institute, are:

• Structural characterisation of thin film and multilayer devices for magnetoelectronics (W. Babik, E. Kentzinger, U. Rücker, Y.-G. Wang, W. Caliebe, G. Goerigk)                                                                                                      
  Within the framework of the HGF strategy project "magnetoelectronics", we collaborate with groups within the IFF, IGV and ISI. By means of x-ray and neutron reflectivity and diffuse scattering measurements, we characterise the interface morphology of technologically relevant multilayers, such as Fe/Cr/Fe, Co/Cu, Al₂O₃/Ni₃Al. Scattering methods are the only technique to access statistical parameters of buried interfaces. We have developed a program package for the analysis of reflectivity and diffuse scattering data and established the measurement procedure employing contrast variation with anomalous scattering at synchrotron based instruments. These techniques have been applied to the above systems and we could identify changes in the interface morphology depending on the preparation process in Fe/Cr/Fe-multilayers. The hope is to correlate these structural parameters with transport properties, such as the magnetoresistance, in order to improve the multilayer production and obtain a better understanding of the magnetoresistance phenomena.
• Magnetism of d -Mn (E. Kentzinger, S. Nerger, U. Rücker, W. Caliebe, G. Goerigk, J. Voigt)                                  
  We were able to stabilise the bct d -Mn phase in the form of a thin epitaxial layer on top of an Fe thin film. The magnetism of this phase, which is thermodynamically stable only at very high temperatures, has recently attracted a lot of attention and several magnetic structures were predicted. We did an intensive characterisation of this multilayer system and could, by means of neutron diffraction, exclude the proposed antiferromagnetic structures. At present, we vary the Mn distance by evaporating Mn on a FeCo-alloy. We intend to study the magnetic order of d -Mn as a function of the lattice parameters.
• Magnetism of rare-earth multilayers (J. Voigt, U. Rücker, E. Kentzinger, S. Nerger, W. Caliebe, G. Goerigk)       
  To investigate proximity effects and the effects of restricted dimensionality, we have started a research program on multilayers consisting of two magnetic rare-earth elements. While multilayers of magnetic rare-earth elements separated by non-magnetic intermediate layers have been studied extensively in the past, little is known about multilayers consisting of two magnetic rare-earth elements. The first system we chose are Tb/Er-multilayers. These systems are being produced by MBE and the growth conditions are optimised by LEED, Auger-analysis and x-ray reflectometry. The magnetic order has been investigated with neutron diffraction, where we found magnetic order with finite correlation lengths. Resonant exchange scattering of synchrotron radiation will be employed to study the magnetic order element-specific.
• Spin, charge, and orbital ordering in manganites (K. Istomin, W. Caliebe, Th. Zeiske)                                           
  Neutron scattering, high energy x-ray diffraction and resonant x-ray diffraction can be used to study the interplay between spin, charge and orbital ordering in the colossal magnetoresistance compounds. Currently, we are optimising the growth conditions for single crystals of LaMnO₃ with different dopings. Orbital and charge ordering in layered manganites has been studied in collaboration.
• Magnetic x-ray scattering (D. Hupfeld, J. Voigt, J. Strempfer, W. Caliebe)                                                              
  Resonant, as well as non-resonant magnetic scattering of synchrotron x-rays has been performed on Sm and Gd compounds, as well as metallic Cr and Tb. By combining high energy x-ray data with neutron diffraction, spin and orbital contributions could be separated in antiferromagnetic Cr. In SmBi, magnetic resonant scattering could be observed and the magnetic structure could be determined. With polarisation analysis, the mechanism of resonance exchange scattering in Tb has been investigated.
• Fourth order exchange interactions (U. Köbler, R. Mueller, B. Olefs)                                                                      
  Fourth order exchange interactions have been shown to be present in many materials. They influence the magnetic phase transitions, the magnetic structure and the low temperature properties. Deviations from the Bloch T^3/2 law could be identified in many materials. Indications of weak first order phase transitions with changes of the critical exponent b for the sublattice magnetisation have been found in several materials.
  
  
  
  
• Structural disorder: CuAu alloys (W. Schweika)                                                                                                        
  By means of surface sensitive x-ray scattering from a 1,0,0 surface of a CuAu alloy, an oscillating concentration profile has been found. Bulk disordered Cu₃Au alloys have been studied with diffuse anomalous x-ray scattering. The effects due to chemical short range order and displacements related to size and charge transfer could be separated.
• Electrochemical process (H.-G. Haubold, Th. Vad, P. Hiller, H. Jungbluth)                                                            
  Size distributions, oxidation states and surface coverage of platin catalyst particles on carbon supports were studied in situ by anomalous small angle scattering (ASAXS) and x-ray absorption spectroscopy (XANES) at the JUSIFA beamline. These processes are important for fuel cells and electrochemical sensor applications. For the direct methanol fuel cells, the best known membrane for use as a solid proton conducting electrolyte is Naphion. A small angle x-ray scattering study of this membrane gives new inside into the nanostructure.

Examples for the work accomplished at the Institute for scattering methods during the year 1999 are given by the progress reports on the following pages.

Thomas Brückel