IFF
Scientific Report 2000/2001

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 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 the fields: "solid state magnetism", "structural disorder", "electrocatalytic processes" 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).

• High Energy Side Station at the Advanced Photon Source in Argonne, USA (Th. Brückel, D. Hupfeld, P. Hiller in collaboration with Ames Laboratory and Argonne National Laboratory)
  During the last three years, an undulator beamline for scattering of high energy x-rays (30 - 130 keV) has been developed and built at the IFF. In fall 2000, the components were transported to Argonne National Laboratory, where the Forschungszentrum participates in the development and operation of a sector at the Advanced Photon Source APS in the framework of the MUCAT (Midwestern Universities Collaborative Access Team) collaboration. The components were installed and first radiation was received in the experimental enclosure during the last storage ring cycle in 2000. The first characterisation of the beam indicates that specifications concerning beam stability could be fulfilled. However, the full expected flux of about 10¹² photons at 80 keV will only be available with the second generation of monochromator crystals (see detailed report).
• Focusing Small Angle Scattering Instrument and Reflectometer KWS-3 (B. Alefeld, L. Dohmen)                             
  This new small angle camera and reflectometer is based on an entirely new principle employing mirrors as focusing elements. The mechanical design and construction of the instrument has been completed. The instrument has been erected in the neutron guide hall between the two small angle scattering instruments KWS-1 and KWS-2. We expect the instrument to be tested with neutrons in the beginning of 2001 after we obtain the permission for operation by the reactor safety commission.

• Reflectometer HADAS (U. Rücker, E. Kentzinger, T. Sorkalla)                                                                                  
  The neutron reflectometer HADAS is the first instrument, which received the new instrument control electronics developed by the ZEL (central electronic department of FZ Jülich) for the new Munich reactor FRMII. The reflectometer has been equipped with polarisation analysis employing remanent supermirrors. With a position sensitive detector, it is now possible to collect the diffuse scattering under grazing incidence with full polarisation analysis. First experiments with polarised supermirrors show a so far unknown spin splitting of the diffuse scattering of these magnetic multilayers (see detailed report).

• Neutron Spectrometer for Polarisation Analysis SV30 (H. Conrad, A. Ioffe, R. Mueller, S. Massalovitch, E. Küssel, B. Schmitz, Ch. Horriar-Esser)                                                                                                                       
  A new spectrometer for neutron polarisation analysis in the thermal neutron energy range up to 110 meV is under construction and will replace the old SV4 triple axis spectrometer at the DIDO reactor. Nuclear spin polarised ³He in filter cells will be employed for polarisation analysis over a large solid angle. We study the possibility to use a neutron sensitive image plate as position sensitive detector. The design of the primary spectrometer has been essentially completed and the mechanical components have been delivered or are being built on side. Two large double focusing monochromators, one with PG 002 and one with Cu 002 crystals, are available. In the beginning of January 2001, SV4 has been dismantled and the new beam tube insert has been approved by the reactor safety commission. In parallel, we started a new approach for ³He polarisation by spin exchange from potassium vapour. The appropriate 20 Watt laser has been purchased and the set-up is presently being installed (collaboration with ZEL, S. Appelt, F. Häsing).
• ESS Target Station (H. Conrad and ESS Collaboration)                                                                                             
  The JESSICA experiment, a full size test stand of the target, moderator and reflector module of the European Spallation Neutron Source ESS, has been taken into operation at the COSY synchrotron. For the first time, thermal neutrons from a spallation source have been produced in Jülich. The measured thermal neutron spectrum coincides perfectly with the predictions of theory. A report about the JESSICA experiment is given in a special section of this annual report.

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

• Theory of Grazing Incidence Diffraction (B. Toperverg, E. Kentzinger)                                                                    
  The scattering theory for grazing incidence diffraction using a supermatrix formalism has been further developed and has been implemented in a software package for simulation and analysis of neutron as well as x-ray data. For the neutron case, full polarisation analysis has been implemented for arbitrary non-collinear magnetic structures of multilayers. The formalism has been extended to laterally structured multilayers within the Distorted Wave Born Approximation DWBA.
• Resonant Exchange Scattering from Ferromagnets (D. Hupfeld, O. Seeck, J. Voigt, J. Bos, B. Schmitz)                  
  Magnetic x-ray scattering is several orders of magnitude weaker than charge scattering and therefore difficult to observe for ferromagnetic materials, where magnetic and charge scattering coincide. The traditional method is to measure the interference term between magnetic and charge scattering. We have developed a new method employing polarisation analysis, where in addition to the interference term, the pure magnetic resonant term (resonance exchange scattering) can be measured. The method has been applied to the ferromagnet EuS (see detailed report).

• Critical Behaviour of the Spin Chirality (V. P. Plakhty, Th. Brückel, W. Schweika)

In helically polarised antiferromagnets, the order parameter consists of not only the spin variables, but also the spin chirality, which describes whether the magnetic order is left or right handed. In a polarised neutron scattering experiment on the helimagnet holmium, we were able to determine the critical exponents for the average chirality b _c and the chiral susceptibility g _c and found experimental evidence that the spin order transitions in chiral systems belong to new chiral universality classes. Note that in order to measure the chiral susceptibility, a four spin correlation function has to be determined, in contrast to the common believe that neutron scattering gives only access to spin pair correlations (see detailed report).

• An Empirical Scheme of Spin Wave Exponents (U. Köbler, R. M. Mueller, K. Fischer)

From spin wave calculations, F. Bloch predicted that in the limit of very small temperatures, the spontaneous magnetisation follows a T^3/2 term. We report here the experimental finding that in an intermediate temperature range up to 70 % of the critical temperature, the temperature dependence of the spontaneous magnetisation exhibits simple power laws with a T² behaviour holding in many cases (see detailed report).

• Transition Metal Multilayers (W. Babik, E. Kentzinger, U. Rücker, J. Voigt, G. Goerigk, O. Seeck)

Transition metal multilayers, relevant for applications in the field of magnetoelectronics, have been prepared by MBE techniques, the interface morphology has been determined by x-ray scattering under grazing incidence and the magnetic structure has been investigated with polarised neutron reflectometry. Trilayers of Co/Cu/Co have been grown on sapphire as well as Si substrates. The interface morphology has been characterised with grazing incidence diffraction and will be linked to the magnetic properties. Similar studies have been performed in the Fe/Cr/Fe system. Fe/Mn/Fe and FeCo/Mn/FeCo trilayers have been investigated by means of polarised neutron reflectometry. It was found that in the FeCo samples, the Mn layer exhibits a spontaneous magnetisation, in contrast to bulk Mn and the Fe/Mn/Fe samples.

• Rare Earth Multilayers (J. Voigt, D. Hupfeld)

The magnetic structure and phase transitions in rare earth Er/Tb multilayers have been investigated by means of neutron diffraction and resonant magnetic x-ray scattering. A rich phase diagram could be established. As a result of proximity effects, the magnetic structures are quite different from the bulk. As an example, we have evidence from magnetic x-ray scattering at the Tb L_II and L_III edges that in the Tb layers a helical magnetic structure exists at low temperatures, where bulk Tb exhibits a basal plane ferromagnetism.

• Structure of Nanosized Metal Particle Networks (H.-G. Haubold, T. Vad, H. Jungbluth)

Nanosized metal particles can be assembled into highly ordered arrays by interconnecting the metal particles with organic spaces. These new materials are expected to exhibit novel collective properties. The structure of such Pt/polymer hybrids has been investigated by anomalous small angle x-ray scattering and particle radii and distances could be determined (see detailed report).

• Optimisation of the Composition and Synthesis of Storage Phosphors (M. Schlapp in collaboration with TU Darmstadt)

The storage phosphor BaFBr:Eu²⁺ finds application in x-ray image plate detectors. The dependence of the photo stimulated luminescence upon doping with alkaline earth metals has been studied and the spectral distribution of the stimulation could be optimised.

• Phason Disorder in Quasicrystals (W. Schweika)

The diffuse scattering of an icosahedral AlPdMn quasicrystal has been investigated. The scattering shows the characteristic behaviour expected for phason disorder. The temperature dependence of the diffuse intensity indicates the existence of a phason instability at lower temperatures.

• Free Standing Polymer Films (O. Seeck, M. Mihaylova)

Free standing polymer films have been investigated by grazing incidence scattering of synchrotron radiation. Films thicker than three times the radius of gyration behave like liquids, while in thinner films additional elastic terms seem to play an important role.

Examples for the work accomplished at the Institute for Scattering Methods during the year 2000 are given by the progress reports on the following pages.

Thomas Brückel