Garching bei München, Germany
Garching bei München, Germany

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Babcock E.,Jülich Research Center | Salhi Z.,Jülich Research Center | Pistel P.,Jülich Research Center | Simeoni G.,Forschungs neutronenquelle Heinz Maier Leibnitz | Ioffe A.,Jülich Research Center
Journal of Physics: Conference Series | Year: 2014

We present progress towards a complete system for neutron polarization analysis on a time-of-flight (TOF) neutron spectrometer with a large area/angle detector array. Finite element calculations have been used to model the field gradients of a newly proposed PASTIS coil set, which uses a wide-angle banana shaped 3He Neutron Spin Filter cell (NSF) to cover a large neutron scattering solid angle. The final goal of this insert is to enable X-Y-Z polarization analysis to be installed on the future hot/thermal time-of flight spectrometers, although the method is also applicable to thermal/cold spectrometers as well. The components of this system, such as the magnetic field coils and design are applicable to neutron spectroscopy with wide angle detector arrays in general, and the 3He wide angle cell developments for polarized inelastic neutron scattering are independent from the methods used to polarize the gas as well. © Published under licence by IOP Publishing Ltd.

Ziegler P.,TU Munich | Paul N.,Forschungs Neutronenquelle Heinz Maier Leibnitz | Muller-Buschbaum P.,TU Munich | Wiedemann B.,TU Munich | And 7 more authors.
Journal of Applied Crystallography | Year: 2014

Fe layers with thicknesses between 5 and 100 nm were sputtered on mesoporous nanostructured anatase TiO2 templates. The morphology of these hybrid films was probed with grazing-incidence small-angle X-ray scattering and X-ray reflectivity, complemented with magnetic measurements. Three different stages of growth were found, which are characterized by different correlation lengths for each stage. The magnetic behavior correlates with the different growth regimes. At very small thicknesses the TiO2 template is coated and a porous Fe film results, with in-plane and out-of-plane magnetization components. With increasing thickness, agglomeration of Fe occurs and the magnetization gradually turns mostly in plane. At large thicknesses, the iron grows independently of the template and the magnetization is predominantly in plane with a bulk-like characteristic. © 2014 International Union of Crystallography.

Rodriguez E.E.,University of Maryland University College | Sokolov D.A.,University of Edinburgh | Stock C.,University of Edinburgh | Green M.A.,University of Kent | And 5 more authors.
Physical Review B - Condensed Matter and Materials Physics | Year: 2013

We construct a phase diagram of the parent compound Fe1+xTe as a function of interstitial iron x in terms of the electronic, structural, and magnetic properties. For a concentration of x<10%, Fe1+xTe undergoes a "semimetal" to metal transition at approximately 70 K that is also first-order and coincident with a structural transition from a tetragonal to a monoclinic unit cell. For x≈14%, Fe1+xTe undergoes a second-order phase transition at approximately 58 K corresponding to a semimetal to semimetal transition along with a structural orthorhombic distortion. At a critical concentration of x≈11%, Fe1+xTe undergoes two transitions: the higher-temperature one is a second-order transition to an orthorhombic phase with incommensurate magnetic ordering and temperature-dependent propagation vector, while the lower-temperature one corresponds to nucleation of a monoclinic phase with a nearly commensurate magnetic wave vector. While both structural and magnetic transitions display similar critical behavior for x<10% and near the critical concentration of x≈11%, samples with large interstitial iron concentrations show a marked deviation between the critical response indicating a decoupling of the order parameters. Analysis of temperature dependent inelastic neutron data reveals incommensurate magnetic fluctuations throughout the Fe1+xTe phase diagram are directly connected to the "semiconductor"-like resistivity above TN and implicates scattering from spin fluctuations as the primary reason for the semiconducting or poor metallic properties. The results suggest that doping driven Fermi surface nesting maybe the origin of the gapless and incommensurate spin response at large interstitial concentrations. © 2013 American Physical Society.

Kalisvaart W.P.,University of Alberta | Luber E.J.,University of Alberta | Poirier E.,Optimal Inc. | Harrower C.T.,University of Alberta | And 6 more authors.
Journal of Physical Chemistry C | Year: 2012

Magnesium hydride has high storage capacity (7.6 wt % H) but very slow sorption kinetics. Addition of catalytic phases on the surface as well as alloying with transition metals is known to improve the properties. In this study, the sorption kinetics of a 50-nm Mg layer and Mg-10%Cr-10%V layer, capped with a CrV/Pd bilayer catalyst, are compared using a combination of neutron reflectometry (NR), X-ray diffraction (XRD), and atomic force microscopy to elucidate the effects of alloying on the hydrogen storage properties of Mg at room temperature. From NR it is found that the Cr-V alloyed layer shows both a delay in expansion in the first absorption cycle and a delay in contraction in the first desorption, which indicates a delay in nucleation of MgD 2 and formation of substoichiometric MgD 2-δ, respectively. Compared to pure Mg, the kinetics are strongly improved as no blocking MgD 2 layer is formed. XRD showed a strong reduction in the Mg grain size for the Cr-V alloyed layer after one cycle. For pure Mg, the grain size is almost unchanged although the film becomes nanocrystalline in the hydrided state. NR is shown to be highly sensitive to both the deuterium distribution as well as the layer thickness, which makes it a valuable tool for studying reaction mechanisms and quantification of the expansion of both crystalline and amorphous energy storage materials. © 2012 American Chemical Society.

Kraus F.,TU Munich | Baer S.A.,TU Munich | Hoelzel M.,Forschungs Neutronenquelle Heinz Maier Leibnitz | Karttunen A.J.,University of Jyväskylä
European Journal of Inorganic Chemistry | Year: 2013

Treatment of BeCl2 with dry liquid ND3 and subsequent removal of the solvent leads to the colourless microcrystalline powder of [Be(ND3)4]Cl2. It crystallises in the orthorhombic space group Pna21 with a = 9.395(4), b = 11.901(6), c = 6.761(3) Å, V = 755.9(6) and Z = 4 at 27°C, and a = 9.3736(8), b = 11.8162(12), c = 6.6596(6) Å, V = 737.62(12) and Z = 4 at -269.6°C. The structure contains the tetrahedral tetraammineberyllium(II) cation which follows the octet rule. It was shown to be stable under ambient conditions and temperatures up to approximately 175°C. We additionally discuss the aid of solid-state quantum chemical calculations for the assignment of proper crystallographic space groups in cases where the choice between centrosymmetry and noncentrosymmetry is not completely obvious, e.g. here between Pna2 1 and Pnma. Thermogravimetric measurements show that [Be(ND 3)4]Cl2 decomposes only above approximately 175°C, giving off two equivalents of ND3 to form [Be(ND 3)2Cl2]. The latter seems to be stable up to approximately 300°C after which it sublimes. These results show unambiguous evidence for the stability of the tetraammineberyllium(II) cations of the title compounds from -269.6°C to 175°C. Treatment of BeCl2 with dry liquid ND3 and subsequent removal of the solvent leads to a colourless microcrystalline powder of [Be(ND3)4]Cl 2. © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Stock C.,University of Edinburgh | Rodriguez E.E.,University of Maryland University College | Sobolev O.,Forschungs Neutronenquelle Heinz Maier Leibnitz | Rodriguez-Rivera J.A.,U.S. National Institute of Standards and Technology | And 5 more authors.
Physical Review B - Condensed Matter and Materials Physics | Year: 2014

Neutron spectroscopy is used to investigate the magnetic fluctuations in Fe1+xTe - a parent compound of chalcogenide superconductors. Incommensurate "stripelike" excitations soften with increased interstitial iron concentration. The energy crossover from incommensurate to stripy fluctuations defines an apparent hourglass dispersion. Application of sum rules of neutron scattering find that the integrated intensity is inconsistent with an S=1Fe2+ ground state and significantly less than S=2 predicted from weak crystal field arguments pointing towards the Fe2+ being in a superposition of orbital states. The results suggest that a highly anisotropic order competes with superconductivity in chalcogenide systems. © 2014 American Physical Society.

Paul A.,TU Munich | Teichert A.,Forschungs Neutronenquelle Heinz Maier Leibnitz | Krist T.,Helmholtz Center Berlin | Steitz R.,Helmholtz Center Berlin
Journal of Applied Crystallography | Year: 2015

Application of a bias voltage can influence the growth kinetics and thereby the stress in a magnetic multilayer. The inherent structural correlations in turn can influence the magnetic domain structures and thereby the overall device performance. Here, prototypical Fe/Si supermirrors are subjected to stress relaxation during the growth of sequential layers by applying a sufficient substrate bias voltage. A change in the coercivity associated with the grain size variation upon biasing is found. Most interestingly, using polarized neutron scattering, it is possible to identify that the conformal roughness becomes nonconformal with the relaxation of stress within the multilayers. The magnetic domains, on the other hand, always remain nonconformal (independent of the structural change) as they undergo spatial fluctuations around a mean magnetization. This study underscores the importance of the substrate biasing in affecting the structural correlation, which is detrimental to the resultant optical (e.g. supermirror) quality. © 2015 International Union of Crystallography.

Toth S.,Helmholtz Center Berlin | Toth S.,TU Berlin | Lake B.,Helmholtz Center Berlin | Lake B.,TU Berlin | And 5 more authors.
Physical Review Letters | Year: 2012

In this Letter, we explore the phase diagram and excitations of a distorted triangular lattice antiferromagnet. The unique two-dimensional distortion considered here is very different from the "isosceles"-type distortion that has been extensively investigated. We show that it is able to stabilize a 120° spin structure for a large range of exchange interaction values, while new structures are found for extreme distortions. A physical realization of this model is α-CaCr 2O 4, which has a 120° structure but lies very close to the phase boundary. This is verified by inelastic neutron scattering which reveals unusual rotonlike minima at reciprocal space points different from those corresponding to the magnetic order. © 2012 American Physical Society.

Brunnet H.,Saarland University | Hofmann M.,Forschungs Neutronenquelle Heinz Maier Leibnitz | Lyubenova N.,Saarland University | Bahre D.,Saarland University
Advanced Materials Research | Year: 2014

Selectively induced compressive residual stress depth profiles are gaining increasing importance as design tool for internally pressurized components. Hydraulic autofrettage (AF) is a well-known manufacturing process to induce pronounced compressive residual stresses. However, AF does not stand alone in the technical process chain. In this paper, results from neutron diffraction experiments performed on thick-walled cylinders are presented and compared to finiteelement simulations with Abaqus/CAE. The impact on the final residual stress depth profile after pre-machining, autofrettage and post-machining is discussed. © (2014) Trans Tech Publications, Switzerland.

Flierl B.,Ludwig Maximilians University of Munich | Hertenberger R.,Ludwig Maximilians University of Munich | Biebel O.,Ludwig Maximilians University of Munich | Zeitelhack K.,Forschungs Neutronenquelle Heinz Maier Leibnitz
Nuclear Instruments and Methods in Physics Research, Section A: Accelerators, Spectrometers, Detectors and Associated Equipment | Year: 2016

Spatial resolution of less than 200μmis challenging for thermal neutron detection. A novel readout scheme based on the time-projection-chamber (TPC) concept is used in a gaseous electron multiplier (GEM) detector [1]. Thermal neutrons are captured in a single 2μmthick Boron-10 converter cathode and secondary Helium and Lithium ions are produced with a combined energy of 2.8 MeV. These ions have sufficient energy to form straight tracks of several mm length. With a time resolving 2-dimensional readout of 400μmpitch in both directions, based on APV25 chips, the ions are tracked and their respective origin in the cathode converter foil is reconstructed. Using an Ar-CO2 93:7% gas mixture, a resolution of 100μm(FWHM 235μm) has been observed with a triple GEM-detector setup at the Garching neutron source (FRMII) for neutrons of 4.7 Å. © 2015 Elsevier B.V.

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