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Garching bei München, Germany

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. Source

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. Source

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. Source

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. Source

Kraus F.,TU Munich | Baer S.A.,TU Munich | Hoelzel M.,Forschungs Neutronenquelle Heinz Maier Leibnitz | Karttunen A.J.,University of Jyvaskyla
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. Source

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