Search

The general performance of diffractometers at the first long pulse spallation source ESS, is compared with their counterparts at J-PARC, a short pulse spallation source. The difference in the inherent pulse structure of these neutron sources presents opportunities for new concepts for instrumentation, where performance does not scale simply with source power. The article describes advantages and disadvantages of those diffractometers, adapting to the very different source characteristics. We find that the two sources offer comparable performance in flux and resolution when operating in high-resolution mode. ESS offers significant advantages in tunability and flexibility, notably in the ability to relax resolution in order to increase flux for a given experiment. The slow repetition rate of ESS favors long instruments. On the other hand, J-PARC instruments perform very well in spite of the lower source power and allow better access to epithermal neutrons, of particular interest for PDF analysis of diffraction data.

Swedenborgite Ca${}_{0.5}$Y${}_{0.5}$BaCo${}_{4}$O${}_{7}$, with geometrically frustrated kagome and triangular lattices, exhibits a disordered ground state in spite of strong antiferromagnetic coupling between the Co spins. Here, the authors perform single-crystal diffuse neutron scattering experiments to fully characterize the short-range spin correlations in this compound and determine that long-range incommensurate spin correlations are suppressed by exchange disorder. This result reveals a general correspondence between unequal interactions and incommensurate spin correlations on the swedenborgite lattice.

A fundamental description of the instrument resolution file is elaborated for the angular- and wavelength-dispersive cases of Rietveld refinement, exemplified for the POWGEN instrument. It is shown how to refine the necessary profile function parameters from a dataset measured on a diamond reference sample. The analysis is performed in a two-dimensional refinement space based on the convenient variables d and d ⊥., The method of angular- and wavelength-dispersive (e.g. two-dimensional) Rietveld refinement is a new and emerging tool for the analysis of neutron diffraction data measured at time-of-flight instruments with large area detectors. Following the approach for one-dimensional refinements (using either scattering angle or time of flight), the first step at each beam time cycle is the calibration of the instrument including the determination of instrumental contributions to the peak shape variation to be expected for diffraction patterns measured by the users. The aim of this work is to provide the users with calibration files and – for the later Rietveld refinement of the measured data – with an instrumental resolution file (IRF). This article will elaborate on the necessary steps to generate such an IRF for the angular- and wavelength-dispersive case, exemplified for the POWGEN instrument. A dataset measured on a standard diamond sample is used to extract the profile function in the two-dimensional case. It is found that the variation of reflection width with 2θ and λ can be expressed by the standard equation used for evaluating the instrumental resolution, which yields a substantially more fundamental approach to the parameterization of the instrumental contribution to the peak shape. Geometrical considerations of the POWGEN instrument and sample effects lead to values for Δθ, Δt and ΔL that yield a very good match to the extracted FWHM values. In a final step the refinement results are compared with the one-dimensional, i.e. diffraction-focused, case.

The hexagonal swedenborgite, CaBaCo2Fe2O7, is a chiral frustrated antiferromagnet, in which magnetic ions form alternating kagome and triangular layers. We observe a long-range 3×3 antiferromagnetic order setting in below TN=160 K by neutron diffraction on single crystals of CaBaCo2Fe2O7. Both magnetization and polarized neutron single crystal diffraction measurements show that close to TN spins lie predominantly in the ab plane, while upon cooling the spin structure becomes increasingly canted due to Dzyaloshinskii-Moriya interactions. The ordered structure can be described and refined within the magnetic space group P31m′. Diffuse scattering between the magnetic peaks reveals that the spin order is partial. Monte Carlo simulations based on a Heisenberg model with two nearest-neighbor exchange interactions show a similar diffuse scattering and coexistence of the 3×3 order with disorder. The coexistence can be explained by the freedom to vary spins without affecting the long-range order, which gives rise to ground-state degeneracy. Polarization analysis of the magnetic peaks indicates the presence of long-period cycloidal spin correlations resulting from the broken inversion symmetry of the lattice, in agreement with our symmetry analysis. (Less)

We present a study that combines polarized and unpolarized neutrons to derive the magnetic structure of the swedenborgite compound $\mathrm{CaBa}({\mathrm{Co}}_{3}\mathrm{Fe}){\mathrm{O}}_{7}$. Integrated intensities from a standard neutron diffraction experiment and polarization matrices from spherical neutron polarimetry have been simultaneously analyzed revealing a complex order, which differs from the usual spin configurations on a kagome lattice. We find that the magnetic structure is well described by a combination of two one-dimensional representations corresponding to the magnetic superspace symmetry $P{2}_{1}^{\ensuremath{'}}$, and it consists of spins rotating around an axis close to the [110] direction. Due to the propagation vector $\mathbf{q}=(\frac{1}{3}00)$, this modulation has cycloidal and helicoidal character rendering this system a potential multiferroic. The resulting spin configuration can be mapped onto the classical $\sqrt{3}\ifmmode\times\else\texttimes\fi{}\sqrt{3}$ structure of a kagome lattice, and it indicates an important interplay between the kagome and the triangular layers of the crystal structure.

Centimetre-sized single crystals of high-quality CaBaCo2Fe2O7were synthesized by the optical floating zone technique. The metal-to-metal stoichiometry and oxygen content were confirmed by spectroscopy and thermal reduction experiments. The hexagonal symmetryP63mc(No. 186) well describes the powder X-ray and neutron diffraction as well as single-crystal neutron diffraction at all measured temperatures. This symmetry is also consistent with optical second harmonic generation data obtained between 10 and 295 K. However, a satisfactory structure description from single-crystal neutron diffraction data needs an oxygen split position. Specific heat, magnetic susceptibility and powder neutron diffraction data indicate a magnetic phase transition atTN= 159 K to an antiferromagnetic ground state, but with a persisting hexagonal symmetry and intrinsic geometric frustration.

The B-10 based Jalousie detector is a replacement for He-3 counter tubes, which are nowadays less affordable for large area detectors due to the He-3 crisis. In this paper we investigate and verify the performance of the new B-10 based detector concept and its adoption for the POWTEX diffractometer, which is designed for the detection of thermal neutrons with predicted detection efficiencies of 75-50% for neutron energies of 10-100 meV, respectively. The predicted detection efficiency has been verified by absolute measurements using neutrons with a wavelength of 1.17 angstrom (59 meV). (C) 2014 Elsevier B.V. All rights reserved.

Journal of Instrumentation 14(10), P10020 - P10020 (2019). doi:10.1088/1748-0221/14/10/P10020, HEIMDAL is a thermal powder diffractometer designed to operate at the European Spallation Source, world's most intense neutron source. The detailed design of the instrument, which is expected to enter user operation in 2024/2025, assumes that the neutrons scattered by the powder under investigation will be collected with hundreds of large-area Multi-Wire Proportional Counters employing a 10B4C-solid converter. The gas counters will consist of large active volumes and tapered trapezoidal shapes that allow for close packing into a cylindrical shell with high solid angle coverage. The whole detector will operate in an air environment within the shielding cave and provide signals with sensitivity for locating detection in three dimensions. This paper presents the results of a GEANT4 study of the baseline design for the HEIMDAL powder diffraction detector. The detector model was used to study the spatial resolution, which in the horizontal scattering plane must be below 3 mm in order to enable high-resolution diffraction studies. The contribution of the detector to the resolving power of the instrument, one of the key figures-of-merit for powder diffractometers, was also investigated. Most of the simulation results reported in this work cannot be validated against a sufficiently similar physical reference until the first segment or module are constructed and tested with a neutron beam. However, these results can help to identify possible ways of optimising the detector design and provide the first glimpse into the expected performance of this technological approach., Published by Inst. of Physics, London

The A-site spinel material, CoAl2O4, is a physical realization of the frustrated diamond-lattice antiferromagnet, a model in which is predicted to contain unique incommensurate or `spin-spiral liquid' ground states. Our previous single-crystal neutron scattering study instead classified it as a `kinetically-inhibited' antiferromagnet, where the long ranged correlations of a collinear Neel ground state are blocked by the freezing of domain wall motion below a first-order phase transition at T* = 6.5 K. The current paper expands on our original results in several important ways. New elastic and inelastic neutron measurements are presented that show our initial conclusions are affected by neither the sample measured nor the instrument resolution, while measurements to temperatures as low as T = 250 mK limit the possible role being played by low-lying thermal excitations. Polarized diffuse neutron measurements confirm reports of short-range antiferromagnetic correlations and diffuse streaks of scattering, but major diffuse features are explained as signatures of overlapping critical correlations between neighboring Brillouin zones. Finally, and critically, this paper presents detailed elastic and inelastic measurements of magnetic correlations in a single-crystal of MnAl2O4, which acts as an unfrustrated analogue to CoAl2O4. The unfrustrated material is shown to have a classical continuous phase transition to Neel order at T_N = 39 K, with collective spinwave excitations and Lorentzian-like critical correlations which diverge at the transition. Direct comparison between the two compounds indicates that CoAl2O4 is unique, not in the nature of high-temperature diffuse correlations, but rather in the nature of the frozen state below T*. The higher level of cation inversion in the MnAl2O4 sample indicates that this novel behavior is primarily an effect of greater next-nearest-neighbor exchange.

The POWTEX instrument will be a high-intensity time-of-flight diffractometer at a continuous reactor source. Its design utilizes several new concepts and is suited for small samples. Herein, the optimization of the guide system is presented. It is based on two coupled elliptic neutron guides intersected by the pulse-chopper as an eye of a needle to increase the time resolution and to reduce the background transported from the source. The optical properties of two coupled ellipses are investigated with respect to their design constraints in terms of geometry and regarding the choice of the super-mirror coating. It is shown that low-divergent neutrons are preferentially transported near the pulse-chopper. Accordingly, a numerical optimization is utilized to tailor the coating to the instrument requirements using the back-tracing method that has been developed for the VITESS Monte-Carlo simulation package for virtual instrumentation. The procedure allows for a cost and background optimization because the m c -value can be kept as low as needed. For a diffractometer, the homogeneity of the divergence profile is crucial. Herein, the new development of neutron guides with octagonal cross sections is investigated. Appendix A shows a step-by-step description of the newly developed optimization features within VITESS as it was used for POWTEX.

The ambitious instrument suite for the future European Spallation Source whose civil construction started recently in Lund, Sweden, demands a set of diverse and challenging requirements for the neutron detectors. For instance, the unprecedented high flux expected on the samples to be investigated in neutron diffraction or reflectometry experiments requires detectors that can handle high counting rates, while the investigation of sub-millimeter protein crystals will only be possible with large-area detectors that can achieve a position resolution as low as 200 {\mu}m. This has motivated an extensive research and development campaign to advance the state-of-the-art detector and to find new technologies that can reach maturity by the time the ESS will operate at full potential. This paper presents the key detector requirements for three of the Time-of-Flight diffraction instrument concepts selected by the Scientific Advisory Committee to advance into the phase of preliminary engineering design. We discuss the available detector technologies suitable for this particular instrument class and their major challenges. The detector technologies selected by the instrument teams to collect the diffraction patterns are briefly discussed. Analytical calculations, Monte-Carlo simulations, and real experimental data are used to develop a generic method to esti- mate the event rate in the diffraction detectors. The proposed approach is based upon conservative assumptions that use information and input parameters that reflect our current level of knowledge and understanding of the ESS project. We apply this method to make predictions for the future diffraction instruments, and thus provide additional information that can help the instrument teams with the optimisation of the detector designs., Comment: 25 pages, 5 figures

VI European Conference on Neutron Scattering (ECNS2015) : 30 August to 4 September 2015, Zaragoza, Spain / Javier Campo (chair of ECNS 2015, Materials Science Institute of Aragón, CSIC - University of Zaragoza) 6th European Conference on Neutron Scattering, ECNS2015, Zaragoza, Spain, 30 Aug 2015 - 4 Sep 2015; [Bristol] : IOP Publishing, Journal of physics / Conference Series, 746, 012013, 1-8 (2016). doi:10.1088/1742-6596/746/1/012013, Published by IOP Publishing, [Bristol]

We have investigated the structural and magnetic properties of (Er|Tb) multilayers by different scattering methods. Diffuse X-ray scattering under grazing incidence reveals the interface structure in (Er|Tb) bilayers and trilayers, indicating vertically correlated roughness between the Er and Tb interfaces. The magnetic properties of (ErnEr |TbnTb ) superlattices have been studied as a function of the superlattice composition (indices denote the number of atomic layers). Coupled ferromagnetic structures exist in all investigated samples. The phase transition temperature varies with the Tb layer thickness. Modulated magnetic order is short range for all samples beside the (Er20|Tb5) superlattice, the sample with the smallest Tb layer thickness. We observe dipolar antiferromagnetic coupling between single ferromagnetic Tb layers in all samples, with the onset of this ordering depending on the Tb layer thickness. Due to competing interactions, exchange coupling is limited to the interface near region. Therefore long range modulated magnetic order is observed in the (Er20|Tb5) superlattice only, where the interface regions overlap. The distinct differences to the magnetic structure of an Er0.8Tb0.2 alloy film are explained by a highly anisotropic arrangement of neighbouring atoms due to the correlated roughness. PACS. 75.75.+a Magnetic properties of nanostructures - 75.25.+z Spin arrangements in magnetically ordered materials (including neutron and spin-polarized electron studies, synchrotron-source X-ray scattering, etc.) - 61.18.Fs Magnetic resonance techniques; Mossbauer spectroscopy

Inelastic neutron scattering with emphasis on low-lying modes as well as cold neutron leakage measurements were performed on four prospective advanced cold moderator materials. Employing the time-of-flight instrument SV29 and DNS at the Julich FRJ-2 reactor, spectra were obtained from synthetic methane clathrate, tetrahydro-furane (THF) clathrate, 1,3,5-trimethyl-benzene (mesitylene) and light water ice at several temperatures between 2 and 70 K. Excitations at ±1,+2 and +3 meV were observed with synthetic methane clathrate. In mesitylene a wealth of low lying excitations were found. In the quenched phase we detected lines at 4.7, 7.2, 9.6, 13.6, 15.4, 18.4, 19.0, 23.0, 29.5 and 34.3 meV, respectively. In the annealed phase, we observed significant shifts with the majority of lines. The lowest lying lines now located at 7.0, 8.5 and 10.5 meV, respectively. In ice at T =2 K up to now unreported low-lying energy levels were found at 1.8 and 2.6 meV. An additional line at 10 meV was detected in THF clathrate. Mesitylene, synthetic methane clathrate and water ice, all at T =20 K, were tested as moderators at the Julich spallation mock-up JESSICA. The expected gain in neutron leakage current at energies around 2 meV as compared to liquid hydrogen moderators was observed for methane clathrate and mesitylene.

Proximity effects in an [Er20|Tb5] superlattice lead to the formation of new magnetic phases. Modulated magnetic order, expected for pure Er, and ferromagnetic order, expected for pure Tb, coexist at low temperatures. Employing X-ray resonance exchange scattering, we could probe directly the respective spin polarisations of the conduction band electrons, providing a mechanism for the interlayer coupling. The different anisotropies of Er and Tb compete with this tendency to long-range magnetic order, leading to substantial thermal-hysteresis effects.

The dynamics of mixed single crystals of AgxNa1−xCl has been investigated by inelastic neutron scattering before and after chemical demixing. In the homogeneous phase, the concentration dependence of acoustic phonons reveals that doping of NaCl with silver chloride leads to a considerable softening of the lattice, while the elastic properties of AgCl are almost independent of sodium chloride additives. After quenching into the miscibility gap, the phase separation is associated with a well-defined splitting of acoustic phonons which provide the most direct information about the underlying mechanism. In contrast to the local dynamical properties, the lattice structure is essentially determined by coherency strains which hinder the relaxation of lattice parameters. Thus diffraction and inelastic scattering yield independent and complementary information about demixing processes in ionic solids.

This paper introduces a novel approach to Rietveld refinements of two-dimensional angular- and wavelength-dispersive powder diffraction data as measured at time-of-flight neutron diffraction instruments. To do so, the authors’ ansatz for diffraction data obtained from the POWGEN diffractometer has been verified, and furthermore its feasibility and benefit in simulations for the novel POWTEX instrument presently under construction are demonstrated., This paper introduces a two-dimensional extension of the well established Rietveld refinement method for modeling neutron time-of-flight powder diffraction data. The novel approach takes into account the variation of two parameters, diffraction angle 2θ and wavelength λ, to optimally adapt to the varying resolution function in diffraction experiments. By doing so, the refinement against angular- and wavelength-dispersive data gets rid of common data-reduction steps and also avoids the loss of high-resolution information typically introduced by integration. In a case study using a numerically simulated diffraction pattern of Rh0.81Fe3.19N taking into account the layout of the future POWTEX instrument, the profile function as parameterized in 2θ and λ is extracted. As a proof-of-concept, the resulting instrument parameterization is then utilized to perform a typical refinement of the angular- and wavelength-dispersive diffraction pattern of CuNCN, yielding excellent residuals within feasible computational efforts. Another proof-of-concept is carried out by applying the same approach to a real neutron diffraction data set of CuNCN obtained from the POWGEN instrument at the Spallation Neutron Source in Oak Ridge. The paper highlights the general importance of the novel approach for data analysis at neutron time-of-flight diffractometers and its possible inclusion within existing Rietveld software packages.

In neutron scattering experiments using polarization analysis we separated the spectra of transverse and longitudinal magnetic fluctuations in the anisotropic antiferromagnet MnF2. While transverse modes are related to single-magnon scattering, the longitudinal part is essentially due to two-magnon scattering. They were measured at T = 30 K and 50 K well below the Neel temperature TN = 67 K. The dynamic magnetic response due to two-magnon creation or annihilation is separated by a gap centered near the spin-wave frequency from the central peak corresponding to neutron-magnon scattering (creation of one magnon and annihilation of another). The longitudinal energy spectrum extends to about twice the frequency of the zone boundary modes. This tail at high energies is fairly independent of momentum transfer. The observed longitudinal spectra are in qualitative agreement with the theory for two-magnon processes and are determined for large energy transfers ω by the density of states D(ω/2).

We report on an investigation of the magnetic properties of Gd x Eu 1 - x S mixed crystals with compositions in the range of 0.6 < x < 1. For the two samples Gd 0.8 Eu 0.2 S and Gd 0.73 Eu 0.27 S a long-range antiferromagnetic order was observed at low temperatures. Element-specific measurements exhibited a different temperature dependence of the reduced sublattice magnetisation of the two magnetic species. A model calculation and Monte Carlo simulations revealed that the different temperature dependence is due to frustration effects. These frustration effects lead to a breakdown of the long-range order for higher europium contents. For the Gd 0.67 Eu 0.33 S-sample we were able to observe a short-range antiferromagnetic order with correlation lengths of a few 10A with X-ray resonance exchange scattering.

The diffuse neutron scattering (DNS) instrument is a time-of-flight spectrometer that similar to the D7 in Grenoble has been equipped with neutron polarizers and analyzers covering a wide angular range. A particular feature of this instrument is its compact size and consequently, a focusing layout of the polarizers. The polarizing devices are benders that are made of a stack of magnetic supermirrors. Details of the instrumental design and its performance will be discussed.

We report on the determination of element-specific magnetic order for a Gd0.8Eu0.2S mixed crystal by means of X-ray resonance exchange scattering. For the first time, a different temperature dependence of the sublattice magnetization of two lanthanide ions was observed. With model calculations we were able to reproduce the measured temperature dependence of the reduced sublattice magnetization. We can show that competing magnetic interactions lead to the observed effects and can reveal the frustration mechanism.

Polycrystalline TbMn2O5 was prepared by the standard solid-state reaction method and characterized by powder X-ray diffraction and magnetization to assure it is of single phase. Heat capacity measurements on the compound reveal an antiferromagnetic phase transition at � 45 K. A broad peak below � 6 K in the heat capacity measurements corresponds to the crossover transition of Tb 3+ ordering. To confirm these magnetic orderings, neutron powder diffractions on TbMn2O5 with XYZ neutron polarization analysis were performed at the diffuse neutron scattering (DNS) spectrometer, FRJ-II, by

Quasielastic neutron scattering was applied to investigate the protonic motion in water-doped Ba[Ca 0.39 Nb 0.61 ]O 2.91 between 5 and 700 K. The backscattering spectra in the μeV-region for small scattering vectors Q lead to the protonic self-diffusion constant. The results were compared with measurements of the electrical conductivity. At larger Q , the spectra reveal two superimposed quasielastic components probably related to two groups of jump rates. Low resolution measurements indicate a rapid rotation of the hydroxyl groups.

Disorder and atomic diffusion are closely connected and these phenomena are the subject of investigations with numerous methods in our laboratory. We will report experiments on diffuse neutron scattering caused by the disorder in a strongly non-stoichiometric iron oxide, which leads to the formation of defect clusters and is related to diffusion and ion conductivity. Secondly, we will present results on quasielastic scattering for a water-doped perovskite with hydrogen atoms randomly bound at OH-ions. This material exhibits the interesting effect of proton conduction. The experiments were carried out on the spectrometers for diffuse scattering (DNS) and for peV-spectroscopy by backscattering (BSS1), both in the FRJ-2 guide laboratory ELLA.

Using extensive Monte Carlo simulations we have studied phase transitions in a fcc model with antiferromagnetic nearest-neighbor couplings $J$ in the presence of different free surfaces which lead either to surface-induced order or to surface-induced disorder. Our model is a prototype for CuAu-type ordering alloys and shows a strong first-order bulk transition at a temperature $\frac{k{T}_{\mathrm{cb}}}{|J|}=1.738005(50)$. For free (100) surfaces, we find a continuous surface transition at a temperature ${T}_{\mathrm{cs}}g{T}_{\mathrm{cb}}$ exhibiting critical exponents of the two-dimensional Ising model. Surface-induced ordering occurs as the temperature approaches ${T}_{\mathrm{cb}}$ and the surface excess order and surface excess energy diverges logarithmically. For a free (111) surface, the surface order vanishes continuously at ${T}_{\mathrm{cb}}$ accompanied by surface-induced disorder (SID). In addition to a logarithmic divergence of the excess quantities of order and energy, we find further critical exponents which confirm the actual theory of SID and critical wetting and which can be understood in terms of rough interfaces. For both cases of free surfaces, the asymptotic behavior of the squared interfacial width shows the expected logarithmic divergence.

We have performed systematic studies of the lattice dynamics in LFeAsO (L = La, Ce, Pr, Nd, Sm) in the parent and in the similar to 10% F-doped compounds as a function of pressure and temperature. We have found that the modifications in the partial Fe density of phonon states are mainly governed by the Fe-As bond length. The change of this bond length explains the change of the Fe density of phonon states above 25 meV. We further observe anomalies in the behavior of the phonon mode near 16 meV. In the parent phase, this mode softens anomalously upon cooling through the structural phase transition. Upon F doping, this mode hardens indicating a strong electron-phonon coupling. This suggests that the corresponding phonons play an important role in the competition between superconductivity and magnetism in these materials. DOI: 10.1103/PhysRevB.87.064302 (Less)

We have investigated the defect structure of nonstoichiometric w\"ustite ${\mathrm{Fe}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$O as a function of temperature and oxygen partial pressure by means of diffuse elastic neutron scattering of a single crystal at thermal equilibrium. Particularly for T=1423 K and x=0.08, a Fourier analysis yielded short-range order and lattice displacement parameters as well as the ratio of the numbers of vacancies to interstitials, \ensuremath{\rho}=4.0\ifmmode\pm\else\textpm\fi{}0.5. The measured short-range order showing a strong correlation between nearest interstitials and vacancies was simulated in a computer model. A further analysis yielded the size distribution of the vacancy-interstitial defect clusters exhibiting a large fraction of 30% of free vacancies, while a further 15% of the defects are bound in isolated so-called 4:1 defect clusters. The entire long-range displacement fields were modeled in a Kanzaki force approach using independent phonon data. Within the usual approach of a single-defect approximation the observed asymmetries of the diffuse scattering around the Bragg peaks were described by using Coulomb-like forces around the cation vacancies of random 4:1 defect clusters. The measured decrease of Huang scattering and the development of diffuse peaks with increasing nonstoichiometry was reproduced by a more general type of Kanzaki model for concentrated solid solutions. Therefore, we used the same defect model of the 4:1 cluster, although surrounded with two occupied cation shells to screen the long-range displacement fields.

We present the results of diffuse neutron scattering experiments on a single crystal of disordered α-VD0.781 at room temperature. The short range order of the deuterium atoms was described quantitatively in terms of SRO parameters. From these the D-D interaction parameters for a metal hydrogen system were determined for the first time by using the inverse Monte Carlo method.

We present a detailed study of magnetism in LuFe(2)O(4), combining magnetization measurements with neutron and soft x-ray diffraction. The magnetic phase diagram in the vicinity of T(N) involves a metamagnetic transition separating an antiferro- and a ferrimagnetic phase. For both phases the spin structure is refined by neutron diffraction. Observed diffuse magnetic scattering far above T(N) is explained in terms of near degeneracy of the magnetic phases.

A single crystal of Ni–20 at% Cr aged at 744 K for 480 h, similar to the case of a previous neutron scattering experiment, is investigated by diffuse X-ray scattering using X-rays at three different wavelengths, λ = 0.2075 nm (5975 eV), 0.1489 nm (8327.2 eV), and 0.1531 nm (8097 eV). The short-range order and displacement scattering are separated based on the 3λ method recently introduced as well as on the Georgopoulos-Cohen method. Both evaluation schemes yield similar Warren-Cowley short-range order parameters with less agreement in the individual static atomic displacements. The nearest-neighbor displacement obtained by the 3A method for NiCr pairs, is consistent with a simple phenomenological model in which the Cr atoms expand the Ni lattice in a short-range ordered alloy. Effective pair interactions calculated from model crystals using the inverse Monte Carlo method are found to be larger than previously deduced from neutron scattering experiments. If these pair interactions are used in Monte Carlo simulations to calculate the transition temperature T0 of Ni2Cr, T0 is underestimated compared with direct experimental findings. Ein Ni–20 At% Cr-Einkristall, der ahnlich wie bei einem fruheren Neutronenstreuexperiment fur 480 h bei 744 K ausgelagert worden ist, wird mit diffuser Rontgenstreuung bei drei Wellenlangen i untersucht: 0,2075 nm (5975 eV), 0,1489 nm (8327,2 eV) und 0,1531 nm (8097 eV). Nahordnungs- und Verzerrungsstreuung werden sowohl mit der kurzlich eingefuhrten 3λ-Methode als auch mit der Georgopoulos-Cohen-Methode separiert. Beide Auswertungen ergeben vergleichbare Warren-Cowley-Nahordnungsparameter mit geringerer Ubereinstimmung in den individuellen statischen atomaren Verschiebungen. Die mit der 3λ-Methode erhaltene Verschiebung nachstbenachbarter Ni-Cr-Paare ist konsistent mit einem einfachen phanomenologischen Modell, bei dem die Cr-Atome das Ni-Gitter einer nahgeordneten Legierung ausdehnen. Effektive Paarpotentiale, die mit der inversen Monte-Carlo-Methode bestimmt wurden, sind groser als die zuvor aus einem Neutronenstreuexperiment erhaltenen. Mit diesen Paarpotentialen wird die Ubergangstemperatur T0 von Ni2Cr in Monte-Carlo-Simulationen unterschatzt.

The transition metal-based oxide YBaCo${}_{3}$FeO${}_{7}$ is structurally related to the mineral Swedenborgite SbNaBe${}_{4}$O${}_{7}$, a polar noncentrosymmetric crystal system. The magnetic Co${}_{3}$Fe sublattice consists of a tetrahedral network containing kagome-like layers with trigonal interlayer sites. This geometry causes frustration effects for magnetic ordering, which were investigated by magnetization measurements, M\"ossbauer spectroscopy, polarized neutron diffraction, and neutron spectroscopy. Magnetization measurement and neutron diffraction do not show long range ordering even at low temperature (1 K), although a strong antiferromagnetic coupling (\ensuremath{\sim}2000 K) is deduced from the magnetic susceptibility. Below 590 K we observe two features, a spontaneous weak anisotropic magnetization hysteresis along the polar crystallographic axis and a hyperfine field on the Fe kagome sites, whereas the Fe spins on the interlayer sites remain idle. Below \ensuremath{\sim}50 K the onset of a hyperfine field shows the development of moments static on the M\"ossbauer time scale also for the Fe interlayer sites. Simultaneously, an increase of spin correlations is found by polarized neutron diffraction. The relaxation part of the dynamic response has been further investigated by high-resolution neutron spectroscopy, which reveals that the spin correlations start to freeze below \ensuremath{\sim}50 K. Monte Carlo simulations show that the neutron scattering results at lower temperatures are compatible with a recent proposal that the particular geometric frustration in the Swedenborgite structure promotes quasi--one-dimensional partial order.

The density of phonon states in the thermoelectric material Yb${}_{14}$MnSb${}_{11}$ has been studied first by inelastic neutron scattering and second in an element-specific way by nuclear inelastic x-ray scattering. The low sound velocity of 1880(50) m/s as obtained from the density of phonon states can be identified as an important reason for the low heat transport in this system. The high melting temperature of Yb${}_{14}$MnSb${}_{11}$ contrasts with the low energy of all phonons ($l$25 meV) and relates to an unusual lack of softening of phonon modes with temperature, when comparing the phonon density of states observed at ambient temperatures and at 1200 K. We have also measured the density of phonon states of the related Eu${}_{14}$MnSb${}_{11}$ compound and of the thermoelectric Zintl phase Zn${}_{4}$Sb${}_{3}$ in order to compare with related thermodynamic properties of Yb${}_{14}$MnSb${}_{11}$ and to elucidate the different mechanisms of the heat conductivity reduction in Zintl phases.

The coherent static structure factor of water has been investigated by polarized neutron diffraction. Polarization analysis allows us to separate the huge incoherent scattering background from hydrogen and to obtain high quality data of the coherent scattering from four different mixtures of liquid H(2)O and D(2)O. The information obtained by the variation of the scattering contrast confines the configurational space of water and is used by the reverse Monte Carlo technique to model the total structure factors. Structural characteristics have been calculated directly from the resulting sets of particle coordinates. Consistency with existing partial pair correlation functions, derived without the application of polarized neutrons, was checked by incorporating them into our reverse Monte Carlo calculations. We also performed Monte Carlo simulations of a hard sphere system, which provides an accurate estimate of the information content of the measured data. It is shown that the present combination of polarized neutron scattering and reverse Monte Carlo structural modelling is a promising approach towards a detailed understanding of the microscopic structure of water.

Polycrystalline samples of ${\mathrm{Ho}}_{\mathrm{x}}{\mathrm{Tb}}_{2\ensuremath{-}\mathrm{x}}{\mathrm{Ti}}_{2}{\mathrm{O}}_{7}$ (0 $x$ 2) have been prepared and characterized for their structural and magnetic properties. The parent compounds of this solid solution are the spin ice ${\mathrm{Ho}}_{2}{\mathrm{Ti}}_{2}{\mathrm{O}}_{7}$ and the spin liquid ${\mathrm{Tb}}_{2}{\mathrm{Ti}}_{2}{\mathrm{O}}_{7}$. Specific-heat measurements on ${\mathrm{HoTbTi}}_{2}{\mathrm{O}}_{7}$ ($x$ $=$ 1.0) reveal the absence of a long-range order state above 0.5 K. The integrated entropies of all ${\mathrm{Ho}}_{\mathrm{x}}{\mathrm{Tb}}_{2\ensuremath{-}\mathrm{x}}{\mathrm{Ti}}_{2}{\mathrm{O}}_{7}$ specimens up to 30 K scale well with the ratio of spin ice and spin liquid in the composition. The neutron diffraction spectrum of ${\mathrm{HoTbTi}}_{2}{\mathrm{O}}_{7}$ exhibits a dipolar spin-ice pattern and can be well described by mean-field theory for $\ensuremath{\langle}111\ensuremath{\rangle}$ Ising spins, nearest-neighbor exchange, and dipolar interactions. Inelastic neutron scattering on ${\mathrm{HoTbTi}}_{2}{\mathrm{O}}_{7}$ reveals two dispersionless excitations, one of \ensuremath{\sim}2.5 meV out of the ground state, and a 4-meV transition out of an excited state. We argue that these data suggest that the very strong single-ion effects of ${\mathrm{Ho}}_{2}{\mathrm{Ti}}_{2}{\mathrm{O}}_{7}$ and ${\mathrm{Tb}}_{2}{\mathrm{Ti}}_{2}{\mathrm{O}}_{7}$ persist in the ${\mathrm{Ho}}_{\mathrm{x}}{\mathrm{Tb}}_{2\ensuremath{-}\mathrm{x}}{\mathrm{Ti}}_{2}{\mathrm{O}}_{7}(0\mathrm{ }\mathrm{ }\mathrm{x}\mathrm{ }\mathrm{ }2)$ solid solution, whereas the Tb-Ho correlations are weak, resulting in small shifts in the energy scales but with no dramatic effect on the bulk properties.

We present systematic ab initio calcultions for the interaction energies of impurity pairs in Cu, Ni, Ag, and Pd. The calculations are based on local-density theory and apply the Korring-Kohn-Rostoker Green's-function method for spherical potentials. The full nonspherical charge density is used to calculate the double-counting contributions to the total energy. In particular, we calculate the nearest- and next-nearest-neighbor interaction energies of impurity pairs of the 3d and 4sp elements in Cu and Ni as well as of similar pairs of the 4d and 5sp elements in Ag and Pd: these interactions determine the ordering or segregation behavior of the alloys at finite temperatures. Comparisons are drawn with experimental phase diagrams, solid solubilities of the impurities, and also with the available information about the interaction from diffuse-scattering experiments. The physical mechanisms determining the nearest-neighbor interactions of the impurity pairs are discussed using simple model calculations based on the tight-binding method and the jellium approach: (1) The 4d-4d interactions in Ag and Pd, being repulsive at the beginning in the 4d series and attractive around the middle, can be understood by considering both the changes of the d bond and the repulsive energies between different atomic rearrangement of isolated impurities and impurity pairs. For 3d impurity pairs with the large local moments (Cr to Co) in Cu and Ni, magnetic effects also become important. (2) The repulsive interactions of 4sp-4sp in Cu as well as of 5sp-5sp in Ag can be understood by the electrostatic interaction between the excess ionic charges of the impurities, being screened by the conduction electrons of the host, while the stronger repulsive interactions of 4sp-4sp in Ni as well as of 5sp-5sp in Pd mainly arise from the breakup of the sp-d bonds between the sp impurities and the transition-metal hosts.

International audience

We have investigated the magnetic ordering and the phase transition in MnO nanoparticles confined in a porous glass using polarized neutron scattering. These MnO nanoparticles are best described as extended wormlike structures with a mean diameter of $70\text{ }\text{\AA{}}$. We observe an apparent continuous magnetic phase transition in MnO nanoparticles, in contrast to the well-known discontinuous phase transition in bulk MnO. By polarization analysis, separating the magnetic scattering, it is found that within the individual MnO nanoparticles about 60% of atoms remain disordered in the low-temperature limit, presumably due to interactions between nanoparticles and glass walls. The continuous character of the phase transition and the unusual temperature dependence suggests a surface-induced disorder phenomenon.

Studies of diffuse magnetic scattering largely benefit from the use of a multi-detector covering wide scattering angles. Therefore, the different contributions to the diffuse scattering that originate from magnetic, nuclear coherent, and nuclear spin-incoherent scattering can be separated by the so-called XYZ-polarization analysis. In the past this method has been successfully applied to the analysis of diffuse scattering by polycrystalline samples of magnetic disordered materials. Single crystal studies that exploit the vector properties of spin correlations are of particular interest for furthering our understanding of frustration effects in magnetism. Based on the symmetry properties of polarised scattering a suitable extension of the conventional XYZ method has been derived, which allows for the complete separation and the analysis of features of diffuse magnetic scattering from single crystals., Comment: 6 pages 2 figures, revised as published, one Eq. removed, minor corrections, typos corrected

In-situ diffuse neutron scattering experiments were carried out to investigate the defect structure of wustite Fe1-xO. The data were collected at various temperatures from room temperature to 1150°C and at deviations from stoichiometry, x, from 0.05 to 0.13. Diffuse peaks were found at Q = 2π/a (1.66, 0, 0) and Q = 2π/a (0.83,0.83, 0.83). Within the stability field of wustite the intensity of these diffuse maxima scales with the defect concentration. Neither the 13:4 defect clusters proposed from earlier X-ray experiments, nor a simple vacancy defect with a spherical displacement field (as found in Mn1-xO) are compatible with our observations.

In the context of a larger research project on amorphous polycondensates we have investigated the short range order in the polymer glass bisphenol-A-polycarbonate and two chemical modifications thereof using spin polarized neutron scattering. The experimental results are compared with model calculations obtained by “amorphous-cell” computer simulations.

The magnetic structure of MnNCN, the nitrogen equivalent of MnO, and the first carbodiimide of a non-d(10) transition metal ever made, has been determined on the basis of spin-polarized neutron-diffraction data close to absolute zero temperature. The S=5/2 moments of the high-spin Mn2+ ions lie within the ab hexagonal sheets and exhibit an antiferromagnetic ordering between the sheets along the c axis. Below the Neacuteel point (28 K), a temperature-dependent ordering pattern exists due to magnetic frustration inside the sheets, characterized by a varying angle between the moments. A comparison with other magnetic structures reveals that the compact NCN2- group is capable of mediating significant spin density needed to ensure relatively strong magnetic communication in view of a large Mn2+-Mn2+ separation.

Y{0.5}$Ca{0.5}BaCo4O7 contains kagome layers of Co ions, whose spins are strongly coupled according to a Curie-Weiss temperature of -2200 K. At low temperatures, T = 1.2 K, our diffuse neutron scattering study with polarization analysis reveals characteristic spin correlations close to a predicted two-dimensional coplanar ground state with staggered chirality. The absence of three dimensional long-range AF order proves negligible coupling between the kagome layers. The scattering intensities are consistent with high spin S=3/2 states of Co2+ in the kagome layers and low spin S=0 states for Co3+ ions at interlayer sites. Our observations agree with previous Monte Carlo simulations indicating a ground state of only short range chiral order., Comment: 4 pages, 4 figures, contact author: w.schweika@fz-juelich.de

Inelastic neutron scattering measurements on thermoelectric Zn4Sb3 reveal a dominant soft local phonon mode at 5.3(1) meV. The form factor of this local mode is characteristic for dumbbells vibrating preferably along the dumbbell axis and can be related to a vibration of Sb dimers along the c axis. The Lorentzian width of the mode corresponds to short phonon lifetimes of 0.39(2) ps and yields an estimate of the thermal conductivity that agrees quantitatively with recent steady state measurements. Heat capacity measurements are consistent with an Einstein mode model describing the local Sb-dimer rattling mode with an Einstein temperature of 62(1) K. Our study suggests that soft localized phonon modes in crystalline solids are not restricted to cagelike structures and are likely to be a universal feature of good thermoelectric materials.

Using a novel polarised neutron scattering technique, the critical exponents for the spin chirality and chiral susceptibility are determined for the triangular lattice antiferromagnet (TLA) CsMnBr3 in the ranges of reduced temperature τ>10−3 and τ>7×10−3, respectively. Their values, βC=0.44(2) and γC=0.85(3), together with the scaling relation α + 2 β C + γ C = 2.13 ( 9 ) , including the critical exponent where α for the specific heat, prove that the spin-ordering transition belongs to the XY chiral universality class. In the case of helimagnet Ho, it is found that β C - 2 β = 0.14 ( 4 ) , where β is the staggered magnetisation exponent. The scaling relation α + 2 β + γ = 2 could be fulfilled with a reasonable α=0.23(4), although for the chiral critical exponents βC=0.90(2) and γC=0.69(5) one needs α=−0.49(5) in contradiction with any experimental data. As the scaling relation always holds, we assume that the spin-ordering transition in Ho is of the first order. In the quantum antiferromagnet CsCuCl3, a triangular spin order coexists with a long-period Dzyaloshinskii helix. The Dzyaloshinskii axial vector should remove the helix chiral degeneracy, which has not been observed in reality. The critical exponent β=0.22(2) is found to be in agreement with the XY chiral scenario for a TLA. Chiral scattering above TN is very weak, probably being masked by zero-point quantum fluctuations. A modulation of the crystal structure with the periodicity of the helix is observed, indicating strong coupling of the Dzyaloshinskii–Moriya interaction with the lattice.

A focusing neutron polariser system was developed for the instrument for diffuse neutron scattering (DNS) located in the neutron guide laboratory at the research centre Julich. The focusing polarisers of the bender type are equipped with polarising FeCoV/Ti:N supermirrors. Results of Monte Carlo calculations of the transmission and focusing properties for different wavelength are given. Comparing the focusing polariser system with a conventional non-focusing set up, there is a considerable gain in intensity. For a wavelength of 3.3 A the largest enhancements are obtained; at the positions of the sample and detectors these are given by the factors 1.8 and 2.7, respectively.

Inelastic neutron scattering measurements on ${\mathrm{Ba}}_{8}{\mathrm{Ga}}_{16}{\mathrm{Ge}}_{30}$ and ${\mathrm{Sr}}_{8}{\mathrm{Ga}}_{16}{\mathrm{Ge}}_{30}$ indicate the presence of soft local modes with Einstein oscillator energies of $4.9\ifmmode\pm\else\textpm\fi{}0.1$ and $4.4\ifmmode\pm\else\textpm\fi{}0.1\phantom{\rule{0.3em}{0ex}}\mathrm{meV}$, modes which are associated with the lowest ``rattling'' mode of the Ba(B) and Sr(B) guests, respectively. Nuclear inelastic scattering measurements on ${\mathrm{Eu}}_{8}{\mathrm{Ga}}_{16}{\mathrm{Ge}}_{30}$ yield Einstein oscillator energies of between $3.0\ifmmode\pm\else\textpm\fi{}0.3$ and $7.5\ifmmode\pm\else\textpm\fi{}1.2\phantom{\rule{0.3em}{0ex}}\mathrm{meV}$ for the different modes of the Eu guests. Further, the resonant character of the measurements on ${\mathrm{Eu}}_{8}{\mathrm{Ga}}_{16}{\mathrm{Ge}}_{30}$ reveals, without question, that neither of the Eu guests exhibit any vibrational modes above $9\phantom{\rule{0.3em}{0ex}}\mathrm{meV}$.