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The incident detector rates that are anticipated for the indirect-geometry cold-neutron spectrometer BIFROST at the European Spallation Source are estimated, and the use of powerful simulation tools for the correct interpretation of neutron transport in crystalline materials is demonstrated., The European Spallation Source (ESS) is intended to become the most powerful spallation neutron source in the world and the flagship of neutron science in upcoming decades. The exceptionally high neutron flux will provide unique opportunities for scientific experiments but also set high requirements for the detectors. One of the most challenging aspects is the rate capability and in particular the peak instantaneous rate capability, i.e. the number of neutrons hitting the detector per channel or cm2 at the peak of the neutron pulse. The primary purpose of this paper is to estimate the incident rates that are anticipated for the BIFROST instrument planned for ESS, and also to demonstrate the use of powerful simulation tools for the correct interpretation of neutron transport in crystalline materials. A full simulation model of the instrument from source to detector position, implemented with the use of multiple simulation software packages, is presented. For a single detector tube, instantaneous incident rates with a maximum of 1.7 GHz for a Bragg peak from a single crystal and 0.3 MHz for a vanadium sample are found. This paper also includes the first application of a new pyrolytic graphite model and a comparison of different simulation tools to highlight their strengths and weaknesses.

Four different methods for mounting HOPG analyzer crystals on Si holders have been evaluated in the design process of the new multiplexing spectrometer CAMEA. Contrary to neutron optics used in standard spectrometers, the new instrument concept employs a series of analyzer segments behind each other where the neutrons have to pass through the bonding compound of the different analyzer crystals. The different methods, namely screws, shellac, indium soldering and clips, have been evaluated with regards to background, transmission, cooling, activation and handling. The results presented here will give valuable input for future CAMEA-type spectrometers currently planned and designed at various neutron sources.

We explore magnetic order in the quantum spin chain compound SrCo2V2O8 up to 14.9 T and down to 50 mK, using single-crystal neutron diffraction. Upon cooling in zero-field, commensurate antiferromagnetic (C-AFM) order with modulation vector k C = (0, 0, 1) develops below T N ≃ 5.0 K. Applying an external magnetic field (H∥c axis) destabilizes this C-AFM order, leading to an order-disorder transition between T N and ∼1.5 K. Below 1.5 K, a commensurate to incommensurate (IC-AFM) transition occurs at 3.9 T, above which the magnetic reflections can be indexed by k IC = (0, 0, 1 ± δl). The incommensurability δl scales monotonically with H until the IC-AFM order disappears around 7.0 T. Magnetic reflections modulated by k C emerge again at higher fields. While the characters of the C-AFM, IC-AFM and the emergent AFM order in SrCo2V2O8 appear to fit the descriptions of the Néel, longitudinal spin density wave and transverse AFM order observed in the related compound BaCo2V2O8, our results also reveal several unique signatures that are not present in the latter, highlighting the inadequacy of mean-field theory in addressing the complex magnetic order in systems of this class.

Placing a compound refractive lens (CRL) as an objective in a neutron beam generates new possibilities for 2D and 3D nondestructive mapping of the structure, strain and magnetic domains within extended objects. A condenser setup is introduced that allows correction for the lateral chromatic aberration. More generally, for full-field microscopy the loss in performance caused by the chromatic aberration can be more than offset by introducing arrays of CRLs and exploiting the fact that the field of view can be much larger than the physical aperture of the CRL. Comments are made on the manufacture of such devices. The potential use is illustrated by comparisons between state-of-the-art instrumentation and suggested approaches for bright-field microscopy, small-angle neutron scattering microscopy, grain mapping and mapping of stresses. Options are discussed for depth-resolved imaging inspired by confocal light microscopy. Finally, experimental demonstrations are given of some of the basic properties of neutron full-field imaging for a single CRL.

We present a systematic study on the heavily Cr doped iron pnictides ${\mathrm{BaFe}}_{1.9\ensuremath{-}x}{\mathrm{Ni}}_{0.1}{\mathrm{Cr}}_{x}{\mathrm{As}}_{2}$ by using elastic neutron scattering, high-resolution synchrotron x-ray diffraction (XRD), resistivity, and Hall transport measurements. When the Cr concentration increases from $x=0$ to 0.8, neutron diffraction experiments suggest that the collinear antiferromagnetism persists in the whole doping range, where the N\'eel temperature ${T}_{N}$ coincides with the tetragonal-to-orthorhombic structural transition temperature ${T}_{s}$, and both of them keeps around 35 K. The magnetic ordered moment, on the other hand, increases within increasing $x$ until $x=0.5$, and then decreases with further increasing $x$. Detailed refinement of the powder XRD patterns reveals that the Cr substitutions actually stretch the ${\mathrm{FeAs}}_{4}$ tetrahedron along the $c$ axis and lift the arsenic height away from the Fe-Fe plane. Transport results indicate that the charge carriers become more localized upon Cr doping, then changes from electron type to hole type around $x=0.5$. Our results suggest that the ordered moment and the ordered temperature of static magnetism in iron pnictides can be decoupled and tuned separately by chemical doping.

We report the discovery of a field driven transition from a single-$q$ to multi-$q$ spin density wave (SDW) in the tetragonal heavy fermion compound ${\mathrm{CeAuSb}}_{2}$. Polarized along $\mathbf{c}$, the sinusoidal SDW amplitude is $1.8(2){\ensuremath{\mu}}_{B}/\mathrm{Ce}$ for $T\ensuremath{\ll}{T}_{N}=6.25(10)\text{ }\text{ }\mathrm{K}$ with a wave vector ${\mathbf{q}}_{1}=(\ensuremath{\eta},\ensuremath{\eta},1/2)$ $[\ensuremath{\eta}=0.136(2)]$. For $\mathbf{H}\ensuremath{\parallel}\mathbf{c}$, harmonics appearing at $2{\mathbf{q}}_{1}$ evidence a striped magnetic texture below ${\ensuremath{\mu}}_{\ensuremath{\circ}}{H}_{1}=2.78(1)\text{ }\text{ }\mathrm{T}$. Above ${H}_{1}$, these are replaced by coupled harmonics at ${\mathbf{q}}_{1}+{\mathbf{q}}_{2}=(2\ensuremath{\eta},0,0)+{\mathbf{c}}^{*}$ until ${\ensuremath{\mu}}_{\ensuremath{\circ}}{H}_{2}=5.42(5)\text{ }\text{ }\mathrm{T}$, where satellites vanish and magnetization nonlinearly approaches saturation at $1.64(2){\ensuremath{\mu}}_{B}/\mathrm{Ce}$ for ${\ensuremath{\mu}}_{\ensuremath{\circ}}H\ensuremath{\approx}7\text{ }\text{ }\mathrm{T}$.

The continuous angle multiple energy analysis concept is a backend for both time-of-flight and analyzer-based neutron spectrometers optimized for neutron spectroscopy with highly efficient mapping in the horizontal scattering plane. The design employs a series of several upward scattering analyzer arcs placed behind each other, which are set to different final energies allowing a wide angular coverage with multiple energies recorded simultaneously. For validation of the concept and the model calculations, a prototype was installed at the Swiss neutron source SINQ, Paul Scherrer Institut. The design of the prototype, alignment and calibration procedures, experimental results of background measurements, and proof-of-concept inelastic measurements on LiHoF4 and h-YMnO3 are presented here.

Inelastic neutron scattering has been used to study the magneto-elastic excitations in the multiferroic manganite hexagonal YMnO$_3$. An avoided crossing is found between magnon and phonon modes close to the Brillouin zone boundary in the $(a,b)$-plane. Neutron polarization analysis reveals that this mode has mixed magnon-phonon character. An external magnetic field along the $c$-axis is observed to cause a linear field-induced splitting of one of the spin wave branches. A theoretical description is performed, using a Heisenberg model of localized spins, acoustic phonon modes and a magneto-elastic coupling via the single-ion magnetostriction. The model quantitatively reproduces the dispersion and intensities of all modes in the full Brillouin zone, describes the observed magnon-phonon hybridized modes, and quantifies the magneto-elastic coupling. The combined information, including the field-induced magnon splitting, allows us to exclude several of the earlier proposed models and point to the correct magnetic ground state symmetry, and provides an effective dynamic model relevant for the multiferroic hexagonal manganites., Comment: 12 pages, 10 figures

We demonstrate that a Ni-Mn oxide partially inverted spinel (Ni1-pMnv)[NivMn2-v]O-4 having inversion degree v approximate to 0.8 and produced by a modified polymeric precursor method exhibits behaviour previously reported only in monophased samples. The structure of the specimen was determined using Rietveld analysis of X-ray and neutron powder diffraction data, showing that at room temperature the material crystallizes in the Fd3(over bar)m space group with a lattice constant a = 8.392 angstrom. Combining magnetization measurements with neutron powder diffraction, we show that the magnetic structure of this spinel is associated with the interplay between the ferromagnetic and antiferromagnetic lattices which coexist due to the cations' presence on both tetrahedral and octahedral sites. Our analysis of the neutron diffraction data confirms the postulated magnetic structure involving a star-like moment arrangement, arising from competition for the B (octahedral) spinel sites by the Ni and Mn cations. Finally, we show that strong magnetic fluctuations are observed in the inelastic neutron scattering data. (C) 2014 Elsevier B.V. All rights reserved. (Less)

We describe a systematic approach for the design of long, ballistic cold, and thermal neutron guides for the European Spallation Source (ESS). The guides investigated in this work are 170 m long and are required to have a narrowing point with room for a pulse shaping chopper placed 6 m from the moderator. In addition, most guides avoid line-of-sight from the moderator to the sample. The guides are optimized in order to find a reasonable trade-off between neutronics performance and construction price. The geometries simulated are closely related to the thermal-neutron multi-length-scale diffractometer HEIMDAL and the cold-neutron multi-analyser spectrometer BIFROST. For the cold-neutron guide an inexpensive solution was found that maintains good transport properties, while avoiding line-of-sight. However, for the thermal-neutron guide the losses when avoiding line-of-sight are large and it seems a good choice to stay in line-of-sight, even though this will increase both the shielding costs and fast-neutron background. The results are of general relevance for the understanding of the relation between transport, background, and price of long neutron guides.

The analyzer detector system continuous angle multiple energy analysis will be installed on the cold-neutron triple-axis spectrometer RITA-2 at SINQ, PSI. CAMEA is optimized for efficiency in the horizontal scattering plane enabling rapid and detailed mapping of excitations. As a novelty the design employs a series of several sequential upward scattering analyzer arcs. Each arc is set to a different, fixed, final energy and scatters neutrons towards position sensitive detectors. Thus, neutrons with different final energies are recorded simultaneously over a large angular range. In a single data-acquisition many entire constant-energy lines in the horizontal scattering plane are recorded for a quasi-continuous angular coverage of about 60°. With a large combined coverage in energy and momentum, this will result in a very efficient spectrometer, which will be particularly suited for parametric studies under extreme conditions with restrictive sample environments (high field magnets or pressure cells) and for small samples of novel materials. In this paper we outline the concept and the specifications of the instrument currently under construction.

We define a virtual neutron experiment as a complete simulation of an experiment, from source over sample to detector. The virtual experiment (VE) will ideally interface with the instrument control software for the input and with standard data analysis packages for the virtual data output. Virtual experiments are beginning to make their way into neutron scattering science with applications as diverse as instrument design/upgrade, experiment planning, data analysis, test of analysis software, teaching, and outreach. In this paper, we summarize the recent developments in this field and make suggestions for future developments and use of VEs.

Researchers found evidence for a quantum phase transition within a superconducting condensate., The application of magnetic fields, chemical substitution, or hydrostatic pressure to strongly correlated electron materials can stabilize electronic phases with different organizational principles. We present evidence for a field-induced quantum phase transition, in superconducting Nd0.05Ce0.95CoIn5, that separates two antiferromagnetic phases with identical magnetic symmetry. At zero field, we find a spin-density wave that is suppressed at the critical field μ0H* = 8 T. For H > H*, a spin-density phase emerges and shares many properties with the Q phase in CeCoIn5. These results suggest that the magnetic instability is not magnetically driven, and we propose that it is driven by a modification of superconducting condensate at H*.

The CAMEA ESS neutron spectrometer is designed to achieve a high detection efficiency in the horizontal scattering plane, and to maximize the use of the long pulse European Spallation Source. It is an indirect geometry time-of-flight spectrometer that uses crystal analysers to determine the final energy of neutrons scattered from the sample. Unlike other indirect gemeotry spectrometers CAMEA will use ten concentric arcs of analysers to analyse scattered neutrons at ten different final energies, which can be increased to 30 final energies by use of prismatic analysis. In this report we will outline the CAMEA instrument concept, the large performance gain, and the potential scientific advancements that can be made with this instrument., Comment: 7 pages, 3 figures, 3 tables

Goethite (α-FeOOH) is a common nanocrystalline antiferromagnetic mineral. However, it is typically difficult to study the properties of isolated single-crystalline goethite nanoparticles, because goethite has a strong tendency to form particles of aggregated nanograins often with low-angle grain boundaries. This nanocrystallinity leads to complex magnetic properties that are dominated by magnetic fluctuations in interacting grains. Here we present a study of the magnetic properties of 5.7 nm particles of goethite by use of magnetization measurements, inelastic neutron scattering and Mossbauer spectroscopy. The ‘ultra-small’ size of these particles (i.e. that the particles consist of one or only a few grains) allows for more direct elucidation of the particles' intrinsic magnetic properties. We find from ac and dc magnetization measurements a significant upturn of the magnetization at very low temperatures most likely due to freezing of spins in canted spin structures. From hysteresis curves we estimate the saturation magnetization from uncompensated magnetic moments to be σs = 0.044 A m2 kg−1 at room temperature. Inelastic neutron scattering measurements show a strong signal from excitations of the uniform mode (q = 0 spin waves) at temperatures of 100–250 K and Mossbauer spectroscopy studies show that the magnetic fluctuations are dominated by ‘classical’ superparamagnetic relaxation at temperatures above ∼170 K. From the temperature dependence of the hyperfine fields and the excitation energy of the uniform mode we estimate a magnetic anisotropy constant of around 1.0 × 105 J m−3.

We here describe the result of simulations of 15 generic neutron instruments for the long-pulsed European Spallation Source. All instruments have been simulated for 20 different settings of the source time structure, corresponding to pulse lengths between 1 ms and 2 ms; and repetition frequencies between 10 Hz and 25 Hz. The relative change in performance with time structure is given for each instrument, and an unweighted average is calculated. The performance of the instrument suite is proportional to (a) the peak flux and (b) the duty cycle to a power of approximately 0.3. This information is an important input to determining the best accelerator parameters. In addition, we find that in our simple guide systems, most neutrons reaching the sample originate from the central 3-5 cm of the moderator. This result can be used as an input in later optimization of the moderator design. We discuss the relevance and validity of defining a single figure-of-merit for a full facility and compare with evaluations of the individual instrument classes. (C) 2013 AIP Publishing LLC.

The MIRACLES team., An overview is provided of the 15 neutron beam instruments making up the initial instrument suite of the European Spallation Source (ESS), and being made available to the neutron user community. The ESS neutron source consists of a high-power accelerator and target station, providing a unique long-pulse time structure of slow neutrons. The design considerations behind the time structure, moderator geometry and instrument layout are presented. The 15-instrument suite consists of two small-angle instruments, two reflectometers, an imaging beamline, two single-crystal diffractometers; one for macromolecular crystallography and one for magnetism, two powder diffractometers, and an engineering diffractometer, as well as an array of five inelastic instruments comprising two chopper spectrometers, an inverse-geometry single-crystal excitations spectrometer, an instrument for vibrational spectroscopy and a high-resolution backscattering spectrometer. The conceptual design, performance and scientific drivers of each of these instruments are described. All of the instruments are designed to provide breakthrough new scientific capability, not currently available at existing facilities, building on the inherent strengths of the ESS long-pulse neutron source of high flux, flexible resolution and large bandwidth. Each of them is predicted to provide world-leading performance at an accelerator power of 2 MW. This technical capability translates into a very broad range of scientific capabilities. The composition of the instrument suite has been chosen to maximise the breadth and depth of the scientific impact of the early years of the ESS, and provide a solid base for completion and further expansion of the facility., Some of the work described here is part of projects that have received funding from the European Union’s Horizon 2020 research programme: SINE 2020 under grant agreement number 654000, BrightnESS under grant agreement number 676548, and SoNDe under grant agreement number 654124. Financial support for the fast-shutter upgrade for FREIA from Swedish Research Council VR, grant no 2018-05013, is acknowledged. The VESPA team gratefully acknowledges the Science & Technology Facilities Council (STFC) and the CNR , within the CNR-STFC Agreement (2014–2020) No. 3420.