Search

With the construction of the ESS, the European neutron user community is eagerly awaiting the commissioning of the brightest neutron source worldwide in 2021. Parallel to this, there is however the ongoing development of neutron science being undertaken at a dwindling number of neutron facilities worldwide. The Julich Centre for Neutron Science has started a project to develop and design compact accelerator-driven high brilliance neutron sources as an efficient and cost effective alternative to the current low- and medium-flux reactor and spallation sources with the potential to offer science and industry access to neutrons. The project aims to deliver a high brilliance neutron source (HBS), consisting of a compact neutron production and moderator system which provides thermal and cold neutrons with high brilliance efficiently extracted in an optimized neutron transport system. By shaping the experiment holistically from the source to the detector, neutron experiments could be set-up for specific scientific requirements in a flexible and efficient way for the neutron user.

Low Energy accelerator-driven Neutron Facilities have the potential to become competitive to research reactors and spallation sources to generate neutron beams for scattering experiments. A low energy accelerator-driven neutron facility is developed at the Julich Centre for Neutron Science. This source is expected to provide thermal and cold neutrons with high brilliance and is therefore called “High Brilliance neutron Source” (HBS). In this work, we study the performance of neutron guide systems at HBS by using neutron ray-tracing simulations. Elliptical and ballistic guides with elliptic diverging/converging section have been used in simulations for various moderator-to-guide distances and guide entrance cross-sections. Results show that the beam properties have a strong dependence on the distance between guide entry and moderator. We demonstrate that the ballistic guide system can achieve a comparable neutron flux and brilliance transfer as the true elliptical guide for thermal neutrons if a proper distance between guide entrance and moderator is chosen. For low-divergence cold neutrons, the selected ballistic guide is showing even better performance than the elliptical one.

The neutron yield of a compact accelerator driven neutron source depends strongly on the target performance. This performance is influenced by the target composition and geometry, the cooling system design and which primary particles are used. We show that the temperature difference inside the target depends directly on the target thickness determined by the ion stopping range and therefore on the type and energy of the primary particle. Deuterons with a larger stopping power show a smaller temperature difference inside the target than protons allowing thus for a better target cooling.

Journal of physics / Condensed matter Condensed matter 32(16), 165801 - (2020). doi:10.1088/1361-648X/ab678c, Published by IOP Publ., Bristol

The High-Brilliance Neutron Source (HBS) project aims to design a scalable compact accelerator driven neutron source (CANS) which is competitive and cost-efficient. The concept allows one to optimize the whole facility including accelerator, target, moderators and neutron optics to the demands of individual neutron instruments. Particle type, energy, timing, and pulse structure of the accelerator are fully defined by the requirements of a given neutron instrument. In the following, we present the current status of the HBS project., Proceedings of the 14th Int. Conf. on Heavy Ion Accelerator Technology, HIAT2018, Lanzhou, China

${\mathrm{Co}}_{2}{\mathrm{V}}_{2}{\mathrm{O}}_{7}$ was recently reported to exhibit remarkable magnetic-field-induced magnetization plateaus and ferroelectricity [R. Chen et al., Phys. Rev. B 98, 184404 (2018)], but its magnetic ground state remains ambiguous. Magnetometry measurements and time-of-flight neutron powder diffraction (NPD) have been employed to study the structural and magnetic properties of ${\mathrm{Co}}_{2}{\mathrm{V}}_{2}{\mathrm{O}}_{7}$, which includes two nonequivalent Co sites. Upon cooling below the N\'eel temperature ${T}_{\text{N}}=6.0(2)$ K, we observe magnetic Bragg peaks at 2 K in NPD, which indicates the formation of long-range magnetic order of ${\mathrm{Co}}^{2+}$ moments. After symmetry analysis and magnetic structure refinement, we demonstrate that ${\mathrm{Co}}_{2}{\mathrm{V}}_{2}{\mathrm{O}}_{7}$ possesses a complicated noncollinear magnetic ground state with Co moments mainly located in the b-c plane and forming a noncollinear spin-chain-like structure along the c-axis. The ab initio calculations demonstrate that the noncollinear magnetic structure is more stable than various ferromagnetic states at low temperature. The noncollinear magnetic structure with a canted $\ensuremath{\uparrow}\ensuremath{\uparrow}\ensuremath{\downarrow}\ensuremath{\downarrow}$ spin configuration is considered to be the origin of magnetoelectric coupling in ${\mathrm{Co}}_{2}{\mathrm{V}}_{2}{\mathrm{O}}_{7}$ because the inequivalent exchange striction induced by the spin-exchange interaction between the neighboring spins could be the driving force of ferroelectricity. It is also found that the deviation of lattice parameters a and b is opposite below ${T}_{\text{N}}$, while the lattice parameter c and $\ensuremath{\beta}$ stay almost constant below ${T}_{\text{N}}$, evidencing the anisotropic magnetoelastic coupling in ${\mathrm{Co}}_{2}{\mathrm{V}}_{2}{\mathrm{O}}_{7}$.

The study of universal critical behavior is a crucial issue in a continuous phase transition, which groups various critical phenomena into universality classes for revealing microscopic electronic behaviors. The understanding of the nature of magnetism in Eu-based ferromagnetic superconductors is largely impeded by the infeasibility of performing inelastic neutron scattering measurements to deduce the microscopic magnetic behaviors and the effects on the superconductivity, due to the significant neutron absorption effect of natural $^{152}\mathrm{Eu}$ and unavailability of large single crystals. However, by systematically combining the neutron diffraction experiment, the first-principles calculations, and the quantum Monte Carlo simulations, we have obtained a perfectly consistent universal critical exponent value of $\ensuremath{\beta}=0.385(13)$ experimentally and theoretically for $\mathrm{Eu}{({\mathrm{Fe}}_{0.75}{\mathrm{Ru}}_{0.25})}_{2}{\mathrm{As}}_{2}$, from which the magnetism in the Eu-based ferromagnetic superconductors is identified as the universal class of a three-dimensional anisotropic quantum Heisenberg model with long-range magnetic exchange coupling. This systematic study points out a suitable microscopic theoretical model for describing the nature of magnetism in the intriguing Eu-based ferromagnetic superconductors.

By performing high-pressure single-crystal neutron-diffraction measurements, the evolution of structure and magnetic ordering in ${\mathrm{EuFe}}_{2}{\mathrm{As}}_{2}$ under hydrostatic pressure were investigated. Both the tetragonal-to-orthorhombic structural transition and the Fe spin-density-wave transition are gradually suppressed and become decoupled with increasing pressure. The antiferromagnetic order of the Eu sublattice is, however, robust against the applied pressure up to 24.7 kbar, without showing any change of the ordering temperature. Under the pressure of 24.7 kbar, the lattice parameters of ${\mathrm{EuFe}}_{2}{\mathrm{As}}_{2}$ display clear anomalies at 27(3) K, well consistent with the superconducting transition observed in previous high-pressure resistivity measurements. Such an anomalous thermal expansion around ${T}_{c}$ strongly suggests the appearance of bulk superconductivity and strong electron-lattice coupling in ${\mathrm{EuFe}}_{2}{\mathrm{As}}_{2}$ induced by the hydrostatic pressure. The coexistence of long-range ordered Eu antiferromagnetism and pressure-induced superconductivity is quite rare in the ${\mathrm{EuFe}}_{2}{\mathrm{As}}_{2}$-based iron pnictides.

Using complementary polarized and unpolarized single-crystal neutron diffraction, we have investigated the temperature-dependent magnetic structures of ${\mathrm{Eu}}_{0.5}{\mathrm{Ca}}_{0.5}{\mathrm{Fe}}_{2}{\mathrm{As}}_{2}$. Upon 50% dilution of the Eu sites with isovalent ${\mathrm{Ca}}^{2+}$, the Eu sublattice is found to be still long-range ordered below ${T}_{\text{Eu}}=10$ K, in the $A$-type antiferromagnetic (AFM) structure. The moment size of ${\mathrm{Eu}}^{2+}$ spins is estimated to be as large as $6.74(4){\ensuremath{\mu}}_{B}$ at 2.5 K. The Fe sublattice undergoes a spin-density-wave transition at ${T}_{\text{SDW}}=192(2)$ K and displays an in-plane AFM structure above ${T}_{\text{Eu}}$. However, at 2.5 K, the ${\mathrm{Fe}}^{2+}$ moments are found to be ordered in a canted AFM structure with a canting angle of $14{(4)}^{\ensuremath{\circ}}$ out of the $ab$ plane. The spin reorientation of Fe below the AFM ordering temperature of Eu provides direct evidence of a strong interplay between the two magnetic sublattices in ${\mathrm{Eu}}_{0.5}{\mathrm{Ca}}_{0.5}{\mathrm{Fe}}_{2}{\mathrm{As}}_{2}$.

The manipulation of the magnetism of self-assembled iron oxide nanoparticle (NP) monolayers on top of BaTiO3 (BTO) single crystals is reported. We observe strain induced magnetoelectric coupling (MEC) as shown by measurements of both the magnetization and magneto-electric AC susceptibility (MEACS). The magnetization, coercivity, remanent magnetization and MEACS signal as a function of temperature show abrupt jumps at the BTO phase transition temperatures. Hereby the jump values are opposite for in-plane and out-of-plane measurements. Grazing incidence small angle X-ray scattering (GISAXS) and scanning electron microscopy (SEM) confirm a hexagonal close-packed supercrystalline order of the NP monolayers. Cross-sectional scanning transmission electron microscopy (STEM) experiments provide information about the layer structure of the sample. This work opens up viable possibilities for fabricating energy-efficient electronic devices by self-assembly techniques.

International audience; After nearly 40 years of successful operation the cold three-axis spectrometer 1N12 at the Institut Laue-Langevin, Grenoble, France, has been relocated to a new position and the primary spectrometer has been upgraded. Latest modern optical components are employed. A new guide in combination with a virtual source concept and a double focusing monochromator guarantee highest flux. With its high unpolarized and polarized neutron flux 1N12 allows for demanding experiments. A velocity selector in the guide ensures a clean beam and a very low background. A gain in flux of about an order of magnitude at the sample position has been achieved compared to the previous instrument and 1N12's wavelength range now extends far into the warmish region. (C) 2016 Elsevier B.V. All rights reserved.

The ground-state magnetic structure of EuNi$_{2}$As$_{2}$ was investigated by single-crystal neutron diffraction. At base temperature, the Eu$^{2+}$ moments are found to form an incommensurate antiferromagnetic spiral-like structure with a magnetic propagation vector of $\mathit{k}$ = (0, 0, 0.92). They align ferromagnetically in the $\mathit{ab}$ plane with the moment size of 6.75(6) $\mu_{B}$, but rotate spirally by 165.6(1){\deg} around the $\mathit{c}$ axis from layer to layer. The magnetic order parameter in the critical region close to the ordering temperature, $\mathit{T_{N}}$ = 15 K, shows critical behavior with a critical exponent of $\beta_{Eu}$ = 0.34(1), consistent with the three-dimensional Heisenberg model. Moreover, within the experimental uncertainty, our neutron data is consistent with a model in which the Ni sublattice is not magnetically ordered., Comment: 6 pages, 5 figures

The magnetic excitation spectrum of the magnetocaloric compound ${\mathrm{MnFe}}_{4}{\mathrm{Si}}_{3}$ has been investigated by means of polarized and unpolarized inelastic neutron scattering on single crystals. Spectra were collected in the ferromagnetic phase (${T}_{C}\ensuremath{\approx}305\phantom{\rule{0.16em}{0ex}}\mathrm{K}$), as well as in the paramagnetic state, in order to understand the nature of the magnetism in ${\mathrm{MnFe}}_{4}{\mathrm{Si}}_{3}$. Spin-wave measurements at 1.5 K reveal a strong anisotropy of the magnetic exchange interactions along the (h00) and (00l) reciprocal directions of the hexagonal system, which also manifests itself in the $q$-dependent linewidths in the paramagnetic state. The correlation lengths indicate a short-range order, while the average linewidth is of the order of ${k}_{B}{T}_{C}$ pointing to a behavior typical of many ferromagnets. In addition, the in- and out-of-plane spin fluctuations are found to be isotropic around ${T}_{C}$ and can be suppressed by a magnetic field of 2 T.

The manipulation of the magnetism of self-assembled iron oxide nanoparticle (NP) monolayers on top of BaTiO

We report about the magnetic properties of antiferromagnetic (AF) MnO nanoparticles (NPs) with different sizes (6--19 nm). Using a combination of polarized neutron scattering and magnetometry, we were able to resolve previously observed peculiarities. Magnetometry, on the one hand, reveals a peak in the zero-field-cooled (ZFC) magnetization curves at low temperatures $(\ensuremath{\sim}25$ K) but $no$ feature around the N\'eel temperature at 118 K. On the other hand, polarized neutron scattering shows the expected behavior of the AF order parameter vanishing around 118 K. Moreover, hysteresis curves measured at various temperatures reveal an exchange-bias effect, indicating a coupling of an AF core to a ferromagnetic (FM)-like shell. ZFC data measured at various fields exclude a purely superparamagnetic (SPM) scenario. We conclude that the magnetic behavior of MnO particles can be explained by a superposition of SPM-like thermal fluctuations of the AF-N\'eel vector inside the AF core and a magnetic coupling to a ferrimagnetic ${\mathrm{Mn}}_{2}{\mathrm{O}}_{3}$ or ${\mathrm{Mn}}_{3}{\mathrm{O}}_{4}$ shell. In addition, we have studied heterodimer (``Janus'') particles, where a FM FePt particle is attached to the AF MnO particle. Via the exchange-bias effect, the magnetic moment of the FePt subunit is stabilized by the MnO.

We discuss the implications of a long pulse neutron source for the chopper system of direct geometry time-of-flight spectrometers. While the same conditions apply for the layout of the resolution defining choppers as on reactor based instruments, we emphasize the multi-chromatic nature of the instruments. The chopper system must not only provide a unique assignment of the wavelength to each pulse, but also provide adopted time frames matching the respective energy of the pulse. We propose a chopper system consisting of disc choppers, heavy T 0 choppers and a newly developed Fan chopper to account for the various challenges due to the long pulse nature.

We present a method to calculate the covariance matrix of wavevector-energy transfer for neutron time-of-flight direct geometry spectrometers, with both pulse shaping and monochromating choppers. As the definition of the wavevector transfer and of its components depends on the detector geometry and on the instrumental parameters, three different geometries have been analyzed: spherical, cylindrical with horizontal and vertical axis. The specific dependence of the matrix elements on the instrumental parameters is explicitly given. The results of example calculations are also presented and discussed.

In order to understand the origin of the huge quasielastic magnetic scattering observed previously with a back-scattering neutron spectrometer, we have re-investigated the low energy excitations in HoCrO3 by inelastic neutron scattering in a much wider energy range with time-of-flight neutron spectrometers. The inelastic signals are due to the excitations between the ground state doublet of the Ho ion. The quasielastic signal is due to the fluctuation of the disordered Ho moments. At low temperature the intensity of quasielastic scattering is small. It starts increasing as the temperature increases above 30 K. At the same temperature, the elastic intensity due to Ho moment ordering decreases in a similar way. This observation strengthens the hypothesis that the quasielastic scattering is due the fluctuations of the disordered Ho moments. The time scale of fluctuations has been determine from the quasielastic scattering and was found to vary from about 22 ps at [Formula: see text] K to about 2.5 ps at [Formula: see text] K. The stretched exponential line shape indicates a distribution of decay rates at low temperatures.

The magnetic ground state of the ${\mathrm{Eu}}^{2+}$ moments in a series of $\mathrm{Eu}{({\mathrm{Fe}}_{1\ensuremath{-}x}{\mathrm{Co}}_{x})}_{2}{\mathrm{As}}_{2}$ single crystals grown from the Sn flux has been investigated in detail by neutron diffraction measurements. Combined with the results from the macroscopic properties (resistivity, magnetic susceptibility and specific heat) measurements, a phase diagram describing how the Eu magnetic order evolves with Co doping in $\mathrm{Eu}{({\mathrm{Fe}}_{1\ensuremath{-}x}{\mathrm{Co}}_{x})}_{2}{\mathrm{As}}_{2}$ is established. The ground-state magnetic structure of the ${\mathrm{Eu}}^{2+}$ spins is found to develop from the A-type antiferromagnetic (AFM) order in the parent compound, via the A-type canted AFM structure with some net ferromagnetic (FM) moment component along the crystallographic $\mathit{c}$ direction at intermediate Co doping levels, finally to the pure FM order at relatively high Co doping levels. The ordering temperature of Eu declines linearly at first, reaches the minimum value of 16.5(2) K around $\mathit{x}=0.100(4)$, and then reverses upwards with further Co doping. The doping-induced modification of the indirect Ruderman-Kittel-Kasuya-Yosida (RKKY) interaction between the ${\mathrm{Eu}}^{2+}$ moments, which is mediated by the conduction $\mathit{d}$ electrons on the (Fe,Co)As layers, as well as the change of the strength of the direct interaction between the ${\mathrm{Eu}}^{2+}$ and ${\mathrm{Fe}}^{2+}$ moments, might be responsible for the change of the magnetic ground state and the ordering temperature of the Eu sublattice. In addition, for $\mathrm{Eu}{({\mathrm{Fe}}_{1\ensuremath{-}x}{\mathrm{Co}}_{x})}_{2}{\mathrm{As}}_{2}$ single crystals with 0.$10\ensuremath{\le}\mathit{x}\ensuremath{\le}0.18$, strong ferromagnetism from the Eu sublattice is well developed in the superconducting state, where a spontaneous vortex state is expected to account for the compromise between the two competing phenomena.

High-resolution inelastic neutron scattering reveals that the elementary magnetic excitations in multiferroic ${\mathrm{MnWO}}_{4}$ consist of low-energy dispersive electromagnons in addition to the well-known spin-wave excitations. The latter can well be modeled by a Heisenberg Hamiltonian with magnetic exchange coupling extending to the 12th nearest neighbor. They exhibit a spin-wave gap of 0.61(1) meV. Two electromagnon branches appear at lower energies of 0.07(1) and 0.45(1) meV at the zone center. They reflect the dynamic magnetoelectric coupling and persist in both the collinear magnetic and paraelectric AF1 phase and the spin spiral ferroelectric AF2 phase. These excitations are associated with the Dzyaloshinskii-Moriya exchange interaction, which is significant due to the rather large spin-orbit coupling.

The physical and structural properties of Fe1.11Te and Fe1.11Te0.5Se0.5 have been investigated by means of X-ray and neutron diffraction as well as physical property measurements. For the Fe1.11Te compound, the structure distortion from a tetragonal to monoclinic phase takes place at 64 K accompanied with the onset of antiferromagnetic order upon cooling. The magnetic structure of the monoclinic phase was confirmed to be of antiferromagnetic configuration with a propagation vector k = (1/2, 0, 1/2) based on Rietveld refinement of neutron powder diffraction data. The structural/magnetic transitions are also clearly visible in magnetic, electronic and thermodynamic measurements. For superconducting Fe1.11Te0.5Se0.5 compound, the superconducting transition with T c = 13.4 K is observed in the resistivity and ac susceptibility measurements. The upper critical field H c2 is obtained by measuring the resistivity under different magnetic fields. The Kim’s critical state model is adopted to analyze the temperature dependence of the ac susceptibility and the intergranular critical current density is calculated as a function of both field amplitude and temperature. Neutron diffraction results show that Fe1.11Te0.5Se0.5 crystalizes in tetragonal structure at 300 K as in the parent compound Fe1.11Te and no structural distortion is detected upon cooling to 2 K. However an anisotropic thermal expansion anomaly is observed around 100 K.

A neutron powder diffraction (NPD) study on the crystal and magnetic structure of a crushed La1-xSrxMnO3 (x ≈ 1/8) single crystal has been performed. The sample belongs to orthorhombic (Pnma, O) above the Jahn-Teller (JT) transition temperature (TJT) and monoclinic (P121/c1, M′) in the JT regime. We have also refined the NPD data below the charge/orbital ordering (CO/OO) temperature (TCO/OO) with a monoclinic (P121/c1, M′′) model because the experimental resolution was insufficient to clearly identify a triclinic $(P\bar {1})$ structure. The refined lattice parameters show an obvious breathing-mode distortion between TCO/OO and TJT, accompanied by a large deviation of the monoclinic angle β from 90°, signifying a very strong cooperative JT distortion. A ferromagnetic (FM) moment of 3.43(5)μ B/Mn besides an A-type antiferromagnetic (A-AFM) moment of 0.54(2) μ B/Mn is directed mainly along the b axis in P121/c1 symmetry at 5 K. With increasing temperature, the A-AFM domains transform into FM ones above ~100 K and the FM spin orientation turns from the b to the c axis in crystallographic b-c plane below Tc = 187(1) K. The magnetization measurements show typical anomalies around TCO/OO and TJT. The measured saturation moment of 3.9(1)μ B/Mn at 70 kOe and 5 K is well consistent with the sum 3.97(5)μ B/Mn of the refined FM and A-AFM moments at 5 K, implying the A-AFM spins are aligned in field direction at 70 kOe. The applied magnetic field can affect the paramagnetic insulating (PMI) state in the range of magnetic polarons. Based on the size of JT distortion and the bond-valence sums (BVS’s), the CO/OO phenomenon is being discussed.

A high intensity triple-axis spectrometer is described, which has been commissioned only a year before the final shut-down of the FRJ-2 reactor in May 2006. It was designed both as an instrument for quasi-elastic scattering with polarisation analysis on an area detector, and as a triple-axis spectrometer with polarization analysis for measurements of magnetic and nuclear inelastic scattering. The instrument features two exchangeable monochromators (Cu [200] and PG [002]), which enable both horizontal and vertical focusing. High monochromatic neutron current densities at the sample position of up to 4 × 107cm− 2 s− 1 (for pyrolytic graphite at 34 meV) and energies of up to 117 meV (with copper) have been achieved at the FRJ-2 (thermal flux density: 3 × 1014cm− 2 s− 1). The utilization of 3He filters both as polarizer and analyzer will enable polarization analysis for high-energy neutrons. The spectrometer is presently being prepared for installation at the new Chinese research reactor CARR at the China Ins...

With the construction of the European Spallation Source ESS, the European neutron user community is looking forward to the brightest source worldwide. At the same time there is an ongoing concentration of research with neutrons to only a few but very powerful neutron facilities. Responding to this situation the Julich Centre for Neutron Science has initiated a project for a compact accelerator driven high-brilliance neutron source, optimized for neutron scattering on small samples and to be realized at reasonable costs. The project deals with the optimization of potential projectiles, target and moderator concepts, versatile accelerator systems, cold sources, beam extraction systems and optimized instrumentation. A brief outline of the project, the achievements already reached, will be presented, as well as a vision for the future neutron landscape in Europe.

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 Publ., Journal of physics / Conference Series, 746, 1, 012036, 1-6 (2016). doi:10.1088/1742-6596/746/1/012036, Published by IOP Publ., Bristol

We report a comprehensive specific heat and inelastic neutron scattering study to explore the possible origin of multiferroicity in HoCrO$_3$. We have performed specific heat measurements in the temperature range 100 mK - 290 K and inelastic neutron scattering measurements were performed in the temperature range 1.5 - 200 K. From the specific heat data we determined hyperfine splitting at 22.5(2) $\mu$eV and crystal field transitions at 1.379(5) meV, 10.37(4) meV, 15.49(9) meV and 23.44(9) meV, indicating the existence of strong hyperfine and crystal field interactions in HoCrO$_3$. Further, an effective hyperfine field is determined to be 600(3) T. The quasielastic scattering observed in the inelastic scattering data and a large linear term $\gamma=6.3(8)$ mJmol$^{-1}$K$^{-2}$ in the specific heat is attributed to the presence of short range exchange interactions, which is understood to be contributing to the observed ferroelectricity. Further the nuclear and magnetic entropies were computed to be, $\sim$$17.2$ Jmol$^{-1}$K$^{-1}$ and $\sim$34 Jmol$^{-1}$K$^{-1}$, respectively. The entropy values are in excellent agreement with the limiting theoretical values. An anomaly is observed in peak position of the temperature dependent crystal field spectra around 60 K, at the same temperature an anomaly in the pyroelectric current is reported. From this we could elucidate a direct correlation between the crystal electric field excitations of Ho$^{3+}$ and ferroelectricity in HoCrO$_3$. Our present study along with recent reports confirm that HoCrO$_3$, and $R$CrO$_3$ ($R=$ Rare earth) in general, possess more than one driving force for the ferroelectricity and multiferroicity., Comment: 17 pages, 6 figures, published in J. Phys.: Condens. Matter

While structural and magnetic lateral correlations in thin film materials can be investigated at the μm length scale by neutron off-specular scattering (OSS) with polarization analysis, they can also be investigated at the nm length scale by grazing incidence small-angle scattering of polarized neutrons (polarized GISANS). We exemplify this issue showing a combined OSS and GISANS study of the lateral correlations in a remanent polarizing supermirror.

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

The spin relaxation processes within the pyrochlore Ho2Ru2O7 have been investigated by neutron scattering and bulk property techniques. A single-ion process, that is thermally activated, dominates the spin–spin relaxation spectrum above 2 K. Assuming Arrhenius behaviour, we found an activation energy Δ = (329 ± 6) K and characteristic relaxation time τ0 = (5.2 ± 0.3) × 10−12 s in the paramagnetic state, akin to those found in the spin ice, Ho2Ti2O7. Atlow temperature (T

Neutron image plates (NIPs) are used for two-dimensional, position-sensitive detection of neutrons. Commercially available neutron image plates consist of a mixture Gd 2 O 3 and BaFBr:Eu 2+ . In this contribution we present a system of converter ( 6 LiF) and storage phosphor (KCl:Eu 2+ ), which exhibits a high efficiency for the detection of neutrons, while at the same time features a low γ -sensitivity due to the low atomic numbers. Two different morphologies of image plates based on KCl:Eu 2+ – 6 LiF were fabricated: ceramic (C-NIPs) and pixelated NIP (Pix-NIPs) . For these NIPs a neutron equivalent of 1/20 neutron per photon for soft γ -rays was found while the equivalent for hard γ -quanta is 1/400 neutron per absorbed photon. The detective quantum efficiency (DQE) of a C-NIP consisting of KCl:Eu 2+ – 6 LiF amounts to 38%, a value which is comparable to commercially available neutron image plates. Pix-NIPs represent a novel structure in which the scattered light is confined to square cells embedded in the image plate. Due to this light confinement, the resolution of such an image plate is primarily independent of scattering properties of the NIP and is mainly a function of the size of the cell structure used.

Ultra-small angle (U-SANS) and small angle neutron scattering (SANS) experiments are performed by two different types of instruments to cover a combined Q-range from ≈10−5A−1 up to ≈1A−1. Bonse-Hart cameras (double crystal diffractometers) are used for U-SANS experiments, whereas the “standard” SANS experiment is performed using a pinhole camera. In principle, the Q-range of both instrument classes overlaps. Typical U-SANS instruments like S18 (ILL), PCD (NIST), or DKD (FZJ) may reach maximum Q-vectors of ≈5 × 10−3. The disadvantage of these instruments is that they do not allow taking a full area image on a 2D position sensitive detector. On the other hand, the well-known pinhole instrument D11 at Institut Laue-Langevin (France) reaches a minimum Q-vector of 5 × 10−4A−1 by use of large wavelengths and sample-to-detector distances (≈40m).

While magnetic multilayers can be regarded as systems that are artificially structured along one dimension, laterally patterned magnetic structures are systems that are structured, in addition, along a second (nanostripes) or third (nanodots) dimension. Fundamentally, novel properties analogous to the interlayer exchange coupling and the giant magnetoresistance effect in magnetic multilayers can be expected if the size of the structures become comparable to or smaller than certain characteristic length scales, such as the spin diffusion length, carrier mean free path, magnetic domain wall width, etc. [1]. Typically, those length scales lie in the several-to-hundreds nm range.

Recent experimental studies on new materials (storage phosphors and neutron converter) to be used in a low γγ-sensitive neutron image plate detector have been performed at the Research Center Julich and Darmstadt University of Technology. It was shown that a mixture LiF+KCl:Eu2+ (or KBr:Eu2+) features a higher neutron/gamma signal ratio compared to BaFBr:Eu2+mixed with Gd as in commercially available neutron image plates. However, the smaller neutron capture cross-section of 6Li in comparison with Gd requires the use of significantly thicker recording layers. This article presents the results of numerical estimations for the performance of such a detector. The use of image plates with a thick recording layer and a regular cell structure allowing for the increase in spatial resolution (pixelated image plate) is discussed.

We have investigated non-resonant high energy X-ray magnetic scattering from EuAs3 both in the antiferromagnetic and in the incommensurate phase by using an X-ray energy of 104 and 106 keV. In the antiferromagnetic phase, we obtained a signal to background ratio of about 10:1 for the magnetic Bragg peak at Q = ( − 1 , 0 , 1 / 2 ) and a maximum count rate of about 200 counts/s at T=3.1 K. To our knowledge this is the first reported observation of the non-resonant magnetic signal from a rare-earth ion at X-ray energy as high as 106 keV. The temperature dependence of the integrated intensity of the (−1,0,1/2); magnetic reflection has been measured and compared with that obtained previously by neutron diffraction. We measured the integrated intensities of several magnetic reflections from the antiferromagnetic phase and have compared them with those calculated from the magnetic structure model derived from neutron diffraction. The intensities of the magnetic satellite reflections from the incommensurate phase have been measured and have been found to be very weak. We also investigated the temperature variation of the lattice spacing close to the magnetic ordering transition and have found a large magnetoelastic anomaly at the lock-in phase transition.

Nuclear spin-polarized 3 He efficiently polarizes neutrons over a wide energy range including cold, thermal and hot neutron spectra by means of its strongly spin-dependent absorption of neutrons. At Forschungszentrum Julich we are studying the optical polarization of 3 He by two methods, metastable exchange and spin-exchange, in order to optimize the neutron polarization at various spectrometers of the FRJ-2 reactor. For the spin-exchange method we designed an open, refillable cell system. In contrast to the conventional sealed cells adopted at many other laboratories, we will be able to vary many crucial parameters in the 3 He cell, such as gas ratios of 3 He and N 2 , total gas pressure, and temperature while always working in the same experimental environment of a single cell. Laser polarized Rb vapor is used to polarize 3 He through the spin-exchange interaction, and NMR is employed to determine the polarization of the 3 He after calibration with a proton (water) NMR signal. In this paper we will report the 3 He polarization and relaxation rate, T 1, observed for various gas conditions.

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 implementation in Julich of 3He spin filters for neutrons has progressed substantially. Apparatus for polarization via metastable 3He atoms in an RF-excited plasma has been constructed and polarization of 3He above 60% has been achieved. The compressors now in construction have been designed with the major goal of bringing of all components containing polarized gas as close as possible, thereby, minimizing the length of connecting lines which otherwise contribute excessively to polarization losses on account of their unfavorable ratio of surface to volume. The compressors are the result of a cooperative effort between groups at HMI (Berlin) and IFF (Julich). For the spin-exchange method we are working on a new approach, namely, the use of potassium instead of the standard choice, rubidium, as the alkali metal. We expect a much higher polarization of the 3He nuclei due to the much higher efficiency of K–3He spin transfer. In addition, we plan tests of filter cells including two separate chambers. One chamber is where the 3He is continuously optically polarized while the filter is in the neutron beam while the other chamber is the actual neutron spin filter. In this way, we can achieve a better environment for each of these vital functions. The transfer of the polarization will be by diffusion and convection.

The surface layer of an epitaxial Fe/Cr multilayer system has been structured lithographically into stripes with 1 μm period. Polarized neutron reflectometry and diffuse scattering under grazing incidence measurements show strongly enhanced domain formation compared to the unstructured sample. The domains are correlated throughout the layers. The lateral domain size is partly the period of the stripe pattern of the surface layer, and partly the double.

A special Microsymposium on polarized neutron methods and technologies on existing and future pulsed neutron sources was held at PNCMI-2002. We present the recommendations formulated by three working groups on “Diffraction,” “Inelastic Scattering” and “Reflectometry.” Needs for method development as well as opportunities for novel polarized neutron instrumentation are identified.

The magnetic order of the localized Eu$^{2+}$ spins in optimally-doped Eu(Fe$_{1-x}$Ir$_{x}$)$_{2}$As$_{2}$ ($\mathit{x}$ = 0.12) with superconducting transition temperature $\mathit{T_{SC}}$ = 22 K was investigated by single-crystal neutron diffraction. The Eu$^{2+}$ moments were found to be ferromagnetically aligned along the $\mathit{c}$-direction with an ordered moment of 7.0(1) $\mu_{B}$ well below the magnetic phase transition temperature $\mathit{T_{C}}$ = 17 K. No evidence of the tetragonal-to-orthorhombic structural phase transition was found in this compound within the experimental uncertainty, in which the spin-density-wave (SDW) order of the Fe sublattice is supposed to be completely suppressed and the superconductivity gets fully developed. The ferromagnetic groud state of the Eu$^{2+}$ spins in Eu(Fe$_{0.88}$Ir$_{0.12}$)$_{2}$As$_{2}$ was supported by the first-principles density functional calculation. In addition, comparison of the electronic structure calculations between Eu(Fe$_{0.875}$Ir$_{0.125}$)$_{2}$As$_{2}$ and the parent compound EuFe$_{2}$As$_{2}$ indicates stronger hybridization and more expanded bandwith due to the Ir substitution, which together with the introduction of electrons might work against the Fe-SDW in favor of the superconductivity.

${\mathrm{La}}_{0.4}{\mathrm{Na}}_{0.6}{\mathrm{Fe}}_{2}{\mathrm{As}}_{2}$ single crystals have been grown out of an NaAs flux in an alumina crucible and characterized by measuring magnetic susceptibility, electrical resistivity, specific heat, as well as single-crystal x-ray and neutron diffraction. ${\mathrm{La}}_{0.4}{\mathrm{Na}}_{0.6}{\mathrm{Fe}}_{2}{\mathrm{As}}_{2}$ single crystals show a structural phase transition from a high-temperature tetragonal phase to a low-temperature orthorhombic phase at ${T}_{s}\phantom{\rule{0.16em}{0ex}}=\phantom{\rule{0.16em}{0ex}}125$ K. This structural transition is accompanied by an anomaly in the temperature dependence of electrical resistivity, anisotropic magnetic susceptibility, and specific heat. Concomitant with the structural phase transition, the Fe moments order along the $a$ direction with an ordered moment of 0.7(1) ${\ensuremath{\mu}}_{\mathrm{B}}$ at $T=5$ K. The low-temperature stripe antiferromagnetic structure is the same as that in other $A{\mathrm{Fe}}_{2}{\mathrm{As}}_{2}$ ($A$ = Ca, Sr, Ba) compounds. ${\mathrm{La}}_{0.5\ensuremath{-}x}{\mathrm{Na}}_{0.5+x}{\mathrm{Fe}}_{2}{\mathrm{As}}_{2}$ provides a material platform for the study of iron-based superconductors where the electron-hole asymmetry could be studied by simply varying the La/Na ratio.

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 have employed a new experimental configuration to measure ferromagnetic X-ray resonance exchange scattering (XRES) from a EuS single crystal. Using polarisation analysis we could achieve a magnetic-scattering intensity stronger than charge scattering resulting in an asymmetry ratio as large as Ra = 0.67 at the EuLII edge. By a combined refinement of the dependencies of the scattered intensity on the energy and the applied magnetic field we could uniquely determine spectroscopic information such as the 5d conduction band exchange splitting = 0.27(1) eV.

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 magnetic structure of the Eu2+ moments in the superconducting EuFe2(As1-xPx)2 sample with x = 0.19 has been determined using neutron scattering. We conclude that the Eu2+ moments are aligned along the c direction below T_C = 19.0(1) K with an ordered moment of 6.6(2) mu_B in the superconducting state. An impurity phase similar to the underdoped phase exists within the bulk sample which orders antiferromagnetically below T_N = 17.0(2) K. We found no indication of iron magnetic order, nor any incommensurate magnetic order of the Eu2+ moments in the sample.