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SESANS data analysis has been implemented in the SasView software package, allowing SESANS experiments to be analyzed using a numerical Hankel transformation of isotropic small-angle scattering (SAS) models. The error of the numerical approximation is three orders of magnitude below typical experimental errors. All advanced data fitting features of SasView (multi-model fitting, batch fitting, and simultaneous/constrained fitting) are now also available for SESANS and this is demonstrated by examples of fitting SAS models to SESANS measurements.

We study the evolution of the low-temperature field-induced magnetic defects observed under an applied magnetic field in a series of frustrated amorphous ferromagnets (Fe$_{1-x}$Mn$_{x}$)$_{75}$P$_{16}$B$_{3}$Al$_{3}$ (a-FeMn). Combining small-angle neutron scattering and Monte Carlo simulations, we show that the morphology of these defects resemble that of quasi-bidimensional spin vortices. They are observed in the reentrant spin-glass (RSG) phase, up to the critical concentration $x_{\rm C} \approx 0.36$ which separates the RSG and "true" spin glass (SG) within the low temperature part of the magnetic phase diagram of a-FeMn. These vortices systematically decrease in size with increasing magnetic field or decreasing the average exchange interaction, and they finally disappear in the SG sample ($x = 0.41$), being replaced by field-induced correlations over finite length scales. We argue that the study of these nanoscopic defects could be used to probe the nature of the critical line between the RSG and SG phases., Comment: 23 pages, 14 figures, includes supplement

Topologically stable field configurations appear in many fields of physics, from elementary particles to condensed matter and cosmology. During the last decade, chiral liquid crystals and chiral magnets took on the role of model objects for experimental investigation of topological solitons and understanding of their nonsingular field configurations. Here we introduce a paradigm of facile skyrmionic networks with mutually-orthogonal orientations of constituent isolated skyrmions. Such networks are envisioned as a novel concept of spintronic devices and are presumably responsible for precursor phenomena near the ordering temperatures of bulk cubic helimagnets. In particular, we demonstrate an interconversion between mutually orthogonal skyrmions: horizontal skyrmions swirl into an intermediate spring-like states and subsequently squeeze into vertical skyrmions with both polarities. Skyrmion tubes are also considered as building blocks for particle-like states with more involved internal structure. A family of target-skyrmions, which includes an overlooked so far type with a multiple topological charge, is formed owing to the tendency to minimize the interaction energy between vertical and horizontal skyrmions. The conical phase serves as a suitable background for considered skyrmion evolution. It substantializes the attracting skyrmion-skyrmion interaction in the skyrmionic networks, shapes their internal structure and guides the nucleation processes. Alternatively, intricate textural changes of isolated skyrmions result not only in the structural deformations of a host conical state, but may lead to the formation of an exotic skyrmion order with pairs of merons being the core of the game. Generically, the fundamental insights provided by this work emphasize a three-dimensional character of skyrmion metamorphoses and can also be extended to three-dimensional solitons, such as hopfions., Comment: 12 pages, 6 figures

In-situ Neutron Diffraction and Small-Angle Neutron Scattering (SANS) are employed for the first time simultaneously in order to reveal the interaction between the austenite to ferrite phase transformation and the precipitation kinetics during isothermal annealing at 650 and at 700 °C in three steels with different vanadium (V) and carbon (C) concentrations. Austenite-to-ferrite phase transformation is observed in all three steels at both temperatures. The phase transformation is completed during a 10 h annealing treatment in all cases. The phase transformation is faster at 650 than at 700 °C for all alloys. Additions of vanadium and carbon to the steel composition cause a retardation of the phase transformation. The effect of each element is explained through its contribution to the Gibbs free energy dissipation. The austenite-to-ferrite phase transformation is found to initiate the vanadium carbide precipitation. Larger and fewer precipitates are detected at 700 than at 650 °C in all three steels, and a larger number density of precipitates is detected in the steel with higher concentrations of vanadium and carbon. After 10 h of annealing, the precipitated phase does not reach the equilibrium fraction as calculated by ThermoCalc. The external magnetic field applied during the experiments, necessary for the SANS measurements, causes a delay in the onset and time evolution of the austenite-to-ferrite phase transformation and consequently on the precipitation kinetics.

This chapter is mainly devoted to the distinctive properties of spiral and skyrmion states accounted by the cubic anisotropy in bulk cubic helimagnets such as MnSi and Cu2OSeO3. We start by systematizing the main features of the universal phase diagrams at the critical fields Hc1 and Hc2. We first discuss the influence of cubic anisotropy on the reversible, or irreversible, alignment of oblique spiral states into the conical phase at Hc1. We then address the phase transition between the conical and field-polarized state at Hc2, which may be of both first order and second order depending on the orientation of the applied magnetic field with respect to easy anisotropy axes. We show that the effect of cubic anisotropy is more subtle than assumed so far, as it may induce new elliptically distorted spiral states and phase transitions between them. Finally, we show how this distortion of the spiral states by cubic anisotropy opens up the way to skyrmion stabilization.

Interphase precipitation occurring during solid-state phase transformations in micro-alloyed steels is generally studied through transmission electron microscopy, atom probe tomography, and ex situ measurements of Small-Angle Neutron Scattering (SANS). The advantage of SANS over the other two characterization techniques is that SANS allows for the quantitative determination of size distribution, volume fraction, and number density of a statistically significant number of precipitates within the resulting matrix at room temperature. However, the performance of ex situ SANS measurements alone does not provide information regarding the probable correlation between interphase precipitation and phase transformations. This limitation makes it necessary to perform in situ and simultaneous studies on precipitation and phase transformations in order to gain an in-depth understanding of the nucleation and growth of precipitates in relation to the evolution of austenite decomposition at high temperatures. A furnace is, thus, designed and developed for such in situ studies in which SANS measurements can be simultaneously performed with neutron diffraction measurements during the application of high-temperature thermal treatments. The furnace is capable of carrying out thermal treatments involving fast heating and cooling as well as high operation temperatures (up to 1200 °C) for a long period of time with accurate temperature control in a protective atmosphere and in a magnetic field of up to 1.5 T. The characteristics of this furnace give the possibility of developing new research studies for better insight of the relationship between phase transformations and precipitation kinetics in steels and also in other types of materials containing nano-scale microstructural features. This work was financially supported equally by the Technology Foundation TTW, as part of the Netherlands Organization for Scientific Research (NWO), and Tata Steel Europe through the Grant No. 14307 under the Project No. S41.5.14548 in the framework of the Materials Innovation Institute (M2i) Partnership Program. The experiments performed at ISIS Neutron and Muon Source were supported by beam-time allocation from the Netherlands Organization for Scientific Research (NWO) through Project No. 721.012.102 (LARMOR) with Experiment No. RB1869024.

Neutron irradiation effects on the microstructure of nuclear graphite have been investigated by X-ray diffraction on virgin and low doses ( ∼ 1.3 and ∼ 2.2 dpa), high temperature (750° C) irradiated samples. The diffraction patterns were interpreted using a model, which takes into account the turbostratic disorder. Besides the lattice constants, the model introduces two distinct coherent lengths in the c-axis and the basal plane, that characterise the volumes from which X-rays are scattered coherently. The methodology used in this work allows to quantify the effect of irradiation damage on the microstructure of nuclear graphite seen by X-ray diffraction. The results show that the changes of the deduced structural parameters are in agreement with previous observations from electron microscopy, but not directly related to macroscopic changes.

We discuss distinctive features of spiral states in bulk chiral magnets such as MnSi and Cu$_2$OSeO$_3$ that stem from the effect of the cubic magneto-crystalline anisotropy. First of all, at both the helical-to-conical and the conical-to-ferromagnetic transitions, taking place at H$_{c1}$ and H$_{c2}$, respectively, the cubic anisotropy leads to reversible or irreversible jump-like reorientations of the spiral wavevectors. The subtle interplay between the easy and hard anisotropy axes gives rise to a phase transition between elliptically distorted conical states almost without any detectable change in the period. We show that the competition between on-site cubic and exchange anisotropy terms can also lead to oblique spiral states. Our work gives clear directions for further experimental studies to reveal theoretically predicted spiral states in cubic helimagnets beyond the aforementioned well-established states thus, can help to understand the magnetic phase diagram of these archetypal skyrmion hosts. In addition, we show that properties of isolated skyrmions such as inter-skyrmion attraction, orientation and/or nucleation are also rooted in the properties of host spirals states, in which skyrmions are stabilized.

Neutron Spin Echo methods (NSE) use Larmor labelling to measure the precession phase of the neutron beam polarization around well-defined magnetic fields. Scattering by a sample can affect the resulting precession phase providing information on the sample's structure and dynamics with high accuracy and resolution. A major limitation for the performance of Neutron Spin Echo instruments is the homogeneity of the precession magnetic fields. Here we investigate the influence of the new 'pancake' moderator, which is being built at the European Spallation Source, on the design of a Neutron Spin Echo spectrometer. The calculations show clear gains when the height to width ratios of the rectangular beam cross-sections mimic those of the ESS 'pancake' moderator beams. In such a case the homogeneity of the magnetic field integrals could improve by at least 30%. However, the calculations show that will not be possible to preserve a high resolution and at the same time reduce the length of the instrument. Consequently, NSE spectrometers will perform better at the ESS but they will not be substantially compacter than at other neutron sources e.g. ILL or FRM2.

The precipitation kinetics of vanadium carbides and its interaction with the austenite-to-ferrite phase transformation is studied in two micro-alloyed steels that differ in vanadium and carbon concentrations by a factor of two, but have the same vanadium-to-carbon atomic ratio of 1:1. Dilatometry is used for heat-treating the specimens and studying the phase transformation kinetics during annealing at isothermal holding temperatures of 900, 750 and 650 °C for up to 10 h. Small-Angle Neutron Scattering (SANS) and Atom Probe Tomography (APT) measurements are performed to study the vanadium carbide precipitation kinetics. Vanadium carbide precipitation is not observed after annealing for 10 h at 900 and 750 °C, which is contrary to predictions from thermodynamic equilibrium calculations. Vanadium carbide precipitation is only observed during or after the austenite-to-ferrite phase transformation at 650 °C. The precipitate volume fraction and mean radius continuously increase as holding time increases, while the precipitate number density starts to decrease after 20 min, which corresponds to the time at which the austenite-to-ferrite phase transformation is finished. This indicates that nucleation and growth are dominant during the first 20 min, while later precipitate growth with soft impingement (overlapping diffusion fields) and coarsening take place. APT shows gradual changes in the precipitate chemical composition during annealing at 650 °C, which finally reaches a 1:1 atomic ratio of vanadium-to-carbon in the core of the precipitates after 10 h.

The archetype cubic chiral magnet MnSi is home to some of the most fascinating states in condensed matter such as skyrmions and a non-Fermi liquid behavior in conjunction with a topological Hall effect under hydrostatic pressure. Using small angle neutron scattering, we study the evolution of the helimagnetic, conical and skyrmionic correlations with increasing hydrostatic pressure. We show that the helical propagation vector smoothly reorients from $\langle 111 \rangle$ to $\langle100\rangle$ at intermediate pressures. At higher pressures, above the critical pressure, the long-range helimagnetic order disappears at zero magnetic field. Nevertheless, skyrmion lattices and conical spirals form under magnetic fields, in a part of the phase diagram where a topological Hall effect and a non-Fermi liquid behavior have been reported. These unexpected results shed light on the puzzling behavior of MnSi at high pressures and the mechanisms that destabilize the helimagnetic long-range order at the critical pressure.

We present a comprehensive investigation of the evolution of helimagnetic correlations in Mn$_{1-x}$Fe$_x$Si with increasing doping. By combining polarised neutron scattering and high resolution Neutron Spin Echo spectroscopy we investigate three samples with $x$=0.09, 0.11 and 0.14, i.e. with compositions on both sides of the concentration $x^* \sim 0.11$ where the helimagnetic Bragg peaks disappear and between $x^*$ and the quantum critical concentration $x_C \sim 0.17$, where $T_C$ vanishes. We find that the abrupt disappearance of the long range helical periodicity at $x^*$, does not affect the precursor fluctuating correlations. These build up with decreasing temperature in a similar way as for the parent compound MnSi. Also the dynamics bears strong similarities to MnSi. The analysis of our results indicates that frustration, possibly due to achiral RKKY interactions, increases with increasing Fe doping. We argue that this effect explains both the expansion of the precursor phase with increasing $x$ and the abrupt disappearance of long range helimagnetic periodicity at $x^*$.

Magnetic skyrmions are topologically protected nanoscale spin textures with particle-like properties. In bulk cubic helimagnets, they appear under applied magnetic fields and condense spontaneously into a lattice in a narrow region of the phase diagram just below the magnetic ordering temperature, the so-called A-phase. Theory, however, predicts skyrmions to be locally stable in a wide range of magnetic fields and temperatures. Our neutron diffraction measurements reveal the formation of skyrmion states in large areas of the magnetic phase diagram, from the lowest temperatures up to the A-phase. We show that nascent and disappearing spiral states near critical lines catalyze topological charge changing processes, leading to the formation and destruction of skyrmionic states at low temperatures, which are thermodynamically stable or metastable depending on the orientation and strength of the magnetic field. Skyrmions are surprisingly resilient to high magnetic fields: the memory of skyrmion lattice states persists in the field polarized state, even when the skyrmion lattice signal has disappeared. These findings highlight the paramount role of magnetic anisotropies in stabilizing skyrmionic states and open up new routes for manipulating these quasi-particles towards energy-efficient spintronics applications.

We present an investigation of the dynamics of calcium caseinate as a function of hydration, solvent isotope (H2O and D2O) and drying methods (roller drying and spray drying), using quasi-elastic neutron scattering (QENS). These factors are key to the formation of fibres in this material which makes it a potential candidate as a next-generation meat analogue. Using a phenomenological model, we find that the relaxation times of the dry spray dried powder decrease with increasing temperatures, while they do not change for the roller dried powder. The spectra of the hydrated samples reveal two independent picosecond processes, both reflecting localized re-orientational motions. We hypothesize that the faster motion is due to the external protein groups that are hydrophilic and the slower motion is due to the internal groups that are hydrophobic. The solvent effect of D2O is not limited to the external groups but prevails to the internal groups where less protons are mobile compared to the H2O hydrated samples. Higher temperatures narrow the number difference in mobile protons, possibly by altering the weak interactions inside the protein aggregates. These findings suggest that a harsh and longer drying process contributes to less active protein side-groups and highlight the hydrophobic effect of D2O on the fibre formation in calcium caseinate.

Recent experimental findings in the bulk cubic helimagnet and Mott insulator ${\mathrm{Cu}}_{2}{\mathrm{OSeO}}_{3}$ highlight the paramount role of cubic anisotropy in stabilizing novel chiral and skyrmion phases. It was indeed found that if a magnetic field is applied along the easy $\ensuremath{\langle}001\ensuremath{\rangle}$ crystallographic direction, competing cubic and exchange anisotropies tilt the wave vector of the conical spiral away from the magnetic field [Qian et al., Sci. Adv. 4, eaat7323 (2018)]. Furthermore, in this configuration skyrmions have been observed in a broad range of temperatures and magnetic fields [Chacon et al., Nat. Phys. 14, 936 (2018)]. Starting from these experimental observations and on the basis of a phenomenological Dzyaloshinskii theory, we investigate additional implications of the cubic anisotropy for this specified field direction. By including cubic anisotropy we show that the phase transition between the conical and field-polarized or homogeneous states becomes first order. Furthermore, we show that this transition is accompanied by the formation of conical droplets---domains of the conical phase in the homogeneous state. We investigate the internal structure of these droplets, which at their boundaries encompass alternating regions of positive and negative energy densities with respect to the homogeneous state. We thus deduce that these droplets may be zipped and unzipped in phase during the first-order phase transition that occurs by either increasing or decreasing the magnetic field. On the other hand, we show that in the conical phase skyrmions may form clusters due to their attractive mutual interaction. However, in the homogeneous state, the skyrmion-skyrmion interaction becomes repulsive, and the skyrmion clusters expand and disperse isolated skyrmions. This mutual skyrmion repulsion prevents the stabilization of skyrmion clusters even if the energy of isolated skyrmions is lower than that of the homogeneous state. Yet this skyrmion dispersal may be prevented if skyrmions are surrounded by the circular spiral state and form skyrmion bags. Such a scenario could explain the existence of skyrmions in the field-polarized state reported experimentally.

We present a comprehensive and systematic magnetization and ac susceptibility study of ${\mathrm{Mn}}_{1\ensuremath{-}x}{\mathrm{Fe}}_{x}\mathrm{Si}$ over an extensive range of ten Fe concentrations between $x=0$ and $x=0.32$. With increasing Fe substitution, the critical temperature decreases but the magnetic phase diagrams remain qualitatively unaltered for $x\ensuremath{\le}{x}^{*}\ensuremath{\approx}0.11$ with clear boundaries between the helical, conical, and skyrmion lattice phase as well as an enhanced precursor phase. A notably different behavior sets in for $x\phantom{\rule{4pt}{0ex}}=0.11,\phantom{\rule{0.28em}{0ex}}0.13$, and 0.14, where certain characteristics of helimagnetic correlations persist, but without clear phase boundaries. Although a qualitative change already sets in at ${x}^{*}$, the transition temperature and spontaneous magnetization vanish only at ${x}_{c}=0.17$ where also the average magnetic interactions change sign. Although the Curie-Weiss temperature reaches $\ensuremath{-}12$ K for $x=0.32$, no signature of long-range magnetic order is found down to the lowest temperature, indicating a possible significant role for quantum fluctuations in these systems.

Tuning the interactions in magnetic materials by pressure or chemical substitution is a much visited route to discover new and exotic phases of condensed matter. A well known example is doping the archetype chiral magnet MnSi with Fe. Here, the authors use small-angle neutron scattering to track the evolution of the helimagnetic order in Mn${}_{1\ensuremath{-}x}$Fe${}_{x}$Si with Fe doping. They discover that the long-range helimagnetic order found in MnSi is suppressed already at ${x}^{*}$ $\ensuremath{\approx}$ 0.11, well before the critical concentration of $x$ $\ensuremath{\approx}$ 0.17. Finite isotropic helimagnetic correlations persist for $x$ $g$ ${x}^{*}$ that can only be partly aligned by magnetic fields through a complex magnetization process.

We have developed and realised a compact high-pressure clamp cell of the piston-cylinder type for Small Angle Neutron Scattering (SANS) and Neutron Spin Echo (NSE) spectroscopy measurements. The cell can pressurise φ5 mm samples up to 1.5 GPa. It is non-magnetic, produces a sufficiently low and flat SANS background with a relatively good neutron transmission and can be used in combination with polarised neutrons. The cell accommodates samples of up to 55 mm3 in volume and adopts a two-layer geometry: a TiZr body (outer layer) and a CuBe2 insert (inner layer). Its design is very compact and can be employed inside cryomagnets featuring a sample bore greater than φ30 mm. We present the cell and illustrate its performances with a series of neutron scattering experiments performed on single-crystalline MnSi.

The lack of inversion symmetry in the crystal lattice of magnetic materials gives rise to complex noncollinear spin orders through interactions of a relativistic nature, resulting in interesting physical phenomena, such as emergent electromagnetism. Studies of cubic chiral magnets revealed a universal magnetic phase diagram composed of helical spiral, conical spiral, and skyrmion crystal phases. We report a remarkable deviation from this universal behavior. By combining neutron diffraction with magnetization measurements, we observe a new multidomain state in Cu

We present a comprehensive structural characterization of two different highly pure nuclear graphites that compasses all relevant length scales from nanometers to sub-mm. This has been achieved by combining several experiments and neutron techniques: Small Angle Neutron Scattering (SANS), high-resolution Spin Echo SANS (SESANS) and neutron imaging. In this way it is possible to probe an extraordinary broad range of 6 orders of magnitude in length from microscopic to macroscopic length scales. The results reveal a fractal structure that extends from ∼0.6 nm to 0.6 mm and has surface and mass fractal dimensions both very close to 2.5, a value found for percolating clusters and fractured ranked surfaces in 3D.

The lack of inversion symmetry in the crystal lattice of magnetic materials gives rise to complex noncollinear spin orders through interactions of a relativistic nature, resulting in interesting physical phenomena, such as emergent electromagnetism. Studies of cubic chiral magnets revealed a universal magnetic phase diagram composed of helical spiral, conical spiral, and skyrmion crystal phases. We report a remarkable deviation from this universal behavior. By combining neutron diffraction with magnetization measurements, we observe a new multidomain state in Cu2OSeO3. Just below the upper critical field at which the conical spiral state disappears, the spiral wave vector rotates away from the magnetic field direction. This transition gives rise to large magnetic fluctuations. We clarify the physical origin of the new state and discuss its multiferroic properties.

Reentrant spin glasses are frustrated disordered ferromagnets developing vortex-like textures under an applied magnetic field. Our study of a Ni$_{0.81}$Mn$_{0.19}$ single crystal by small angle neutron scattering clarifies their internal structure and shows that these textures are randomly distributed. Spin components transverse to the magnetic field rotate over length scales of 3-15 nm in the explored field range, decreasing as field increases according to a scaling law. Monte-Carlo simulations reveal that the internal structure of the vortices is strongly distorted and differs from that assumed for "frustrated" skyrmions, built upon a competition between symmetric exchange interactions. Isolated vortices have small non-integer topological charge. The vortices keep an anisotropic shape on a 3 dimensional lattice, recalling "croutons" in a "ferromagnetic soup". Their size and number can be tuned independently by the magnetic field and concentration x (or heat treatment), respectively. This opens an original route to understand and control the influence of quenched disorder in systems hosting non trivial spin textures., Comment: 23 pages, 15 figures, includes supplement

Magnetic skyrmions are nano-sized topologically protected spin textures with particle-like properties. They can form lattices perpendicular to the magnetic field and the orientation of these skyrmion lattices with respect to the crystallographic lattice is governed by spin-orbit coupling. By performing small angle neutron scattering measurements, we investigate the coupling between the crystallographic and skyrmion lattices in both Cu$_2$OSeO$_3$ and the archetype chiral magnet MnSi. The results reveal that the orientation of the skyrmion lattice is primarily determined by the magnetic field direction with respect to the crystallographic lattice. In addition, it is also influenced by the magnetic history of the sample which can induce metastable lattices. Kinetic measurements show that these metastable skyrmion lattices may or may not relax to their equilibrium positions under macroscopic relaxation times. Furthermore, multidomain lattices may form when two or more equivalent crystallographic directions are favored by spin-orbit coupling and oriented perpendicular to the magnetic field., Comment: Supplementory movie 4 is not uploaded due to size restrictions but is available upon request

We describe theoretical and practical aspects of spin-echo modulated small-angle neutron scattering (SEMSANS) as well as the potential combination with SANS. Based on the preliminary technical designs of SKADI (a SANS instrument proposed for the European Spallation Source) and a SEMSANS add-on, we assess the practicability, feasibility and scientific merit of a combined SANS and SEMSANS setup by calculating tentative SANS and SEMSANS results for soft matter, geology and advanced material samples that have been previously studied by scattering methods. We conclude that lengths from 1 nm up to 0.01 mm can be observed simultaneously in a single measurement. Thus, the combination of SANS and SEMSANS instrument is suited for the simultaneous observation of a wide range of length scales, e.g. for time-resolved studies of kinetic processes in complex multiscale systems., accepted manuscript

Chiral magnetism plays an increasingly important role in condensed matter investigations driven by the discovery of exotic spin textures, such as the topologically protected chiral skyrmions that can form a lattice under magnetic fields. The nature and universality of the phase diagram in cubic chiral magnets has been the subject of numerous experimental and theoretical investigations, including the transition to the helimagnetic state. In MnSi, the archetypal system in this family, this transition is of first order and involves a precursor phase, where strong chiral fluctuating correlations build up. This work presents an experimental investigation of the structural and dynamical aspects of the phase transition in Fe${}_{1-x}$Co${}_{x}$Si, a system that belongs to the same family as MnSi but with the additional possibility to tune important physical interactions and parameters through variation of the Fe and Co concentration. In this system, the combination of small-angle neutron scattering and neutron spin echo spectroscopy uncovers that the scenario of the transition is qualitatively very different from that in MnSi. This goes beyond what can be expected from a comparison of the relevant length scales and thus challenges the validity of a universal approach to the helimagnetic transition in cubic chiral magnets.

The atomic structure of several nuclear graphite samples, an essential moderator material for nuclear reactors, has been investigated by X-ray diffraction. The patterns were analyzed by the conventional Rietveld refinement approach as well as by the CARBONXS model, which takes into account disorder and stacking faults. The refined parameters compiled with those from literature reveal a generic picture for the structure of all graphite specimens.

The reference chiral helimagnet MnSi is the first system where skyrmion lattice correlations have been reported. At zero magnetic field the transition at $T_C$ to the helimagnetic state is of first order. Above $T_C$, in a region dominated by precursor phenomena, neutron scattering shows the build up of strong chiral fluctuating correlations over the surface of a sphere with radius $2\pi/\ell$, where $\ell$ is the pitch of the helix. It has been suggested that these fluctuating correlations drive the helical transition to first order following a scenario proposed by Brazovskii for liquid crystals. We present a comprehensive neutron scattering study under magnetic fields, which provides evidence that this is not the case. The sharp first order transition persists for magnetic fields up to 0.4 T whereas the fluctuating correlations weaken and start to concentrate along the field direction already above 0.2 T. Our results thus disconnect the first order nature of the transition from the precursor fluctuating correlations. They also show no indication for a tricritical point, where the first order transition crosses over to second order with increasing magnetic field. In this light, the nature of the first order helical transition and the precursor phenomena above $T_C$, both of general relevance to chiral magnetism, remain an open question.

We present a systematic study of the ac susceptibility of the chiral magnet ${\mathrm{Fe}}_{1\ensuremath{-}x}{\mathrm{Co}}_{x}\mathrm{Si}$ with $x=0.30$ covering four orders of magnitude in frequencies from 0.1 Hz to 1 kHz, with particular emphasis to the pronounced history dependence. Characteristic relaxation times ranging from a few milliseconds to tens of seconds are observed around the skyrmion lattice A phase, the helical-to-conical transition and in a region above ${T}_{C}$. The distribution of relaxation frequencies around the A phase is broad, asymmetric, and originates from multiple coexisting relaxation processes. The pronounced dependence of the magnetic phase diagram on the magnetic history and cooling rates as well as the asymmetric frequency dependence and slow dynamics suggest more complicated physical phenomena in ${\mathrm{Fe}}_{0.7}{\mathrm{Co}}_{0.3}\mathrm{Si}$ than in other chiral magnets.

Small angle neutron scattering measurements on a bulk single crystal of the doped chiral magnet ${\mathrm{Fe}}_{1\ensuremath{-}x}{\mathrm{Co}}_{x}\mathrm{Si}$ with $x=0.3$ reveal a pronounced effect of the magnetic history and cooling rates on the magnetic phase diagram. The extracted phase diagrams are qualitatively different for zero and field cooling and reveal a metastable skyrmion lattice phase outside the $A$ phase for the latter case. These thermodynamically metastable skyrmion lattice correlations coexist with the conical phase and can be enhanced by increasing the cooling rate. They appear in a wide region of the phase diagram at temperatures below the $A$ phase but also at fields considerably smaller or higher than the fields required to stabilize the $A$ phase.

Polarized neutron imaging has recently been introduced as an efficient method to three-dimensionally visualize and measure magnetic fields even in the bulk of massive objects. Here we introduce a spin-echo approach for polarized neutron imaging, which has the potential to overcome some drawbacks of the first attempts, namely to deal with strong fields, arbitrary field directions and quantification. Furthermore our novel approach increases the efficiency of quantitative studies due to relaxed monochromatisation requirements for spin-echo neutron imaging and with respect to additional information available in the recorded images, which allows for straightforward quantification in many cases.

With the new brilliant neutron sources and the developments of novel optical elements, neutron spin echo (NSE) spectroscopy evolves to tackle new problems and scientific fields. The new developments pave the way to complex experimental set-ups such as the intensity modulated variant of NSE (IMNSE), a powerful technique which was introduced some 20 years ago but found limited use up to now. With the new compact supermirror or He3 polarizers IMNSE becomes attractive for a broad range of applications in magnetism, soft matter and biology. A novel development along this line is the polarimetric NSE technique, which combines IMNSE and the zero-field polarimeter Cryopad to access components of the scattered polarization that are transverse to the incoming polarization. Polarimetric NSE is the method of choice for studying chiral fluctuations, as illustrated by new results on the reference helimagnet MnSi.

The Helmholtz-Zentrum Berlin (HZB, former Hahn-Meitner-Institut) operates more than 20 instruments at the medium-flux research reactor BER II, a modern neutron source which was completely renewed during the years 1985–1992. The creation of the Berlin Neutron Scattering Center (BENSC) in 1993 marked the beginning of an extended user program, offering access to a great variety of new instruments with some unique features suited for research in many fields of science. In the course of time the Helmholtz-Zentrum Berlin has established itself as a major neutron science facility with an exceptionally high demand from international, in particular European users.

The performance of the new neutron powder diffraction instrument PEARL that is installed at the research reactor of Delft University of Technology is reported. It is based on the optimization concepts developed by Cussen [Nucl. Instrum. Methods Phys. Res. Sect. A (2007), 583, 394-406], which lead to high performance competing with existing constant-wavelength neutron powder diffractometers, despite the relatively low source brightness of the 2MW reactor of Delft University of Technology.This manuscript describes the novel neutron powder diffractometer PEARL at Delft University of Technology and shows the first experimental results.

We present an investigation of the magnetic-field-temperature phase diagram of Cu2OSeO3 based on dc magnetization and ac susceptibility measurements covering a broad frequency range of four orders of magnitude, from very low frequencies reaching 0.1 Hz up to 1 kHz. The experiments were performed in the vicinity of T-c = 58.2 K and around the skyrmion lattice A phase. At the borders between the different phases the characteristic relaxation times reach several milliseconds and the relaxation is nonexponential. Consequently the borders between the different phases depend on the specific criteria and frequency used and an unambiguous determination is not possible.

This paper describes design and optimization for the wide angle spin-echo spectrometer (WASP) presently being planned at the ILL. The spectrometer will be a replacement for the high intensity spectrometer IN11 and will enhance its signal by more than one order of magnitude while maintaining the resolution of the present spectrometer. The paper outlines the magnetic field calculations and the considerations about possible limitations. The projected performance has been verified carefully by Monte Carlo raytracing and Biot–Savart magnetic field calculations. The maximum momentum transfer of the new spectrometer is to be extended to 4 A− 1.

Neutron dark-field imaging constitutes a seminal progress in the field of neutron imaging as it combines real space resolution capability with information provided by one of the most significant neutron scattering techniques, namely small angle scattering. The success of structural characterizations bridging the gap between macroscopic and microscopic features has been enabled by the introduction of grating interferometers so far. The induced interference pattern, a spatial beam modulation, allows for mapping of small-angle scattering signals and hence addressing microstructures beyond direct spatial resolution of the imaging system with high efficiency. However, to date the quantification in the small angle scattering regime is severely limited by the monochromatic approach. To overcome such drawback we here introduce an alternative and more flexible method of interferometric beam modulation utilizing a spin-echo technique. This novel method facilitates straightforward quantitative dark-field neutron imaging, i.e. the required quantitative microstructural characterization combined with real space image resolution. For the first time quantitative microstructural reciprocal space information from small angle neutron scattering becomes available together with macroscopic image information creating the potential to quantify several orders of magnitude in structure sizes simultaneously.

Measures to optimally operate a neutron spin-echo (NSE) spectrometer at a pulsed source are discussed. Gain factors and resolution functions are addressed.

Neutron spin echo (NSE) is characterized by a broad variety of new developments. The obvious trend is towards the highest possible energy resolution at the highest possible wavelengths by further developing the generic IN11 design. The planned NSE spectrometer at the SNS will further push the limits of NSE reaching Fourier times as long as 1 μs. On the other hand, simultaneous measurements over higher total solid angles require the real cylindrical symmetry of the magnetic field configuration realized with the spectrometer SPAN at BENSC. The state-of-the-art neutron optical elements lead to considerable gains in neutron flux and allow for extending the wavelength range up to very high wavelengths as well as down to the thermal spectrum. With thermal neutrons NSE reaches energy transfers, which up to now were reserved to TOF spectroscopy, while keeping the advantage of the most straightforward data analysis combine with the most wide dynamic range also in the sub-picosecond Fourier time range. This is illustrated by a detailed analysis of both the alpha and beta processes of glass forming polymers close to the glass transition.

In the near future, the SNS will be the most powerful spallation neutron sources worldwide. The neutron instruments at these sources are being planned and many of them are currently under construction. To cover the domain of ultra-high-resolution spectroscopy a neutron spin-echo (NSE) spectrometer is foreseen. Here, we present the layout of the planned instrument with a Fourier time range that covers τ=1 ps…1 μs and high effective neutron flux. A huge field of application will be the investigation of soft condensed matter and complex fluids. However, easily accessible optional modes for a ferromagnetic and intensity modulated NSE, respectively, offers access also to magnetic samples.

For a polycrystalline sample of (Fe 0.65 Ni 0.35 ) 1− x Mn x with x = 0.113 the intermediate scattering function s ( Q , t ) was observed by paramagnetic neutron spin echo techniques at scattering vectors 0.26 nm −1 Q −1 and at temperatures ranging from 120 to 40 K, thus covering both the ferromagnetic phase and the RSG phase of the alloy. The normalized intermediate scattering function S ( Q , t ) = s ( Q , t )/ s ( Q ) strongly depended on Q in the FM phase as would be expected. However, even in the mixed RSG phase the Q -dependence was appreciable, and thus it differed from the behaviour observed in classical spin glasses. It deviated from an exponential decay and was represented by a stretched exponential (with exponent n = 0.4–0.9 typically). The spin freezing, as observed by the increase of the inverse time constant, changed by more than two orders of magnitude between the onset of the RSG phase at 90 K and the strongly irreversible RSG phase at low temperature (⩽ 50 K).

We present the results of the first TOFLAR (Time Of Flight and LARmor precession) experiment with a Larmor modulation device positioned after the sample. Using a setup based on the configuration of the SNS-NSE spectrometer (Spallation Neutron Source, USA), we did TOFLAR measurements on water at 5 and 20 degree Celsius, analyzed the spectra, and obtained diffusion coefficient values that are in a reasonable agreement with literature. Thus, our results demonstrate that TOFLAR may become a standard technique to study quasielastic neutron scattering in future.

We report a neutron spin-echo (NSE) study of deuterated atactic polypropylene (aPP) using the multi-detector option on the spectrometer IN11 at the Institut Laue-Langevin (France). Experiments were performed in the temperature range 300–360 K i.e. above the polymer glass transition ( T g =253 K). Neutron spectra were collected in the Q -region (0.8 Q −1 ) around the first interference peak of the static structure factor S ( Q ) at 1.1 A −1 which corresponds to intermolecular correlations. As shown recently for polyisobutylene and polybutadiene, the NSE data of aPP display the same temperature dependence as obtained from viscosity measurements.

Simultaneous NSE measurements over a wide angular range are for the first time made possible with the new magnetic field configuration realized at the spectrometer SPAN. Three pairs of coils with diameters field 1, 3 and 4.8 m, respectively, create the NSE precession. Each pair is mounted in a Helmholtz-like fashion, with the electric currents oriented oppositely to each other, one coil above and one coil below the horizontal scattering plane. The set-up has a symmetry axis, which is perpendicular to the scattering plane and crosses the sample position so that the resulting magnetic field integral does not change with the scattering angle. The open construction also allows for medium resolution TOF measurements. Both TOF and NSE can be done for a wide incident wavelength band including parts of both the cold and the thermal neutron spectrum.

Simultaneous measurements over a large angular range are commonly used to improve the data acquisition rate of neutron spectrometers. Almost thirty years after the discovery of Neutron Spin Echo (NSE) [l], however, all NSE spectrometers, with the exception of INllC [2], are single-detector type. This is due to the strict conditions for the homogeneity of the magnetic field integral (10−4 − 10−5) along all neutron trajectories. A substantial enhancement of the NSE data collection rate has been achieved with the magnetic field configuration of the spectrometer SPAN at BENSC. We developed a novel setup with cylindrical symmetry, which allows for NSE data to be taken simultaneously over the whole range of scattering angles accessible by a spectrometer, typically from −150° to 150°. The high symmetry and homogeneity of the magnetic field configuration allows for using a large incoming beam cross section, of actually 58 mm × 100 mm on SPAN, and a large vertical beam divergence of up to 3°, leading to a...

The magnetic field configuration of the spectrometer SPAN allows simultaneous NSE measurements to be performed over a particularly large angular range. The NSE precession field is created by three pairs of coils with diameters 1, 3 and 4.8 m, respectively, and the set-up has a symmetry axis, which is perpendicular to the scattering plane and crosses the sample position. The resulting magnetic field integral does not change with the scattering angle and thus fulfils the condition for multidetector NSE. In order to preserve the symmetry of the magnetic field, remanent supermirrors are used as analyzers. These supermirrors perform in reflection, in the stray magnetic field of the precession coils, and are mounted on analyzer frames with an angular opening of 25.7° each. The neutron optical elements (analyzer, polarizing guide) of SPAN have a high transmission over a large wavelength band. As a consequence of this, NSE can be done for incident wavelengths ranging from 2.5 to 9 A. At the lowest wavelengths high q values can be reached, which were not previously accessible for NSE experiments (at λ=3 A: 0.08 A−1⩽q⩽4 A−1).

A common feature of all glassy systems is the decisive role of dynamics in the evolution of macroscopic properties, while the short and eventually partial long-range order shows little difference between the different phases. Our recent neutron scattering results on a Cu–Mn spin glass dramatically illustrate this point, and provide unambiguous evidence against recent suggestions that behaviour in Cu–Mn is driven by spin density waves rather than glassy dynamics. Recent findings in Au–Fe provide the first circumstantial evidence for the theoretically predicted extremely long correlation lengths at the spin-glass transition. Recent experimental advances opened up the way to investigate dynamics in glasses and liquids on extended length scales, where only spin correlations could be studied by now.

The neutron spin–echo spectroscopy (NSE) precession field, which is created by three pairs of coils with diameters 1, 3 and 4.8 m, respectively, is the peculiarity in the design of SPAN. Each pair is mounted in a Helmholtz-like fashion, with the electric currents oriented oppositely to each other, one coil above and one coil below the horizontal scattering plane. The centers of all coils are placed on the same vertical axis, which crosses the sample position. The NSE field is horizontal with axial symmetry. Particular care has been taken to shape the magnetic field around the sample, which is weak, vertical and very homogeneous. We present a first series of NSE measurements, which show that only little field adjustment is required to satisfy the exact echo condition (phase) simultaneously for a range of scattering angles and confirm the high axial symmetry of the precession field. The results show that the new magnetic field configuration of SPAN allows for multidetector NSE over a wide angular range.