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Compact accelerator-driven neutron sources allow to operate multiple optimised target-moderator-reflector (TMR) units adapted to the requirements of the respective instruments. The compact design of the TMR units allows an efficient coupling of neutron production, neutron moderation and extraction, but requires a novel way of optimisation. The neutronic performance of different TMR units based on polyethylene, heavy water and a mixture of heavy and light water moderators together with Pb and Be reflectors and a borated polyethylene absorber is discussed. Extraction channels for thermal and cold neutrons are investigated regarding the energy and time spectra.

Applied physics / A 127(8), 576 (2021). doi:10.1007/s00339-021-04713-4, Published by Springer, New York

Magnetic thin films and multilayers offer many novel physical phenomena with tremendous potential for applications. Various growth processes and methods are in place to make homogeneous films. However, dispersion in local easy axes is an inherent issue which may give rise to an effective random anisotropy in thin film. It is important to investigate the effect of random anisotropy on the magnetic properties of the thin films. In this report we address the aspect of local dispersion induced random anisotropy on magnetization reversal in ferromagnetic (FM)/non-magnetic(NM)/ferromagnetic(FM) stacks. We show that the magnetization reversal is not only governed by the inter-layer interaction but the intrinsic anisotropies also play a crucial role. In this context we have studied the magnetization reversal of Co/ AlO X /Co trilayers with various thicknesses of AlO X which acts as a spacer between the two Co layers. Presence of random anisotropy in addition to growth induced uniaxial anisotropy was observed in all the samples. Magneto-optic Kerr effect (MOKE) based microscopy revealed dipolar-coupled layer-by-layer magnetization reversal of the Co layers which was corroborated by polarized neutron reflectometry (PNR) experiments. Micromagnetic simulations confirmed that the presence of random anisotropy in addition to uniaxial anisotropy leads to layer-by-layer magnetization reversal.

[8th International Meeting of the Union for Compact Accelerator-driven Neutron Sources, UCANS-8, 2019-07-08 - 2019-07-10, Paris, France] 8th International Meeting of the Union for Compact Accelerator-driven Neutron Sources, UCANS-8, Paris, France, 8 Jul 2019 - 10 Jul 2019; Les Ulis : EDP Sciences, The European physical journal / Web of Conferences Web of Conferences : proceedings proceedings 231, 04001 pp. (2020). doi:10.1051/epjconf/202023104001, Published by EDP Sciences, Les Ulis

[8th International Meeting of the Union for Compact Accelerator-driven Neutron Sources, UCANS-8, 2019-07-08 - 2019-07-10, Paris, France] 8th International Meeting of the Union for Compact Accelerator-driven Neutron Sources, UCANS-8, Paris, France, 8 Jul 2019 - 10 Jul 2019; Les Ulis : EDP Sciences, The European physical journal / Web of Conferences Web of Conferences : proceedings proceedings 231, 03006 pp. (2020). doi:10.1051/epjconf/202023103006, Published by EDP Sciences, Les Ulis

Journal of radioanalytical and nuclear chemistry 325(2), 641-655 (2020). doi:10.1007/s10967-020-07271-w, Published by Springer Science + Business Media B.V, Dordrecht [u.a.]

Organic aromatic cold neutron moderators - like mesitylene ( C 9 H 12 ) - are often much more convenient to handle and to commission than cryogenic methane or ortho/para hydrogen moderators. Although this benefit comes at the cost of reduced brilliance, mesitylene moderators are suited to enable cold neutron applications at sources where a complex traditional cold moderator system is not feasible. Developing the Julich High Brilliance neutron Source (HBS) project, we have investigated the use of such a low-dimensional mesitylene moderator with MCNP and ANSYS simulations and validated the simulations with experiments at TU Dresden's AKR-2 reactor. Here we will document the feasibility, advantages and drawbacks of such a system and give an outlook on future optimization potentials.

The advent of research reactors in the middle of the last century opened the opportunity for research with neutrons. Cliff Shull and Bert Brockhouse were awarded the Nobel prize for physics in 1994 for pioneering contributions to the development of neutron scattering techniques for studies of condensed matter. Since then, research with neutrons has emerged as an indispensable technique for materials research and development of functional materials, which is key to solving many of the grand challenges of our modern societies. With its broad user community, hierarchical structure of neutron sources and the Institute Laue-Langevin as the leading neutron facility, Europe has acquired an excellent standing in this field in terms of publication output. In its network of sources, the low flux sources provide the foundation by educating the next generation of neutron users and offer a platform for method development. Medium flux sources with broader instrumentation cover similar aspects, but additionally provide capacity and capability, e.g., by specializing in certain aspects like a distinctive sample environment or by addressing a specific local or regional user group. Finally, the flagship facilities are essential for the most demanding, flux-hungry experiments.

We present bulk and neutron scattering measurements performed on the isotopically enriched $^{154}\mathrm{Sm_2Ti_2O_7}$ and $^{154}\mathrm{Sm_2Sn_2O_7}$ samples. Both compounds display sharp heat capacity anomalies, at 350 mK and 440 mK, respectively. Inelastic neutron scattering measurements are employed to determine the crystalline electric field (CEF) level scheme, which includes transitions between the ground-state and first excited $J$ multiplets of the $\mathrm{Sm}^{3+}$ ion. To further validate those results, the single-ion magnetic susceptibility of the compounds is calculated and compared with the experimental DC-susceptibility measured in low applied magnetic fields. It is demonstrated that the inclusion of intermultiplet transitions in the CEF analysis is fundamental to the understanding of the intermediate and, more importantly, low temperature magnetic behaviour of the Sm-based pyrochlores. Finally, the heat capacity anomaly is shown to correspond to the onset of an all-in-all-out long-range order in the stannate sample, while in the titanate a dipolar long-range order can be only indirectly inferred., 13 pages, 10 Figures

The interface between the organic semiconductor (OSC)/ferromagnetic (FM) material can exhibit ferromagnetism due to their orbital hybridization. Charge/spin transfer may occur from the FM to OSC layer leading to the formation of “spinterface,” i.e., the interface exhibiting a finite magnetic moment. In this work, the magnetic properties of the Co / C 60 bilayer thin film have been studied to probe the interface between the Co and C60 layer. Polarized neutron reflectivity (PNR) measurement indicates that the thickness and moment of the spinterface are ∼2 ± 0.18 nm and 0.8 ± 0.2 μB/cage, respectively. The comparison of the magnetization reversal between the Co / C 60 bilayer and the parent single layer Co thin film reveals that spinterface modifies the domain microstructure. Further, the anisotropy of the bilayer system shows a significant enhancement (∼two times) in comparison to its single layer counterpart which is probably due to an additional interfacial anisotropy arising from the orbital hybridization at the Co / C 60 interface.The interface between the organic semiconductor (OSC)/ferromagnetic (FM) material can exhibit ferromagnetism due to their orbital hybridization. Charge/spin transfer may occur from the FM to OSC layer leading to the formation of “spinterface,” i.e., the interface exhibiting a finite magnetic moment. In this work, the magnetic properties of the Co / C 60 bilayer thin film have been studied to probe the interface between the Co and C60 layer. Polarized neutron reflectivity (PNR) measurement indicates that the thickness and moment of the spinterface are ∼2 ± 0.18 nm and 0.8 ± 0.2 μB/cage, respectively. The comparison of the magnetization reversal between the Co / C 60 bilayer and the parent single layer Co thin film reveals that spinterface modifies the domain microstructure. Further, the anisotropy of the bilayer system shows a significant enhancement (∼two times) in comparison to its single layer counterpart which is probably due to an additional interfacial anisotropy arising from the orbital ...

[8th International Meeting of Union for Compact Accelerator-Driven Neutron Sources, UCANS-8, 2019-07-08 - 2019-07-10, Paris, France] 8th International Meeting of Union for Compact Accelerator-Driven Neutron Sources, UCANS-8, Paris, France, 8 Jul 2019 - 10 Jul 2019; Les Ulis : EDP Sciences, The European physical journal / Web of Conferences Web of Conferences : proceedings proceedings 231, 02002 pp. (2020). doi:10.1051/epjconf/202023102002, Published by EDP Sciences, Les Ulis

Scientific reports 8(1), 5515 (2018). doi:10.1038/s41598-018-23864-8, Published by Nature Publishing Group, London

We present high-resolution thermal-expansion and specific-heat measurements of single crystalline alpha-RuCl3. An extremely hysteretic structural transition expanding over 100 K is observed by thermal- expansion along both crystallographic axes, which we attribute to a change of stacking sequence of the RuCl3 layers. Three magnetic transitions are observed, which we link to the different stacking sequences. Using our data and thermodynamic relations, we derive the uniaxial and hydrostatic pressure derivatives of all three magnetic transitions. Our results demonstrate that magnetic order should be totally suppressed by very moderate pressures of 0.3 GPa to 0.9 GPa. Finally, we discuss why our results differ from recent hydrostatic pressure measurements and suggest a possible route to reaching the spin-liquid state in alpha-RuCl3., Comment: 6 pages, 3 figures

Inelastic neutron scattering measurements are performed on single crystals of the antiferromagnetic compound Mn_{5}Si_{3} in order to investigate the relation between the spin dynamics and the magnetothermodynamics properties. It is shown that, among the two stable antiferromagnetic phases of this compound, the high temperature one has an unusual magnetic excitation spectrum where propagative spin waves and diffuse spin fluctuations coexist. Moreover, it is evidenced that the inverse magnetocaloric effect of Mn_{5}Si_{3}, the cooling by adiabatic magnetization, is associated with field induced spin fluctuations.

We report an extensive study on the zero field ground state of a powder sample of the pyrochlore ${\mathrm{Yb}}_{2}{\mathrm{Ti}}_{2}{\mathrm{O}}_{7}$. A sharp heat capacity anomaly that labels a low temperature phase transition in this material is observed at 280 mK. Neutron diffraction shows that a quasicollinear ferromagnetic order develops below ${T}_{\mathrm{c}}$ with a magnetic moment of $0.87(2){\ensuremath{\mu}}_{\mathrm{B}}$. High resolution inelastic neutron scattering measurements show, below the phase transition temperature, sharp gapped low-lying magnetic excitations coexisting with a remnant quasielastic contribution likely associated with persistent spin fluctuations. Moreover, a broad inelastic continuum of excitations at $\ensuremath{\sim}0.6$ meV is observed from the lowest measured temperature up to at least 2.5 K. At 10 K, the continuum has vanished and a broad quasielastic conventional paramagnetic scattering takes place at the observed energy range. Finally, we show that the exchange parameters obtained within the framework of linear spin-wave theory do not accurately describe the observed zero field inelastic neutron scattering data.

We have studied the crystal and magnetic structures of Fe-doped hexagonal manganites $\mathrm{LuM}{\mathrm{n}}_{1\ensuremath{-}x}\mathrm{F}{\mathrm{e}}_{x}{\mathrm{O}}_{3}(x=0,0.1,0.2$, and 0.3) by using bulk magnetization and neutron powder diffraction methods. The samples crystalize consistently in a hexagonal structure and maintain the space group $P{6}_{3}cm$ from 2 to 300 K. The N\'eel temperature ${T}_{\mathrm{N}}$ increases continuously with increasing Fe doping. In contrast to a single ${\mathrm{\ensuremath{\Gamma}}}_{4}$ representation in $\mathrm{LuMn}{\mathrm{O}}_{3}$, the magnetic ground state of the Fe-doped samples can only be described with a combination of ${\mathrm{\ensuremath{\Gamma}}}_{3}(P{{6}_{3}}^{\ensuremath{'}}c{m}^{\ensuremath{'}})$ and ${\mathrm{\ensuremath{\Gamma}}}_{4}(P{{6}_{3}}^{\ensuremath{'}}{c}^{\ensuremath{'}}m)$ irreducible representations, whose contributions have been quantitatively estimated. The drastic effect of Fe doping is highlighted by composition-dependent spin reorientations. A phase diagram of the entire composition series is proposed based on the present result and those reported in literature. Our result demonstrates the importance of tailoring compositions in increasing magnetic transition temperatures of multiferroic systems.

Ferrimagnetic iron oxide nanoparticle monolayers on top of ferroelectric BaTiO3 substrates were prepared and a magnetoelectric coupling effect was observed. We employed hereby a magnetoelectric AC susceptibility setup as modification of a commercial superconducting quantum interference device magnetometer. The magnetoelectric coefficient shows two jumps at the BaTiO3 phase transition temperatures. Moreover, the magnetic depth profile of the nanoparticle monolayer was probed by polarized neutron reflectivity. The data recorded at various electric field values show that the electric field is able to alter the magnetism of the nanoparticle monolayer by a strain mediated magnetoelectric coupling effect.

Zero field cooled (ZFC)/Field Cooled (FC) magnetization curves of a bulk MnO single crystal show a peculiar peak at low temperatures (~ 40 K) similar to the low temperature peak observed in MnO nanoparticles. In order to investigate the origin of this peak, the spin structure of a MnO single crystal has been studied and compared with a single phase powder sample using magnetometry and polarized neutron scattering. Both magnetometry and polarized neutron diffraction results confirm the antiferromagnetic (AF) phase transition at the Néel temperature TN of 118 K, in both powder and single crystal form. However, the low temperature peak in the ZFC/FC magnetization curves is not observed in single phase MnO powder. To better understand the observed behavior, ac susceptibility measurements have been employed. We conclude that the clear peak in the magnetic signal from the single crystal originates from a small amount of ferrimagnetic (FiM) Mn2O3 or Mn3O4 impurities, which is grown at the interfaces between MnO crystal twins.

We report on a versatile mini ultra-high vacuum (UHV) chamber which is designed to be used on the MAgnetic Reflectometer with high Incident Angle of the J\"ulich Centre for Neutron Science at Heinz Maier-Leibnitz Zentrum in Garching, Germany. Samples are prepared in the adjacent thin film laboratory by molecular beam epitaxy and moved into the compact chamber for transfer without exposure to ambient air. The chamber is based on DN 40 CF flanges and equipped with sapphire view ports, a small getter pump, and a wobble stick, which serves also as sample holder. Here, we present polarized neutron reflectivity measurements which have been performed on Co thin films at room temperature in UHV and in ambient air in a magnetic field of 200 mT and in the Q-range of 0.18 \AA$^{-1}$. The results confirm that the Co film is not contaminated during the polarized neutron reflectivity measurement. Herewith it is demonstrated that the mini UHV transport chamber also works as a measurement chamber which opens new possibilities for polarized neutron measurements under UHV conditions., Comment: 5 pages, 5 figures