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DiffMod is a simulation program for the evolution of a neutron ensemble in a thermal target – moderator – reflector assembly of a pulsed neutron source based on the statistical description of diffusion, scattering, moderation, and absorption processes. The spatial resolution, the energy resolution and the diffusion directions are strongly restricted to achieve calculation times in a realistic moderator – reflector assembly below 1 hour. In comparison with Monte-Carlo simulations describing the geometry and interactions between neutrons and moderator material exactly, we prove that the DiffMod approach can deliver intensities and pulse shapes that are exact within 10% compared to the Monte-Carlo simulations that require much more computing power. In addition, a time-resolved illustration of the spatial distribution of the neutrons at different energy levels is provided.

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.

Iron oxide nanoparticles are presently considered as main work horses for various applications including targeted drug delivery and magnetic hyperthermia. Several questions remain unsolved regarding the effect of size onto their overall magnetic behavior. One aspect is the reduction of magnetization compared to bulk samples. A detailed understanding of the underlying mechanisms of this reduction could improve the particle performance in applications. Here we use a number of complementary experimental techniques including neutron scattering and synchrotron X-ray diffraction to arrive at a consistent conclusion. We confirm the observation from previous studies of a reduced saturation magnetization and argue that this reduction is mainly associated with the presence of antiphase boundaries, which are observed directly using high-resolution transmission electron microscopy and indirectly via an anisotropic peak broadening in X-ray diffraction patterns. Additionally small-angle neutron scattering with polarized neutrons revealed a small non-magnetic surface layer, that is, however, not sufficient to explain the observed loss in magnetization alone.

Double-stranded DNA (dsDNA) fragments exhibit noncovalent attractive interactions between their tips. It is still unclear how DNA liquid crystal self-assembly is affected by such blunt-end attractions. It is demonstrated that stiff dsDNA fragments with moderate aspect ratio can specifically self-assemble in concentrated aqueous solutions into different types of smectic mesophases on the basis of selectively screening of blunt-end DNA stacking interactions. To this end, this type of attractions are engineered at the molecular level by constructing DNA duplexes where the attractions between one or both ends are screened by short hairpin caps. All-DNA bilayer and monolayer smectic-A type of phases, as well as a columnar phase, can be stabilized by controlling attractions strength. The results imply that the so far elusive smectic-A in DNA rod-like liquid crystals is a thermodynamically stable phase. The existence of the bilayer smectic phase is confirmed by Monte-Carlo simulations of hard cylinders decorated with one attractive terminal site. This work demonstrates that DNA blunt-ends behave as well-defined monovalent attractive patches whose strength and position can be potentially precisely tuned, highlighting unique opportunities concerning the stabilization of nonconventional DNA-based lyotropic liquid crystal phases assembled by all-DNA patchy particles with arbitrary geometry and composition.

By controlling the chemical composition and the spatial organization of nanoparticles, hybrid nanocomposites incorporating ordered arrangements of nanoparticles could be endowed with exotic physical and chemical properties to fulfill demands in advanced electronics or energy-harvesting devices. However, a simple method to fabricate hybrid nanocomposites with precise control of nanoparticle distribution is still challenging. We demonstrate that block copolymer-based nanocomposites containing well-ordered nanoparticles with various morphologies can be readily obtained by adjusting the nanoparticle concentration. Moreover, the structural evolution of nanocomposite thin films as a function of nanoparticle loading is unveiled using grazing-incidence transmission small-angle X-ray scattering and atomic force microscopy. The morphological transformation proceeds through a phase transition from perforated lamellae to in-plane cylinder layout, followed by structural changes. The successful achievement of a variety of morphologies represents an effective and straightforward approach to producing functional hybrid nanocomposites for potential applications in various functional devices.

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.

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

The High-Brilliance Neutron Source project (HBS) aims at developing a medium-flux accelerator-driven neutron source based on a 70 MeV, 100 mA proton accelerator. The concept optimizes the facility such that it provides high-brilliance neutron beams for instruments operating at different time structures. This can be realized by generating an interlaced proton pulse structure, which is unraveled and sent to three different target stations by a multiplexer system. In the following we present the developments of a multiplexer system at the JULIC accelerator at Forschungszentrum Jülich GmbH (FZJ), which serves as test facility for HBS. The main components of the JULIC multiplexer system are designed to be scalable to the HBS parameters.

Advanced materials 33(24), 2008683 (2021). doi:10.1002/adma.202008683, Self‐assembly of iron oxide nanoparticles (IONPs) into 1D chains is appealing, because of their biocompatibility and higher mobility compared to 2D/3D assemblies while traversing the circulatory passages and blood vessels for in vivo biomedical applications. In this work, parameters such as size, concentration, composition, and magnetic field, responsible for chain formation of IONPs in a dispersion as opposed to spatially confining substrates, are examined. In particular, the monodisperse 27 nm IONPs synthesized by an extended LaMer mechanism are shown to form chains at 4 mT, which are lengthened with applied field reaching 270 nm at 2.2 T. The chain lengths are completely reversible in field. Using a combination of scattering methods and reverse Monte Carlo simulations the formation of chains is directly visualized. The visualization of real‐space IONPs assemblies formed in dispersions presents a novel tool for biomedical researchers. This allows for rapid exploration of the behavior of IONPs in solution in a broad parameter space and unambiguous extraction of ​the parameters of the equilibrium structures. Additionally, it can be extended to study novel assemblies formed by more complex geometries of IONPs., Published by Wiley-VCH, Weinheim

The manipulation of magnetic states in nanoparticle supercrystals promises new pathways to design nanocrystalline magnetic materials and devices. Trench-patterned silicon substrates were used as templates to guide the self-assembly of iron oxide nanoparticles. Grazing incidence small angle X-ray scattering shows that the nanoparticles form a long-range ordered structure along the trench direction while in the direction perpendicular to the trenches, no coherent structure is observable. Electron holography provides evidence of an ordered magnetic state of nanoparticle moments in the remanent state after the application of a saturation magnetic field parallel to the trenches. However, a disordered magnetic state was observed in a perpendicular geometry. Hysteresis loops indicate that the nanoparticle moments form a superferromagnetic state for the geometry parallel to the trenches. Memory effect investigations reveal that the disordered magnetic state corresponds to a collective superspin glass state in the perpendicular geometry, while the superferromagnetic state in the parallel geometry suppresses the superspin glass state.

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.

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.

This paper reports on the formation of large area, self assembled, highly ordered monolayers of stearyl alcohol grafted silica nanospheres of ≈50 nm diameter on a silicon substrate based on the drop-casting method. Our novel approach to achieve improved order uses stearyl alcohol as an assistant by adding it to the colloidal NanoParticle (NP) dispersion from which the monolayers are formed. Additionally, a heat treatment step is added, to melt the stearyl alcohol in the monolayer and thereby give the particles more time to further self-assemble, leading to additional improvement in the monolayer quality. The formation of the monolayers is significantly affected by the concentration of the NPs and the stearyl alcohol, the volume of the drop as well as the time of the heat treatment. A high surface coverage and uniform monolayer film of SiO

[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

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

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.

In the present work we demonstrate, how the self-assembly of nanoparticles provides a simple and straightforward way to fabricate ordered nanocomposites on length scales of up to 300…1000 µm. We realize this by employing a centrifugation assisted sedimentation technique. We start with magnetic iron oxide nanoparticles (NPs) with an average diameter of 15 nm, which are dispersed in toluene. The particles are coated with an organic shell to prevent unordered agglomeration of particles. After centrifugation and subsequent drying large macroscopic assemblies of NPs are obtained. The characterization of the samples using scanning electron microscopy (SEM), x-ray diffraction (XRD) and small angle x-ray scattering (SAXS) reveals that the samples are NP macro-polycrystals. By variation of the fabrication parameters as e.g. centrifugation speed and temperature we obtain a systematic study on how NP ordering, crystallinity and morphology depend on the fabrication parameters. Moreover, the magnetic properties of the NP macro-polycrystals are studied using superconducting quantum interference device (SQUID) magnetometry.

Adding shape and interaction anisotropy to a colloidal particle offers exquisitely tunable routes to engineer a rich assortment of complex-architected structures. Inspired by the hierarchical self-assembly concept with block copolymers and DNA liquid crystals and exploiting the unique assembly properties of DNA, we report here the construction and self-assembly of DNA-based soft-patchy anisotropic particles with a high degree of modularity in the system's design. By programmable positioning of thermoresponsive polymeric patches on the backbone of a stiff DNA duplex with linear and star-shaped architecture, we reversibly drive the DNA from a disordered ensemble to a diverse array of long-range ordered multidimensional nanostructures with tunable lattice spacing, ranging from lamellar to bicontinuous double-gyroid and double-diamond cubic morphologies, through the alteration of temperature. Our results demonstrate that the proposed hierarchical self-assembly strategy can be applied to any kind of DNA nanoarchitecture, highlighting the design principles for integration of self-assembly concepts from the physics of liquid crystals, block copolymers, and patchy colloids into the continuously growing interdisciplinary research field of structural DNA nanotechnology.

International Conference on Neutron Optics, NOP2017, Nara, Japan, 5 Jul 2017 - 8 Jul 2017; JPS conference proceedings 22, 011025 (2018). doi:10.7566/JPSCP.22.011025 special issue: "Proceedings of the International Conference on Neutron Optics (NOP2017)", Published by The Physical Society of Japan, Tokyo

[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

A formalism is presented which allows the quantitative evaluation of data from grazing-incidence small-angle neutron and X-ray scattering – GISANS and GISAXS – in the framework of the distorted wave Born approximation. While several aspects have been reported previously, this formalism combines solutions for scattering intensities in both reflection and transmission hemispheres, taking into account instrumental resolution effects. This formalism is applied to the case of GISANS from self-organized diblock copolymers, ordered in perpendicular lamellar structures on an Si wafer in randomly oriented short-range-ordered regions. The periodicity ofD= 85 (9) nm found for deuterated polystyrene–polybutadiene of molecular weight Mw= 165 kg mol−1and a molecular weight fraction of the deuterated polystyrene block of 52% is consistent with atomic force microscopy and specular neutron reflectivity results.

The vacancy distribution of oxygen and its dynamics directly affect the functional response of complex oxides and their potential applications. Dynamic control of the oxygen composition may provide the possibility to deterministically tune the physical properties and establish a comprehensive understanding of the structure-property relationship in such systems. Here, an oxygen-vacancy-induced topotactic transition from perovskite to brownmillerite and vice versa in epitaxial La0.7 Sr0.3 MnO3-δ thin films is identified by real-time X-ray diffraction. A novel intermediate phase with a noncentered crystal structure is observed for the first time during the topotactic phase conversion which indicates a distinctive transition route. Polarized neutron reflectometry confirms an oxygen-deficient interfacial layer with drastically reduced nuclear scattering length density, further enabling a quantitative determination of the oxygen stoichiometry (La0.7 Sr0.3 MnO2.65 ) for the intermediate state. Associated physical properties of distinct topotactic phases (i.e., ferromagnetic metal and antiferromagnetic insulator) can be reversibly switched by an oxygen desorption/absorption cycling process. Importantly, a significant lowering of necessary conditions (temperatures below 100 °C and conversion time less than 30 min) for the oxygen reloading process is found. These results demonstrate the potential applications of defect engineering in the design of perovskite-based functional materials.

A precise control over the meso- and microstructure of ordered and aligned nanoparticle assemblies, i.e., mesocrystals, is essential in the quest for exploiting the collective material properties for potential applications. In this work, we produced evaporation-induced self-assembled mesocrystals with different mesostructures and crystal habits based on iron oxide nanocubes by varying the nanocube size and shape and by applying magnetic fields. A full 3D characterization of the mesocrystals was performed using image analysis, high-resolution scanning electron microscopy and Grazing Incidence Small Angle X-ray Scattering (GISAXS). This enabled the structural determination of e.g. multi-domain mesocrystals with complex crystal habits and the quantification of interparticle distances with sub-nm precision. Mesocrystals of small nanocubes (l = 8.6–12.6 nm) are isostructural with a body centred tetragonal (bct) lattice whereas assemblies of the largest nanocubes in this study (l = 13.6 nm) additionally form a simple cubic (sc) lattice. The mesocrystal habit can be tuned from a square, hexagonal to star-like and pillar shapes depending on the particle size and shape and the strength of the applied magnetic field. Finally, we outline a qualitative phase diagram of the evaporation-induced self-assembled superparamagnetic iron oxide nanocube mesocrystals based on nanocube edge length and magnetic field strength.

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

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.

The structural properties of an epitaxically grown Fe/Cr/Fe trilayer were studied with anomalous X-ray scattering. Two different X-ray energies have been used: (i)E1= 5985 eV to match the maximum contrast of the Fe/Cr interface close to the Cr absorptionKedge; (ii)E2= 6940 eV where the Fe/Cr interface displays the lowest contrast. The specular reflectivity and longitudinal diffuse scans together with ω scans for both energies were measured. The simulations within the frame of the distorted-wave Born approximation allowed a quantitative description of the morphology of each interface. The roughness, Hurst parameter and the thickness of every layer, as well as an oxidation effect at the surface of the sample, are derived. The strength and limitations of the method are discussed.

We used polarized neutron reflectometry for a quantitative study of the field dependent arrangement of the magnetization vector in the different regions of an ion beam imprinted stripe pattern in a magnetic film. For the magnetic patterning of the Co70Fe30 film we took advantage of the exchange bias to an antiferromagnetic Mn83Ir17 layer which was changed locally by He-ion bombardment. The exchange bias was set to be antiparallel in the two different striped regions. We found that after magnetization reversal of half of the stripes the magnetization in neighbouring regions is periodically canted with respect to the stripe axis so that the net magnetization of the ferromagnetic film tums almost perpendicular to the stripes. At the same time the projection of the magnetization vector onto the stripe axis has a periodically alternating sign. (c) 2007 Published by Elsevier Ltd

We investigate an exchange biased polycrystalline IrMn/CoFe sample, measuring along each full magnetization loop as we vary the direction θ of the applied field Ha with respect to the unidirectional anisotropy direction or the field cooling HFC direction. Measurements are done using specular and off-specular polarized neutron scattering for increasing (negative to positive) and decreasing (positive to negative) field sweeping directions of Ha with respect to the negative direction of HFC. For both the angles θ = 45° and 90°, remagnetization behaviours of all ferromagnetic layers occur simultaneously in a uniform mode (via coherent rotation) only. This is in contrast to the nonuniform reversal (via domain wall motion) previously observed by us for θ = 0°. These variations of the relative strengths of the uniaxial and exchange anisotropies are thus found to be responsible for the reversal of magnetization via coherent rotation or via domain wall motion. Interestingly, off-specular spin-flip scattering shows that coherent rotation (where a significant specular spin-flip signal is observed) is accompanied by underlying concomitant in-plane magnetization fluctuations during reversal for both loop branches. These fluctuations—linked to the magnetization reversal—indicate the fluctuations from domain to domain in the system.

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

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.

We report on the magnetization reversal in series of exchange-biased multilayers NiFe(10.0 nm)/[ Ir20Mn80(6.0 nm)/Co80Fe20(3.0 nm)] N studied by specular reflection and off-specular scattering of polarized neutrons. All specimens are sputtered and post-annealed at 530 K (i.e. above the IrMn Neel temperature of 520 K) in Ar atmosphere before cooling to room temperature in the presence of a field of 130 Oe which induces the unidirectional anisotropy. We find HEB is dependent upon the number of bilayers N as it gradually increases from 0.33 kOe for N=1 to a considerably higher value of upto ≈0.9 kOe for N=10. X-ray specular and diffuse scattering data reveal no significant variation of the lateral correlation length and only a weak dependence of the vertical rms interface roughness on N. Atomic and magnetic force microscopy, however, show a strong reduction of the grain size accompanied by distinct changes of the ferromagnetic domain structure. The enhancement of the exchange bias effect is presumably related to the shrinking of the related domain size in the antiferromagnet due to the structural evolution in the multilayers. Polarized neutron reflectometry (PNR) measurements are done at different applied fields sweeping both branches of the hysteresis loop. The spin-flip (SF) cross section of both the N=10 and 3 samples show diffusely scattered intensity appears gradually as the field approaches HEB and is most intense where the net magnetization vanishes. The disappearance of diffuse scattering in saturation indicates that the off-specular intensity is related to the reversal process. The reversal proceeds sequentially starting with the bottom (top) CoFe layer for decreasing (increasing) field and is related to the evolution of the grain size along the stack. The reversal of each CoFe layer is for both field branches due to domain wall motion. Thus as a main result, we observe a sequential and symmetric magnetization reversal in exchange-biased multilayers. The concomitant in-plane magnetization fluctuations revealed by off-specular spin-flip scattering indicate a more complex reversal mechanism than hitherto considered. Moreover, although the grain size decreases from N=3 to 10 by a factor of about four the reversal mechanism remains similar.

The magnetic coupling in Co/Cu multilayers with individual layer thicknesses tuned to the 2nd maximum of the giant magnetoresistance (GMR) is investigated by soft X-ray magnetic scattering. A series of half-integer order magnetic peaks appears in the specularly scattered intensity especially with the higher orders being strongly sensitive to magnetic effects. Diffuse magnetic scattering, performed with polarized neutrons, indicates long-range magnetic correlation in the remanent multilayer state and a corresponding magnetic domain structure sensitive to the magnetic field applied before.

Specular reflection and off-specular scattering of polarized neutron has been used to study the magnetization reversal in polycrystalline [ Ir 20 Mn 80 / Co 80 Fe 20 ] N = 3 exchange-biased multilayers. The reversal proceeds sequentially starting from the bottom (top) CoFe layer for decreasing (increasing) field. This behavior is related to the evolution of the grain size along the stack. The reversal of each CoFe layer is due to domain wall motions for both the decreasing and increasing field branch. The reversals are accompanied by fluctuations of the in-plane magnetization component perpendicular to the external field as evidenced by off-specular spin-flip scattering. The observed reversal mode is very similar to that of [ Ir 20 Mn 80 / Co 80 Fe 20 ] N multilayers with N = 10 although the grain size decreases from N = 3 to 10 by a factor of about four.

We report on the measurement and simulation of the field-dependent polarized neutron reflectivity (specular) and scattering (off-specular) for an antiferromagnetically coupled Co/Cu multilayer (ML) with number of bilayers N = 40 . The first-order Bragg peak is visible in the non-spin flip (NSF) channels representing the bilayer periodicity. We observe off-specular intensity around the AF Bragg peak at the half-order position in both SF and NSF channels. These Bragg sheets are most intense at lower fields and gradually disappearing at higher fields as the system attains saturation. We attribute the Bragg sheets to vertically correlated AF domains along the ML stack which get bigger in size with field strength. Those interpretations are confirmed by simulation of the data within the distorted wave born approximation (DWBA).

Ferromagnetic interlayer coupling through a semiconducting spacer has been studied in epitaxial layered structures Fe/EuS/Fe(001). The coupling strength has been determined as a function of temperature and interlayer thickness using magneto-optical Kerr effect magnetometry. A strong rise of the coupling strength with increasing temperature has been observed in the temperature range, where EuS is paramagnetic. The observations match the theoretical description of interlayer exchange coupling between two metallic layers mediated by the electronic states in a semiconductor. Near the Curie temperature of EuS, long-range ferromagnetic coupling is observed, that is related to ferromagnetic order inside the EuS layer.

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 direction of the magnetization of each Fe layer in an Fe/Cr multilayer with uniaxial anisotropy was determined with polarized neutron reflectometry. The vectors of the layer magnetization of the multilayer transit from an antiferromagnetic alignment into a nearly ferromagnetic one with increasing magnetic field. In the transition region the system consists of an antiferromagnetically aligned part and a ferromagnetically aligned part. The magnetization curve is characterized by the subsequent switching of the antiferromagnetically aligned bilayers into the nearly ferromagnetically aligned state. Via this mechanism the antiferromagnetic part of the multilayer reduces in favor of the ferromagnetically aligned part with increasing magnetic field.

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.

We report on the first results of a structural and magnetic investigation of Fe 1-x Co x /Mn/Fe 1-x Co x trilayers (x = 0-0.75). In situ low energy electron diffraction shows that Mn grows epitaxially on bcc Fe 1-x Co x (x ≤ 0.75) with a good crystallinity up to 30 A. Reflectivity and small angle diffuse scattering of synchrotron radiation reveal sharp interfaces, each of them having two lateral characteristic lengths in the height-height correlation function that differ by one order of magnitude. The long-range height-height correlation function is vertically correlated and the short-range one is not. From magneto-optical Kerr effect measurements, evidence for non-collinear coupling between Fe 1-x Co x layers (x = 0 and 0.5) was found for Mn spacer thicknesses below 12 A. Polarized neutron reflectometry performed at 300 K reveal a net magnetization of 0.8 ± 0.3 μ B /at in the Mn layer in Fe 0.5 Co 0.5 /Mn/Fe 0.5 Co 0.5 and no net magnetization in the Mn layer in Fe/Mn/Fe.