Small-angle neutron scattering

Small-angle neutron scattering (SANS) is an experimental technique that uses elastic neutron scattering at small scattering angles to investigate the structure of various substances at a mesoscopic scale of about 1–100 nm. Small angle neutron scattering is in many respects very similar to small-angle X-ray scattering (SAXS); both techniques are jointly referred to as small-angle scattering (SAS). The most important feature of the SAS method is its potential for analyzing the inner structure of disordered systems, and frequently the application of this method is a unique way to obtain direct structural information on systems with random arrangement of density inhomogeneities in such large-scales. Advantages of SANS over SAXS are its sensitivity to light elements, the possibility of isotope labelling, and the strong scattering by magnetic moments. Technique During a SANS experiment a beam of neutrons is directed at a sample, which can be an aqueous solution, a solid, a po

Visualisation of magnetic domain structures and magnetisation processes in Goss-oriented, high permeability steels using neutron grating interferometry

Benedikt Karl-Josef Gerhard Betz

Industrial transformers cores are built from stacked sheets of an iron silicon alloy, called laminations. The magnetic domain structures of these highly anisotropic electrical steels with a sharp (110)[001]-texture, the so-called Gosstexture, determines the magnetic properties of transformers. Commonly used investigation techniques for these laminations are inductive B-H-measurements. This technique reveals global magnetic properties such as hysteresis, remanence, saturation and losses. Locally resolved information about the underlying domain structure cannot be obtained. In this thesis, the neutron grating interferometer (nGI) is used to investigate the domain structures in Goss-oriented (GO) electrical steels at the Swiss Spallation Neutron Source using the cold neutron imaging facility ICON. In contrast to the attenuation based transmission image, the dark-field image (DFI) is related to multiple refraction of unpolarised neutrons at magnetic domain walls. Thereby the use of the DFI allows for the visualisation of bulk magnetic domain structures in two and three dimensions with a spatial resolution of down to 70 um in a field of view of 64mm x 64mm. The DFI is used for the visualisation of the locally resolved response of the bulk magnetic domain structures under the influence of externally applied magnetic fields. In the first part, the domain formation and growth along the initial magnetisation curve (0-6000 A/m) up to saturation in static DC magnetic fields was studied. For decreasing field values, the visualisation of the recurrence of the hysteretic domain structure down to the remanent (0 A/m) state is given. A correlation of the grain orientation and the corresponding basic and supplementary domain structure is given. In the second part, the DFI is used to investigate the response of magnetic domain walls to dynamic AC magnetic excitations. The visualisation of the domain wall motion under influence of alternating magnetic fields is performed. In detail, scans combining varying levels of an offset DC (0-30 A/m), oscillation amplitude A (0-1500 A/m), and oscillation frequency f (0-200 Hz) are conducted. By increasing the amplitude while maintaining constant values of DC and f, the transition from a frozen domain wall structure to a mobile one is recorded. Vice versa, increasing f while keeping A and DC constant led to the reverse transition from a mobile domain wall structure into a frozen one. It is shown that varying both, A and f shifts the position of transition region. Higher frequencies require higher oscillation amplitudes to overcome the freezing. The DFI allows for the analysis of the laminations coatings impact to the magnetic structures. To visualise the stress effect of the coating to the underlying domain formation an uncoated lamination is investigated with stepwise increasing applied external tensile stresses up to 20MPa. The domain configurations of the intermediate stress states are imaged and the original domain structure of the coated state is reproduced. To verify the results, complementary experiments using Kerr microscopy, Faraday imaging, small-angle neutron scattering and Laue X-ray diffraction were performed. The here presented findings allow for new insights into macromagnetism and enable new approaches in the field of descriptive models for bulk macromagnetism. Furthermore the results have the potential to further improve the properties of GO-steels used in industrial transformer applications.

EPFL2016

Advanced characterization methods for self-assembled organization on nanoparticles

Zhi Luo

The complex organizations of amino acid residues on protein surfaces give rise to a wide space of functions for proteins. Great efforts are devoted in biology to understand such structure-property relationships. Many advanced characterization techniques have been developed for this purpose. Nanomaterials are regarded as a close resemblance to proteins, as they share similar sizes and, when coated with different types of ligand molecules, can present patchy surfaces (e.g. stripe-like, patchy, or Janus) that bear some similarities to the surfaces of proteins. So far, most characterization methods developed for nanoparticles come from classic material sciences and consequently deliver a precise description of the inorganic and well defined core of the nanoparticles. Yet big challenges remain to accurately describe the morphologies of the ligand shell of nanoparticles that is closer to a biological entity rather than small sized materials. This thesis establishes two new techniques for the characterization of the morphology of the ligand shell of nanoparticles. The first stems directly from structural biology, i.e. small angle neutron scattering combined with ab initio reconstructions. 3D models could be obtained by fitting the scattering data, allowing for the quantitative determination of different morphologies. The second one is based on the statistical analysis of the MALDI-TOF MS spectra. Adopting Monte Carlo calculations, the structures of mixed ligands could be visualized by fitting the fragmentation patterns that represents a sampling of the nanoparticle surfaces. Thanks to the advancement in characterization provided by these newly development methods, two phenomena are studied in this thesis. First, the evolution of the ligand shell morphology during a ligand replacement reaction called place-exchange is uncovered for the first time. Also the evolution of the ligand shell morphology upon gentle heating is uncovered. These results provide a new way to control the morphology of the nanoparticle ligand shells. Second, an in-depth study of the water structure around patchy surfaces is presented. Two nanoparticles with the same core size and composition but varying in the ligand shell structures are synthesized. The morphologies of the heterogeneous nanoparticle surfaces on top of chemical nature are thus directly correlated to the structure of interfacial water. This work offers molecular explanation on the structural factor in the interfacial energy of multiple component systems, which has not been discussed in physical chemistry textbooks before. This latter work offers new insights on how patchy nanoparticles could be used as model systems to understand the interfacial properties of proteins, which are often very difficult to be studied directly due to the complex and dynamic structures of the proteins.

EPFL2018

Small-angle neutron scattering study of mesoscale magnetic disordering and skyrmion phase suppression in the frustrated chiral magnet Co6.75Zn6.75Mn6.51

Henrik Moodysson Rønnow, Jonathan White

Co-Zn-Mn chiral cubic magnets display versatile magnetic skyrmion phases, including equilibrium phases stable far above and far below room temperature, and the facile creation of robust far-from-equilibrium skyrmion states. In this system, compositional disorder and magnetic frustration are key ingredients that have profound effects on the chiral magnetism. Reported here are studies of the magnetism in Co6.75Zn6.75Mn6.5 by magnetometry, small-angle neutron scattering (SANS), magnetic diffuse neutron scattering and Lorentz transmission electron microscopy (LTEM). While features in magnetometry and LTEM often give standard indications for skyrmion formation, they are not readily observed from the measurements on this system. Instead, skyrmion lattice correlations are only revealed by SANS, and they are found to form an orientationally disordered structure in a minority fraction of the sample. The majority fraction of the sample always displays orientationally disordered helical spin correlations, which undergo further disordering along the radial direction on cooling below the critical temperature (T-c similar or equal to 102 K). The near-complete suppression of the skyrmion phase, and the process of disordering on cooling, are attributed to competing magnetic interactions that dominate over the ferromagnetic interaction expected to favour chiral magnetism in this system. These competing interactions start to develop above T-c and become further enhanced towards low temperatures. The present observations of co-existing and disordered magnetic correlations over multiple length scales are not unique to Co6.75Zn6.75Mn6.5 but are seemingly common to the family of Co-Zn-Mn compounds with finite Mn, and their accurate description presents a challenge for theoretical modelling. In addition, this study highlights a need for neutron instrumentation capable of the comprehensive measurement of magnetic correlations over expanded ranges of momentum transfer in such multiplelength-scale magnets.

INT UNION CRYSTALLOGRAPHY2022