Small-angle neutron scattering (SANS) is one of the most important techniques for microstructure determination, being utilized in a wide range of scientific disciplines, such as materials science, physics, chemistry, and biology. The reason for its great significance is that conventional SANS is probably the only method capable of probing structural inhomogeneities in the bulk of materials on a mesoscopic real-space length scale, from roughly 1 − 300 nm. Moreover, the exploitation of the spin degree of freedom of the neutron provides SANS with a unique sensitivity to study magnetism and magnetic materials at the nanoscale. As such, magnetic SANS ideally complements more real-space and surface-sensitive magnetic imaging techniques, e.g., Lorentz transmission electron microscopy, electron holography, magnetic force microscopy, Kerr microscopy, or spinpolarized scanning tunneling microscopy. In this review article we summarize the recent applications of the SANS method to study magnetism and magnetic materials. This includes a wide range of materials classes, from nanomagnetic systems such as soft magnetic Fe-based nanocomposites, hard magnetic Nd−Fe−B-based permanent magnets, magnetic steels, ferrofluids, nanoparticles, and magnetic oxides, to more fundamental open issues in contemporary condensed matter physics such as skyrmion crystals, noncollinar magnetic structures in noncentrosymmetric compounds, magnetic/electronic phase separation, and vortex lattices in type-II superconductors. Special attention is paid not only to the vast variety of magnetic materials and problems where SANS has provided direct insight, but also to the enormous progress made regarding the micromagnetic simulation of magnetic neutron scattering.

Grazing incidence small-angle scattering and electron microscopy have been used to show for the first time that nonspherical nanoparticles can assemble into highly ordered body-centered tetragonal mesocrystals. Energy models accounting for the directionality and magnitude of the van der Waals and dipolar interactions as a function of the degree of truncation of the nanocubes illustrated the importance of the directional dipolar forces for the formation of the initial nanocube clusters and the dominance of the van der Waals multibody interactions in the dense packed arrays.

By means of polarized small-angle neutron scattering, we have resolved the long-standing challenge of determining the magnetization distribution in magnetic nanoparticles in absolute units. The reduced magnetization, localized in non-interacting nanoparticles, indicates strongly particle shapedependent surface spin canting with a 0.3(1) and 0.5(1) nm thick surface shell of reduced magnetization found for ∼9 nm nanospheres and ∼8.5 nm nanocubes, respectively. Further, the reduced macroscopic magnetization in nanoparticles results not only from surface spin canting, but also from drastically reduced magnetization inside the uniformly magnetized core as compared to the bulk material. Our microscopic results explain the low macroscopic magnetization commonly found in nanoparticles.

Synthesis and structural characterization of a turbostratically disordered polymorph of (PbSe)1.18(TiSe2)2 is reported. The structure of this compound consists of an intergrowth between one distorted rock salt structured PbSe bilayer and two transition metal dichalcogenide structured Se–Ti–Se trilayers. In addition to the lattice mismatch, there is extensive rotational disorder between these constituents. The electrical resistivity of (PbSe)1.18(TiSe2)2 is a factor of 9 lower at room temperature, and the Seebeck coefficient is almost double that reported for the crystalline misfit layered compound analogue.

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The synthesis and characterization of a new layered compound with the composition (PbSe)1·16TiSe2 in thin-film form is reported in this study. The structure of the new compound was characterized by specular and in-plane synchrotron x-ray diffraction studies, which indicate that the compound can be described as a layered intergrowth of PbSe and TiSe2 in which the individual constituents are precisely layered yet rotationally (turbostratically) disordered with an average in-plane domain size in the order of 10 nm. In contrast to crystalline (PbSe)1·16(TiSe2)2 prepared by solid-state reaction at high temperature, the electrical resistivity in the range 20–300 K is nearly temperature independent. The Seebeck coefficient at room temperature was measured to be S = −66(1) μV/K at the carrier concentration of n = 2·1(5) × 1021 cm−3, indicating behavior characteristic of a heavily doped semiconductor. The electrical transport properties for the (PbSe)1·16TiSe2 compound are compared and contrasted to those of other misfi t-layered and turbostratically disordered (MX)1+δ(TX2)n compounds.

The mesostructure of ordered arrays of anisotropic nanoparticles is controlled by a combination of packing constraints and interparticle interactions, two factors that are strongly dependent on the particle morphology. We have investigated how the degree of truncation of iron oxide nanocubes controls the mesostructure and particle orientation in drop cast mesocrystal arrays. The combination of grazing incidence small-angle X-ray scattering and scanning electron microscopy shows that mesocrystals of highly truncated cubic nanoparticles assemble in an fcc-type mesostructure, similar to arrays formed by iron oxide nanospheres, but with a significantly reduced packing density and displaying two different growth orientations. Strong satellite reflections in the GISAXS pattern indicate a commensurate mesoscopic superstructure that is related to stacking faults in mesocrystals of the anisotropic nanocubes. Our results show how subtle variation in shape anisotropy can induce oriented arrangements of nanoparticles of different structures and also create mesoscopic superstructures of larger periodicity.

Since the initial observation of melting-point reduction with size for gold nanoparticles, [1] the relative stability of crystalline phases has also been demonstrated to be size-dependent in a number of technologically important materials, including CdSe [2] and CdS, [3] Al 2 O 3 , [4] and various other metal oxides [5] and chalcogenides. [6] As the surface free energy contribution to the total free energy becomes increasingly important as the size of a system is decreased, a crystalline phase with lower surface free energy may be favored with respect to the thermodynamically stable bulk phase when the crystallite size is smaller than a critical value in one or more dimensions. [2][3][4][5][6][7] The role of size as an effective thermodynamic parameter is of fundamental importance, but also provides a mechanism for controlling the crystal structure of a material, and therefore its properties.Although precise control of surface chemistry and nanocrystal size in well-defined material systems is prerequisite to understanding and controlling size-induced phenomena, it nevertheless remains challenging to achieve. Although impressive progress has been made in the preparation of ensembles of inorganic nanocrystals with relatively narrow size distributions, [8] the preparation of nanocrystal ensembles of completely uniform size that can be tuned with atomic precision would constitute a significant synthetic advance, in particular for the application of size-dependent structural and physical properties. Using a combination of experimental and computational techniques, we demonstrate herein that chemically designed nanolaminates, consisting of an intergrowth of chemically and structurally distinct components, comprise a class of materials in which this level of control can be achieved for one crystallographic direction. As a consequence of the ability to precisely control size, the crystal structure of the components can be tuned via a size-induced transformation. In contrast to epitaxial superlattices, which experience structural distortions due to strain induced by epitaxial interfaces, [9] the intergrowths reported on in the present work lack an epitaxial relationship between the components. This structural independence of the constituents allows the effect of size on crystal structure to be delineated from strain effects.It was recently discovered [10] that [(MSe) 1+d ] m [TSe 2 ] n intergrowths can be prepared by the modulated elemental reactants (MER) synthetic route.[11] Here M = {Pb, Sn, Bi, Ce}, T is an early transition metal, and the integers m and n denote the number of consecutive layers of the respective components in the repeating unit of the intergrowth (Figure 1). The value of d reflects the difference in the inplane atomic packing density for the independent component structures in the intergrowth (hereafter we use the designation [MSe] m [TSe 2 ] n for convenience).We chose to explore the SnSe-MoSe 2 system for several reasons. Along with the prospect of developing novel nanocrystalline SnSe materials fo...