Investigation of Nanostructure and Interactions in Water-in-Xylene Microemulsions Using Small-Angle X-ray and Neutron Scattering.

The ability to modulate the size, the nanostructure, and the macroscopic properties of water-in-oil microemulsions is useful for a variety of technological scenarios. To date, diverse structures of water-in-alkane microemulsions stabilized by sodium bis(2-ethylhexyl) sulfosuccinate (AOT) have been extensively studied. Even though the decisive parameter which dictates the phase behavior of micremulsions is the nature of the continuous phase, relatively very few reports are available on the structure and interactions in the microemulsions of aromatic oil. Here, we present a fundamental investigation on water-in-xylene microemulsions using small-angle neutron scattering (SANS) at a fixed molar ratio (ω) of water to AOT. We elucidate the microstructural changes in the water-AOT-xylene ternary system at dilute volume fractions (Φ = 0.005, 0.01, 0.03), where the droplet-droplet interactions are absent, to moderately concentrated systems (Φ = 0.05, 0.10, 0.15, and 0.20), where colloidal interactions become important. We also characterize the reverse microemulsions (RMs) for thermally induced microstructural changes at six different temperatures from 20 to 50 °C. Depending on the magnitude of Φ, the scattering data is found to be well described by considering the RMs as a dispersion of droplets (with a Schulz polydispersity) which interact as sticky hard spheres. We show that while the droplet diameter remains almost constant with increase in the volume fraction, the attractive interactions become prominent, much like the trends observed for water-in-alkane microemulsions. With increase in temperature, the RMs showed a marginal decrease in the droplet size but no pronounced dependence on the interactions was observed with the overall structure remaining intact. The fundamental study on a model system presented in this work is key to understanding the phase behavior of multiple component microemulsions as well as their design for applications at higher temperatures, where the structure of most RMs breaks down.

Modulating shape transition in surfactant stabilized reverse microemulsions.

The formation of reverse microemulsions (RMs) of spherical shape in the oil/water/surfactant ternary mixture at high molar ratio of water to surfactant (ω) is well established. Using dynamic light scattering, small-angle X-ray and neutron scattering, we elucidate the formation of non-spherical reverse microemulsions stabilised by sodium bis(2-ethylhexyl) sulfosuccinate (AOT) at ω = 10 and volume fractions of the dispersed phase, Φ, ranging from 0.005 to 0.20. In addition, we propose a strategy to tune the aspect ratio of non-spherical droplets and colloidal interactions by (i) varying the volume fraction of the dispersed phase (ii) changing the temperature, and (iii) by substituting the aliphatic oil with a mixture of aliphatic and aromatic hydrocarbons. This tunability of anisotropy along with a precise control of the interactions in the RMs, their ability to form spontaneously and their thermodynamic stability is crucial to provide a handle on reaction kinetics, synthesis of anisotropic nanoparticles as well as for their application as lubricants and viscosity modifiers.

Toward Understanding Complex Spin Textures in Nanoparticles by Magnetic Neutron Scattering.

In the quest to image the three-dimensional magnetization structure we show that the technique of magnetic small-angle neutron scattering (SANS) is highly sensitive to the details of the internal spin structure of nanoparticles. By combining SANS with numerical micromagnetic computations we study the transition from single-domain to multidomain behavior in nanoparticles and its implications for the ensuing magnetic SANS cross section. Above the critical single-domain size we find that the cross section and the related correlation function cannot be described anymore with the uniform particle model, resulting, e.g., in deviations from the well-known Guinier law. In the simulations we identify a clear signature for the occurrence of a vortexlike spin structure at remanence. The micromagnetic approach to magnetic SANS bears great potential for future investigations, since it provides fundamental insights into the mesoscale magnetization profile of nanoparticles.

The benefits of a Bayesian analysis for the characterization of magnetic nanoparticles.

Magnetic nanoparticles offer a unique potential for various biomedical applications, but prior to commercial usage a standardized characterization of their structural and magnetic properties is required. For a thorough characterization, the combination of conventional magnetometry and advanced scattering techniques has shown great potential. In the present work, we characterize a powder sample of high-quality iron oxide nanoparticles that are surrounded with a homogeneous thick silica shell by DC magnetometry and magnetic small-angle neutron scattering (SANS). To retrieve the particle parameters such as their size distribution and saturation magnetization from the data, we apply standard model fits of individual data sets as well as global fits of multiple curves, including a combination of the magnetometry and SANS measurements. We show that by combining a standard least-squares fit with a subsequent Bayesian approach for the data refinement, the probability distributions of the model parameters and their cross correlations can be readily extracted, which enables a direct visual feedback regarding the quality of the fit. This prevents an overfitting of data in case of highly correlated parameters and renders the Bayesian method as an ideal component for a standardized data analysis of magnetic nanoparticle samples.

Phase-Transfer and Stabilization of Highly Monodisperse Ferrite Nanoparticles into Polar Solvents by Ligand Exchange Synthesis.

Cobalt ferrite nanoparticles with controlled particle size are transferred from nonpolar into polar solvent by exchange of the as-synthesized oleic acid ligand shell with polyacrylic acid (PAA). The nanoparticles are highly monodisperse (σlog < 6%), and the ligand exchange synthesis has no measurable effect on particle size, shape, or size distribution. The stability of the aqueous particle dispersion without significant interparticle correlations as observed using small-angle X-ray scattering confirms the successful phase-transfer.

Uniaxial polarization analysis of bulk ferromagnets: theory and first experimental results.

On the basis of Brown's static equations of micromagnetics, the uniaxial polarization of the scattered neutron beam of a bulk magnetic material is computed. The approach considers a Hamiltonian that takes into account the isotropic exchange interaction, the antisymmetric Dzyaloshinskii-Moriya interaction, magnetic anisotropy, the dipole-dipole interaction and the effect of an applied magnetic field. In the high-field limit, the solutions for the magnetization Fourier components are used to obtain closed-form results for the spin-polarized small-angle neutron scattering (SANS) cross sections and the ensuing polarization. The theoretical expressions are compared with experimental data on a soft magnetic nanocrystalline alloy. The micromagnetic SANS theory provides a general framework for polarized real-space neutron methods, and it may open up a new avenue for magnetic neutron data analysis on magnetic microstructures.

Multiscale magnetization in cobalt-doped ferrite nanocubes.

The magnetization of cobalt ferrite nanocubes of similar size, but with varying Co/Fe ratio, is extensively characterized on atomistic and nanoscopic length scales. Combination of X-ray diffraction, Mössbauer spectroscopy, magnetization measurements and polarized small-angle neutron scattering (SANS) reveals that a lower amount of cobalt leads to an enhanced magnetization. At the same time, magnetic SANS confirms no or negligible near-surface spin disorder in these highly crystalline, homogeneously magnetized nanoparticles, resulting in an exceptionally hard magnetic material with high coercivity.

Robust approaches for model-free small-angle scattering data analysis.

The small-angle neutron scattering data of nanostructured magnetic samples contain information regarding their chemical and magnetic properties. Often, the first step to access characteristic magnetic and structural length scales is a model-free investigation. However, due to measurement uncertainties and a restricted q range, a direct Fourier transform usually fails and results in ambiguous distributions. To circumvent these problems, different methods have been introduced to derive regularized, more stable correlation functions, with the indirect Fourier transform being the most prominent approach. Here, the indirect Fourier transform is compared with the singular value decomposition and an iterative algorithm. These approaches are used to determine the correlation function from magnetic small-angle neutron scattering data of a powder sample of iron oxide nanoparticles; it is shown that with all three methods, in principle, the same correlation function can be derived. Each method has certain advantages and disadvantages, and thus the recommendation is to combine these three approaches to obtain robust results.

Controlling the rotation modes of hematite nanospindles using dynamic magnetic fields.

The magnetic field-induced actuation of colloidal nanoparticles has enabled tremendous recent progress towards microrobots, suitable for a variety of applications including targeted drug delivery, environmental remediation, or minimally invasive surgery. Further size reduction to the nanoscale requires enhanced control of orientation and locomotion to overcome dominating viscous properties. Here, control of the coherent precession of hematite spindles via a dynamic magnetic field is demonstrated using nanoscale particles. Time-resolved small-angle scattering and optical transmission measurements reveal a clear frequency-dependent variation of orientation and rotation of an entire ensemble of non-interacting hematite nanospindles. The different motion mechanisms by nanoscale spindles in bulk dispersion resemble modes that have been observed for much larger, micron-sized elongated particles near surfaces. The dynamic rotation modes promise hematite nanospindles as a suitable model system for field-induced locomotion in nanoscale magnetic robots.

Magnetic nanoprecipitates and interfacial spin disorder in zero-field-annealed Ni50Mn45In5 Heusler alloys as seen by magnetic small-angle neutron scattering.

Shell ferromagnetism is a new functional property of certain off-stoichiometric Ni-Mn-In Heusler alloys, with a potential application in non-volatile magnetic memories and recording media. One key challenge in this field remains the determination of the structural and magnetic properties of the nanoprecipitates that are the result of an annealing-induced segregation process. Thanks to its unique mesoscopic length scale sensitivity, magnetic small-angle neutron scattering appears to be a powerful technique to disclose the microstructure of such annealing-induced nanoprecipitates. In this study, the microstructure of a zero-field-annealed off-stoichiometric Ni50Mn45In5 Heusler alloy is investigated by unpolarized magnetic small-angle neutron scattering. The neutron data analysis reveals a significant spin-misalignment scattering, which is mainly related to the formation of annealing-induced ferromagnetic nanoprecipitates in an antiferromagnetic matrix. These particles represent a source of perturbation which, due to dipolar stray fields, gives rise to canted spin moments in the surroundings of the particle-matrix interface. The presence of anticorrelations in the computed magnetic correlation function reflects the spatial perturbation of the magnetization vector around the nanoprecipitates. The magnetic field dependence of the zero crossing and the minima of the magnetic correlation function are qualitatively explained using the law of approach to ferromagnetic saturation for inhomogeneous spin states. More specifically, at remanence, the nanoprecipitates act magnetically as one superdefect with a correlation length that lies outside the experimental q range, whereas near saturation the magnetization distribution follows each individual nanoprecipitate. Analysis of the neutron data yields an estimated size of 30 nm for the spin-canted region and a value of about 75 nm for the magnetic core of the individual nanoprecipitates.

Using small-angle scattering to guide functional magnetic nanoparticle design.

Magnetic nanoparticles offer unique potential for various technological, biomedical, or environmental applications thanks to the size-, shape- and material-dependent tunability of their magnetic properties. To optimize particles for a specific application, it is crucial to interrelate their performance with their structural and magnetic properties. This review presents the advantages of small-angle X-ray and neutron scattering techniques for achieving a detailed multiscale characterization of magnetic nanoparticles and their ensembles in a mesoscopic size range from 1 to a few hundred nanometers with nanometer resolution. Both X-rays and neutrons allow the ensemble-averaged determination of structural properties, such as particle morphology or particle arrangement in multilayers and 3D assemblies. Additionally, the magnetic scattering contributions enable retrieving the internal magnetization profile of the nanoparticles as well as the inter-particle moment correlations caused by interactions within dense assemblies. Most measurements are used to determine the time-averaged ensemble properties, in addition advanced small-angle scattering techniques exist that allow accessing particle and spin dynamics on various timescales. In this review, we focus on conventional small-angle X-ray and neutron scattering (SAXS and SANS), X-ray and neutron reflectometry, gracing-incidence SAXS and SANS, X-ray resonant magnetic scattering, and neutron spin-echo spectroscopy techniques. For each technique, we provide a general overview, present the latest scientific results, and discuss its strengths as well as sample requirements. Finally, we give our perspectives on how future small-angle scattering experiments, especially in combination with micromagnetic simulations, could help to optimize the performance of magnetic nanoparticles for specific applications.

Clustering in ferronematics-The effect of magnetic collective ordering.

Clustering of magnetic nanoparticles can dramatically change their collective magnetic properties, and it consequently may influence their performance in biomedical and technological applications. Owing to tailored surface modification of magnetic particles such composites represent stable systems. Here, we report ferronematic mixtures that contain anisotropic clusters of mesogen-hybridized cobalt ferrite nanoparticles dispersed in liquid crystal host studied by different experimental methods-magnetization measurements, small-angle X-ray scattering (SAXS), small-angle neutron scattering (SANS), and capacitance measurements. These measurements reveal non-monotonic dependencies of magnetization curves and the Fréedericksz transition on the magnetic nanoparticles concentration. This can be explained by the formation of clusters, whose structures were determined by SAXS measurements. Complementary to the magnetization measurements, SANS measurements of the samples were performed for different magnetic field strengths to obtain information on the orientation of the liquid crystal molecules. We demonstrated that such hybrid materials offer new avenues for tunable materials.

Magnetic Guinier law.

Small-angle scattering of X-rays and neutrons is a routine method for the determination of nanoparticle sizes. The so-called Guinier law represents the low-q approximation for the small-angle scattering curve from an assembly of particles. The Guinier law has originally been derived for nonmagnetic particle-matrix-type systems and it is successfully employed for the estimation of particle sizes in various scientific domains (e.g. soft-matter physics, biology, colloidal chemistry, materials science). An important prerequisite for it to apply is the presence of a discontinuous interface separating particles and matrix. Here, the Guinier law is introduced for the case of magnetic small-angle neutron scattering and its applicability is experimentally demonstrated for the example of nanocrystalline cobalt. It is well known that the magnetic microstructure of nanocrystalline ferromagnets is highly nonuniform on the nanometre length scale and characterized by a spectrum of continuously varying long-wavelength magnetization fluctuations, i.e. these systems do not manifest sharp interfaces in their magnetization profile. The magnetic Guinier radius depends on the applied magnetic field, on the magnetic interactions (exchange, magnetostatics) and on the magnetic anisotropy-field radius, which characterizes the size over which the magnetic anisotropy field is coherently aligned into the same direction. In contrast to the nonmagnetic conventional Guinier law, the magnetic version can be applied to fully dense random-anisotropy-type ferromagnets.

Sub-millisecond time-resolved small-angle neutron scattering measurements at NIST.

Instrumentation for time-resolved small-angle neutron scattering measurements with sub-millisecond time resolution, based on Gähler's TISANE (time-involved small-angle neutron experiments) concept, is in operation at NIST's Center for Neutron Research. This implementation of the technique includes novel electronics for synchronizing the neutron pulses from high-speed counter-rotating choppers with a periodic stimulus applied to a sample. Instrumentation details are described along with measurements demonstrating the utility of the technique for elucidating the reorientation dynamics of anisometric magnetic particles.

Probing the stability and magnetic properties of magnetosome chains in freeze-dried magnetotactic bacteria.

Magnetospirillum gryphiswaldense biosynthesize high-quality magnetite nanoparticles, called magnetosomes, and arrange them into a chain that behaves like a magnetic compass. Here we perform magnetometry and polarized small-angle neutron scattering (SANS) experiments on a powder of freeze-dried and immobilized M. gryphiswaldense. We confirm that the individual magnetosomes are single-domain nanoparticles and that an alignment of the particle moments along the magnetic field direction occurs exclusively by an internal, coherent rotation. Our magnetometry results of the bacteria powder indicate an absence of dipolar interactions between the particle chains and a dominant uniaxial magnetic anisotropy. Finally, we can verify by SANS that the chain structure within the immobilized, freeze-dried bacteria is preserved also after application of large magnetic fields up to 1 T.

Magnetic ordering of the martensite phase in Ni-Co-Mn-Sn-based ferromagnetic shape memory alloys.

The magnetic state of low temperature martensite phase in Co-substituted Ni-Mn-Sn-based ferromagnetic shape memory alloys (FSMAs) has been investigated, in view of numerous conflicting reports of occurrences of spin glass (SG), superparamagnetism (SPM) or long range anti-ferromagnetic (AF) ordering. Combination of DC magnetization, AC susceptibility and small angle neutron scattering (SANS) studies provide clear evidence for AF order in the martensitic phase of Ni45Co5Mn38Sn12 alloy and rule out SPM and SG orders. Identical studies on another alloy of close composition, Ni44Co6Mn40Sn10, point to the presence of SG order in the martensitic phase and the absence of SPM behavior, contrary to earlier reports. SANS results do show the presence of nanometre-sized clusters but they are found to grow in size from 3 nm at 30 K to 11 nm at 300 K, and do not correlate with magnetism in these alloys.

Using the singular value decomposition to extract 2D correlation functions from scattering patterns.

The truncated singular value decomposition (TSVD) is applied to extract the underlying 2D correlation functions from small-angle scattering patterns. The approach is tested by transforming the simulated data of ellipsoidal particles and it is shown that also in the case of anisotropic patterns (i.e. aligned ellipsoids) the derived correlation functions correspond to the theoretically predicted profiles. Furthermore, the TSVD is used to analyze the small-angle X-ray scattering patterns of colloidal dispersions of hematite spindles and magnetotactic bacteria in the presence of magnetic fields, to verify that this approach can be applied to extract model-free the scattering profiles of anisotropic scatterers from noisy data.

Morphological and crystallographic orientation of hematite spindles in an applied magnetic field.

The magnetic response of spindle-shaped hematite (α-Fe2O3) nanoparticles was investigated by simultaneous small-angle and wide-angle X-ray scattering (SAXS/WAXS) experiments. The field-dependent magnetic and nematic order parameters of the magnetic single-domain nanospindles in a static magnetic field are fully described by SAXS simulations of an oriented ellipsoid with the implemented Langevin function. The experimental scattering intensities of the spindle-like particles can be modeled simply by using the geometrical (length, radius, size distribution) and magnetic parameters (strength of magnetic field, magnetic moment) obtained from isotropic SAXS and macroscopic magnetization measurements, respectively. Whereas SAXS gives information on the morphological particle orientation in the applied field, WAXS texture analysis elucidates the atomic scale orientation of the magnetic easy direction in the hematite crystal structure. Our results strongly suggest the tendency for uniaxial anisotropy but indicate significant thermal fluctuations of the particle moments within the hematite basal plane.

Transverse and longitudinal spin-fluctuations in INVAR Fe0.65Ni0.35.

The presence of spin-fluctuations deep within the ordered state of ferromagnetic [Formula: see text] alloy [Formula: see text] has long been suspected but seldom directly observed. Inhomogeneities of one type or another have been cited as important in stabilizing [Formula: see text] behaviour-either longitudinal spin-fluctuations associated with the [Formula: see text]-state (local environment) model or transverse magnetisation arising from non-collinear spin structures. In this study we employ small-angle neutron scattering with neutron polarization analysis to distinguish between the two possibilities. Surprisingly we in fact find evidence of dominant but uncorrelated longitudinal spin-fluctuations coexisting with transverse magnetisation which exists in short-range clusters of size ~[Formula: see text]. This finding supports recent first principles calculations of [Formula: see text] in which both longitudinal spin-fluctuations and magnetic short-range order are identified as important ingredients in reproducing the equilibrium [Formula: see text] lattice.