Small-angle Neutron Scattering Signal Research Articles

Long-wavelength correlations in ferromagnetic titanate pyrochlores as revealed by small-angle neutron scattering

We have carried out small angle neutron scattering measurements on single crystals of two members of the family of cubic rare-earth titanate pyrochlores that display ferromagnetic Curie-Weiss susceptibilities,Yb$_2$Ti$_2$O$_7$ and Ho$_2$Ti$_2$O$_7$. Ho$_2$Ti$_2$O$_7$ is established as displaying a prototypical classical dipolar spin ice ground state, while Yb$_2$Ti$_2$O$_7$ has been purported as a candidate for a quantum spin ice ground state. While both materials have been well studied with neutron scattering techniques, neither has been previously explored in single crystal form with small angle neutron scattering (SANS). Our results for Yb$_2$Ti$_2$O$_7$ show distinct SANS features below its $\Theta_{CW}$$\sim$ 0.50 K, with rods of diffuse scattering extending along $\langle 111 \rangle$ directions in reciprocal space, off-rod scattering which peaks in temperature near $\Theta_{CW}$, and quasi-Bragg scattering at very small angles which correlates well with T$_C$ $\sim$ 0.26 K. The quasi-Bragg scattering corresponds to finite extent ferromagnetic domains $\sim$ 140 \AA$~$ across, at the lowest temperatures. We interpret the $\langle 111\rangle$ rods of diffuse scattering as arising from domain boundaries between the finite-extent ferromagnetic domains. In contrast the SANS signal in Ho$_2$Ti$_2$O$_7$ is isotropic within the (HHL) plane around $\textbf{Q}$=0. However the strength of this overall SANS signal has a temperature dependence resembling that of the magnetic heat capacity, with a peak near 3 K. Below the break between the field-cooled and the zero-field cooled susceptibility in Ho$_2$Ti$_2$O$_7$ at $\sim$ 0.60 K, the SANS signal is very low, approaching zero.

Intermicellar Interactions and the Viscoelasticity of Surfactant Solutions: Complementary Use of SANS and SAXS.

In ionic surfactant micelles, basic interactions among distinct parts of surfactant monomers, their counterion, and additives are fundamental to tuning molecular self-assembly and enhancing viscoelasticity. Here, we investigate the addition of sodium salicylate (NaSal) to hexadecyltrimethylammonium chloride and bromide (CTAC and CTAB) and 1-hexadecylpyridinium chloride and bromide (CPyCl and CPyBr), which have distinct counterions and headgroup structures but the same hydrophobic tail. Different contrasts are obtained from small-angle neutron scattering (SANS), which probes differences between the nucleus of atoms, and X-rays SAXS, which probes differences in electron density. If combined, this contrast allows us to define specific intramicellar length scales and intermicellar interactions. SANS signals are sensitive to the contrast between the solvent (D2O) and the hydrocarbonic tails in the micellar core (hydrogen), and SAXS can access the inner structure of the polar shell because the headgroups, counterions, and penetrated salt have higher electron densities compared to the solvent and to the micellar core. The number density, intermicellar distances, aggregation number, and inter/intramicellar repulsions are discussed on the basis of the dependence of the structure factor and form factor on the micellar aggregate morphology. Therefore, we confirm that micellar growth can be tuned by variations in the flexibility and size of the the headgroup as well as the ionic dissociation rate of its counterion. Additionally, we show that the counterion binding is even more significant to the development of viscoelasticity than the headgroup structure of a surfactant molecule. This is a surprising finding, showing the importance of electrostatic charges in the self-assembly process of ionic surfactant molecules.

Magnetic SANS study of a sintered Nd–Fe–B magnet: Estimation of defect size

The magnetic microstructure of an isotropic sintered Nd–Fe–B permanent magnet was studied at room temperature by means of unpolarized magnetic-field-dependent small-angle neutron scattering (SANS). Although the microstructure of the material consists of micron-sized Nd2Fe14B crystallites that are separated by a (low volume fraction) nanometer-sized Nd-rich phase, the SANS signal contains a significant contribution due to spin misalignment, which is evidenced by an enhanced scattering signal along the direction of the applied field. The origin of this spin misalignment is presumably related to the perturbing effect of the Nd-rich interphases. Analysis of the SANS data in terms of the autocorrelation function of the spin misalignment provides an almost field-independent length scale lC≅40nm, which, for soft magnetic materials, is usually taken as a measure for the size of inhomogeneously magnetized regions around lattice imperfections. Comparison of our results with a phenomenological expression for lC based on micromagnetic theory suggests that the large anisotropy field of the Nd2Fe14B phase (which is of the order of 8T) reduces the size of gradients in the spin microstructure, so that, for hard magnetic materials, lC represents the (field-independent) size of the characteristic defect in the microstructure.

Structural and magnetic properties of amorphous iron oxide

The structural and magnetic properties of amorphous Fe 2 O 3 have been studied by polarized neutron small angle scattering, transmission electron microscopy (TEM) and dc magnetization techniques. The small angle neutron scattering (SANS) study shows two different lognormal distributions of particle sizes with mean diameters of ∼ 4.14 and 1.21 nm with standard deviations 0.33 and 0.40, respectively, and the structure factor corresponds to a mass fractal with a fractal dimension of 2.42. A short-range crystalline nature for these nanoparticles has been confirmed from the high resolution TEM study. The magnetic field and temperature dependent magnetization has been found to scale very well with the SANS signal. It is evident from the field dependent polarized SANS study that the spin clusters do not grow in size under applied field; rather a larger spin alignment occurs under field.

Alternative non-aqueous water-miscible solvents for surfactants

Adsorption and aggregation properties of model non-ionic C i E 8 surfactants (octaethyleneoxide mono- n-alkyl ethers C 12E 8, C 14E 8 and C 16E 8) were studied in different solvents: water, water/ethylene glycol and water/propylene glycol mixtures, as well as pure ethylene and propylene glycol. Significant changes in critical micellar concentrations, surface tensions and surface excess were observed using surface tension, as a function of solvent type and surfactant tail length. Aggregation structures in these solvents were studied using small-angle neutron scattering (SANS). The SANS results were consistent with ellipsoidal, or cylindrical micelles, depending on solvent. Importantly, pure ethylene glycol is identified as a solvent in which C i E 8 surfactants exhibit classic behavior, similar to that which has been well documented in water. On the other hand, C 12E 8 exhibits only very weak surfactant properties in pure propylene glycol: surface tension depressions were very small, and SANS signals were weak, suggesting only minimal adsorption and aggregation. Hence, surfactant behavior of a model non-ionic like C 12E 8 is strongly dependent on the type of glycol.

Medium Range Structure of Hydrogenated Amorphous Ti84Si16

Abstract Amorphous melt-spun Ti84Si16 was investigated by small angle neutron scattering (SANS) in the blank state as well as after loading with hydrogen or deuterium. The integrated intensity and the fractal dimension of the SANS signal change strongly with increasing H, D content. A model is proposed which explains the changes of the scattering signal. By means of this model it is concluded that melt-spun amorphous Ti84Si16 consists of two types of regions. The first type contains pure Ti and the composition of the second type is Ti80Si20-The inner surfaces between these types of regions have fractal features.

Performance of a New Small-Angle Neutron Scattering Instrument at the Malaysian TRIGA Reactor

The set-up and alignment of a new small-angle neutron scattering (SANS) instrument, installed at the 1 MW light-water-moderated MINT TRIGA research reactor, are described. The wavelength distribution and the flux at the sample position have been determined. First neutron scattering measurements were made on two reference samples with strong scattering power; the results prove that the SANS signal is well reproduced on the instrument when samples of typical size are used, despite the high level of the background of fast and epithermal neutrons.

Microstructural Characterization of Materials for Nuclear Applications Using Small-Angle Neutron Scattering

The results of a small-angle neutron scattering (SANS) investigation of microstructural evolution in martensitic steels, developed for use in future fusion reactors, are presented. Specifically, modified martensitic steel DIN 1.4914 (MANET type), subjected to different thermal treatments, was studied. The separate analysis of the size distribution functions determined from the nuclear and magnetic SANS components provides information on the presence of different inhomogeneities: large carbides and very small (~1–nm) C–Cr elementary aggregates. In the latter case, since they are such small particles at the resolution limit of the technique, the general problem of extracting their SANS signal from the background has been addressed.

Temperature-Dependent Structural Changes of Asphaltenes in 1-Methylnaphthalene

Small angle neutron scattering (SANS) was used to investigate the structural changes of a 5 wt % asphaltene solution in perdeuterated 1-methylnaphthalene (1MN-d 10 ) as a function of temperature. A special stainless steel cell was constructed and used for the measurements. The SANS data measured at various temperatures from 20 to 400 °C show that the scattering intensity continuously decreases with increasing temperature. The data at 20 °C suggested rodlike morphology for the particles, and a nonlinear curve in the Guinier plot implied polydispersity in their sizes. Maximum entropy analysis using the form factor for a cylinder allowed extraction of particle size distributions in the radius and length space. At 20 °C the asphaltenes self-associate in 1MN-d 10 forming long rod-shaped particles whose radius was around 18 A but vary in length over 500 A. At 50 °C these aggregates break down, as evidenced by the decrease in signal intensity and the radius of gyration. In the temperature range of 100-320 °C the maximum length of the particles decreases and the polydispersity varies in both the radius and length dimensions. Between 340 and 400 °C, the particles become smaller having a spherical shape with a radius around 12 A. Upon returning the sample to 20 °C, the SANS signal was too weak to derive any structural information, implying irreversible thermochemistry. We also carried out similar studies on the deasphalted oil (DAO) in 1MN-d 10 at 20, 100, and 200 °C, but the SANS signals were very weak and no changes in the scattering behavior were seen as a function of temperature.

Dodecylsulfate-induced dissociation of human alpha2-macroglobulin. An investigation using small-angle neutron scattering and the equilibrium dialysis technique

The dodecylsulfate-induced dissociation of the tetrameric alpha 2-macroglobulin molecule from human plasma has been investigated by the small-angle neutron scattering (SANS) method. The great advantage with the SANS method is that, by using deuterated dodecylsulfate, and contrast variation by changing the D2O/H2O ratio of the solvent, we can selectively study just the protein part, or the dodecylsulfate part, of the protein-dodecylsulfate complex. More than a thousandfold excess of dodecylsulfate (on a molar basis) is needed in order to dissociate alpha 2-macroglobulin to particles with, on average, half the original molecular mass. By combining the SANS data with results obtained by the equilibrium dialysis technique it follows that, under these circumstances, approximately one thousand dodecylsulfate molecules are associated per alpha 2-macroglobulin molecule. From the significant increase in the radius of gyration, which accompanies the dissociation process, we can conclude that the dissociation is associated with a drastic change in conformation of the protein molecule. From measurements where the dodecylsulfate part of the complex dominates the SANS signal we also get an indication that the dodecylsulfate is randomly distributed along the polypeptide chain, rather than being arranged in large clusters at certain regions of the protein molecule. By fitting the parameters of a binding model to the experimental data we obtain the result that most of the more than one thousand bound dodecylsulfate molecules, necessary for dissociation, are involved in the change in conformation, and the dissociation process is, in fact, driven by the binding of a very few extra dodecylsulfate molecules to the dissociation products. These data indicate that the dodecylsulfate-induced dissociation of alpha 2-macroglobulin is probably more complicated than just breaking, for instance, a hydrophobic interaction.