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We have used the neutron spin-echo technique to measure the small energy change of neutrons which are diffracted by a moving vortex lattice in a low-pinning Nb-Ta superconducting sample. A transport current was passed in the mixed state to cause flux line movement. In the case of uniform motion, the flux velocity ${\mathbf{v}}_{L}$ was given as expected by the values of electric and magnetic fields, via $\mathbf{E}\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}\ensuremath{-}{\mathbf{v}}_{L}\ensuremath{\wedge}\mathbf{B}$. We show that with a nonuniformly moving vortex lattice, one can measure the dispersion of the velocities, opening up new possibilities for investigating moving vortex lines.

We report further on an investigation of magnetic vortex lattice (VL) structures in a single crystal of pure niobium with the magnetic field applied along a fourfold symmetry axis. New VL structures are observed, all of which spontaneously break some crystal symmetry. This symmetry-breaking causes multiple degenerate VL domains to exist within the sample. Through small rotations of the sample away from the fourfold axis, we can lift the degeneracy and associate diffraction spots with VL domains.

We have studied the adsorption of lysozyme layers at a hydrophobic silicon water interface using specular neutron reflection. The hydrophobic surface was obtained by self-assembly of a densely packed monolayer of octadecyltrichlorosilane (OTS) onto the natural silica layer on the smooth surface of a (111) silicon block. The effect of pH on the adsorbed lysozyme layer was examined at a constant lysozyme concentration of 0.03 g dm-3 and at a constant ionic strength of 0.02 M. Reflectivity profiles at different pH show that adsorption is irreversible with respect to pH, the composition and structure of the final layer being dependent on the route by which the pH was achieved. The adsorbed protein layer was found to divide into approximately two regions, a densely packed thin layer next to the OTS surface and a diffuse thicker layer extending into the bulk solution. None of the dimensions of this structure corresponds to those of the globular protein in solution, suggesting that, unlike its adsorption at the hydrophilic silica/water interface, lysozyme is denatured at the OTS/water surface. The irreversible adsorption is explained by the combined interaction of the hydrophobic attraction of the hydrophobic fragments in lysozyme to the OTS surface and electrostatic repulsion within the adsorbed layer. The hydrophobic surface induces the exposure of hydrophobic fragments from the lysozyme assembly. The thickness of the dense layer suggests that the denatured protein adsorbs in the form of peptide chains with the hydrophobic amino acid side chains attached to the OTS surface with the hydrophilic side chains extending into the bulk solution. Since lysozyme is more stable at pH 7 than at pH 4, the difference in initial adsorption is dominated by the greater relative stability of lysozyme to denaturation at the higher pH. A change of pH from 7 to 4 reduces the stability of the protein to unfolding and results in more adsorption than when the pH is changed in the opposite direction. Solution pH also affects the net charges within the hydrophilic tail region and the structural distribution of the tail region was found to vary with pH. Copyright 1998 Academic Press.

Systematic measurements of the magnetic moment per Ni atom in Cu/Ni/Cu/Si(001) structures have been made using polarized neutron reflection (PNR) for Ni thicknesses in the range 30Å, , perpendicular anisotropy strength, and surface in-plane Ni lattice constant, respectively, during epitaxial growth. RHEED measurements show that the in-plane lattice constant falls by 1.7% in the Ni thickness range 30 Å, , and total moment per atom versus Ni thickness as found for the total moment by PNR. Polar MOKE measurements confirmed the transition from a perpendicular easy axis towards an in-plane magnetic easy axis as has already been extensively studied in the literature. Comparison of the PNR results with RHEED measurements reveal a striking correlation between the increase of in-plane strain and reduction in magnetic moment per atom with decreasing Ni thickness. While a direct strain-induced variation of the moment based on bulk phase calculations cannot account for the magnitude of the moment variations we observe, we show that the results cannot be attributed to sample contamination, interdiffusion, or a reduction of the Curie temperature with decreasing Ni thickness. Furthermore, the presence of a magnetically dead layer in the samples is not consistent with the PNR results. The strong moment variation partially explains the large thickness range for which perpendicular anisotropy is observed in this system.

The drying process of solution cast statistical copolymer films is recorded in situ with neutron reflectometry as a function of molecular weight. P(nBA-stat-MA) with a majority of n-butyl acrylate (nBA) and a minority of methyl acrylate (MA) is dissolved in deuterated toluene and the polymer dries out due to the evaporation of toluene. The evolution of the depth dependence of the near-surface solvent concentration profile is monitored in situ. The detected kinetics deviates from a simple case II diffusion. For all probed samples we find an acceleration of the solvent evaporation for the freshly cast films which then slows down towards an equilibrium stage. With increasing molecular weight the maximum evaporation velocity decreases and equilibrium is reached later. The solvent concentration in the near-surface region is constant as a function of distance from the film–air interface except at times near the maximum evaporation speed, where solvent enrichment at the surface is found. The surface roughness depends neither on the solvent content nor on the chain length of the statistical copolymer. In the final dried films, independent of the molecular weight, a very small amount of solvent remains. Gravimetric experiments concerning the weight loss during drying show that the near-surface kinetics is different from its bulk behavior.

Small Angle Neutron Scattering has been used to investigate the structure of the flux line lattice in the isotropic ( K , Ba ) BiO 3 high T c compound. The flux lines are well ordered into a hexagonal lattice at low field ( H H e ∼ 0.6 T ⪡ H g (0) where H g is the second order vortex glass transition field) but dislocations become energetically favorable above H e leading to a glassy structure. Moreover, the orientationnal order is partially lost in the vicinity of the entanglement transition but can be recovered by slightly oscillating the field during the cooling process.

We have used off-specular neutron reflectometry to study the rupture of a thin, deuterated polystyrene layer confined between a silicon substrate and a thick poly(methyl methacrylate) upper layer. After total rupture of the buried layer, the resulting roughness at the interface that controls the total reflection edge gives rise to Yoneda off-specular scattering. This hypothesis is confirmed by specular reflectivity data that can be fitted to an effective error function subsequent to the rupture of the buried layer. The observed increase in the Yoneda peak intensities, integrated along the path of constant momentum transfer qz, can be directly related to the formation of an interdigitated structure upon rupture of the buried layer. A distorted-wave Born approximation is used to quantitatively simulate the form of the off-specular data in terms of an equivalent error function, with fitting parameters in good agreement with the physical properties of the samples and key experimental results.

We describe small angle neutron scattering measurements of the vortex structure within a current-carrying wire in the presence of a longitudinal magnetic field. This situation involves the combined effects of the applied field and the self-field of the current and has attracted a great deal of theoretical and experimental work as exotic helical vortex structures have been predicted. We find that the voltage characteristics, a complete absence of spreading of the Bragg peaks and loss of scattered intensity with increasing current are inconsistent with a helical vortex state. Our data are qualitatively consistent with a model where the crossing of self-field loops with vortex lines results in a ring of kinks leaving the remaining longitudinal lattice unperturbed.

We report on the first small-angle neutron scattering measurements from the flux line lattice (FLL) in the high-Tc cuprate superconductor Sr0.9La0.1CuO2. Using a polycrystalline sample, the scattered intensity decreases monotonically with scattering angle away from the undiffracted beam, independently of the azimuthal angle around the beam. The absence of clear peaks in the intensity suggests the establishment of a highly disordered FLL within the grains. We find that the intensity distribution may be represented by the form factor for a single flux line in the London approximation, with some contribution from crystal anisotropy. Most interestingly however, we find that, over the observed field range, the temperature dependence of the diffracted intensity is best represented by s-wave pairing, with lower limits of the gap values being very similar to the Bardeen–Cooper–Schrieffer value of Δ(0) = 1.76 kBTc. However, a qualitative consideration of corrections to the observed intensity suggests that these gap values are likely to be higher, implying strong-coupling behaviour.

A micromagnetic simulation of the magnetic morphology of CoCrPtB based longitudinal recording media is used to model small-angle neutron scattering data and provide a quantitative description of the local flux density in these nanostructured magnetic materials. The model addresses several aspects of the experimental measurements and, in particular, explains an unusual increase in scattering along the applied field direction for certain values of the scattering vector q. The increased scattering along the field direction results in a change of the phase of the angular variation of the scattering by 90∘. This 90∘ change in scattering phase arises naturally from the influence of stray fields when modeling the media as a heterogeneous alloy consisting of grains with a high saturation magnetization core and a permeable shell.

Small-angle neutron scattering SANS has been used to measure the local magnetic structure of perpendicular media consisting of granular CoCrPt– SiOx thin films. The dimensions of the magnetic structures determined by SANS are consistent with the physical grain sizes suggested by transmission electron microscopy measurements, but yield additional information on magnetic structure within the grains, including the existence of a much smaller magnetic core. The results are similar to those recently obtained on longitudinal magnetic recording media, but with the addition of strong interference terms due to the narrower distribution of grain sizes in these samples. © 2006 American Institute of Physics. DOI: 10.1063/1.2165798

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We have used a variety of microscopic techniques to reveal the structure and motion of flux line arrangements, while the flux lines are caused to move by a transport current. Using small-angle neutron scattering, we are able to demonstrate direaly the alignment of the nearest-neighbour direction of flux lines while they are moving. The results are similar but not identical to those from two-dimensional simulations. We also see the destruction of the flux lattice by plastic flow and the bending of flux lines. We have also employed μSR, which measures the distribution of field values within the mixed state, to obtain information about the speed and degree of order of the motion of flux lines past the muons. Such measurements give complementary information to that obtained by neutron techniques. The general aspects of these measurements, of interest to those working in other areas, are the creation of order in non-equilibrium situations and the interplay of the ordering cccuning during dissipation and the ordering caused by the crystal structure of the superconductor which is host to the flux lines.

Neutron scattering has been used to observe diffraction from the flux-line lattice in high temperature superconductors. This technique is particularly useful in such systems as it allows observations to be made over a wide range of temperature and magnetic field. Experiments on the systems K0.4Ba0.6BiO3 and YBa2Cu3O7−δ will be described with applied magnetic fields of up to 4 T and for temperatures from 2 K to Tc.

Nature 396, 242–245 (1998) In this Letter, the X-ray Laue patterns used to give the orientation of our crystal were misinterpreted by 45°. With the magnetic field perpendicular to the RuO2 planes, the nearest-neighbour directions in the square flux-line lattice (FLL) are actually at 45° to the Ru–O–Ru directions in the crystal lattice.

A Comment on the Letter by J. W. Lynn, [ital et] [ital al]., Phys. Rev. Lett. [bold 72], 3413 (1994).

Flux lattice melting in Bi 2 Sr 2 CaCu 2 O 8+x (Bi-2212) single crystals has recently been studied by various techniques. In this contribution to the I. Schegolev Memorial Volume we review some experiments and compare their results. Muon-spin rotation and small angle neutron scattering (SANS) gave the first indications for an abrupt change of the flux lattice structure along a line in the B - T phase diagram. Experiments with Hall-sensor arrays proved that the flux line lattice experiences a first order phase transition (FOT) at this line. Measured as a function of field the resistance shows an abrupt kink at the FOT line and flux flow through narrow channels reveals a sudden drop of the shear modulus. Subsequent studies showed that the location of the FOT line in the phase diagram strongly depends on oxygen stoichiometry. For decreasing temperature the FOT line terminates at a critical point and continues into an almost field independent line which represents the location of a sharp peak in the magnetization (second peak). The second-peak line coincides with a crossover region in the low temperature range of the B - T diagram where both the muon spin rotation and SANS experiments reveal a transition from a lattice of flux lines to a lattice of pancake vortices. We compare our results with theoretical scenarios.

The magnetic phase diagram of high-temperature superconductors can contain many exotic vortex phases not observed in conventional superconducting materials. For example, the familiar vortex lattice may melt at high temperatures into a vortex liquid. The influence of defects, which pin the vortices, is of particular interest from both a theoretical and an experimental point of view. We have used small angle neutron scattering (SANS) to probe the order of the vortex system on a microscopic scale and have succeeded, for the first time, to measure a well-ordered vortex lattice (VL) structure at all doping regimes of La2−xSrxCuO4. In the optimally to overdoped regime a field-induced transition from hexagonal to square coordination is reported. The possible connections of our SANS results to photoemission data, as well as the implications for various competing theoretical models will be discussed.

We have performed detailed SANS investigations on a large, high quality, twinned YBCO single crystal. The crystal was oriented with H 51 degrees to c to reduce the effect of twin plane pinning. The diffracted intensity was found to disappear at a temperature well below Hc2(T). The data is consistent with a Lindemann melting criterion giving a physically reasonable value forcL of 0.15 that is in good agreement with previous work. The disappearance of the diffracted intensity coincides with the irreversibility lines as measured by an inductance technique.

The surface structure of thin polymer blend films of deuterated polystyrene (dPS) and polyparamethylstyrene (PpMS) after annealing above the glass transition temperature was investigated. With scanning force microscopy (SFM) the surface topography originated by a dewetting process is detected. The sample surface is covered with small droplets consisting of several polymer molecules. Utilizing grazing incidence small angle neutron scattering (GISANS) the topographical information as well as the in-plane composition is probed. For thin confined blend films a substructure of the droplets resulting from an additional phase separation process at different length scales is detected.

We report the first dynamic neutron reflectivity measurements on redox switching of a modified electrode. Neutron reflectivity with isotopic substitution of the solvent was used to probe the dynami...

Reflectometry is a powerful technique for determining many physical quantities of stratified media, including length scales, densities and magnetism. However, experimentally neutron reflectometry in particular suffers from the relatively feeble brilliance of the sources compared with those of X-rays, for example. In this paper, a simple modification of existing data-reduction methods is demonstrated, allowing quantitative improvements in the quality of the data. Using the same algorithm, reflections from non-flat surfaces can be treated, leading to a full recovery of the resolution. The method involves re-binning of the data in the linear coordinates of the raw data, which leads to substantial gains in statistical quality, equivalent to a significant flux increase, and also improved resolution.

We have performed detailed SANS investigations of the flux lattice in BSCCO. At fields above 600G, the diffracted intensity increases as a function of temperature above 30K. This increase in intensity indicates that the flux lattice becomes more perfect at higher temperatures. We attribute this to a weakening of pinning at higher temperatures, which allows the flux lines to relax into a more ordered configuration. This decrease in pinning strength is due to thermal fluctuations smearing out the pinning wells seen by the flux pancakes. If our interpretation is correct, our work represents the first experimental evidence for this phenomenon.