Dynamics of field-induced ordering in magnetic colloids studied by new time-resolved small-angle neutron-scattering techniques

The reversal of magnetic moments of nanoparticles in concentrated Co ferrofluids was monitored in an oscillating magnetic field by new time-resolved stroboscopic small-angle neutron-scattering techniques. Time resolution in the micros range was achieved by using a pulsed beam technique, TISANE, while in continuous mode resolution was limited by the wavelength spread to about 1 ms. The frequency dependence of anisotropic scattering patterns has been modeled using Langevin dynamics. The dynamics follows a two step mechanism: field-induced ordering is governed by fast Brownian rotation of nanoparticles with a characteristic time of about 160 micros. Magnetic relaxation of locally ordered domains of about 100 nm in size takes place within a few seconds by Brownian rotation or by Néel type rotation of magnetic moments.     

Small-angle neutron scattering of nanocrystalline terbium with random paramagnetic susceptibility

We report magnetic small-angle neutron scattering (SANS) data for the nanocrystalline rare earth metal Terbium in its paramagnetic state. Whereas critical scattering dominates at large momentum transfer, q, the (magnetic-) field response of the scattering at small q arises from the spatial nonuniformity of the paramagnetic susceptibility tensor. The finding of an interrelation between SANS and the susceptibility suggests a way for characterizing the nonuniform magnetic interactions in hard magnets by neutron scattering.     

Time dependent neutron optics — Quantum mechanical effects on beam chopping and a new type of high resolution neutron spectrometer (FOTOF)

We calculate the time dependent wave function and energy spectrum resulting from the passage of a monoenergetic (plane wave) neutron beam through a chopper consisting of a slit with time dependent width. Experimental possibilities of observing the predicted effects are discussed. A new type of inelastic scattering spectrometer involving the use of frame overlap in a time of flight instrument is proposed.     

Vortex imaging in Co-doped BaFe2As2

We review superconducting vortex imaging in Co-doped BaFe₂As₂ by Bitter decoration and small-angle neutron scattering (SANS). At all measured fields a highly disordered vortex configuration is observed, which is attributed to strong pinning. Further support of this conclusion comes from the absence of a Meissner rim in decoration images obtained close to the sample edge. The evolution of the SANS scattering vector with increasing applied field indicates vortex lattice domains of (distorted) hexagonal symmetry. This is consistent with the decoration images which show small, six fold coordinated ordered vortex domains. The SANS scattered intensity is found to decrease rapidly with increasing field, exceeding the rate expected from estimates of the upper critical field. This is consistent with the large degree of vortex “lattice” disorder.     

Pulse inversion ultrasound modulated optical tomography

Pulse inversion acoustic imaging is useful as it allows second harmonic imaging to be obtained with short acoustic pulses. This allows high axial resolution, but removes any overlap in the frequency spectra of fundamental and harmonic. We demonstrate pulse inversion ultrasound modulated optical tomography using an optical speckle based detection method. Inverted and non-inverted acoustic pulses combined with synchronized strobed illumination are applied to an optically scattering medium. Over the acquisition time of a camera, multiple pulses are summed and at the next frame the phase of the ultrasound is shifted by π/2 and the process repeated. Combining the two frames allows a second harmonic signal to be obtained. A reduction in linewidth is observed (DC=9.26 mm, fundamental=4.02 mm, second harmonic=2.43 mm) in line scans of optically absorbing objects embedded in a scattering medium (thickness=16 mm, scattering coefficient=2.3 mm(-1), anisotropy factor=0.938).     

Gas-phase thermometry in a high-pressure cell using BaMgAl<Subscript>10</Subscript>O<Subscript>17</Subscript>:Eu as a thermographic phosphor

We demonstrate the feasibility of laser-induced phosphorescence thermography for gas-phase temperature field measurement in a high-pressure cell. BaMgAl₁₀O₁₇:Eu (BAM) was used as a thermographic phosphor; it shows a blue-shifted laser-induced emission spectrum with increasing temperature. Local temperature was determined from the intensity ratio of two disjunctive emission bands. A new seeding device was developed to suspend the solid thermographic phosphor particles in a gas environment. The particle suspension time was modeled and validated by experiments. The influence of multiple scattering and other aspects of quantitative measurement were examined. The technique is currently capable of measuring up to 650 K, limited by signal intensity. The 2D temperature distributions were measured with a precision better than 60 K at 650 K. Multiple scattering limits the spatial resolution to only about 10 mm along the line of sight.     

Design of a neutron polarizer using polarizing super mirrors for the TOF-SANS instrument at the J-PARC

Small-angle neutron scattering technique using polarized neutrons is powerful for studying structures in the range between nm and μm of magnetic materials. In addition, they have been used for the incident beam of focusing-geometry SANS instruments using a magnetic neutron lens, where a high polarization degree of about 99.9% is necessary because the imperfectness of the neutron polarization increases the background level. We are going to install such a magnetic focusing system on the new time-of-flight SANS (TOF-SANS) instrument at the J-PARC so as to make q_min smaller than 10⁻³ Å⁻¹ and improve the resolution of the conventional TOF-SANS at low q. As a polarizing device of the instrument, two V-shaped polarizing super mirrors arranged in crossed geometry to enhance the polarization degree has been considered. In this paper, we present the concept and the detailed design of this device and its performance estimated by Monte Carlo simulations.     

Transition from unilamellar to bilamellar vesicles induced by an amphiphilic biopolymer

We report some unusual structural transitions upon the addition of an amphiphilic biopolymer to unilamellar surfactant vesicles. The polymer is a hydrophobically modified chitosan and it embeds its hydrophobes in vesicle bilayers. We study vesicle-polymer mixtures using small-angle neutron scattering (SANS) and cryotransmission electron microscopy (cryo-TEM). When low amounts of the polymer are added to unilamellar vesicles of ca. 120 nm diameter, the vesicle size decreases by about 50%. Upon further addition of polymer, lamellar peaks are observed in the SANS spectra at high scattering vectors. We show that these spectra correspond to a co-existence of unilamellar and bilamellar vesicles. The transition to bilamellar vesicles as well as the changes in unilamellar vesicle size are further confirmed by cryo-TEM. A mechanism for the polymer-induced transitions in vesicle morphology is proposed.     

Anodic aluminium oxide: Concurrent SEM and SANS characterisation. Influence of AAO confinement on PEO mean-square displacement

Anodic aluminium oxide (AAO) membranes are macroscopically highly ordered systems made of oriented nano-scale parallel cylindrical channels. The membrane topology is defined by the pore radius, the inter-pores distance and the pore length. We show by a combined SANS and SEM study that the membrane morphology can be tailored by the main anodization parameters: voltage, temperature and anodization duration. We show that the confinement of an hydrophilic polymer (poly(ethylene oxide)) inside the pores is possible. We observe a striking effect of the confinement on the polymer mean-square displacement as measure by high resolution neutron quasi-elastic scattering.     

THE EMC EFFECT AND THE LIGHT-CONE QUANTUM FIELD THEORY

We indicate that final state interactions can be neglected if one calculate deep inelastic lepton nucleus scattering in the time-ordered perturbation theory in the infinite momentum frame,or its equivalent,the light-cone perturbation theory in an ordinary frame.We thus extend the light-cone quantum field theory to baryon-meson field to establish a relativistic composite model of nuclei,and then apply the impulse approximation to deep inelastic lepton nucleus scattering in this model.     

Ultraviolet tip‐enhanced nanoscale Raman imaging

We have constructed an ultraviolet (UV)‐apertureless near‐field scanning optical microscope‐Raman spectroscopy system by using an aluminum tip for the simultaneous measurement of topography and Raman scattering of nanomaterials with high spatial resolution. The topography, Rayleigh scattering image, and tip‐enhanced Raman scattering image of the carbon nanotube film showed that a spatial resolution of around 19 nm was achieved. This spatial resolution of UV‐Raman mapping image exceeds that of previous approaches, which have several hundred nanometers of spatial resolution. Copyright © 2012 John Wiley & Sons, Ltd.     

Time-resolved nuclear spin-dependent small-angle neutron scattering from polarised proton domains in deuterated solutions

We have investigated the process of dynamic proton polarisation by means of time-resolved polarised small-angle neutron scattering (SANS) on frozen solutions of EHBA-Cr^V molecules in glycerol-water mixtures as a function of the concentration of EHBA-Cr^V and for different degrees of deuteration of the solvent. In the EHBA-Cr^V complex, the spins of the 20 protons which surround the paramagnetic Cr^V can be oriented using the method of dynamic nuclear polarisation (DNP), thereby offering the possibility to create locally a nuclear spin-dependent contrast for SANS. The time constants which describe the build-up of polarisation around the paramagnetic centre and the subsequent diffusion of polarisation in the solvent were determined by analysing the temporal evolution of the nuclear polarisation, which in turn was obtained by fitting a core-shell model to the time-dependent SANS curves. The results on the spin dynamics obtained using the scattering function of a core-shell could be independently confirmed by evaluating the integrated SANS intensity. A thermodynamic one-centre model is presented which is able to reproduce the observed dependence of the proton polarisation times on the proton concentration of the solvent.     

Self-assemblies of magnetic nanoparticles and di-block copolymers: Magnetic micelles and vesicles

Magnetic nanocomposites are obtained by the self-assembly in water of polypeptide-based di-block copolymers polybutadiene-b-poly(glutamic acid) combined with hydrophobic γ-Fe₂O₃ nanoparticles. These hybrid supramolecular objects are either—(3D) spherical micelles filled with a hydrophobic ferrofluid at a concentration as high as 45 vol% or—hollow vesicles with a (2D) magnetic membrane. In this last case, the organic amphiphile copolymers are able to confine the hydrophobic nanoparticles within the thin layer of polybutadiene blocks. We probe these objects by atomic force microscopy, by small-angle neutron scattering (SANS) and by light scattering. Furthermore, anisotropic SANS data bring the experimental evidence of the capability to modify the shape of the mineralized membranes in response to a magnetic field intensity as low as 290 G.     

Structural response of polyelectrolyte dendrimer towards molecular protonation: the inconsistency revealed by SANS and NMR

Polyamidoamine (PAMAM) dendrimers and their charged state in deuterium oxide have been investigated with proton pulsed field gradient diffusion nuclear magnetic resonance (PFG-NMR) and small-angle neutron scattering (SANS) techniques. NMR measurements suggest that significant variation of the hydrodynamic radius, calculated by the Stokes-Einstein relation with appropriate surface conditions, is observed upon increasing the molecular protonation. However, a comparative SANS experiment indicates little dependence of the dendrimer global size, in terms of its radius of gyration, on molecular protonation. The inconsistency indicates the necessity of incorporating the effect of molecular interface modification and molecular porosity provided by dressed counterions, when dynamical measurements are used for the determination of the structural characteristics of ionic soft colloids even in dilute suspensions.     

SANS response of VVER440-type weld material after neutron irradiation,post-irradiation annealing and reirradiation

It is well accepted that the reirradiation behaviour of reactor pressure vessel (RPV) steel after annealing can be different from the original irradiation behaviour. We present the first small-angle neutron scattering (SANS) study of neutron irradiated, annealed and reirradiated VVER440-type RPV weld material. The SANS results are analyzed both in terms of the size distribution of irradiation-induced defect/solute atom clusters and in terms of the ratio of total and nuclear scattering intensity in a saturation magnetic field (A ratio). The measured A ratio is compared with calculations performed on the basis of the cluster composition reported for a similar weld material investigated by means of three-dimensional atom probe field ion microscopy. The observed deviation between both estimates and possible reasons for the discrepancy are discussed. Special emphasis is placed on the differences between the materials response to the original irradiation and to reirradiation after annealing. The results indicate that reirradiation-induced clusters are slightly different in their average composition and their formation saturates at a lower volume fraction than in the case of the original irradiation.     

Ultrasensitive femtosecond two-photon detector with resonantly enhanced nonlinear absorption

We report on two-photon detection based on nonlinear absorption between subbands in quantum wells. Resonantly enhanced nonlinear absorption, 6 orders of magnitude higher than that of typical bulk semiconductors, leads to a threshold power density for quadratic detection as low as 0.1 W/cm2 and to femtosecond time resolution. The approach facilitates dynamic characterization of the optical light field of infrared emitters with unprecedented sensitivity. We also present a new method for determining the intersubband and intrasubband scattering times by means of autocorrelation measurements.     

Small angle neutron scattering observation of chain retraction after a large step deformation

The process of retraction in entangled linear chains after a fast nonlinear stretch was detected from time-resolved but quenched small angle neutron scattering (SANS) experiments on long, well-entangled polyisoprene chains. The statically obtained SANS data cover the relevant time regime for retraction, and they provide a direct, microscopic verification of this nonlinear process as predicted by the tube model. Clear, quantitative agreement is found with recent theories of contour length fluctuations and convective constraint release, using parameters obtained mainly from linear rheology. The theory captures the full range of scattering vectors once the crossover to fluctuations on length scales below the tube diameter is accounted for.     

Magnetic correlations in Cr–Fe–Mn alloy

Small angle neutron scattering (SANS) measurement has been performed on the reentrant ferromagnet Cr₆₀Fe₂₀Mn₂₀ alloy. An increase in the SANS intensity is observed below 100 K coinciding with an increase in the magnetization. It exhibits a maximum at 40 K and shifts to lower temperatures with increase in Q. The magnetic part of the SANS intensity fits to a Lorentzian at all temperatures. The temperature dependence of the inverse correlation length exhibits a minimum at 40 K. In the antiferromagnetic spin-glass phase at 15 K ferromagnetic correlations of ~40 Å is observed.     

Intrinsic bulk vortex dynamics revealed by time resolved small angle neutron scattering

Measurements of intrinsic vortex lattice (VL) dynamics in superconductors as for instance VL melting or Bragg glass transitions are typically performed by e.g. macroscopic transport or surface sensitive measurement techniques. Therefore, usually thin superconducting films are used for microscopic measurements of VL dynamics. A direct consequence of using thin films is the strong influence of surface effects and defects, sample quality and geometry. We succeeded to combine time resolved stroboscopic small angle neutron scattering (SANS) with an advanced, time varying magnetic field setup allowing to extend the time window for slow dynamical processes to the range of 10 ms up to several minutes. The new results demonstrate that it is possible to observe directly the intrinsic dynamics of the VL in a bulk niobium single crystal on a microscopic scale without limitations due to surface effects. Field and temperature dependent relaxation times of the VL from 100 to 500 ms could be observed for the first time, allowing to directly measure the VL of the tilt modulus. This new experimental technique provides the possibility to study also the dynamical magnetic properties of various strongly correlated electron systems.