Macroscopic ordering of polystyrene carboxylate-modified nanospheres self-assembled at the water-air interface

Copyright © 2010 American Chemical SocietyThis document is the Accepted Manuscript version of a Published Work that appeared in final form in Langmuir, copyright © American Chemical Society after peer review and technical editing by the publisher. To access the final edited and published work see http://dx.doi.org/10.1021/la1009658We present results from an experimental study of ordering characteristics in monolayers of polystyrene nanospheres self-assembled at a water-air interface. We demonstrate that the interaction of spheres, governed by the dissemination of surface charge, leads to the formation of macroscopic close-packed ordered areas or "domains" with a well-defined orientation of the lattice axes over areas of 25 mm(2). It was found that by changing the surface chemistry of the spheres it is possible to modify the balance between the attractive and repulsive forces and thus to control the ordering characteristics. We implemented a model that simulates the process of self-assembly and examines the ordering characteristics for layers with different ratio between attractive and repulsive forces. A good qualitative agreement was found between the simulations and experiment. These studies are technologically relevant as a method of producing nanosphere templates for large area patterned materials

Theory of ferromagnetic microswimmers

Copyright © 2011 Oxford University Press. This is a pre-copy-editing, author-produced PDF of an article accepted for publication in The Quarterly Journal of Mechanics and Applied Mathematics following peer review. The definitive publisher-authenticated version [Volume 64, Issue 3, pp. 239-263] is available online at: http://qjmam.oxfordjournals.org/content/64/3/239This paper considers the dynamics of a microscale swimmer based on two magnetic beads that are elastically coupled together. A time-varying external magnetic field is imposed that has two principal effects: one is to exert a torque on the magnetic beads. The second is to change the orientation of the magnetic field dipoles in one or both beads, depending on their ferromagnetic properties. This then creates an attraction or repulsion between the two dipoles. The combination of dipole attraction/repulsion, moderated by the elastic coupling, and torque gives motions that are not generally time reversible and can lead to unidirectional swimming, that is persistent motion in one direction, in a Stokes flow regime. The equations of motion for the swimmer are set up using a Lagrangian formulation and supplemented by equations giving the dipole orientation of the magnetic fields of the beads in the external field. The equations are non-dimensionalized and key parameters determined. Numerical simulations reveal a number of regimes that are studied using simplified models and multiple scale analysis. Approximate thresholds are obtained above which the swimmer moves in a closed path and below which the orientation is `trapped' giving unidirectional motion. Three mechanisms for such trapping are isolated and discussed

Small angle x-ray and neutron scattering study of disordered and three dimensional-ordered magnetic protein arrays

Copyright © 2009 American Institute of PhysicsProceedings of the 53rd Annual Conference on Magnetism and Magnetic Materials, Austin, Texas, 11-14 November 2008The magnetic nanoparticles of Fe3O4-γ–Fe2O3 grown inside the cavity of globular proteins (apoferritin)-magnetoferritin proved to be a useful model system for studying the fundamental effects of magnetostatic interactions in nanoparticle assemblies. In this work the main focus is on structural characterization of such new nanocomposites by small angle x-ray scattering (SAXS) and small angle neutron scattering to evaluate interparticle separation (center to center) in two types of assemblies: three dimensional periodic arrays and disordered (amorphous) assemblies. Straightforward analysis of the face-centered cubic pattern of periodic arrays revealed that the interparticle spacing is 9.9 nm, whereas the SAXS pattern of disordered assembly reveals three correlation lengths, one of which is 10.5 nm and corresponds to the interparticle (center-to-center) nearest neighbor distance. The magnetic behaviors of the two systems are distinctly different. Given that the interparticle separation differs by only ∼ 0.6 nm, the main structural factor contributing to the observed differences in magnetic properties is likely to be the array order

Graded index confined spin waves in a mixed Bloch-Néel domain wall

This is the final version. Available from the American Physical Society via the DOI in this recordWe propose a mathematical model for describing propagating confined modes in domain walls of intermediate angle α ( 0 < α < π / 2 radians ) between domains. The model is obtained from the linearized Bloch equations of motion and under reasonable assumptions that can apply to the scenario of a thick (80 nm) magnetic patch, which simplifies the calculations without a high impact on the model accuracy. The model shows that there is a clear dependence of the local wave number of the confined spin wave on the local angle of domain magnetization with respect to the wall and on the excitation magnetic field frequency. From this model, we can define a local mode index in the wall as a function of such angle and excitation frequency. Therefore, the model can be applied to 1D propagating modes, although it also has physical implications for 2D scenarios where a domain wall merges with a saturated magnetic region. Micromagnetic simulations are in good agreement with the predictions of the model. Our model can also give insight on the effects that curved edge structures may have on the propagating characteristics of spin waves bounded in domain walls.Engineering and Physical Sciences Research Council (EPSRC

Direct observation of the flux-line vortex glass phase in a type II superconductor

The order of the vortex state in La_{1.9} Sr_{0.1} CuO_{4} is probed using muon spin rotation and small-angle neutron scattering. A transition from a Bragg glass to a vortex glass is observed, where the latter is composed of disordered vortex lines. In the vicinity of the transition the microscopic behavior reflects a delicate interplay of thermally-induced and pinning-induced disorder.Comment: 14 pages, 4 colour figures include

Muons as Local Probes of Three-body Correlations in the Mixed State of Type-II Superconductors

The vortex glass state formed by magnetic flux lines in a type-II superconductor is shown to possess non-trivial three-body correlations. While such correlations are usually difficult to measure in glassy systems, the magnetic fields associated with the flux vortices allow us to probe these via muon-spin rotation measurements of the local field distribution. We show via numerical simulations and analytic calculations that these observations provide detailed microscopic insight into the local order of the vortex glass and more generally validate a theoretical framework for correlations in glassy systems.Comment: 4+ pages, high-quality figures available on reques

Exploring carbon nanotubes / BaTiO3 / Fe3O4 Nanocomposites as microwave absorbers

This is the final version of the article. Available from EM Academy via the link in this record.Open access journalWe report the modelling and characterization of microwave absorbing materials specially designed for 26–37 GHz frequency range (Ka-band). Composite materials based on carbon nanotubes/BaTiO3/Fe3O4 in a phosphate ceramic matrix were produced, and their electromagnetic response was investigated. Both theoretical and experimental results demonstrate that this material can absorb up to 100% of the power of an incident plane wave at a normal incidence angle. The physics underlying such absorption level is discussed in terms of refractive index of the material.This work was supported in part by FP7-PEOPLE-2013-IRSES-610875 NAmiceMC, FP7 Twinning Grant Inconet EaP 004. P. Kuzhir is thankful for support by Tomsk State University Competitiveness Improvement Program. Lab-STICC is UMR CNRS 6285

Hollow carbon spheres in microwaves: Bio inspired absorbing coating

This is the final version of the article. Available from American Institute of Physics (AIP)] via the DOI in this record.The electromagnetic response of a heterostructure based on a monolayer of hollow glassy carbon spheres packed in 2D was experimentally surveyed with respect to its response to microwaves, namely, the Ka-band (26-37 GHz) frequency range. Such an ordered monolayer of spheres mimics the well-known "moth-eye"-like coating structures, which are widely used for designing anti-reflective surfaces, and was modelled with the long-wave approximation. Based on the experimental and modelling results, we demonstrate that carbon hollow spheres may be used for building an extremely lightweight, almost perfectly absorbing, coating for Ka-band applications.This work was supported in part by FP7-PEOPLE-2013- IRSES-610875 NAmiceMC, FP7 Twinning Grant Inconet EaP_004