Magnetic Field Control of Cycloidal Domains and Electric Polarization in Multiferroic BiFeO_{3}.

The magnetic field induced rearrangement of the cycloidal spin structure in ferroelectric monodomain single crystals of the room-temperature multiferroic BiFeO_{3} is studied using small-angle neutron scattering. The cycloid propagation vectors are observed to rotate when magnetic fields applied perpendicular to the rhombohedral (polar) axis exceed a pinning threshold value of ∼5 T. In light of these experimental results, a phenomenological model is proposed that captures the rearrangement of the cycloidal domains, and we revisit the microscopic origin of the magnetoelectric effect. A new coupling between the magnetic anisotropy and the polarization is proposed that explains the recently discovered magnetoelectric polarization perpendicular to the rhombohedral axis.

Polarized neutron imaging at NeXT (neutron and x-ray tomograph) at Institut Laue Langevin.

This work describes the implementation of polarized neutron imaging capabilities at the neutron and x-ray tomograph (NeXT) imaging station of the Institut Laue Langevin. This development enhances the capacity of this instrument to study advanced magnetic materials, which are crucial in a variety of engineering applications. Here, the feasibility of polarized neutron imaging at NeXT is demonstrated by visualizing the magnetic field generated by a simple bar magnet. The use of a double-crystal monochromator for wavelength-resolved imaging is also shown to enable both quantitative and qualitative analyses of magnetic materials. This is demonstrated through the determination of magnetization strength in a sample of electric steel (FeSi) in addition to the distribution of its components. Polarimetric imaging is also implemented for the first time to characterize the magnetic field generated by a current-carrying cylindrical wire. These findings collectively underscore the value of incorporating polarized neutron imaging into the already cutting-edge imaging station.

Adherence of the rotating vortex lattice in the noncentrosymmetric superconductor Ru7B3to the London model.

The noncentrosymmetric superconductor Ru7B3has in previous studies demonstrated remarkably unusual behaviour in its vortex lattice (VL), where the nearest neighbour directions of the vortices dissociate from the crystal lattice and instead show a complex field-history dependence, and the VL rotates as the field is changed. In this study, we look at the VL form factor of Ru7B3during this field-history dependence, to check for deviations from established models, such as the London model. We find that the data is well described by the anisotropic London model, which is in accordance with theoretical predictions that the alterations to the structure of the vortices due to broken inversion symmetry should be small. From this, we also extract values for the penetration depth and coherence length.

Small-angle neutron scattering study of mesoscale magnetic disordering and skyrmion phase suppression in the frustrated chiral magnet Co6.75Zn6.75Mn6.5.

Co-Zn-Mn chiral cubic magnets display versatile magnetic skyrmion phases, including equilibrium phases stable far above and far below room temperature, and the facile creation of robust far-from-equilibrium skyrmion states. In this system, compositional disorder and magnetic frustration are key ingredients that have profound effects on the chiral magnetism. Reported here are studies of the magnetism in Co6.75Zn6.75Mn6.5 by magnetometry, small-angle neutron scattering (SANS), magnetic diffuse neutron scattering and Lorentz transmission electron microscopy (LTEM). While features in magnetometry and LTEM often give standard indications for skyrmion formation, they are not readily observed from the measurements on this system. Instead, skyrmion lattice correlations are only revealed by SANS, and they are found to form an orientationally disordered structure in a minority fraction of the sample. The majority fraction of the sample always displays orientationally disordered helical spin correlations, which undergo further disordering along the radial direction on cooling below the critical temperature (T c ≃ 102 K). The near-complete suppression of the skyrmion phase, and the process of disordering on cooling, are attributed to competing magnetic interactions that dominate over the ferromagnetic interaction expected to favour chiral magnetism in this system. These competing interactions start to develop above T c and become further enhanced towards low temperatures. The present observations of co-existing and disordered magnetic correlations over multiple length scales are not unique to Co6.75Zn6.75Mn6.5 but are seemingly common to the family of Co-Zn-Mn compounds with finite Mn, and their accurate description presents a challenge for theoretical modelling. In addition, this study highlights a need for neutron instrumentation capable of the comprehensive measurement of magnetic correlations over expanded ranges of momentum transfer in such multiple-length-scale magnets.

A neutron reflection study of surface enrichment in nematic liquid crystals.

The interfacial adsorption properties of several different dopants in cyanobiphenyl liquid crystals have been measured using specular neutron reflection. It was found that a partly fluorinated analogue of 11OCB, called F17, adsorbed strongly at the interface between 5CB and air but it was not adsorbed at the interface between 5CB and a solid substrate treated with cetyl trimethyl ammonium bromide (CTAB). The concentration dependence of the adsorption at the air interface was well described by the Brunauer, Emmett and Teller (BET) model, adapted for solutions rather than the gas phase. The isotherms are determined by two equilibrium constants: K(S) for adsorption of the dopant directly at the interface and K(L) for adsorption onto previously adsorbed dopant. The temperature dependence of K(S) indicated that the adsorption enthalpy is not influenced by the phase of the 5CB and its value of -29 kJmol(-1) is consistent with physical adsorption. The value of K(L) is zero in the isotropic phase but increases rapidly on cooling in the nematic phase suggesting that the F17 is less compatible with nematic than isotropic 5CB. The smallest layer thicknesses (~18 Å) suggest that the F17 molecules are approximately perpendicular to the surface. The other dopants studied were components of the E7 mixture: 8OCB and 5CT. No adsorption was found for 8OCB but 5CT showed adsorption at a CTAB treated solid interface when present in 5CB at the 10% level. In this case, the value of K(S) was much smaller than for F17 but the value of K(L) was such that an exponential concentration profile (predicted by the BET model) was observed with characteristic thickness of ~200 Å. The results demonstrate the potential for very precise control of surface properties in liquid crystal devices by using appropriate dopants.

Field-induced vortex-like textures as a probe of the critical line in reentrant spin glasses.

We study the evolution of the low-temperature field-induced magnetic defects observed under an applied magnetic field in a series of frustrated amorphous ferromagnets (Fe[Formula: see text]Mn[Formula: see text])[Formula: see text]P[Formula: see text]B[Formula: see text]Al[Formula: see text] ("a-Fe[Formula: see text]Mn[Formula: see text]"). Combining small-angle neutron scattering and Monte Carlo simulations, we show that the morphology of these defects resemble that of quasi-bidimensional spin vortices. They are observed in the so-called "reentrant" spin-glass (RSG) phase, up to the critical concentration [Formula: see text] which separates the RSG and "true" spin glass (SG) within the low temperature part of the magnetic phase diagram of a-Fe1-xMnx. These textures systematically decrease in size with increasing magnetic field or decreasing the average exchange interaction, and they finally disappear in the SG sample ([Formula: see text]), being replaced by field-induced correlations over finite length scales. We argue that the study of these nanoscopic defects could be used to probe the critical line between the RSG and SG phases.

Realignment of the flux-line lattice by a change in the symmetry of superconductivity in UPt3

In 1957, Abrikosov described how quanta of magnetic flux enter the interior of a bulk type II superconductor. It was subsequently predicted that, in an isotropic superconductor, the repulsive forces between the flux lines would cause them to order in two dimensions, forming a hexagonal lattice. Flux-line lattices with different geometry can also be found in conventional (type II) superconductors; however, the ideal hexagonal lattice structure should always occur when the magnetic field is applied along a hexagonal crystal direction. Here we report measurements of the orientation of the flux-line lattice in the heavy-fermion superconductor UPt3, for this special case. As the temperature is increased, the hexagonal lattice, which is initially aligned along the crystal symmetry directions, realigns itself with the anisotropic superconducting gap. The superconductivity in UPt3 is unusual (even compared to unconventional oxide superconductors) because the superconducting gap has a lower rotational symmetry than the crystal structure. This special feature enables our data to demonstrate clearly the link between the microscopic symmetry of the superconductivity and the mesoscopic physics of the flux-line lattice. Moreover, our observations provide a stringent test of the theoretical description of the unconventional superconductivity in UPt3.

Equilibrium Skyrmion Lattice Ground State in a Polar Easy-plane Magnet.

The skyrmion lattice state (SkL), a crystal built of mesoscopic spin vortices, gains its stability via thermal fluctuations in all bulk skyrmion host materials known to date. Therefore, its existence is limited to a narrow temperature region below the paramagnetic state. This stability range can drastically increase in systems with restricted geometries, such as thin films, interfaces and nanowires. Thermal quenching can also promote the SkL as a metastable state over extended temperature ranges. Here, we demonstrate more generally that a proper choice of material parameters alone guarantees the thermodynamic stability of the SkL over the full temperature range below the paramagnetic state down to zero kelvin. We found that GaV4Se8, a polar magnet with easy-plane anisotropy, hosts a robust Néel-type SkL even in its ground state. Our supporting theory confirms that polar magnets with weak uniaxial anisotropy are ideal candidates to realize SkLs with wide stability ranges.

Approaching criticality in polymer-polymer systems.

The interfacial width of polyolefins blends has been probed as a function of distance away from the critical point by using neutron reflectivity. For strongly immiscible polymer pairs, the width of the interface increases slowly when the degree of immiscibility is decreased and the interfacial width varies with the interaction parameter chi of the polymers. Closer to the critical point the dependence on the degree of miscibility becomes stronger and the way in which the interfacial width diverges, as criticality is approached, is related to both the chain length and chi. The self-consistent field theory numerical calculations, with the additional contribution due to capillary waves, provides a good description of the width of the interface between two polymer bulk phases in particular at intermediate values of the degree of immiscibility.

Measurement of vortex motion in a type-II superconductor: A novel use of the neutron spin-echo technique

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 v(L) was given as expected by the values of electric and magnetic fields, via E = -v(L)wedgeB. 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.

Interaction of cationic lipid/DNA complexes with model membranes as determined by neutron reflectivity.

Transfection of cells by DNA for the purposes of gene therapy can be effectively engineered through the use of cationic lipid/DNA "lipoplexes", although the transfection efficiency of these complexes is sensitive to the neutral "helper" lipid included. Here, neutron reflectivity has been used to investigate the role of the helper lipid present during the interaction of these lipoplexes with model membranes composed primarily of zwitterionic lipid 1,2-dimyristoylphosphatidylcholine (DMPC) together with 10 mol % 1,2-dipalmitoylphosphatidylserine (DPPS). Dimethyldioctadecylammonium bromide (DDAB) vesicles were formed with two different helper lipids, 1,2-dioleoyl-sn-glycero-3-phosphatidylethanolamine (DOPE) and cholesterol, and complexed with a 1:1 charge ratio of DNA. The interaction of these complexes with the supported phospholipid bilayer was determined. DOPE-containing lipoplexes were found to interact faster with the model cell membrane than those containing cholesterol, and complexes containing either of the neutral helper lipids were found to interact faster than when DDAB alone was present. The interaction between the lipoplexes and the model membrane was characterized by an exchange of lipid between the membrane and the lipid/DNA aggregates in solution; the deposition of(additional) lipid on the surface of the model cell membrane was not apparent.

Interface-induced morphology transition in triblock copolymer films swollen with low-molecular-weight homopolymer.

The morphology transition due to midblock swelling with low-molecular-weight homopolymer polystyrene of an ABA-type triblock copolymer polyparamethylstyrene-block-polystyrene-block-polyparamethylstyrene at the buried silicon substrate interface is studied as a function of different substrate surface treatments. With grazing incidence small-angle neutron scattering (GISANS), high interface sensitivity is reached. The powderlike oriented lamellar structure in the bulk becomes oriented along the surface normal in the vicinity of the substrate. A transition of the lamellar into a cylinder phase at the polymer-silicon interface is probed with GISANS. The transition is induced by the addition of the homopolymer, but the modification of the short-ranged interface potential of the substrate influences the amount of homopolymer that is necessary for this transition. Without and with 0.1 vol % added homopolymer, the lateral spacing is stretched at the interface as compared to the bulk whereas for a higher added amount of homopolymer no stretching occurs.

Hydration behavior of casein micelles in thin film geometry: a GISANS study?

The water content of casein micelle films in water vapor atmosphere is investigated using time-resolved grazing incidence small-angle neutron scattering (GISANS). Initial dry casein films are prepared with a spin-coating method. At 30 degrees C, the formation of a water-equilibrated casein protein film is reached after 11 min with a total content of 0.36 g of water/g of protein. With increasing water vapor temperature up to 70 degrees C, an increase in the water content is found. With GISANS, lateral structures on the nanometer scale are resolved during the swelling experiment at different temperatures and modeled using two types of spheres: micelles and mini-micelles. Upon water uptake, molecular assemblies in the size range of 15 nm (mini-micelles) are attributed to the formation of a high-contrast D2O outer shell on the small objects that already exist in the protein film. For large objects (>100 nm), the mean size increases at high D2O vapor temperature because of possible aggregation between hydrated micelles. These results are discussed and compared with various proposed models for casein micelle structures.

Uncovering flux line correlations in superconductors by reverse monte carlo refinement of neutron scattering data.

We describe the use of reverse Monte Carlo refinement to extract structural information from angle-resolved data of a Bragg peak. Starting with small-angle neutron scattering data, the positional order of an ensemble of flux lines in superconducting Nb is revealed. We discuss the uncovered correlation functions in the light of topical theories, in particular, the "Bragg glass" paradigm.