C. D. Dewhurst

Néel-type skyrmion lattice with confined orientation in the polar magnetic semiconductor GaV4S8.

Following the early prediction of the skyrmion lattice (SkL)--a periodic array of spin vortices--it has been observed recently in various magnetic crystals mostly with chiral structure. Although non-chiral but polar crystals with Cnv symmetry were identified as ideal SkL hosts in pioneering theoretical studies, this archetype of SkL has remained experimentally unexplored. Here, we report the discovery of a SkL in the polar magnetic semiconductor GaV4S8 with rhombohedral (C3v) symmetry and easy axis anisotropy. The SkL exists over an unusually broad temperature range compared with other bulk crystals and the orientation of the vortices is not controlled by the external magnetic field, but instead confined to the magnetic easy axis. Supporting theory attributes these unique features to a new Néel-type of SkL describable as a superposition of spin cycloids in contrast to the Bloch-type SkL in chiral magnets described in terms of spin helices.

Fabrication of large grain YBCO by seeded peritectic solidification

The ability to process large grain, uniform high temperature superconducting ceramics that exhibit high critical current densities at 77 K is essential if the enormous potential of these materials for a range of permanent magnet-type applications is to be realized. We report a study of the fabrication of large grain YB{sub 2}Cu{sub 3}O{sub 7{minus}{delta}} by seeded peritectic solidification in which key processing parameters such as the peritectic melting process, the seed-YBCO reaction, and the YBCO solidification kinetics are investigated in detail. Evolution of the sample microstructure during various stages of the growth process, in particular, has been studied extensively. The superconducting properties of specimens cut from different regions of large grain samples have been measured using vibrating sample magnetometry, and the results correlated with the microstructure of the materials. {copyright} {ital 1996 Materials Research Society.}

Direct Evidence for an Intrinsic Square Vortex Lattice in the Overdoped High- Tc Superconductor La1.83Sr0.17CuO4+δ

We report here the first direct observations of a well ordered vortex lattice in the bulk of a La(2-x)Sr(x)CuO(4+delta) single crystal (slightly overdoped, x = 0.17). Our small angle neutron scattering investigation of the mixed phase reveals a crossover from triangular to square coordination with increasing magnetic field. The existence of an intrinsic square vortex lattice has never been observed in high-temperature superconductors and is indicative of the coupling of the vortex lattice to a source of anisotropy, such as those provided by a d-wave order parameter or the presence of stripes.

Low-temperature magnetoresistance and magnetic ordering in

In zero field, shows thermally activated behaviour for all values of x. For charge ordering occurs at 250 K, with long-range antiferromagnetic ordering at lower temperatures. For a range of compositions a first-order magnetic-field-induced insulator - metal transition produces changes in resistivity of up to 12 orders of magnitude at low temperatures. These conducting states are metastable, resulting in a large hysteresis in resistivity with changes in both magnetic field and temperature. There is a marked relationship between the resistivity and the magnetic state of the material. Below the charge-ordering temperature, metamagnetic transitions occur which transform the magnetic correlations from either paramagnetic or antiferromagnetic to ferromagnetic. The range of compositions for which these metamagnetic transitions occur, the fields required to induce the ferromagnetic state, and the irreversible changes in the nature of the magnetic ordering all correlate with the field-induced changes in resistivity.

The influence of process parameters on the growth morphology of large-grain Pt-doped YBCO fabricated by seeded peritectic solidification

We report an investigation of the processing of large-grain Y - Ba - Cu - O (YBCO) containing 0.1, 0.2 and 0.5 wt% platinum (Pt) doping using seeded peritectic solidification in a vertical tube furnace. In this study differential thermal analysis of the Pt-doped YBCO powders and careful temperature calibration of the furnace were performed in order to determine optimum values of the thermal process parameters for each composition. The seeded melt process was observed to be extremely sensitive to the melting temperature, peritectic solidification temperature and cooling rate during grain growth, and failure to optimize these parameters can result in various undesirable effects such as incomplete melting, the nucleation of multiple grains and the formation of local regions of impurity concentration within the sample. Scanning electron microscopy and vibrating sample magnetometry were employed to investigate the microstructure and current-carrying capabilities of the samples. From these, 0.1 wt% Pt doping was determined to be sufficient for the fabrication of melt-processed samples containing fine, sub-micron-sized Y-211 particles which exhibited high critical current densities. No improvement in sample properties was observed for higher levels of Pt doping.

Direct Observation of the Flux-Line Vortex Glass Phase in a Type II Superconductor

The order of the vortex state in La1.9Sr0.1CuO4 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.

Gap in KFe2As2studied by small-angle neutron scattering observations of the magnetic vortex lattice

We report the observation, by small-angle-neutron-scattering (SANS), of magnetic flux lines “vortices” in super-clean KFe2As2 single crystals. The results show clear Bragg spots from a well ordered vortex lattice, for the first time in a FeAs superconductor. These measurements can give important information about the pairing state in this material, because the spatial variation of magnetic field in the vortex lattice reflects this pairing. With field parallel to the fourfold c-axis, nearly isotropic hexagonal packing of vortices was observed without VL-symmetry transitions up to high fields, indicating rather small anisotropy of the superconducting properties around this axis. This rules out gap nodes parallel to the c-axis, and thus d-wave and also anisotropic s-wave pairing. The strong temperature-dependence of the scattered intensity down to T Tc further indicates either widely different full gaps on different Fermi surface sheets, or nodal lines perpendicular to the axis. PACS numbers: 74.25.Uv, 74.70.Xa, 74.20.Rp, 74.25.-q

Controlled processing and properties of large Pt-doped Y-Ba-Cu-O pseudocrystals for electromagnetic applications

The quality and preparation of Sm–Ba–Cu–O (SmBCO) seeds, the level of Pt-doping, and the temperature distribution in the thermal processing system have been investigated in detail and identified as key processing factors in the fabrication of large melt processed Pt dope Y–Ba–Cu–O pseudocrystals. Optimization of these process variables and features has been performed to enable controlled processing of these materials up to several centimeters in diameter. Inhomogeneities and second phases present in the YBCO-Pt pseudocrystal microstructure have been analyzed and correlated with a variation of superconducting properties within the bulk of the sample. These observations have clear implications for the controlled processing of large YBCO-Pt pseudocrystals for electromagnetic applications.

Homogeneity of superconducting properties in SmBa2Cu3O7 − δ-seeded melt processed YBCO

The homogeneity of the superconducting properties of a large grain (2 cm diameter) YBa 2 Cu 3 O 7-δ (YBCO) sample fabricated by SmBa 2 Cu 3 O 7 - δ -seeded melt growth has been investigated in detail and correlated with variations in sample composition and microstructure. Magnetic characterisation of small ( 8 mm 3 ) individual specimens cut from various positions in the bulk sample indicates a significant variation in the superconducting transition temperature, critical current density and irreversibility field within the large YBCO grain. The transition temperature increases smoothly with sample radius to a distance of the order of several millimetres from the seed which correlates well with an observed local Sm contamination of the YBCO matrix. The observed variation of the critical current density and irreversibility field with position is associated primarily with the reduced transition temperature in the vicinity of the seed, a degradation in the microstructure towards the sample edge and a variation in the Y 2 BaCuO 5 inclusion density with sample radius.

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

The 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...