National Institute Of Materials Physics - Romania

Four Fe-Mn-Si alloys, Fe(62)Mn(32)Si(6), Fe(62)Mn(20)Si(5)Cr(8)Ni(5), Fe(62)Mn(16)Si(5)Cr(12)Ni(5) and Fe(65)Mn(9)Si(7)Cr(10)Ni(9), were obtained by the melt-spinning method. The samples were structurally, magnetic and shape memory effect (SME) investigated, both ''as quenched'' and thermally treated. The Mn-rich compositions show different phase, magnetic behavior and SME in comparison with Mn-poor compositions. The thermal treatments generate transformation between the two existing majority phases (alpha and gamma), related magnetization and SME behavior. The features are derived from the corroboration of structural, magnetic interaction and magnitude of SME data. (c) 2008 Elsevier B.V. All rights reserved.

Structural, magnetic and transport properties of CeAg1-xNixSb2 compounds are investigated on polycrystalline samples. The experimental results show the formation of ferromagnetic compounds with sharp transitions decreasing from 9.5 K in the 100% Ag compound to 8.1 K for 33% Ni substitution, 7 K for 67% Ni substitution to 6 K for the 100% Ni compound. The characteristic temperatures, considered as the temperature of the resistivity maximum To increases from 26 K to 44 K, 54 K and 65 K, respectively. These results indicate Ce(AgNi)Sb2 compounds as a ferromagnetic Kondo system.

The pressure response of CeAg(1-x)Ni(x)Sb(2) is Studied by means of electrical resistivity measurements. The ferromagnetic ordering temperatures T(c) of this series decrease as pressure is applied, except CeNiSb(2), showing a pressure driven continuous increase of T(c). This refers to crystalline electric field effects originating distinct different magnetic ground states. Concomitantly, the pressure response of various material dependent parameters also behave in the opposite direction.

A typical facility for fabrication of devices based on high temperature superconducting (HTS) films grown on the less-expensive CeO2-buffered sapphire substrates comprises a transfer chamber, a chamber for CeO2 growth, a chamber for YBa2Cu3Oy (YBCO) growth, and a chamber for Au deposition at room temperature. Here we present the adaptation of our previously-reported method to create artificial pinning centres in HTS films through self-assembled nano-scale islands for this type of facility without making any modifications. We succeeded to grow Au nano-dots from an ultra-thin layer of Au grown at room temperature subsequently annealed in the chamber for YBCO deposition, in the same conditions as for YBCO growth. AFM and RBS were used to investigate the formation of Au nano-dots, while XRD and susceptibility measurements were used to study the properties of nano-structured YBCO films.

In this work we focus on the microstructural and magnetic characterization of CuNb/Nb3Sn wires with different architectures (design and reinforcement). The microstructural characterization was performed using scanning electron microscopy. AC magnetic susceptibility was measured with field applied both parallel and perpendicular to the wire axis. The heat treatment performed to form the A-15 superconducting phase leads to partial spheroidization followed by coarsening of the Nb filaments in the reinforcement material. The differences concerning the microstructure of the reinforcement material among the investigated wires were reflected in the broadening of the superconducting transition of Nb, more evident for a field applied parallel to the wire axis. From the magnetic data the wires were also compared in terms of the superconducting volume fraction.

The tensile strain dependence of the upper critical field B-c2 for Nb3Sn wires was measured using a new apparatus for applying tension at low temperature. The critical current I-c calculated from the strain-dependent B-c2 measurements is the almost consistent with the measured I-c. In the measurements by a new apparatus, the axial and the lateral applied strains were examined by the strain gauges attached onto the wire. The deviatoric strain epsilon(dev) and the hydrostatic strain epsilon(hyd) were estimated by the axial and the lateral strain. We found the deviatoric strain dependence of B-c2 is different between the tensile and compressive strain states. This means that the hydrostatic strain also should be taken into account in order to understand the strain effects on the superconducting properties of Nb3Sn.

HgBa2Can-1CunOy (Hg: 12(n -1)n) phases have the same T-c independent of n and the same vortex melting line for n >= 6. In these materials, there are two types of pancake pairs: those separated by the thick block of (n-2) inner CuO2 planes, which are weakly coupled by the magnetic interaction, and, respectively, those separated by the thin HgBa2Ox charge reservoir layer, which are much stronger coupled by the Josephson coupling. The vortex melting line common to all Hg:12(n-1)n phases (n >= 6), separates two new vortex phases, pancake-molecule-glass, and pancake-molecule-liquid. (c) 2008 Elsevier B. V. All rights reserved.

Scanning Hall probe microscopy with an effective spatial resolution of similar to 1 mu m has been used to investigate the vortex structures in superconducting Bi2Sr2CaCu2O8+delta single crystals in the temperature range 77.3-81.3 K and zero applied field (in the presence of the earth field). The vortex images were obtained in real time mode as the temperature increased slowly for 3.36 h. At 77.3 K, the vortices were arranged in a chain structure. With the increase of the temperature, two jumps in the vortex array occur at 77.3 K, immediately when the temperature starts to rise, and at 79.2 K with a good stability between jumps. The second jump is accompanied by the jump in the average magnetic induction when bundles of 4-5 additional vortices enter the scanning area and the vortex array get disordered. These directly visualized transitions in the vortex lattice are consistent with a vortex creep over the surface barriers at high temperatures. A short movie is presented. (c) 2008 Elsevier B.V. All rights reserved.

Structural, thermodynamic and,transport, properties of CeAg1-xNixSb2 compounds are investigated. The Ni substitution of Ag decreases the magnetic ordering temperature from approximate to 9.5 to approximate to 6 K, while the characteristic temperatures, considered as the temperature of the resistivity maximum, increase from 26 to 65 K. (c) 2007 Elsevier B.V. All rights reserved.

Ba2CaCu2O4(O2-yFy) (F-0212), a new high-T-c superconductor (HTSC) including fluorine at an apical site, is an attractive material for application because it shows a highest T-c of 108 K and has thin spacing between conduction layers that is favorable for high critical current density (J(c)) and irreversibility field (B-irr). The polycrystalline samples of F-0212 were synthesized under high pressure. A doping state was controlled from under doping (T-c = 82 K) to slightly-over doping (T-c = 106 K) via optimally-doping (T-c = 108 K). From the DC magnetization hysteresis loops, we determined the intragrain J(c) and B-irr using Bean's critical state model. The J(c) and B-irr properties of F-0212 were found to improve rapidly as the doping state changes from under doping to slightly-over doping to via optimally doping state. The irreversibility lines for the optimally- and slightly-over doped F-0212 are higher than optimally-doped Bi-2212. However, they are much lower than that of Cu-1212 (Y-123) though the spacing of F-0212 is thinner than that of Cu-1212. This indicates that property of the spacing is important to enhance B-irr as well as its thickness. An anisotropy factor for under doped F-0212 was estimated to 118 from a 3D-2D crossover field of about 0.28 T. (C) 2008 Elsevier B.V. All rights reserved.