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Abstract: Cu sheathed polycrystalline Ba0.6K0.4Fe2As2 superconducting wire was prepared by a two-step powder-in-tube method. A pressure of 2GPa was applied to a short sample before heat treatment. Magnetization and transport measurements were performed to investigate the effects of cold high pressure densification on the microstructure and superconductivity. The cold pressed sample shows an improved self-field transport critical current density of 2.8×104 A/cm2 (I c =83A) at 4.2K, which is nearly as twice as the unpressed sample. However, both samples manifest pronounced weak-link behavior, suggesting the technique need to be further optimized. The comparison of properties between pressed and unpressed sample and related mechanism was discussed. [Copyright &y& Elsevier]

Abstract: Polycrystalline iron-based superconducting NdFeAsO0.88F0.12 was prepared via solid-state reaction in ambient pressure. Small amounts of ferromagnetic impurities were detected by Scanning Electron Microscopy (SEM) analysis. To study the influence of magnetism on superconducting properties of the sample, magnetization M(T, H) measurements were performed in fields up to 1.5T and in the temperature range of 5–60K. The abnormal behavior was observed in the χ(T) curves, and the magnetization hysteresis loops (MHLs) can be described by a sum of the contributions of superconductive hysteresis, ferromagnetic hysteresis of the impurities and the paramagnetic background of the isolated Nd3+ ions. The influence of the ferromagnetic impurities and the paramagnetic background were investigated and discussed. After the corrections of the magnetism contributions, the intrinsic superconductive MHLs as well as the critical current density were obtained. [Copyright &y& Elsevier]

Abstract: Resistivities of polycrystalline iron-based NdFeAsO0.88F0.12 superconductors prepared via solid-state reaction in ambient pressure (AP) and high pressure (HP) were measured in various magnetic fields ranging from 0 to 9T. Different resistivity broadening behaviors of the AP and HP samples, which may originate from the anisotropic superconductivity, the vortex motion, or the connectivity, were investigated in detail. The effective pinning barriers of both the AP and HP samples were obtained from the Arrhenius plot of the resistivity data by two analytic methods of which one assuming the prefactor 2ρ c U/T of the thermally activated flux flow (TAFF) resistivity ρ=(2ρ c U/T)exp(−U/T) is a constant, while the other assuming the prefactor is temperature dependent. The results determined from the two methods were compared and analysed, and the true effective pinning barriers were obtained. The origin of different pinning barriers of the AP and HP samples is discussed. [Copyright &y& Elsevier]

Abstract: The superconducting properties of polycrystalline NdFeAsO0.88F0.12 oxypnictide were studied by both DC and AC magnetization measurements. The zero-field cooled (ZFC) magnetic susceptibility, χ(T), measured under the magnetic field of 0.5T shows a dramatic decrease at about 11K. The imaginary component of the first harmonics of the AC magnetic susceptibility, χ″(T), increases with the increasing DC field H dc below 11K. These results indicate the onset of robust intergranular superconductivity at low temperatures. The magnetic hysteresis loops show an anomalous double central peak effect at low temperatures, with one peak at positive fields in decreasing positive external fields. This phenomenon suggests the granular character of this system. The paramagnetism of Nd3+ ions tilts the magnetic hysteresis loops and broadens the hysteresis width ΔM. After correction for the paramagnetism, the temperature and field dependence of the average magnetization critical current density J cm was obtained. The related mechanism was discussed. [ABSTRACT FROM AUTHOR]

Abstract: Polycrystalline NdFeAsO0.88F0.12 superconductors prepared by high pressure (HP) and ambient pressure (AP) methods were comparatively studied by magnetization and transport measurements. Upper critical field H c2, irreversibility field H irr and the anisotropy parameter Γ were estimated from resistance transition curves. The broadening of transition width was observed, and was ascribed to both H c2 anisotropy and superconductivity inhomogeneity of samples. Magnetic hysteresis loops (MHLs) in low fields were measured to detect the trace of weak-link behavior. The reclosed hysteresis loops in low fields indicate that there are weak links in both samples. Magnetization critical current density J cm were derived from MHLs. Sample HP shows higher J cm than sample AP. Direct transport I–V measurements show that the transport critical current density J ct are very low but persist up to 9T, suggesting intrinsic strong-link existing in both samples. [ABSTRACT FROM AUTHOR]