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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: 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]