We report on isofield magnetic relaxation data on a single crystal of Ba 1−x K x Fe 2 As 2 with superconducting transition temperature T c = 32.7 K which exhibit the so-called fish-tail effect. A surface map of the superconducting transition temperature shows that the superconducting properties are close to homogeneous across the sample. Magnetic relaxation data, M͑t͒, was used to obtain the activation energy U͑M͒ in order to study different vortex-dynamics regimes. Results of this analysis along with time-dependent measurements as a function of field and temperature extended to the reversible region of some M͑H͒ curves demonstrate that the irreversibility as well the second magnetization peak position, H p ͑T͒, are time dependent and controlled by plastic motion of the vortex state. In the region delimited by a characteristic field Hon

The influence of the nature of the boron precursor on the superconducting properties of polycrystalline MgB 2 was studied. Critical current densities (J c 's) for the MgB 2 made from high purity amorphous boron are at least a factor of three higher than typical values measured for standard MgB 2 samples made from amorphous precursors. Two possible mechanisms are proposed to account for this difference. Samples made from crystalline boron powders have around an order of magnitude lower J c 's compared to those made from amorphous precursors. X-ray, T c and resistivity studies indicate that this is as a result of reduced current cross section due to the formation of (Mg)B-O phases. The samples made from amorphous B contain far fewer Mg(B)-O phases than crystalline B despite the fact that the amorphous B contains more B 2 O 3 . The different reactivity rates of the precursor powders accounts for this anomaly.

We report a detailed study of isofield magnetic relaxation and isothermal magnetization measurements with H c on an underdoped Ba0.75K0.25Fe2As2 pnictide single crystal, with superconducting transition temperature Tc = 28 K. The second magnetization peak (SMP) has been observed at temperatures below Tc/2 and vanished at higher temperatures. The observed behaviour of the SMP has been studied by measuring the magnetic field dependence of relaxation rate, R(H) and by performing the Maley's analysis. The results suggest that the crossover from collective to plastic pinning observed in the SMP disappears above 12 K with plastic pinning replacing collective pinning. An interesting H-T phase diagram is obtained. The critical current density (Jc) was estimated using Bean's model and found to be ∼ 3.4 × 10 9 A/m 2 at 10 K in the SMP region, which is comparable to an optimally doped Ba-KFe2As2 superconductor and may be exploited for potential technological applications. The pinning mechanism is found to be unconventional and does not follow the usual δl and δTc pinning models, which suggest the intrinsic nature of pinning in the compound.

Here we use global and local magnetometry and Hall probe imaging to investigate the electromagnetic connectivity of the superconducting current path in the oxygendeficient fluorine-free Nd-based oxypnictides. High resolution transmission electron microscopy and scanning electron microscopy show strongly-layered crystallites, evidence for a ~ 5nm amorphous oxide around individual particles, and second phase neodymium oxide which may be responsible for the large paramagnetic background at high field and at high temperatures. From global magnetometry and electrical transport measurements it is clear that there is a small supercurrent flowing on macroscopic sample dimensions (mm), with a lower bound for the average (over this length scale) critical current density of the order of 10 3 A/cm 2 . From magnetometry of powder samples and local Hall probe imaging of a single large conglomerate particle ~120 microns it is clear that on smaller scales, there is better current connectivity with a critical current density of the order of 5 x 10 4 A/cm 2 . We find enhanced flux creep around the second peak anomaly in the magnetisation curve and an irreversibility line significantly below H c2 (T) as determined by ac calorimetry.

The vortex phase diagrams of NdFeAsO 0.85 F 0.15 and NdFeAsO 0.85 superconductors are determined from the analysis of resistivity and current-voltage (I-V) measurements in magnetic fields up to 9 T. A clear vortex glass to liquid transition is identified only in the oxygen deficient NdFeAsO 0.85 , in which I-V curves can be well scaled onto liquid and glass branches consistent with the vortex glass theory. With increasing magnetic field, the activation energy U 0 , deduced from the Arrhenius plots of resistivity based on the thermally activated flux-flow model (TAFF), decays more quickly for NdFeAsO 0.85 F 0.15 than for NdFeAsO 0.85. Moreover, the irreversibility field H irr of NdFeAsO 0.85 increases more rapidly than that of NdFeAsO 0.85 F 0.15 with decreasing temperature. These observations evidence the strong vortex pinning effects, presumably caused by the enhanced defects and disorders in the oxygen deficient NdFeAsO 0.85. It is inferred that the enhanced defects and disorder can be also responsible for the vortex glass to liquid * 1 transition in the NdFeAsO 0.85 .