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 superconducting fluctuations well inside the normal state of Fe-based superconductors were studied through measurements of the in-plane paraconductivity and magnetoconductivity in high quality BaFe 2−x Ni x As 2 crystals with doping levels from the optimal level (x = 0.10) up to highly overdoped (x = 0.20). These measurements, performed in magnetic fields up to 9 T perpendicular to the ab (Fe) layers, allowed a reliable check of the applicability to iron-based superconductors of Ginzburg-Landau approaches for 3D anisotropic compounds, even at high reduced temperatures and magnetic fields. Our results also allowed us to gain valuable insight into the dependence on the doping level of some central superconducting parameters (coherence lengths and anisotropy factor).

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.

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We report on isofield magnetization curves obtained as a function of temperature in two single crystals of Ba1−xKxF e2As2 with superconducting transition temperature Tc=28K and 32.7 K. Results obtained for fields above 20 kOe show a well-defined rounding effect on the reversible region extending 1-3 K above Tc(H) masking the transition. This rounding appears to be due to threedimensional critical fluctuations, as the higher field curves obey a well know scaling law for this type of critical fluctuations. We also analyzed the asymptotic behavior of √ M vs.T curves in the reversible region which probes the shape of the gap near Tc(H). Results of the analysis suggests that phase fluctuations are important in Ba1−xKxF e2As2 which is consistent with nodes in the gap.