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

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.

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a b s t r a c tWe report on systematic field-cooled magnetization experiments in an Y 0.98 Ca 0.02 Ba 2 Cu 3 O 7Àd single crystal and melt-textured YBa 2 Cu 3 O 7Àd samples. Magnetic fields up to 50 kOe were applied and a paramagnetic response related to the superconducting state was observed. We discuss our results in terms of the flux compression scenario into the sample.Ó 2010 Elsevier B.V. All rights reserved.The paramagnetic Meissner effect (PME) was first observed in polycrystalline samples of the high-T C superconductors (HTSC) [1]. The effect is attributed to the occurrence of a net paramagnetic moment when the materials are cooled through T C in the presence of low and high magnetic fields. This unusual effect seems to be superimposed to the conventional diamagnetism. Several models and explanations have been proposed. The PME has been attributed to spontaneous orbital currents which should occur in granular networks, where p junctions are randomly distributed [2], sample geometry and surface [3], nonequilibrium compressed flux states stabilized by inhomogeneous cooling [4], the formation of a giant vortex on the sample surface [5], etc. Recently, the PME has been observed in YBaCuO/LaCaMnO superlattices [6].In this paper, we report on field-cooled-cooling (FCC) magnetization measurements as functions of the temperature in an Y 0.98 Ca 0.02 Ba 2 Cu 3 O 7Àd single crystal (YCaBaCuO) and melt-textured YBa 2 Cu 3 O 7Àd (YBaCuO) samples. Magnetic fields up to H = 50 kOe were applied parallel and perpendicular to the c-axis. The melttextured YBaCuO sample was prepared by top-seeding technique [7], with 25 wt.% of Y 2 BaCuO 5 (Y211), and the single crystalline YCaBaCuO sample was grown by the self-flux method [8]. The magnetization measurements were done with a Quantum Design SQUID magnetometer. The SQUID signal and the sample centering were monitored during the experimental runs. Magnetic fields up to 50 kOe were applied either parallel or perpendicular to the ab plane in both samples. The results were corrected for the demagnetizing factor and the spin contribution from the Y211 particles was subtracted from the raw data in melt-textured sample. Fig. 1 shows the FCC magnetization measurements for the YCaBaCuO single crystalline sample in fields between 0.01 and 50 kOe applied parallel to the c-axis. In Fig. 1 we observe that, at the lowest applied field, the moment is diamagnetic, but the diamagnetism decreases when the field is augmented. When the field magnitude reaches about 5 kOe the magnetic moment shows a sign reversal. For higher fields, the FC magnetization becomes increasingly positive, as illustrated by results obtained at H = 50 kOe. A small diamagnetic dip always occurs nearly below T C in experiments performed in this field range. The experiments were repeated and the results were confirmed. Similar results were obtained when the magnetic field was applied parallel to the ab plane. Fig. 2 shows the FCC magnetization measurements for the melt-textured YBaCuO sample in fields between 0.05 and 50 k...

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