The local ac and dc magnetic response in an untwinned YBa2Cu3O7-delta crystal has been measured using a microscopic Hall sensor. Due to the low pinning energy at elevated temperatures (near T-c), the ac response of the platelet-shaped crystal is governed by the geometrical barrier. From the field where the first vortex penetrates into the center of the crystal we estimated the first critical field (H-c1). The temperature variation of H-c1 near T-c is compatible with a strongly type-H superconductor, where the fluctuations in the order parameter (Psi) are those of an uncharged superfluid of the three dimensional XY model. In order to extract a reasonable value for the London penetration length at T=0 K, a value c(0)=3 is needed for the core contribution of the line energy of the vortex.

Local Hall probe ac permeability and global dc SQUID magnetization measurements were used to construct the phase diagram of the vortex matter for two HgBa2CuO4+delta single crystals with different amounts of disorder. Emphasis was given near T-c and for low magnetic fields H-dc < 1 kOe. For the nearly optimally doped crystal with T-c = 94.8 K, the second peak line H-sp and the respective onset line H-min of the magnetization loops end at (T*, H*) = (88 K, 2 kOe) on the irreversibility line, while for the crystal of lower T-c (= 89.9 K) decreases monotonically and terminates just below T-c In addition, for the crystal with T-c = 89.9 K the local permeability measurements (as a function of temperature or dc magnetic field) revealed a peak in the screening current just below the onset of the diamagnetic screening. The possible sublimation character of the melting transition, the two-stage melting scenario and a Bragg to vortex glass transition are discussed. The different behaviours observed between the two crystals is attributed to the different amount of disorder.

In this work we report on the crystal growth process, the crystal structure and the phase diagram of HgBa2CuO4+chi high T-e superconductor. Precise global DC magnetization measurements, using a SQUID magnetometer, have been performed, on two single crystals in magnetic fields parallel to the c-axis (H \ \ c) with T-c = 95 and 89.9 K, respectively. The irreversibility curve H-irr(T) decreases exponentially up to a temperature T* and after it changes slope abruptly. The magnetization curves at a fixed temperature as a function of magnetic field show two peaks, namely H-fp(T) and H-sp(T). (C) 2001 Elsevier Science B.V. All rights reserved.

We present a modified method of preparation of the new superconductor MgB2. The polycrystalline samples were characterized using X-ray and magnetic measurements. The surface barriers control the isothermal magnetization loops in powder samples. In bulk as prepared samples we always observed symmetric magnetization loops indicative of the presence of a bulk pinning mechanism. Magnetic relaxation measurements in the bulk sample reveal a crossover of surface barrier to bulk pinning.