We performed magnetic moment (m) relaxation measurements at low temperatures (5 less than or equal to T less than or equal to 20 K) in two single phase, c-axis-oriented polycrystalline Bi1.7Pb0.3Sr2Ca2Cu3O10+delta (2223) samples prepared under different conditions with different T-c's. The m versus t data show a logarithmic relation and by fitting them the J(c)(H, T) and U-c(H, T) variation were extracted. (C) 2000 Elsevier Science B.V. All rights reserved.

The relaxation of the magnetic moment of a thin disk of YBa2Cu3O7-delta superconductor in a perpendicular magnetic field shows a logarithmic dependence on time for the examined temperature range T: 10-60 K. Data are analyzed within an analytical model. The temperature dependencies of the critical current j(c), and the pinning potential U-0, are estimated by fitting the relaxation data. Furthermore, the E versus j characteristics are quantitatively derived from the relaxation data at the circumference of the disk. (C) 2000 Elsevier Science B.V. All rights reserved.

The magnetic critical behavior in the series La-0.67(BaxCa1-x)(0.33)MnO3 (0 less than or equal to x less than or equal to 1) of manganese perovskites is studied by means of DC magnetic measurements. As Ca ions are substituted by the larger Ba ions, the transition temperature increases from 265 to 340 K as a result of structural changes. Arrott plots show that only in samples with x greater than or equal to 0.25 the magnetic properties follow the behavior expected for a conventional second-order ferromagnetic transition. The values of critical exponents (beta, gamma) estimated by modified Arrott plots are found to lie between those predicted for a 3D-Heisenberg model and those of mean field theory. (C) 2000 Elsevier Science B.V. All rights reserved.

We used local AC-susceptibility and global DC magnetization measurements to determine the magnetic phase diagram of two HgBa2CuO4+delta single crystals with T-c = 95 and 89.9 K, respectively. The corresponding lines for the more anisotropic crystal (T-c = 89.9 K, crystal A) lie lower than those of the nearly optimally doped crystal (T-c = 95 K, crystal B). The irreversibility line (IL) decreases exponentially in the temperature range 20 less than or equal to T less than or equal to T*. At T* = 82 and 88 K the IL change slope and varies with a power law up to the T-c. Certain features in these measurements are also discussed. (C) 2000 Elsevier Science B.V. All rights reserved.

Local Hall dc-susceptibility and global dc-magnetization measurements are used in order to construct the vortex matter phase diagram for a HgBa2CuO4+x single crystal with T similar to 89.9 K. For T > T* = 82 K, the points where the step in rile local ac-susceptibility measurements and the anomaly in the de magnetization measurements occur fall on the same curve, indicating that the same phenomenon is observed in both cases. We modeled these points by a power law H-m(T) = H-o(1-T/T-c)(a), with a = 2.4 +/- 0.1 and H-o = 267 +/- 125 kOe. The local Nail ac-susceptibility data, below the "step" temperature, show a behavior which can be explained if we suppose that at the vicinity of the step a local peak in the J(c)(T) curve exists (so-called peak effect). (C) 2000 Elsevier Science B.V. All rights reserved.