We report on the magnetic properties of an insulating cubic compound Cu3TeO6 studied by ac and dc susceptibility, torque magnetometry and neutron powder diffraction. A novel three-dimensional magnetic lattice composed of almost planar regular hexagons of Cu2+ S = 1/2 spins is present in Cu3TeO6. The magnetic susceptibility in the paramagnetic state obeys the Curie–Weiss law in the 200–330 K regime with ΘCW = −148 K and at TN = 61 K system undergoes an antiferromagnetic phase transition. Above TN the susceptibility is isotropic. Below TN a large anisotropy develops in fields H≥500 Oe. Torque measurements reveal the presence of antiferromagnetic domains below TN. In a rather low magnetic field ( Oe) switching of domains is observed. The dynamics related to movement of domain walls is very slow at low temperatures (of the order of 102 s) and interferes with all torque measurements. The presence of domains is a consequence of the symmetry of the underlying magnetic lattice. Neutron powder diffraction reveals that antiferromagnetic long-range order is associated with the wavevector . The dominant component of the magnetic moment is along one of the space diagonals of the cubic unit cell, but it is not possible to resolve whether the structure is collinear or canted.

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Various aspects of the problem of current transfer in high- superconductors (HTSs) are reviewed. The spatial inhomogeneities of various types are identified as a primary cause of non-uniformity of both normal currents and supercurrents in real samples of HTSs. The role these inhomogeneities play in transport features of the samples is discussed. The case of grain boundaries in polycrystalline samples is elaborated in detail. The local structural and transport properties of isolated grain boundaries are first reviewed and then integrated into the knowledge of global (macroscopic) charge transport. The paper emphasizes the common ingredients characterizing the transport in various forms and families of HTS samples in small magnetic fields. The phenomenon of percolation is identified as the most obvious one and is shown to dominate a large number of observations covered by this report. The experimental results focused on by this report elaborate primarily the problems of critical currents, initial dissipation and current-voltage characteristics, penetration depth, resistive and metal-insulator transition, resistance noise and magneto-optical studies of current paths. Various models for current transfer (disordered bonds, brick wall and railway switch) are also reviewed and discussed.

We report high-sensitivity ac susceptibility measurements of the penetration depth in the Meissner state of the layered organic superconductor -(BEDT-TTF) 2 Cu͓N(CN) 2 ͔Br. We have studied nominally pure single crystals from the two different syntheses and employed controlled cooling procedures in order to minimize intrinsic remnant disorder at low temperatures associated with the glass transition, caused by ordering of the ethylene moieties in BEDT-TTF molecule at T G ϭ75 K. We find that the optimal cooling procedures ͑slow cooling of Ϫ0.2 K/h or annealing for 3 days in the region of T G ) needed to establish the ground state depend critically on the sample origin, indicating different relaxation times of terminal ethylene groups. We show that, in the ground state, the behavior observed for nominally pure single crystals from both syntheses is consistent with unconventional d-wave order parameter. The in-plane penetration depth in (T) is strongly linear, whereas the out-of-plane component out (T) varies as T 2 . In contrast, the behavior of single crystals with long relaxation times observed after slow (Ϫ0.2 K/h) cooling is as expected for a d-wave superconductor with impurities ͓i.e., in (T)ϰ out (T)ϰT 2 ] or might be also reasonably well described by the s-wave model. Our results might reconcile the contradictory findings previously reported by different authors.

We report ac susceptibility data on RuSr2(Eu,Ce)2Cu2O10−y (Ru-1222, Ce content x=0.5 and 1.0), RuSr2GdCu2O8 (Ru-1212) and SrRuO3. Both Ru-1222 (x=0.5, 1.0) sample types exhibit unexpected magnetic dynamics in low magnetic fields: logarithmic time relaxation, switching behavior, and 'inverted' hysteresis loops. Neither Ru-1212 nor SrRuO3 exhibit such magnetic dynamics. The results are interpreted as evidence of the complex magnetic order in Ru-1222. We propose a specific multilayer model to explain the data, and note that superconductivity in the ruthenocuprate is compatible with both the presence and absence of the magnetic dynamics.