Doping dependent Irreversible Magnetic Properties of Ba(Fe1-xCox)2As2 Single Crystals

We discuss the irreversible magnetic properties of self-flux grown Ba(Fe1-xCox)2As2 single crystals for a wide range of concentrations covering the whole phase diagram from the underdoped to the overdoped regime, x=0.038, 0.047, 0.058, 0.071, 0.074, 0.10, 0.106 and 0.118. Samples were characterized by a magneto-optical method and show excellent spatial uniformity of the superconducting state. The overall behavior closely follows classical Bean model of the critical state. The field-dependent magnetization exhibits second peak at a temperature and doping - dependent magnetic field, Hp. The evolution of this fishtail feature with doping is discussed. Magnetic relaxation is time-logarithmic and unusually fast. Similar to cuprates, there is an apparent crossover from collective elastic to plastic flux creep above Hp. At high fields, the field dependence of the relaxation rate becomes doping independent. We discuss our results in the framework of the weak collective pinning and show that vortex physics in iron-based pnictide crystals is much closer to high-Tc cuprates than to conventional s-wave (including MgB2) superconductors.Comment: for the special issue of Physica C on iron-based pnictide superconductor

Magneto-optical Imaging of Transport Current Densities in Superconductors

Direct imaging of the paths of transport currents in superconductors creates many new possibilities for exploring the basic features of vortex pinning mechanisms and for improving the performance of superconducting materials. A technique for imaging the path and magnitude of the transport current density flowing in superconductors is described. Results are given for a 37-filament BSCCO 2223 powder-in-tube wire, showing a highly inhomogeneous current path within the filaments