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 down to at least the micrometer scale. The in-plane properties are isotropic, as expected for the tetragonal symmetry, and 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(T, x). The evolution of this fishtail feature with doping is discussed. In particular we find that Hp, measured at the same reduced temperature for different x, is a unique monotonic function of the superconducting transition temperature, Tc(x), across all dopings. 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.