Flux dynamics and vortex phase diagram in Ba(Fe1-xCox)2As2 single crystals revealed by magnetization and its relaxation

Magnetization and its relaxation have been measured in Ba(Fe1-xCox)(2)As-2 single crystals at various doping levels ranging from very underdoped to very overdoped regimes. A sizable magnetization-relaxation rate has been observed in all samples, indicating a moderate vortex motion and relatively small characteristic pinning energy. Detailed analysis leads to the following conclusions. (1) A prominent second-peak (SP) effect was observed in the samples around the optimal doping level (x approximate to 0.08), but it becomes invisible or very weak in the very underdoped and overdoped samples. (2) The magnetization-relaxation rate is inversely related to the transient superconducting current density revealing the nonmonotonic field and temperature dependence through the SP region. (3) A very sharp magnetization peak was observed near zero field which corresponds to a much reduced relaxation rate. (4) A weak temperature dependence or a plateau of relaxation rate was found in the intermediate-temperature region. Together with the treatment of the generalized inversion scheme, we suggest that the vortex dynamics is describable by the collective pinning model. Finally, vortex phase diagrams were drawn for all the samples showing a systematic evolution of vortex dynamics.