Irreversibility lines and anomalous Meissner effect in Ba(Fe1-xCox)2As2 superconducting crystals

Magnetization curves were measured in Ba(Fe_1-xCo_x)₂As₂ crystals grown by an indium flux method. A very different vortex dynamics behavior was found when comparing results for crystals in the underdoped and slightly above the optimal doped regions. The irreversibility line is closer to the upper critical field line in the latter case, which indicates a much strong pinning of vortices for the near optimally doped crystal. Also, field cooled magnetization curves revealed an anomalous Meissner effect, where a diamagnetic response increases with the applied field, in both crystals. Possible mechanisms involving vortex pinning at interfaces or by local magnetic ions are presented.