Similarity Between Fe-based Pnictide Superconductors and High-T$_c$ Cuprates Revealed by the Irreversible Magnetic Behavior in the Vortex State

Static and dynamic measurements of DC magnetization and direct real-time magneto-optical imaging have revealed unconventional vortex behavior in NdFeAsO$_{0.9}$F$_{0.1}$\footnote{R. Prozorov \textit{et al.} arXiv:0805.2783 (2008)} and Ba(Fe$_{0.93}$Co$_{0.07}$)$_2$As$_2$\footnote{R. Prozorov \textit{et al.}, Phys. Rev. B in print (2008). arXiv:0810.1338} single crystals. In particular, the fishtail effect, giant magnetic relaxation and a crossover from elastic to plastic vortex creep, reminiscent of similar features found in pure single crystals of Y-Ba-Cu-O and Nd-Ce-Cu-O cuprates. Moreover, despite weak anisotropy of the London penetration depth and second critical field, vortex properties of the pnictides are shown to be very anisotropic. We discuss possible explanations of the observed behavior and conclude that pnictide superconductors bridge a gap between s-wave multiband MgB$_2$ and highly anisotropic d-wave cuprates.