Irreversibility lines of layered Fe-based superconductors with thick blocking layers

Irreversibility lines of the layered iron-arsenide superconductors with thick perovskite-type oxide blocking layers have been systematically studied. Five compounds with different interlayer distance between iron layers from 1.5 to 3.0 nm were selected for the magnetization measurements. These compounds showed very large magnetically reversible region. The irreversibility lines of these compounds were found to locate at very low fields compared to the other iron-based superconductors and shift towards lower fields with an increase in the thickness of the blocking layer. In addition, the analysis on the irreversibility line suggested that the change of dimensionality of the vortex system occurs at T / T c ∼ 0.6 . These results indicate that the electromagnetic anisotropy originated from the layered crystal structure plays a crucial role to determine the irreversibility line and magnetic phase diagram of layered iron arsenides as is the case in the layered cuprates.