Defects related vortex dynamics behaviors in the ‘1144’-type iron-based superconductors

Abstract We have synthesized ‘1144’- type (Ca1-xPrxKFe4As4 and CaKFe4As4), and ‘122’-type (Ba0.6K0.4Fe2As2 and Ba0.6Rb0.4Fe2As2) iron-based superconductors. Based on these crystals, we have investigated the vortex dynamics of the ‘1144’- and ‘122’-type iron-based superconductors. The results indicate that the novel flux dynamics behaviors of ‘1144’-type iron-based superconductors are closely related to their defect structures. Firstly, the intergrowths in ‘1144’-type superconductors can function as columnar defects under the applied magnetic field H // ab-plane, which results in a dip structure in the magnetization hysteresis loops (MHLs) near zero field due to the self-field effect. Secondly, the intergrowths as strong pinning sites provide a large pulling force to the flux lines and cause a small magnetization peak at temperatures below 10 K under magnetic fields from about 3 to 7 T in MHLs for the ‘1144’-type superconductors. Thirdly, there may be a vortex entanglement due to the presence of strong pinning centers, which impedes the Elastic to Plastic motion of vortices (E-P phase transition) resulting in a weak second magnetization peak (SMP) effect in the bulk ‘1144’-type superconductors. Fourthly, the intergrowths introduce disorder into the superconductors and accelerate the magnetic 3D-to-2D transition, which causes the thickness-dependent Jc and SMP effect due to the rigidity of flux lines with the reducing sample thickness. The present results are significant for us to understand the relationship between the flux pinning behaviors and the defect structures for a superconductor.