The iron-based superconductors (IBSs) have attracted tremendous attention in the past decade due to their high critical temperature and upper critical fields as well as low anisotropy. Among IBSs, AeAFe4As4 (AeA1144, where Ae and A are alkaline earth and alkaline elements, respectively), is recently developed which is composed of alternating AeFe2As2 (Ae122) and AFe2As2 (A122) components. 1144 superconductors are considered to be clean and stoichiometric along with superior superconducting performance, hence, regarded as another promising candidate for developing high-field applications. While stacking faults were reported in the single crystalline CaK1144, those of polycrystalline samples are not yet studied. In the present work, we explored the defect structures that are responsible for superior flux pinning nature of the polycrystalline CaK1144 polycrystals by means of extensive scanning transmission electron microscopy (STEM). Two types of stacking faults, (i) straight and (ii) step-like single-layered CaFe2As2 (Ca122), of ~ 1 nm size (c-axis) were observed in the CaK1144 matrix. Geometrical phase analysis indicated that the strains were generated and are localized near the Ca122 defect layers due to fine sized lattice mis-match defects in the matrix of 1144. A self-field critical current density (Jc) of 15.2 kA/cm2 was obtained at 5 K, and the curves were sustained above 30 K with a considerable Jc value of 1.4 kA/cm2. The superior field and temperature dependence Jc in Cak1144 are supported by the effective nanoscale defects of fine-sized stacking faults, a lattice mismatch, and stress fields and their different sizes. Present work gives a correlation between the local microstructure and superconductivity of the polycrystalline CaK1144 materials. The intrinsically intergrown planar defects, even in polycrystalline samples, are expected to be advantageous for various high-field applications of bulk CaK1144 superconductors.
Keywords: CaKFe4As4 superconductor, STEM, Defect structures, Critical current density