  The lossless current-carrying capacity of a superconductor is limited by its
critical current density (Jc). A key to enhance Jc towards real-life
applications is engineering defect structures to optimize the pinning
landscape. For iron-based superconductors (IBSs) considered as candidate
materials for high-field applications, high Jc values have been achieved by
various techniques to introduce artificial pinning centres. Here we report
extraordinary vortex pinning properties in CaKFe4As4 (CaK1144) arising from the
inherent defect structure. Scanning transmission electron microscopy revealed
the existence of nanoscale intergrowths of the CaFe2As2 phase, which is unique
to CaK1144 formed as a line compound. The Jc properties in CaK1144 are found to
be distinct from other IBSs characterized by a significant anisotropy with
respect to the magnetic field orientation as well as a remarkable pinning
mechanism significantly enhanced with increasing temperature. We propose a
comprehensive explanation of the Jc properties based on the unique intergrowths
acting as pinning centres.
