“DEVELOPMENT OF IRON-BASED SUPERCONDUCTING MATERIALS AND THEIR ELECTROMAGNETIC CHARACTERISATION”

This doctoral thesis investigates the structural and superconducting properties of 1144 iron-based superconductors, with particular focus on CaKFe₄As₄, in the broader context of the development of advanced materials for high-field applications. Superconductors are key enablers for technologies such as lossless power transmission, medical imaging, and magnetic confinement in fusion reactors, including projects like ITER/DEMO. In this framework, iron-based superconductors—and especially 1144 compounds—offer a promising balance of relatively high critical temperature, large upper critical fields, reduced anisotropy, and mechanical robustness, making them attractive for next-generation superconducting devices. The work combines the synthesis and characterization of both polycrystalline samples and high-quality single crystals, enabling a systematic exploration of the effects of chemical substitution and defect engineering. Aliovalent doping and rare-earth substitutions reveal that structural distortions play a dominant role in determining superconducting properties. In particular, the c/a ratio emerges as the key parameter controlling the superconducting critical temperature (Tc), which exhibits a universal dome-like dependence. This finding indicates that Tc suppression is primarily governed by structural effects rather than carrier concentration. Detailed single-crystal analysis further shows that doping modifies the local Fe–As coordination, establishing a clear and distinctive structure–property relationship compared to other iron pnictide families. The study also highlights the importance of microstructural features in polycrystalline materials. Grain boundaries and environmental exposure lead to oxidation processes confined at intergranular regions, forming insulating layers that degrade grain connectivity and significantly suppress the critical current density (Jc), especially under high magnetic fields. Optimized synthesis routes improve phase purity and connectivity, supporting the potential of these materials for technological processing. A major focus is devoted to vortex pinning and its enhancement through both chemical doping and particle irradiation. Rare-earth substitutions introduce nanoscale strain fields that act as effective pinning centers, enhancing Jc, particularly in high-field conditions. Complementary defect engineering using protons, Xe ions, and γ-rays allows controlled tuning of the pinning landscape: proton irradiation generates homogeneous point defects that enhance Jc and reduce anisotropy; Xe irradiation introduces correlated columnar defects that maximize Jc at low and intermediate fields but become less effective at high fields; γ-ray irradiation produces negligible changes. Overall, this work demonstrates that chemical substitution and irradiation provide powerful and complementary strategies to tailor the superconducting performance of CaKFe₄As₄. By establishing a unified correlation between structural parameters, defect landscape, and vortex pinning, the thesis contributes to the fundamental understanding of 1144 superconductors and provides practical guidelines for their optimization in high-field and energy-related applications