Magnetic Vortex Phase Diagram for a Non-Optimized CaKFe4As4 Superconductor Presenting a Wide Vortex Liquid Region and an Ultra-High Upper Critical Field

Featured Application This work concerns the study of the vortex phase diagram in CaKFe4As4, whose knowledge can be helpful for the understanding of the applicative ranges in field and temperature of these materials with not-optimized fabrication characteristics, as usually is found in superconducting wires and cables for power applications. To draw a complete vortex phase diagram for a CaKFe4As4 polycrystalline iron-based superconductor, different kinds of magnetic measurements have been performed focusing on the critical parameters of the sample. Firstly, magnetic moment versus field measurements m(H) were performed at low fields in order to evaluate the lower critical field H-c1. After that, by performing relaxation measurements m(t), a field crossover H-cross was detected in the framework of a strong pinning regime. The irreversibility field H-irr as a function of the temperature curve was then drawn by plotting the critical current densities J(c) versus the field for temperatures near T-c. J(c)(H) has demonstrated a second magnetization peak effect phenomenon, and the second peak field H-sp has been identified and plotted as a function of temperature, providing information about an elastic to plastic transition in the vortex lattice. Finally, the upper critical field H-c2 as a function of the temperature has been obtained. H-c1, H-cross, H-sp, H-irr, H-c2 have been fitted and used for drawing the complete vortex phase diagram of the sample. It can be helpful for the understanding of the applicative ranges in the field and temperature of the materials with not-optimized fabrication characteristics, as usually is found in superconducting wires and cables for power applications.