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

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 Hc1. After that, by performing relaxation measurements m(t), a field crossover Hcross was detected in the framework of a strong pinning regime. The irreversibility field Hirr as a function of the temperature curve was then drawn by plotting the critical current densities Jc versus the field for temperatures near Tc. Jc(H) has demonstrated a second magnetization peak effect phenomenon, and the second peak field Hsp 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 Hc2 as a function of the temperature has been obtained. Hc1, Hcross, Hsp, Hirr, Hc2 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.