Second magnetization peak, anomalous field penetration, and Josephson vortices in KCa2 Fe4As4F2 bilayer pnictide superconductor

We performed magnetization measurements in a single crystal of the anisotropic bilayer pnictide superconductor KCa2 Fe4As4F2, with Tc similar or equal to 34 K, for H vertical bar vertical bar c-axis and H vertical bar vertical bar ab-planes. A second magnetization peak (SMP) was observed in the isothermal M(H) curves measured below 16 K for H vertical bar vertical bar ab-planes. A peak in the temperature variation of the critical current density, J(c)(T), at 16 K, strongly suggests the emergence of Josephson vortices at lower temperatures, which leads to the SMP in the sample. In addition, it is noticed that the appearance of Josephson vortices below 16 K renders easy magnetic flux penetration. A detailed vortex dynamics study suggests that the SMP can be explained in terms of elastic pinning to plastic pinning crossover. Furthermore, contrary to the common understanding, the temperature variation of the first peak field, H-1, below and above 16 K, behaves non-monotonically. A highly disordered vortex phase, governed by plastic pinning, has been observed between 17 and 23 K, within a field region around an extremely large first peak field. Pinning force scaling suggests that the point defects are the dominant source of pinning for H vertical bar vertical bar ab-planes, whereas, for H vertical bar vertical bar c-axis, point defects in addition to surface defects are at play. Such disorder contributes to the pinning due to the variation in charge carrier mean free path, delta l-pinning. Moreover, the large J(c) observed in our study is consistent with the literature, which advocates this material for high magnetic field applications.

Automated summary

Magnetization measurements were performed on KCa2Fe4As4F2, an anisotropic bilayer pnictide superconductor with Tc≈34 K. A second magnetization peak (SMP) was observed below 16 K, suggesting the emergence of Josephson vortices. Vortex dynamics study revealed that the SMP can be explained by elastic pinning to plastic pinning crossover. Disorder caused by point defects contributed to pinning, and the material showed potential for high magnetic field applications.