Second magnetization peak, anomalous field penetration, and Josephson vortices in KCa[Formula: see text]Fe[Formula: see text]As[Formula: see text]F[Formula: see text] bilayer pnictide superconductor.

ABSTRACT

We performed magnetization measurements in a single crystal of the anisotropic bilayer pnictide superconductor KCa[Formula see text]Fe[Formula see text]As[Formula see text]F[Formula see text], with [Formula see text] [Formula see text] 34 K, for [Formula see text] [Formula see text] [Formula see text]-axis and [Formula see text] [Formula see text] [Formula see text]-planes. A second magnetization peak (SMP) was observed in the isothermal M(H) curves measured below 16 K for [Formula see text] [Formula see text] [Formula see text]-planes. A peak in the temperature variation of the critical current density, [Formula see text](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, [Formula see text], 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 [Formula see text] [Formula see text]-planes, whereas, for H [Formula see text] [Formula see text]-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, [Formula see text] -pinning. Moreover, the large [Formula see text] observed in our study is consistent with the literature, which advocates this material for high magnetic field applications.