Second magnetization peak, anomalous field penetration, and Josephson vortices in KCa$_2$Fe$_4$As$_4$F$_2$ bilayer pnictide superconductor

TL;DR

This study investigates vortex dynamics and pinning mechanisms in the bilayer pnictide superconductor KCa$_2$Fe$_4$As$_4$F$_2$, revealing a second magnetization peak linked to Josephson vortices and disorder effects, with implications for high-field applications.

Contribution

It provides the first detailed analysis of vortex behavior, pinning mechanisms, and the emergence of Josephson vortices in KCa$_2$Fe$_4$As$_4$F$_2$, highlighting novel insights into its magnetic properties.

Findings

Observation of second magnetization peak (SMP) below 16 K for H//ab.

Identification of Josephson vortices contributing to SMP.

Pinning dominated by point defects and surface defects depending on field orientation.

Abstract

We performed magnetization measurements in a single crystal of the anisotropic bilayer pnictide superconductor KCa$_2$Fe$_4$As$_4$F$_2$, with $T_c$ $\simeq$ 34 K, for $H$$\parallel$$c$-axis and $H$$\parallel$$ab$-planes. A second magnetization peak (SMP) was observed in the isothermal $M(H)$ curves measured below 16 K for $H$$\parallel$$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 K 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 $H$ $\parallel$$ab$-planes, whereas, for H $\parallel$$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.