Plastic pinning replaces collective pinning as the second magnetization peak disappears in the pnictide superconductor Ba-KFe$_2$As$_2$

TL;DR

This study investigates the disappearance of the second magnetization peak in Ba-KFe2As2 superconductors, revealing a transition from collective to plastic pinning above 12 K and highlighting the material's potential for technological use.

Contribution

It provides detailed magnetic relaxation and magnetization analysis showing a pinning mechanism transition and unconventional pinning behavior in Ba-KFe2As2.

Findings

Second magnetization peak vanishes above 12 K.

Plastic pinning replaces collective pinning at higher temperatures.

Critical current density is comparable to optimally doped samples.

Abstract

We report a detailed study of isofield magnetic relaxation and isothermal magnetization measurements with $H$$\parallel$c on an underdoped Ba$_{0.75}$K$_{0.25}$Fe$_2$As$_2$ pnictide single crystal, with superconducting transition temperature $T_c$ = 28 K. The second magnetization peak (SMP) has been observed at temperatures below $T_c$/2 and vanished at higher temperatures. The observed behaviour of the SMP has been studied by measuring the magnetic field dependence of relaxation rate, $R(H)$ and by performing the Maley's analysis. The results suggest that the crossover from collective to plastic pinning observed in the SMP disappears above 12 K with plastic pinning replacing collective pinning. An interesting $H$-$T$ phase diagram is obtained. The critical current density ($J_c$) was estimated using Bean's model and found to be $\sim$ $3.4 \times 10^9$ A/m$^2$ at 10 K in the SMP region, which is comparable to an optimally doped Ba-KFe$_2$As$_2$ superconductor and may be exploited for potential technological applications. The pinning mechanism is found to be unconventional and does not follow the usual $\delta l$ and $\delta T_c$ pinning models, which suggest the intrinsic nature of pinning in the compound.