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

摘要

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