Plastic pinning replaces collective pinning as the second magnetization peak disappears in the pnictide superconductor Ba0.75K0.25Fe2As2

We report a detailed study of isofield magnetic relaxation and isothermal magnetization measurements with H parallel to c on an underdoped Ba0.75K0.25Fe2As2 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 behavior of the SMP has been studied by measuring the magnetic field dependence of relaxation rateR(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 similar to 3.4 x 10(9) A/m(2) at 10 K in the SMP region, which is comparable to an optimally doped Ba-KFe2As2 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.