  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.
