  Isothermal magnetic field dependence of magnetization and the magnetic
relaxation measurements were performed for $H$$\parallel$c axis on single
crystal of Ba(Fe$_{0.935}$Co$_{0.065}$)$_2$As$_2$ pnictide superconductor
having $T_c$ = 21.7 K. The second magnetization peak (SMP) for each isothermal
$M(H)$ was observed in a wide temperature range from $T_c$ to the lowest
temperature of measurement (2 K). Magnetic field dependence of relaxation rate
$R(H)$, shows a peak (H$_{spt}$) between H$_{on}$ (onset of SMP in $M(H)$) and
H$_p$ (peak field of SMP in $M(H)$), which is likely to be related with a
vortex-lattice structural phase transition, as suggested in literature for
similar sample. In addition, the magnetic relaxation measured for magnetic
fields near H$_{spt}$ show some noise which might be the signature of the
structural phase transition of the vortex lattice. Analysis of the magnetic
relaxation data using Maley's criterion and the collective pinning theory
suggests that the second magnetization peak (SMP) in the sample is due to the
collective (elastic) to plastic creep crossover, which is also accompanied with
a rhombic to square vortex lattice phase transition. Analysis of the pinning
force density suggests single dominating pinning mechanism in the sample and is
not showing the usual $\delta$l and $\delta T_c$ nature of pinning. The
critical current density ($J_c$) estimated using the Bean's critical state
model is found to be 5 $\times$ 10$^5$ A/cm$^2$ at 2 K in the zero magnetic
field limit. Surprisingly, the maximum in the pinning force density is not
responsible for the maximum value of the critical current density in the
sample.

Superconductivity