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We report on isofield magnetic relaxation data on a single crystal of$Ba_{1-x}K_xFe_2As_2$ with superconducting transition temperature $T_c$= 32.7 Kwhich exhibit the so called fish-tail effect. A surface map of thesuperconducting transition temperature shows that the superconductingproperties are close to homogeneous across the sample. Magnetic relaxationdata, M(t), was used to obtain the activation energy U(M) in order to studydifferent vortex dynamics regimes. Results of this analysis along with timedependent measurements as a function of field and temperature extended to thereversible region of some M(H) curves demonstrate that the irreversibility aswell the second magnetization peak position, $H_p(T)$, are time dependent andcontrolled by plastic motion of the vortex state. In the region delimited by acharacteristic field Hon (well below $H_p$), and $H_p$, the vortex dynamics iscontrolled by collective pinning. For fields below Hon the activation energy,$U_0$, increases with field as expected for collective pinning, but the pinningmechanism is likely to be in the single vortex limit.

Flux-dynamics associated with the Second Magnetisation Peak in iron-pnictide Ba_{1-x}K_xFe_2As_2