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We study the vortex-dynamics of a BaFe{1.82}Ni_{0.18}As_2 crystal with Tc = 8K, by measuring flux-creep over the second magnetisation (or fish-tail) peakfor both H//c-axis and H//ab planes. Magnetic relaxation data show ananomalously long initial stage of relaxation, lasting approximately10 minutesfor H//c-axis and 2-3 min for H//ab, resembling a transient effect with a lowerrelaxation rate, which is followed by the usual log(time) relaxation.Interestingly, study of the relaxation rate R vs H for both stages ofrelaxation and for both field directions, are featureless over the field rangeassociated with the fish-tail. The same trend was confirmed by plotting R vs Tobtained from flux-creep data measured as a function of temperature for a fixedfield (H//$c-axis). A plot of the activation energy U(M,T) calculated from thetime relaxation of the magnetisation at a fixed field also shows a smoothbehavior further supporting the view that the fish-tail peak is not associatedwith a crossover in vortex pinning regime within the collective pinningscenario.

Fish-tail and vortex dynamics in Ni doped iron-pnictide BaFe{1.82}Ni_{0.18}As_2