Fish-tail and vortex dynamics in Ni doped iron-pnictide BaFe{1.82}Ni_(0.18)As₂

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