Fishtail and vortex dynamics in the Ni-doped iron pnictide BaFe1.82Ni0.18As2

Abstract

We study the vortex dynamics of a ${\text{BaFe}}_{1.82}{\text{Ni}}_{0.18}{\text{As}}_{2}$ crystal with ${T}_{c}=8$ K by measuring flux creep over the second magnetization (or fishtail) peak for both $H\ensuremath{\parallel}c$ axis and $H\ensuremath{\parallel}ab$ planes. Magnetic relaxation data show an anomalously long initial stage of relaxation, lasting $\ensuremath{\sim}$10 min for the $H\ensuremath{\parallel}c$ axis and 2--3 min for $H\ensuremath{\parallel}ab$, 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 fishtail. 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 ($H\ensuremath{\parallel}c$ axis). A plot of the activation energy $U(M,T)$ calculated from the time of relaxation of the magnetization at a fixed field also shows a smooth behavior, further supporting the view that the fishtail peak is not associated with a crossover in a vortex pinning regime within the collective pinning scenario.