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

Abstract

We study the vortex dynamics of a BaFe1.82Ni0.18As2 crystal with Tc=8 K by measuring flux creep over the second magnetization (or fishtail) peak for both H∥c axis and H∥ab planes. Magnetic relaxation data show an anomalously long initial stage of relaxation, lasting ∼10 min for the 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 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∥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.

Source-reported materials — not catalogue approval

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