Flux dynamics associated with the second magnetization peak in the iron pnictide Ba_1−xK_xFe₂As₂

We report on isofield magnetic relaxation data on a single crystal of Ba_1−xK_xFe₂As₂ with superconducting transition temperature T_c=32.7  K which exhibit the so-called fish-tail effect. A surface map of the superconducting transition temperature shows that the superconducting properties are close to homogeneous across the sample. Magnetic relaxation data, M(t), was used to obtain the activation energy U(M) in order to study different vortex-dynamics regimes. Results of this analysis along with time-dependent measurements as a function of field and temperature extended to the reversible region of some M(H) curves demonstrate that the irreversibility as well the second magnetization peak position, H_p(T), are time dependent and controlled by plastic motion of the vortex state. In the region delimited by a characteristic field Hon (well below H_p), and H_p, the vortex dynamics is controlled by collective pinning. For fields below Hon the activation energy, U₀, increases with field as expected for collective pinning, but the pinning mechanism is likely to be in the single vortex limit. ©2010 The American Physical Society