Thickness dependence of the second magnetization peak effect in Ba 0.6 K 0.4 Fe 2 As 2 single crystals

The second magnetization peak (SMP) effect has been observed widely in many type-II superconductors, but the reason remains elusive. This effect manifests an enhanced critical current density with magnetic field and should be very useful for applications. By measuring the magnetization of optimally doped Ba 0.6 K 0.4 Fe 2 As 2 single crystals with different thickness, it is found the SMP effect exists in thick samples and gradually becomes invisible when the sample thickness is reduced to the scale of micrometer. Detailed investigation on the vortex dynamics on samples with different thickness clearly show that there is a common behavior of vortex dynamics in the low field region, which may be characterized by the Bragg glass like elastic vortex motion. This feature holds on in the whole field region for the thin samples, while it turns into the SMP effect for thicker samples when the field is increased. The results suggest that the SMP effect may be induced by the entanglement of the vortex system, and the absence of the SMP effect in thin samples is attributed to the cutoff of the entangled vortex length along c-axis.