On the nature of the second magnetization peak in FeSe1?xTex single crystals

The occurrence of the second peak (SP) on the dc magnetization curves of iron pnictide and chalcogenide single crystals (the ‘fishtail’ effect) has been interpreted in surprisingly different ways, involving, for example, a thermally induced square-to-rhombic vortex lattice transition, a crossover between single vortex pinning in the low magnetic field range and pinning of interacting vortices at higher fields, or has been treated similarly to the peak effect close to the upper critical magnetic field. Here we show that the SP in FeSe1−xTex single crystals is related to the well documented order–disorder transition in the vortex system, and its evolution in the low-temperature T domain is described by a dynamic energy balance relation. The main argument supporting this approach is the observed strong increase of the field for the onset of the SP with decreasing T down to 2 K, with an inflection-like point located between 3 and 4 K, in agreement with the two-band superconductivity reported for FeSe1−xTex.