The role of the pinning crossover in the increase of the superconducting critical current density.

In the framework of the applications of superconductivity, the critical current density $J_c$ plays an important role since it is the maximum current that a superconductor can carry without dissipation. In this context, the Second Magnetization Peak (SMP) phenomenon produces an increase in $J_c$ as a function of the field ($H$). Another important parameter is the creep rate $S$ which gives information about the vortex activity. Here, the correlation in type-II superconductors between $S$ and the SMP phenomenon has been investigated at different temperatures by starting from the minimum in $S(H)$ and the onset of the SMP phenomenon detected on a $FeSe_{0.5}Te_{0.5}$ sample. We have found that the flux dynamic mechanisms behind the appearance of the SMP phenomenon in $J_c(H)$ are activated at fields well below those where the critical current starts effectively to increase. Moreover, both the $S(H)$ and the SMP phenomenon can be attributed to a sequential crossover between a less effective pinning (losing its effectiveness at low fields) to a more effective pinning (still acting at high fields), regardless of the type-II superconductor taken into consideration.