Thermomagnetic instability effects vs. vortex matter phase transitions in type-II superconductors

Thermomagnetic flux-jump instabilities in type-II superconductors reduce the absolute irreversible magnetization relative to the isothermal critical state value at low enough magnetic fields. The recovering of the isothermal critical state with increasing field leads to the second magnetization peak (SMP) in the magnetization curve M(H) as has been unambiguously demonstrated for Nb films. The low-field SMP takes place in many other conventional and unconventional superconductors, and is often attributed to a critical current enhancement associated with a phase transition(s) in the vortex matter. The analysis of the most recent experimental results given in this communication provides clear evidence that the restoration of the isothermal critical state is responsible for the SMP in both conventional and unconventional superconductors and casts doubts about the interpretation based on phase transition(s) of the vortex matter. (C) 2002 Elsevier Science Ltd. All rights reserved.12241671333