Experimental data on the field and time dependencies of the isothermal magnetization hysteresis for Tl 2 Ba 2 CaCu 2 O 8 single crystals with fields applied parallel to the c-axis are reported. From the M–H measurements, we determined the first peak field (Hp), the onset field of the second peak (Hon), the second peak field (Hsp), and the irreversible field (Hirr). Using these physical parameters, we have drawn a vortex phase diagram for this system. As observed in BSCCO(2212) system, below the irreversibility line, the H–T plane is divided into three distinct phases: the vortex lattice, the entangle phase, and the quasi 2D pancake vortex states in the temperature interval 30 K ≤T≤60 K .1 From the relaxation of remanent magnetization measurements, we have obtained the flux creep activation barrier U(j) as a function of j using the method developed by Malay et al. Within the collective pinning theory, it is argued that T≤30 K , the pancake vortices are pinned individually, while for T≥42 K , the vortex state is characterized by individually pinned 3D vortex strings. At T~30 K , a crossover from 2D pancake to 3D collectively pinned state was observed and shows a smooth crossover to 3D single vortex state at higher temperatures.
Flux dynamics and vortex phase diagram of the new superconductor MgB2
