PINNING ENERGY AND VORTEX-PHASE DIAGRAM OF MgB2 BULK MATERIALS

Abstract

By using ac susceptibility measurements at different frequencies and dc fields we investigated the creep properties of MgB 2 bulk samples. Two independent analysis have been used in order to determine the behavior of the pinning energy in the whole investigated temperature and field ranges. The main result of the first analysis (χ′′-peak analysis) is that the attempt frequency v 0 in the Arrhenius formula is about 104 Hz , a value significantly lower than that usually reported for HTS's. By means of the second analysis a field dependent maximum of the pinning energy as a function of temperature has been found. It turns out that the phase points corresponding to the peak of the pinning energy are coincident with the irreversibility line, as determined by magnetization cycles. The results coming from different and independent analysis lead to the determination of a vortexphase diagram for bulk MgB 2, which seems to be largely dominated by creep phenomena.