AC susceptibility of (Bi,Pb) 2Sr 2Ca 2Cu 3O y/Ag tape in perpendicular ac fields

Abstract

The ac susceptibility (ACS) of (Bi,Pb) 2Sr 2Ca 2Cu 3O y /Ag tape was measured as a function of temperature, frequency and amplitude of ac field applied perpendicular to the tape plane ( H⊥ a b). Significant broadening of ACS transition was observed even for a small ac field (0.28 G). The transition width in the real and imaginary parts of ACS is strongly dependent on the amplitude of the ac field. This behaviour of ACS was examined systematically through a scaling method. We found that all measured ACS curves [ χ′( T) and χ″( T)] for ac field amplitudes from 0.28 to 30 G (square root value) can be scaled onto a single curve using the peak temperature ( T p) of its imaginary part as the scaling parameter. The scaling behaviour below T p can be understood on the critical state model. However, the conventional method to estimate the critical current density J c using T p as a reference point cannot be applied due to the large thermal fluctuation. In fact, T p is related to the irreversibility temperature. The frequency effect of ACS was also measured at a small ac field (0.28 G). As the frequency increases, both χ′( T) and χ″( T) curves shift to higher temperature and T p increases exponentially with the frequency. This effect can be interpreted in terms of flux creep.