STUDY OF THE FREQUENCY AND LOW-FIELD DEPENDENCE OF AC SUSCEPTIBILITY IN YBACUO

The frequency dependence of the ac susceptibility in polycrystalline s amples of YBaCuO has been studied at the ac field range 0.06-1.2 Oe. T he curve of the real part of the susceptibility chi' exhibits two drop s, the first one close to T-c and the other well below T-c which could correspond to screening behavior of the grains and grain boundaries, respectively. The imaginary part of the susceptibility shows only a pe ak below T-c which is related to hysteresis losses at the grain bounda ries. The peak corresponding to intragrains hysteresis losses is very small and appears masked by the losses of the intergrains. In the rang e of 1-20 kHz, the onset temperature of chi' showed no frequency depen dence. However, the width of the transition decreases lightly and ther e is a small shift in the peak of chi'' to higher temperature as the f requency increases. This behavior could be explained in terms of the t hermally activated Anderson flux creep. The shift depends on the ampli tude of the measuring field. The activation energy for flux creep rang es from 6.4 eV at 0.02 Oe to 3.23 eV at 1.2 Oe in the zero-field-coole d measurements and from 3.41 eV at 0.02 Oe to 1.41 eV at 1.2 Oe in the field-cooled measurements.