FREQUENCY, FIELD AND THERMOMAGNETIC DEPENDENCE OF AC SUSCEPTIBILITY IN YBCO

We have investigated the AC field and frequency dependence under a superimposed DC field along with the thermomagnetic history of AC losses for polycrystalline YBCO. Considering the pinning potential as a function of temperature and field, the behaviour Of AC losses, i.e. peak position in chi'', T(p), can be explained in terms of thermally activated flux creep. Activation energy decreases with increasing AC field. The frequency dependence of T(p) is used to test the validity of flux creep and vortex glass models. The low-field irreversibility line is also plotted as a function of temperature. The effect of thermomagnetic history cannot be explained by critical state models in which J(c)(H) is assumed to be a single-valued function. The demagnetization effect and magnetization of grains affects the intergranular field and explains the hysteretic nature Of AC susceptibility under the thermal history of a magnetic field.