A STUDY OF THE NONLINEARITY IN MAGNETIC RESPONSE OF BI2SR2CACU2O8

We have carried out low-field ac measurements of harmonic susceptibili ties and ac magnetization of a single-phase partially melt-textured Bi 2Sr2CaCu2O8 high-temperature superconductor over a wide range of tempe ratures, field amplitudes and measuring frequencies. Both the fundamen tal and third-harmonic susceptibility versus temperature and ac magnet ization at fixed temperatures close to T-c exhibit strong ac field amp litude and small frequency dependences. In the third-harmonic suscepti bility, we observed a double-peak feature in the vicinity of T-c as a function of the applied ac field. This strong field dependence is asso ciated with depinning of vortices at some B and T in different superco nducting matrices, which involves different ac loss mechanisms. Experi mental results are compared with predictions of some existing theoreti cal models.