A study of the nonlinearity in magnetic response of

We have carried out low-field ac measurements of harmonic susceptibilities and ac magnetization of a single-phase partially melt-textured high-temperature superconductor over a wide range of temperatures, field amplitudes and measuring frequencies. Both the fundamental and third-harmonic susceptibility versus temperature and ac magnetization at fixed temperatures close to exhibit strong ac field amplitude and small frequency dependences. In the third-harmonic susceptibility, we observed a double-peak feature in the vicinity of as a function of the applied ac field. This strong field dependence is associated with depinning of vortices at some B and T in different superconducting matrices, which involves different ac loss mechanisms. Experimental results are compared with predictions of some existing theoretical models.