The magnetic response of Bi-based superconductors of nominal composition Bi 1.8 Pb 0.2 Sr 2 Ca 3 Cu 4 O 12 against low AC field amplitudes is studied in terms of fundamental and third harmonic AC susceptibilities to investigate various physical parameters. Optimum conditions of a wet preparation technique have been determined. Both transport and inductive critical current densities have increased considerably on samples subjected to a single intermediate grinding.The magnetic data accompanied by XRD measurements support the theory that the minor superconducting phase 2212 is mostly screened by a dominating matrix of 2223 high-T c phase.

In the present work, the harmonic magnetic susceptibilities of the high-T c superconductors are examined and corresponding harmonic susceptibilities are calculated employing a model by Kim and coworkers. According to Kim's model, the critical-current density, J c is a function of the internal magnetic fieldwhere k and H o are constants. In the model calculations, full penetrations field, H p , is taken to beAs a result of the comparison of model calculations with experiments, we conclude that the harmonic susceptibilities for lower p values (p <10) correspond to the similar characteristic magnetic behaviour of granular superconductors, while the harmonic susceptibilities for higher p values correspond to the behaviour of homogeneous superconductors such as single crystal or samples with contiguous oriented grains.

We have carried out low-field ac measurements of harmonic susceptibilities and ac magnetization of a single-phase partially melt-textured Bi 2 Sr 2 CaCu 2 O 8 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 T c exhibit strong ac field amplitude and small frequency dependences. In the third-harmonic susceptibility, 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 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.

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The fundamental and third harmonic ac susceptibility measurements as a function of temperature (20–120 K), frequency (5 Hz–1 kHz) and ac magnetic field amplitude (1–1600 A m−1) on a bulk Bi1.84Pb0.34Sr1.91Ca2.03Cu3.06O10 high-Tc superconductor prepared by the ammonium nitrate fusion technique are reported. The height and temperature of the peak in the out-of-phase component of the fundamental susceptibility increase with increasing field amplitude and the measuring frequency employed. In the presence of a superimposed Hdc = 1520 A m−1, an additional small peak appears and the larger peak broadens even at lower ac fields of 20 A m−1. The third harmonic susceptibility exhibits an interesting temperature dependence with increasing ac field amplitude. Experimental results are qualitatively discussed in the framework of the critical state model. The results are in good agreement with the theory at fields lower than the least field needed for full flux penetration, Hp, below which ac losses are essentially due to the hysteretic intergranular coupling. The critical state model fails to explain the data at high temperatures with ac fields larger than Hp, where the ac losses are interfered by the irreversible flux penetration into the grains. This is presumably due to the weakening of the intergrain coupling with magnetic field penetration.