The field and temperature dependencies of the first and third harmonics of the ac magnetic susceptibilities of a granular Bi‐2223 superconductor have been investigated. Model calculations have been performed on the basis of a theoretical model that incorporates collective creep concepts in a critical‐state equation by considering the temperature‐dependent effective volume fraction. The predictions of the model for the field and temperature dependencies of the first and third harmonics were found to be in good agreement with experimental data. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

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 interaction forces between a permanent disk and a superconductor disk are calculated for a cylindrical system by using the Anderson-Kim model. Model calculations show that both the vertical force and the stability exhibit a peak-like behavior in the repulsive branch. These peaks in the calculations are dependent on the model parameter and the ratio between the penetration and the applied fields, H p /H m . The features of hysteretic force and magnetic stiffness are discussed.