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Abstract-Low frequency ac magnetic response of the Nb thin film in perpendicular oscillating applied fields is detected by continuously reading SQUID magnetometer. Harmonic analysis of the temperature dependence of the nonlinear ac susceptibility gives excellent agreement with the susceptibility calculated on basis of model of the hysteretic critical state in 2D disk. This complete analytical model relates to Bean's critical state described by the critical depinning current density. We map the experimental and model data and we trace the temperature dependence of the critical current density. The model fits up to temperature 0.9997T c above which nonlinear response turns to linear one. This technique is also suitable for characterization of HTS thin films.

The measurement of magnetic properties of superconductors is usually understood in context with flux penetration length and in case of type-II superconductors also with vortex matter. We investigate the behavior of superconductors in very weak magnetic fields, typically µ0H < 1µT, near the critical temperature. The aim is to explore the details of SN transition and phase diagram (B, T ) of vortex matter in extreme type-II superconductors like cuprates. The data recorded on HTS, even if extensively challenged with theory, are hard to understand as a vortex matter behavior. On the other hand, interpretation on the basis of electron conductivity arising from electron scattering is consistent even with data obtained by x-ray diffraction, photoemission, tunnelling spectroscopy and specific heat. PACS : 74.25.Fy, 74.25.Ha,In type-I superconductors, like Pb or Ga which are "good" BCS superconductors, the superconducting-normal (SN) transition measured by the ac susceptibility in weak magnetic fields is very sharp. In pure (6N) long thin Ga single crystal it is 90% complete in a temperature interval of 2×10 −6 K [1]. On the other hand, the magnetic properties of type-II superconductors, particularly of cuprates which are extreme type II, are expected to be strongly affected by thermodynamic fluctuations in the vicinity of the critical temperature T ≈ T c . Fluctuations should considerably broaden the transition and influence the phase diagram (B, T ) of vortex matter [2]. However, recent low field high-resolution magnetic measurements show a distinct character of SN transition in conventional BCS superconductors and cuprates [3]. Since the transition shape depends on the sample geometry (demagnetization * ) Presented at 12

We compare results of high-resolution magnetic flux (susceptibility) measurements in very weak magnetic field, performed of three classes of superconductors. They reveal astonishing details of the transition to the superconducting state. Although Pb behaves also on this scale according to BCS predictions, La is more reminiscent of HTS materials, which exhibit anomalous features. We suggest that known structure peculiarities are due to a strong electron-lattice instability and lead to a resonance electron back scattering