Analysis of Critical State Response in Thin Films by AC Susceptibility Measurements

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 $0.9997T_{\rm c}$ above which nonlinear response turns to linear one. This technique is also suitable for characterization of HTS thin films.