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.

We have measured magnetic response of second-generation high temperature superconductor YBa2Cu3Ox wire in perpendicular ac field as a function of temperature. We compare complex ac susceptibility to the calculated susceptibility based on the model of Bean’s critical-state response in two-dimensional (2D) disk in perpendicular field and infinitely long cylinder in parallel field. We present a method of linking model and experimental susceptibility. Good agreement of experimental susceptibility with model susceptibility of 2D disk allows noncontact estimation of critical depinning current density and its temperature dependence.

We have measured magnetic response of second-generation high temperature superconductor YBa 2 Cu 3 O x wire and Nb thin film in perpendicular ac field as a function of temperature. We compare experimental complex ac susceptibility to the calculated susceptibility based on the model of Bean's critical-state response in two-dimensional disk in perpendicular field. The harmonic analysis is needed for comparison between the model and the experimental data. We present a method of linking model and experimental susceptibility. Good agreement of experimental susceptibility with model susceptibility of 2D disk allows contactless estimation of critical depinning current density and its temperature dependence.

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We have measured temperature dependence of magnetic and dielectric properties in high quality stoichiometric single-crystals of magnetite (Fe3O4). At low temperatures, below the Verwey transition, both ac susceptibility and ac permittivity reveal similar relaxation processes. We discuss these effects in connection with domain structure and variable range hopping mechanism of electrical conductivity.PACS numbers: 71.30.+h, 75.60.−d, 75.85.+t 1. Introduction Magnetite -a mixed valence iron oxide, Fe 3 O 4 , with cubic inverse spinel structure at room temperature -is a prototype ferrimagnetic system that still attracts considerable attention mainly due to the presence of the so-called Verwey transition -a structural phase transition accompanied by substantial changes of magnetic and electrical properties [1]. Although Mott's metalinsulator transition theory [2] was well suited to explain the sharp increase of resistivity of magnetite upon cooling through the Verwey temperature (T V ≈ 120 K), the exact nature of the transition is still unresolved and the charge ordering on the Fe ions has not been sufficiently clarified [3][4][5][6]. To complicate the matter even further, upon cooling through T V the structure changes to monoclinic [5] and the monocrystal splits into structural domains which are strongly linked to the instantaneous magnetic domain structure of the sample (i.e. random in weak magnetic fields). The interplay of spin, lattice and electronic degrees of freedom makes this strongly correlated system a potential candidate for novel applications [7].In this work we focus not on the Verwey transition itself but rather on the magnetic and dielectric behavior of magnetite below T V .2. Experimental Stoichiometric single crystals of magnetite were grown by floating zone technique [8] and cut to disk shape 5 mm in diameter and 1 mm thick, the disk axis corresponds to 100 crystallographic direction. Two silver electrodes (4 mm in diameter) were deposited on the flat surfaces of the disk by means of magnetron sputtering. A custom sample holder with copper and brass contacts was used to measure both electrical and magnetic properties simultaneously. To measure the magnetic properties (complex ac susceptibility, dc magnetization) a home--built continuously reading SQUID magnetometer with immobile sample was used [9]. The magnetometer oper-