Discrete model analysis of the critical current-density measurements in superconducting thin films by a single-coil inductive method

The critical current density of a superconducting film can be easily determined by an inductive and contactless method. Driving a sinusoidal current in a single coil placed in front of a superconducting sample, a nonzero third-harmonic voltage V3 is induced in it when the sample goes beyond the Bean critical state. The onset of V3 marks the value of current beyond which the sample response to the magnetic induction is no more linear. To take into account, in a realistic way, the magnetic coupling between the film and the coil, we have developed a discrete model of the inducing and induced currents distribution. In the framework of this model the magnetic-field profile on the sample surface and the coefficient linking the current flowing in the coil and the critical current density JC of superconducting thin films are evaluated. The numerical results are checked by measuring JC of several thin films of YBa2Cu3O7−δ of known superconducting properties, used as a control material.