Critical current density measurement of thin films by AC susceptibility based on the penetration parameter h

We have numerically proved that the dependence of AC susceptibility χ of a E(J) power law superconducting thin disc on many parameters can be reduced to one penetration parameter h, with E the electric field and J the current density. Based on this result, we propose a way of measuring the critical current density Jc of superconducting thin films by AC susceptibility. Compared with the normally used method based on the peak of the imaginary part, our method uses a much larger range of the AC susceptibility curve, thus allowing determination of the temperature (T) dependence of Jc from a normally applied χ(T) measurement. A fitting equation Jc = 1.9Ha∣χ′∣0.69/d, −0.4 < χ′ < −0.001 derived from the critical state case (Bean model) can be used in most situations, where Ha is the amplitude of the applied AC field, χ′ is the real part of the normalized susceptibility and d is the thickness of the film. The method is valid for the cases where the film is fully penetrated. We also discuss how the finite London penetration depth affects the susceptibility when the film is screened. Measurements with varying T, Ha and DC background field Hdc are performed to support the arguments.

► A method for calculating Jc from AC susceptibility. ► Experiments comparing the method and traditional DC magnetization method showed agreement. ► Dimensionless parameter h for describing the AC susceptibility of a E–J power-law superconductor. ► The validity of h was proved numerically in thin discs.