A contactless method for measurement of the critical current density and critical temperature of superconducting films

Abstract

A new method is described for measuring the critical current density and transition temperature of a superconducting film without making contact to it or modifying it in any way. This technique is particularly well suited for use with high transition temperature oxide films which are notoriously irreproducible and sensitive to patterning. It consists of positioning a flat, multiturn coil near the film surface and driving the coil with an audio frequency sine-wave current. Induced shielding currents flow in the film. We have calculated the radial dependence of the induced currents and show that the induced current density is zero at the coil center, rises to a maximum near the mean radius of the drive coil, and then falls off rapidly as the radius continues to increase. A measurement of the critical current per length can be obtained by monitoring the development of odd harmonic voltage components across the coil as the drive current is increased. We find that this measure of nonlinearity in the coil–film system increases abruptly when the maximum induced current equals the critical current. The critical current obtained by this inductive measurement has been demonstrated to give nearly the same value as that obtained by a transport measurement. Other advantages to this inductive measurement approach include: sensitivity only to intergranular critical current, and an ability to accommodate large substrates. Finally, with only minor modification to the electronics used to measure Jc, the superconducting transition temperature of the sample may be measured as well.

Upper limit of metastability of the vortex-free state of a two-dimensional superconductor in a nonuniform magnetic field

“…This field is orders of magnitude larger than the thermodynamic applied field where vortices would appear if not for the high freeenergy barrier for breaking nascent vortex-antivortex (V-aV) pairs. 1,2 The calculation is useful for determination of the inplane Ginzburg-Landau coherence length, ξ, from the onset of strong non-linear effects in two-coil experiments [3][4][5][6][7][8][9] for measuring the magnetic penetration depth, λ.…”

Flux domes in weak-pinning superconducting films without edges

“…When the current amplitude exceeds the value at which the maximum induced sheet-current density reaches K c = j c d, a third-harmonic voltage appears in the coil. 26,27,28,29 A similar technique was introduced by Hochmuth and Lorenz. 30 The effects discussed in this paper and applied to type-II superconducting films are quite general and also should be observed in type-I superconductors.…”

“…II. Such calculations illuminate the fundamental physics underlying the ac technique introduced by Claassen et al 26 to determine the critical current density j c in superconducting films. This technique employs a small coil, placed just above the film, carrying a sinusoidal current.…”

Numerical Investigation on Accuracy Improvement of Permanent Magnet Method for Measuring jC in High-Temperature Superconducting Film

“…For this reason, a noncontact method has been so for desired for measuring the critical current density. Currently, the popular method is the inductive method proposed by Claassen et al [1].…”