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Local assessment of the current flow in large bulk superconductors and in coated conductors by the newly established magnetoscan technique - theory and experiment

Martin Zehetmayer ,  Rene Fuger ,  Florian Hengstberger ,  Michael Eisterer ,  Harald W. Weber 

Atomic Institute of the Austrian Universities, Stadionallee, Wien 1020, Austria


Large (RE-123) bulk superconductors (with a size of several centimeters) are promising materials for applications, since they provide high trapped magnetic fields. Scanning such fields by a Hall probe leads to rather "global" information on the sample quality, since currents from the whole sample volume contribute to the trapped field. Inhomogeneities and defects can strongly reduce the trapped fields, but details usually cannot be identified by this method. The situation is similar in coated conductors. Currently, a lot of research on Y-123 based conductors is carried out with the aim to produce long tapes with high critical current densities (jc's). Measuring jc over large lengths results in a global value determined by that cross section of the conductor that carries the weakest current. Local properties are not addressed in this way. The position dependence of the superconducting parameters can be obtained from small samples cut from larger pieces. To access the local properties without destroying the original samples, a new method, called magnetoscan, was recently introduced. A small permanent magnet scans the sample surface and locally induces currents. The field of these currents is recorded by a Hall probe fixed below the magnet. We show that the magnetoscan technique indeed reveals a significant local structure of the field map, indicating inhomogeneities and defects in both large bulk samples and long coated conductors. Several experimental results are shown and discussed. Artificial defects are introduced to analyze their effects. Additionally, we provide a theoretical description to confirm our interpretation. We numerically calculate the current dynamics during a magnetoscan applying a 3 dimensional model in the case of bulk materials, and a 2 dimensional model in the case of coated conductors. Defects and inhomogeneities are introduced by a position dependent jc. Experiment and theory are in good agreement for both types of application.


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Presentation: Invited oral at E-MRS Fall Meeting 2006, Symposium K, by Martin Zehetmayer
See On-line Journal of E-MRS Fall Meeting 2006

Submitted: 2006-05-08 07:04
Revised:   2006-08-01 15:39