Noncontact method for measurement of the critical current densities and diagnostics of superconductors

Abstract

The role of the geometric factor of a superconducting sample in magnetic measurements of the critical current densities Jc is investigated. It is shown that a small difference of the shape of the sample from cylindrical does not have much effect on the value of Jc. It is found that the error in evaluating Jc for single crystals with the use of the standard expression for determining Jc of a ceramic is due to the low trapped magnetic flux density Btr in single crystals as compared to ceramics. A technique is proposed for determining Jc for single-crystal and ceramic samples from measurements of the spatial distribution of Btr produced by the Bean shielding supercurrents upon the transition of the whole volume of the sample to the critical state. A formula is obtained for determining Jc for single-crystal and ceramic superconducting samples in the form of polygonal and cylindrical slabs. To increase the accuracy of determining Jc the difference B0*−Bc1 was used instead of Btrmax(0) (B0* is the field that induces Btrmax(0) at the geometric axis of the sample, and Bc1 is the first critical magnetic field of the sample). The values of Jc and of the pinning force in YBCO and BSCCO single-crystal samples are estimated. It is established that the magnetic flux trapping mechanisms in these layered structures are different.