NUMERICAL MODELING OF A 2-COIL APPARATUS FOR MEASURING THE MAGNETIC PENETRATION DEPTH IN SUPERCONDUCTING FILMS AND ARRAYS

We determine the magnetic penetration depth lambda(T) in superconducti ng films by measuring the mutual inductance of two coils located on op posite sides of the films. The apparatus is designed to produce an acc urate value for lambda(O) without any assumptions about the dependence of lambda(T/T-c)/lambda(O) on T/T-c. In a typical configuration of co ils and a 100 +/- 14 nm thick by 6 mm radius film, lambda can be measu red to better than +/- 12% as long as lambda > 1500 Angstrom. The nois e level is typically +/- 2 Angstrom. If two films are made the same wa y, so that they have the same thickness d, and they are measured in th e same apparatus, then the relative uncertainty in lambda between the two films is only +/- 9%, because uncertainties in d, the coil dimensi ons, etc., are eliminated. This article describes the apparatus and a detailed numerical model which illustrates the induced current density in the film and establishes the sources of uncertainties. The accurac y of the model is demonstrated through comparison with measurements on 0.15 mm thick circular Pb disks. (C) 1996 American Institute of Physi cs.