DETERMINATION OF CURRENT AND FLUX DISTRIBUTION IN SQUARES OF THIN-FILM HIGH-TEMPERATURE SUPERCONDUCTORS

An experimental determination of the flux distribution in square samples of thin-film superconductors has been conducted by the use of a scanning Hall probe and a magneto-optical thin film. Two 10 mm x 10 mm samples, one YBa2Cu3O7 and one TlBaCaCuO, have been examined at 77 K with a field applied perpendicular to the surface of the film. For both samples, the current distribution was calculated from the measured field data. At low fields, flux penetrates at the centres of the edges giving rise to the distinctive star pattern often noted in magneto-optical studies and creating a complex current distribution. As the field penetrates fully, the current distribution pattern evolves into simple concentric squares. The evolution of the screening currents was found to be qualitatively the same in both materials, although the magnitudes of the critical currents differed by a factor of up to 10. For both samples, in full field penetration, the current is found to follow the outline of the sample. Lines of constant current are to a good approximation concentric squares. The remnant magnetization, M(rem), is found to be J(c)D/3, where 2D is the size of the square, in complete agreement with the critical-state model.