Magneto-optic images of flux distribution and creep in YBa2Cu3O7

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Abstract
A magneto‐optic imaging technique was used to examine the flux distributions at 50, 60, and 70 K in a single‐crystal platelet of YBa2Cu3O7. The superconducting sample was covered with a layer of iron garnet film, and a polarization microscope was used to examine the magnetic domains in this imaging film. Since the spacing of these domains was proportional to the local magnetic field in the underlying superconductor, it was possible to determine field profiles directly from the optical density of the image. Field profiles indicated that the distributions of remnant flux in the sample were well described by the critical state model. It was also possible, through the use of simple linear models of initial flux penetration, to measure the flux trapped in the sample as a function of time. The flux creep data derived from analysis of the digitized images showed a logarithmic decrease of magnetization with time, in good agreement with SQUID magnetometer measurements performed on the sample. There was no evidence of an axial critical state.