Magneto Optical Studies of Ybco Thick Films in the Critical State

We present magneto optical studies of the critical state in YBa2Cu3O7-δ (YBCO) thick films prepared using BaF2 ex-situ post reaction process. A fractal-like propagation was observed as the magnetic flux entering the superconducting films, in striking contrast to the behavior expected of a uniform type II superconducting film in the critical state. However, by averaging the flux density over a certain length scale, we found that flux penetration can be described with the standard Bean critical state model. The temperature dependence of critical current density Jc was obtained by applying the Bean model to the averaged flux profiles, and found in good agreement with those obtained by direct transport measurements.