  Magnetic flux front and induction contours in superconducting YBa2Cu3O7-d
films with defect size s ~ l (superconducting coherence length) and s > l are
studied by magneto-optical imaging. Robust self-affine spatial correlation was
observed using scaling analysis in the small pinning-dominated (s ~ l)
disorderd films. The roughness exponent was determined to be ~ 0.66,
independent of numbers of defects (or the film thickness). When the disorder
landscape also included a distribution of large defects (s >> l), the flux
front and induction contours exhibited self-similarity, with a fractal
dimension to be ~ 1.33 using the box-counting method. The remarkably different
flux penetration patterns were shown to be the manifestation of self-organized
criticality at different length scales.
