Magnetic‐field‐induced damage in a superconducting YBa 2 Cu 3 O 7−x film

During investigations of the magnetic flux dynamics in thin superconducting YBa 2 Cu 3 O 7−x films we have observed the spontaneous formation of a damage in the film, apparently induced by the applied magnetic field of 0.1 T . The damage developed within less than 40 ms (the time resolution of the experiment) and showed up in the magnetiooptically recorded image of the flux above the sample as a path for massive flux penetration. A subsequent analysis revealed a 1 μm wide gap in the YBa 2 Cu 3 O 7−x film which had developed over a major portion of the 1 · 1 cm 2 sample. It is suggested that the film locally melted as a result of an instability of the superconductor in the critical state.