Topological Hysteresis in the Intermediate State of Type-I   Superconductors
  Magneto-optical imaging of thick stress-free lead samples reveals two distinct topologies of the intermediate state. Flux tubes are formed upon magnetic field penetration (closed topology) and laminar patterns appear upon flux exit (open topology). Two-dimensional distributions of shielding currents were obtained by applying an efficient inversion scheme. Quantitative analysis of the magnetic induction distribution and correlation with magnetization measurements indicate that observed topological differences between the two phases are responsible for experimentally observable magnetic hysteresis. 
Ruslan Prozorov, Russell W. Giannetta, Anatolii A. Polyanskii, Garry   K. Perkins