An investigation of the very incomplete Meissner effect

Since the discovery by Meissner and Ochsenfeld’ that a magnetic field is expelled from the bulk of a superconductor in the transition from the normal to the superconducting state, this effect (named the Meissner effect) has been considered a crucial property of superconductivity. Small deviations from complete flux exclusion have generally been observed, starting with Meissner and Ochsenfeld’ who noted that up to 10% of the flux remained frozen into their 3mm diameter Pb and Sn cylinders. Several explanations for existence of the trapped flux (incomplete Meissner effect) have been proposed. In the first one, Mendelssohn:! postulated inhomogeneity in the form of a multiconnected system of thin filaments having critical fields above that of the majority of material within the superconductor. The high critical fields of these connected filaments, known as the Mendelssohn sponge, can be caused by strains, impurity concentration gradients, or lattice imperfections. If such a specimen is placed in a magnetic field sufficient to make it entirely normal, and the field is subsequently reduced, the anomalous regions will become superconducting first, trapping flux by virtue of their