Critical currents, magnetisation and network models of polycrystalline YBa2Cu3O7

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Abstract
The magnetisation and the field dependence of the transport critical current have been studied in detail at 77 K in three well characterised samples of polycrystalline YBa2Cu3O7. Measurements have been made over a wide range of field, from below 10 mu T to above 5 T. The differences in the Jcs between samples are about a factor of 3, but are reflected only in a rather subtle fashion in the magnetisations. There is a strong dependence of Jc on field history, particularly at fields between 10 and 500 mT. Most of the observed behaviour at low fields, less than about 100 mT, can be understood in terms of a network model of a lattice of weak links, provided that the field dependence of the weak-link critical currents is itself included. Such improvement in Jc as can be gained by careful fabrication of polycrystalline YBa2Cu3O7 seems to involve either the enhancement of a relatively small fraction of macroscopic current paths, or possibly intergranular flux pinning through entanglement with intragranular pinned flux.