FLUX CREEP IN TYPE-2 SUPERCONDUCTORS

We have made measurements of the evanescent decay of the irreversible magnetization induced by magnetic cycling of solid superconducting cylinders in order to elucidate the mechanisms of Anderson's thermally activated flux-creep process. A superconducting quantum interferometer device coupled to the creep specimen by a superconducting flux transformer made possible observations of flux changes with a resolution of one part in 109. The general applicability of Anderson's theory of flux creep was confirmed and the results were analyzed to show that: (1) The total flux in the specimen changed logarithmically in time, i.e.,lntt0. (2) The logarithmic creep rate ddlnt is proportional to the critical current density Jc and to the cube of the specimen radius. (3) The logarithmic creep rate appears to be only weakly temperature-dependent because a proportionality to T is nearly compensated by the proportionality to Jc, which decreases as T increases. (4) The creep process is a bulk process that is not surface-limited (in this case). (5) Flux enters and leaves the surface in discrete events containing from about one flux quantum up to at least 103 flux quanta. (6) On departing from the critical state to a subcritical condition, the creep process tends to remain logarithmic in time, but the rate is decreased exponentially by decreasing T and is decreased extremely rapidly by backing off of the applied field from the critical state. (7) At magnetic fields H<Hc1 on the initial magnetization curve, no flux creep was observed, but the logarithmic creep rate showed a modest increase above Hc1 and a broad rise as H approached Hc2. The creep process is characterized by a dimension parameter VX consisting of a flux bundle volume V and pinning length X, and by an energy U0, both of which are supposed to be material-sensitive parameters characteristic of the irreversible processes. These parameters were determined from the experiments. Bundle volumes V10-12 cm3 and energies U01 eV were found, indicating that groups of fluxoids must be pinned and must move cooperatively. The results are found compatible with a recent model for flux pinning that includes these cooperative effects. © 1969 The American Physical Society.