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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 10^9. 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. , {t₀}. (2) The logarithmic creep rate ddlnt is proportional to the critical current density J₂ 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 J₂, 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 10^3 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<H₂₁ on the initial magnetization curve, no flux creep was observed, but the logarithmic creep rate showed a modest increase above H₂₁ and a broad rise as H approached H₂₂. 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 cm^3 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.
Beasley et al. (Sat,) studied this question.