Conventional relaxation measurements on various samples (a YBa 2Cu 3O 7-δ thin film, a YBa 2Cu 3O 7-δ-coated conductor, and a Bi 2Sr 2Ca 2Cu 3O 10+δ tape) using a levitation balance are presented that are measured after a departure from the critical state by means of a small retraction Δ d of the permanent magnet, which corresponds to a small field reduction ΔH a. In the case of YBa 2Cu 3O 7-δ, flux creep rates S are found to decrease with increasing ΔH a until a small plateau is reached, where flux creep is effectively stopped. A further increase in Δ H a before the relaxation measurement leads to a change of sign of the creep rate. For the Bi 2Sr 2Ca 2Cu 3O 10+δ tape, only a continuous decrease in S is obtained. An explanation of this effect is given based on calculations of flux density profiles and critical current densities in perpendicular geometry. The differences to the Bean model (i.e., longitudinal geometry) are discussed.
ASJC Scopus subject areas
- Electronic, Optical and Magnetic Materials
- Condensed Matter Physics
- Electrical and Electronic Engineering