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Conventional relaxation measurements on various samples (a YBa2Cu3O7-δ thin film, a YBa2Cu3O7-δ-coated conductor, and a Bi2Sr2Ca2Cu3O10+δ tape) using a levitation balance are presented that are measured after a departure from the critical state by means of a small retraction Δd of the permanent magnet, which corresponds to a small field reduction ΔHα. In the case of YBa2Cu3O7-δ, flux creep rates S are found to decrease with increasing ΔHα until a small plateau is reached, where flux creep is effectively stopped. A further increase in ΔHα before the relaxation measurement leads to a change of sign of the creep rate. For the Bi2Sr2Ca2Cu3O10+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.
Date of Publication: April 2012