Influence of field decrements on the relaxation behavior of thin high- T _c superconductors measured using a levitation balance

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.