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 YBa₂Cu₃O_7-δ thin film, a YBa₂Cu₃O_7-δ-coated conductor, and a Bi₂Sr₂Ca₂Cu₃O_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 Δd of the permanent magnet, which corresponds to a small field reduction ΔH_α. In the case of YBa₂Cu₃O_7-δ, flux creep rates S are found to decrease with increasing ΔH_α until a small plateau is reached, where flux creep is effectively stopped. A further increase in ΔH_α before the relaxation measurement leads to a change of sign of the creep rate. For the Bi₂Sr₂Ca₂Cu₃O_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.