Influence of Field Decrements on the Relaxation Behavior of Thin High- $T_{c}$ Superconductors Measured Using a Levitation Balance

Abstract

Conventional relaxation measurements on various samples (a YBa 2 Cu 3 O 7−δ thin film, a YBa 2 Cu 3 O 7−δ -coated conductor, and a Bi 2 Sr 2 Ca 2 Cu 3 O 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 a . In the case of YBa 2 Cu 3 O 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 ΔH a before the relaxation measurement leads to a change of sign of the creep rate. For the Bi 2 Sr 2 Ca 2 Cu 3 O 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.