STRONG MAGNETIC HISTORY DEPENDENCE OF MAGNETIC-RELAXATION IN HIGH-TC SUPERCONDUCTORS

Very detailed flux creep and relaxation measurements of intragrain magnetization in sintered high-T(c) superconductors at 4.2 K are presented. The M-relaxation rate is not a single valued function of the applied B-field but depends strongly on the prior magnetic field cycling procedure. The largest relaxation rate is measured on the envelope curves of the M-hysteresis area. On the transition curves (minor loops) the relaxation rate is reduced and even disappears locally. This is of great importance for applications of high-T(c) superconductors in electronics. The results can also be applied to low-T(c) superconductors like NbTi and are of interest for accelerator magnets with superconducting windings. Theoretical expressions for the relaxation process are derived for all physically relevant parts of the M-hysteresis curve on the basis of Bean's model. As the intragrain M influences also the intergrain critical transport current and microwave absorption in granular high-T(c) material, our results on M-relaxation behavior are a basis to explain the already partly observed time dependence of those phenomena.