REVERSIBLE SUPPRESSION OF THE SO-CALLED FISHTAIL EFFECT IN ULTRA-PURESINGLE-CRYSTALS OF YBA2CU3O7-DELTA

We report on the absence of the so-called fishtail or peak effect in t he magnetisation curves of properly oxygenated high purity (5N) single crystals of YBa2Cu3O7-delta. AS we recently reported [1] high pressur e oxygenation procedures ale suitable to produce a monotonous dependen ce of the critical current density with the magnetic field. In this ar ticle we report furthermore how to reversibly produce or remove this e ffect on the same crystal by proper oxygenation methods. Along with th e high purity of the samples, due to crystal growth in non reactive Ba ZrO3 crucibles -, this proves that at least in pure samples the fishta il effect is due to different oxygenation states and/or oxygen distrib utions. In less pure samples, however, other sources for this anomaly like metallic impurities call act as an additional source for the fish tail anomaly.