Formation of the low-field peak in magnetization loops of high-T-c superconductors

The positions of the central (low-field) peak in the magnetization hysteres is loops (MHLs) are analyzed in various high-T-c superconducting samples co mprising several RBa2Cu3O7-delta (RBCO; R = rare earths) single crystals of different thicknesses, a laser-ablated YBa2Cu3O7-delta (YBCO) thin film, A g-sheathed (Pb,Bi)(2)Sr2Ba2Cu3O10+delta (Bi-2223) mono- and multifilamentar y tapes, and a model sample designed to reproduce a layer of grains [M.R. K oblischka et al., Appl. Phys. Lett. 70 (1997) 514]. The single crystals and the thin film show the peak at zero-field or at negative applied fields on the descending field branch according to the critical state models, the Bi -2223 tapes are found to exhibit the peak anomalously in positive applied f ields. In order to better understand the magnetization processes leading to the formation of the central peak in the MHLs, the local field distributio ns in applied fields close to zero were studied using magneto-optic (MO) fl ux visualization on the same samples. These flux patterns show how the vort ices are rearranged when sweeping through zero-field. A large demagnetizing effect ("perpendicular geometry") facilitates the penetration of vortices of opposite polarity, especially along structural defects, thus, forcing th e central peak towards zero or even to very small positive fields. To expla in the anomalous behaviour found in Bi-2223 tapes, effects of granularity h ave to be considered additionally. Further, we discuss the interaction of t he central peak with other "peak effects" observed in MHLs. (C) 1999 Publis hed by Elsevier Science B.V. All rights reserved.