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 hysteresis loops (MHLs) are analyzed in various high-T_c superconducting samples comprising several RBa₂Cu₃O_7-δ (RBCO; R = rare earths) single crystals of different thicknesses, a laser-ablated YBa₂Cu₃O_7-δ (YBCO) thin film, Ag-sheathed (Pb,Bi)₂Sr₂Ba₂Cu₃O_10+δ (Bi-2223) mono- and multifilamentary tapes, and a model sample designed to reproduce a layer of grains [M.R. Koblischka 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 fields. In order to better understand the magnetization processes leading to the formation of the central peak in the MHLs, the local field distributions in applied fields close to zero were studied using magneto-optic (MO) flux visualization on the same samples. These flux patterns show how the vortices 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 the central peak towards zero or even to very small positive fields. To explain the anomalous behaviour found in Bi-2223 tapes, effects of granularity have to be considered additionally. Further, we discuss the interaction of the central peak with other 'peak effects' observed in MHLs.