Field-cooled magnetization of YBa2Cu3O7-delta superconductors

DC susceptibility as a function of temperature was measured in both field-c ooled cooling and warming modes in magnetic fields in the range of 10 mT le ss than or equal to mu H-0(a) less than or equal to 7 T on a variety of YBa 2Cu3O7-delta samples, comprising different single crystals, melt-processed bulks and polycrystalline samples, with and without KClO3 addition. The Mei ssner curves measured at a field of 1 mT reveal that all the samples are of a single phase. However, with increasing applied fields, the superconducti ng transitions of the single crystals and the melt-processed samples develo p kinks, and above 4 T, even a secondary transition can be observed. At low temperatures, the field-cooled moment increases in all samples, except the KClO3-doped samples, where a clear decrease is observed. These features ar e discussed in detail. Further, we discuss the correlation between the kink s in the DC susceptibility and the appearance of the fishtail shape of magn etization hysteresis loops. (C) 2000 Elsevier Science B.V. All rights reser ved.