Field-cooled magnetization of YBa₂Cu₃O_7-δ superconductors

DC susceptibility as a function of temperature was measured in both field-cooled cooling and warming modes in magnetic fields in the range of 10 mT≤μ₀H_a≤7 T on a variety of YBa₂Cu₃O_7-δ samples, comprising different single crystals, melt-processed bulks and polycrystalline samples, with and without KClO₃ addition. The Meissner curves measured at a field of 1 mT reveal that all the samples are of a single phase. However, with increasing applied fields, the superconducting transitions of the single crystals and the melt-processed samples develop 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 are discussed in detail. Further, we discuss the correlation between the kinks in the DC susceptibility and the appearance of the fishtail shape of magnetization hysteresis loops.