PARAMAGNETIC MEISSNER EFFECT OF HIGH-TEMPERATURE GRANULAR SUPERCONDUCTORS - INTERPRETATION BY ANISOTROPIC AND ISOTROPIC MODELS

The field-cooled magnetization of high-T-c superconducting ceramics me asured in low magnetic fields exhibits the paramagnetic Meissner effec t (PME), i.e., the diamagnetic signal initially increases with decreas e in temperature but reaches a maximum at temperature T-d and later de creases with decrease in temperature. Even in some samples the signal is ultimately able to transform inversely into a paramagnetic regime o nce the sample is cooled below a temperature T-p as long as the applie d field is sufficiently small. This PME has been observed in various h igh-T-c cuprates and is explained by disparate aspects. An anisotropic model, in which the granular superconductors are assumed to be ideall y anisotropic, was first alternatively proposed in the present work so as to theoretically account for this effect. On the other hand, an is otropic model, suitable for granular superconductors with randomly ori ented grains, was proposed to deal with the samples prepared by a conv entional solid-state reaction method. The anomalous magnetization beha vior in the present model was demonstrated to be the superposition of the diamagnetic signal, which occurs as a result of the intragranular shielding currents, over the paramagnetic one due to the induction of the intergranular component induced by these currents where the interg ranular one behaved as the efective pinning centers. The PME was demon strated by this model to exist parasitically in granular superconducto rs. This intergranular effect is therefore worthy of remark when evalu ating the volume fraction of superconductivity for the samples from th e Meissner signal, in particular, at a low magnetic field.