Estudo da relaxação magnética em supercondutores de YBa2Cu3O7- que exibem o efeito Meissner paramagnético

Abstract

The present work consists of an experimental study on the behavior the FC (Field Cooled) magnetic moment relaxation in samples showing the Paramagnetic Meissner Effect (PME). Were investigated melt-textured samples containing 30 wt% of phase (Y211), grown by top-seeding technique and provided by the Department of Materials Science and Metallurgy Engineering of the University of Barcelona, Spain. The samples were investigated by scanning electron microscopy (SEM), where it was possible the observation of a long platelet structure of superconducting layers stacked along the c-axis, typical of melt-textured materials. It was also observed the presence of the Y211 phase dispersed into the superconducting matrix. In order to characterize the PME, FC magnetization measurements in function of the temperature were performed according to FCC (Field Cooled Cooling) and FCW (Field Cooled Warming) prescriptions, for magnetic fields from 0 up to 50 kOe applied parallel and perpendicular to the ab planes. Additionally magnetization measurements were performed according to ZFC (Zero Field Cooled) prescription, that shown the usual diamagnetic behavior for all applied magnetic fields. The occurrence of the PME was observed for FC measurements, for applied magnetic fields above 1 kOe, which apparently increases as the magnetic field is increased. The PME was discussed in terms of the flux pinning caused by the Y211 particles associated with a strong flux compression into the samples. The measurements of the FC magnetic relaxation showed an anomalous paramagnetic response increasing with time, contrasting to the expected diamagnetic behavior, however, with a strong correlation with the PME mechanism. In order to understand the paramagnetic relaxation, we performed a mathematical analysis of the relaxation measurements adopting an double exponential function, which showed two distinct relaxation regimes. We attribute the regime in the early stages to a Josephon-type dynamics (intergranular vortices), while the relaxation regime for long time to an Abrikosov-type dynamics (intragranular vortices). Finalizing, this work presents a simple analysis of the paramagnetic relaxation, however, in our view these results may provide important informations on a interesting subject whose comprehension is under development yet.