The effect of thickness on the magnetic properties of melt-processed YBCO thick films

Abstract

Magnetic properties of melt-processed YBa2Cu3O7- delta thick films have been measured and correlated with features in the microstructure at 4.2 and 77 K for film thicknesses between 50 and 140 mu m. A pronounced peak has been observed in both the measured volume magnetization and calculated length scale over which current flows at a film thickness of approximately 53 mu m and 4.2 K in fields of up to 10 T. An intra 'hub-spoke' (H-S) type grain current dominates the volume magnetization at this film thickness. Measurements at 77 K, on the other hand, exhibit a peak at 80 mu m, the magnitude of which varies significantly with applied field. This correlates well with observed increased connectivity between individual H-S grains and may be accounted for by the flow of inter H-S grain current. The H-S grains transform to a more granular microstructure for films greater than approximately 100 mu m thick which is characterized by the presence of smaller diameter current-carrying loops. This is observed as a decrease in the volume magnetization at 4.2 K and a levelling off of this parameter at 77 K with increasing film thickness. Further evidence for the presence of intra H-S and inter H-S grain critical current densities at 4.2 K in films up to a thickness of 80 mu m has been observed from length-scale analysis as a function of the difference between maximum and applied magnetic field. A qualitative model for the volume magnetization of the films at 4.2 K in terms of individual contributions from intra H-S grain, inter H-S grain and granular Jc components is proposed.