Visualization of magnetization processes in HTSCs

A novel method for visualizing the magnetic flux distribution in superconductors using iron-garnet films operating in a wide temperature range is developed. Processes of magnetization in Y, Bi, and Tl cuprate single crystals and YBaCu films and ceramics were studied using this method. Screening patterns and local susceptibility data revealed inhomogeneity of superconducting properties of the samples associated with weak links due to defects. Magnetization of Y, Bi, and Tl cuprates appears to proceed in two stages. At smaller fields (but above a critical value) the magnetization is reversible and apparently determined by entry of the magnetic flux into samples along channels of weak links. At higher fields the flux begins to penetrate into regions of stronger superconductivity and is trapped there. With increasing temperature the flux pinning decreases and a considerable creep takes place. At T>0.6 T_c noticeable flux trapping is revealed only after field cooling in most of samples. High-temperature superconducting films demonstrate essentially stronger pinning in comparison with single crystals, indicating that anchoring at micrograin boundaries is perhaps the most effective pinning mechanism