Scanning SQUID microscopy and introduction of external pinning centers to yttrium barium copper oxide thin film

Vortex motion dynamics, vortex phase transition, pinning mechanism and depinning process are among the most important topics in high-Tc superconductors. Recently significant researches have been devoted to introducing external pinning centers to superconductors in an effort to improve transport properties of those materials.; There are two major parts in the research work presented in this dissertation: Part I - Scanning SQUID microscopy, and Part II - Introduction of external pinning centers to YBCO thin film.; In the first part, we developed a promising type of SQUID microscope that can be used to detect the motion of a single flux quantum Φ ₀ in a local area of micron size. Our system can work in magnetic field and has a large working temperature range up to 90 K. The microscope detecting loop can scan to different locations on the sample surface. The design and development of the instrumentation are described in detail. Experimental results of vortex motion obtained by the microscope on several YBCO samples are presented and discussed.; In the second part, we developed a theoretical model to describe the interaction between the flux lines in high-Tc superconductors and magnetic dots deposited on the superconductor's surface in a regular pattern. Numerical calculation results of pinning force induced by the dots are given. A detailed procedure of depositing magnetic dots of submicron sizes is described. Effects of the magnetic dots on the transport properties of some YBCO thin film samples are observed and discussed.