Scanning SQUID Microscopy for Magnetic Flux Systems

摘要

Recently,vortices confined into micro-scale superconductors with shapes like a disk,triangle,square,etc.,have attracted much attention because of the quantum phase transition of the self-organized vortex arrangement occurring within such geometrical constraints.Such a transition can be observed using a scanning SQUID microscope with high spatial resolution.We have successfully improved spatial resolution by incrporating a microfabrication technique that reduces both the size of the pick-up coil of the micro DC-SQUID and the standoff distance between the pick-up coil and the sample surface.Using this microscope,we have studied vortex arrangements in micro-scale superconductors made of Nb and Yba_2Cu_3O_7-delta films with various sizes and geometrical shapes.A peculiar oscillating behavior of diamagnetic magnetization corresponding to the particular vortex state was observed.