Microscopic study of vortex pinning in YBCO thin films

The direct microscopic imaging of flux lines by means of low-temperature MFM was applied to study the vortex pinning mechanism at natural and artificial defects in high temperature superconducting films. A low-temperature scanning probe microscope (Omicron Cryogenic SFM) allows measurements in UHV combined with magnetic fields of 7 T (vertical) and 3 T (transversal). Flux lines have been successfully imaged on a pure YBCO film as well as on a YBCO film grown on a template with gas phase prepared Y2O3 nanoparticles. The investigated films with a mean roughness less than 10 nm were deposited by off-axis PLD and cooled down in the microscope to 7.7 K in a magnetic field prior to imaging. The number of vortices observed corresponds to the theoretically expected one. The vortex distribution was compared with the topography. The in situ transport measurements allow an estimation of the pinning strength at the different defects present in the sample. For future studies on chemical deposited films a mechanical polishing procedure is developed to obtain sufficiently smooth sample surfaces. (orig.)