Study of pinning mechanisms in YBCO thin films by means of magnetic force microscopy

Low-temperature magnetic force microscopy is used for the direct visualization of vortex distribution in high-temperature superconducting films. With the scanning probe microscope (Omicron Cryogenic SFM) magnetic fields up to 7 T (vertical) and 3 T (transversal) can be applied to the sample during in situ imaging at temperatures ranging from 7 K to 300 K in ultra high vacuum. The samples investigated were 300 nm thick YBa2Cu3O7-δ films grown epitaxially on SrTiO3 (0 0 1) substrates by off-axis pulsed laser deposition with a mean roughness less than 10 nm. Prior to imaging, the samples were cooled down in the microscope to 7.7 K in a magnetic field of 3 mT. Flux lines have been successfully imaged by measuring their stray field in the scan height of 40 nm above the sample surface. The number of vortices observed corresponds to the theoretically expected one. Whereas a constant current leads to unacceptable heating of the sample, the use of current pulses allows transport measurements without thermal movement of vortices