Local studies of magnetic flux line (vortex) distribution in superconducting thin films and
their pinning by natural and artificial defects have been performed using low-temperature
magnetic force microscopy (LT-MFM).
Taken a 100 nm thin NbN film as an example, the depinning of vortices from natural
defects under the influence of the force that the MFM tip exerts on the individual vortex was
visualized and the local pinning force was estimated. The good agreement of these results with
global transport measurements demonstrates that MFM is a powerful and reliable method to
probe the local variation of the pinning landscape. Furthermore, it was demonstrated that the
presence of an ordered array of 1-μm-sized ferromagnetic permalloy dots being in a magneticvortex
state underneath the Nb film significantly influences the natural pinning landscape of
the superconductor leading to commensurate pinning effects. This strong pinning exceeds the
repulsive interaction between the superconducting vortices and allows vortex clusters to be
located at each dot. Additionally, for industrially applicable YBa$_2$Cu$_3$O$_{7-\delta} thin films the main
question discussed was the possibility of a direct correlation between vortices and artificial
defects as well as vortex imaging on rough as-prepared thin films. Since the surface roughness
(droplets, precipitates) causes a severe problem to the scanning MFM tip, a nanoscale wedge
polishing technique that allows to overcome this problem was developed. Mounting the sample
under a defined small angle results in a smooth surface and a monotonic thickness reduction
of the film along the length of the sample. It provides a continuous insight from the film
surface down to the substrate with surface sensitive scanning techniques.
| Identifer | oai:union.ndltd.org:DRESDEN/oai:qucosa:de:qucosa:25239 |
| Date | 26 January 2010 |
| Creators | Shapoval, Tetyana |
| Contributors | Schultz, Ludwig, Metlushko, Vitali, Technische Universität Dresden |
| Publisher | Leibniz-Institut für Festkörper- und Werkstoffforschung Dresden |
| Source Sets | Hochschulschriftenserver (HSSS) der SLUB Dresden |
| Language | English |
| Detected Language | English |
| Type | doc-type:doctoralThesis, info:eu-repo/semantics/doctoralThesis, doc-type:Text |
| Rights | info:eu-repo/semantics/openAccess |
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