Vortex pinning in superconducting thin films using ordered nanoporous templates

(eng) The vortex motion in type-II superconductors strongly limits their critical current. Currently, ordered arrays of pinning centers in the superconductor are fabricated by lithographic process, which is very expensive and time consuming. In this work, we developed the so-called "template approach", as a simple and low-cost alternative way to create large-area ordered arrays of pinning centers. This approach uses nanoporous alumina templates in order to artificially imprint a dense periodic lattice of defects into a thin superconducting film. On the one hand, a regular array of defects was created in a superconducting film by depositing it onto either the “front face” (periodic pore arrangement) or the “rear face” (array of ordered bumps at the barrier layer) of the porous alumina. Taking advantage of the two opposite faces of the template and of various deposition techniques we introduced three different lattices of pinning centers in NbN and Nb superconducting thin films. Using electrical transport measurement, we investigated the motion and stability of these diverse configurations of vortex lattices. Pronounced high magnetic field matching effects were observed in these superconducting films. The second approach is based on the magnetic pinning of the vortices, i.e. the magnetic interaction between the local field of the vortices and the stray magnetic field of nanoscale ferromagnets. More precisely, we studied the interaction between a superconducting thin film and a highly ordered array of Ni nanowires obtained by electrodeposition into the pores of the porous template. Enhanced pinning and matching effects, as well as pronounced hysteretic behavior and tunability of the superconducting properties were observed at low fields.