Superconducting properties of perforated NbN films using ordered arrays of ferromagnetic nanowires

Abstract

The superconducting properties of NbN thin films deposited perpendicularly and embedding a 100-nm-spaced triangular array of ferromagnetic nanowires have been studied. Matching effects are found to persist up to 3 T in these superconductor-ferromagnet hybrids. Other interesting matching features have been also observed. The first matching field depends on the magnetic history. It can be shifted practically at will between the theoretical matching field H 1 and a lower field H * 1 that depends on the shape of the magnetization hysteretic loop of the nanowires array. Therefore, this shift is strongly influenced by the nanowires packing, in particular by the nanowires diameter in the present system. The reduction of the first matching field is associated with an enhancement of the superconducting state that is the strongest when the first matching field is shifted down to H * 1 . Finally, misleading field-induced superconductivity has been observed in fields up to 1.2 T.

Artificial vortex pinning arrays in superconducting films deposited on highly ordered anodic alumina templates

“…Starting from negative saturation, a pronounced dip in resistance is observed at H = −0.235 T, in accordance with the theoretical matching condition for a triangular lattice and a distance between pinning centers of about 100 nm (H 1 = 0.239 T). The resistance drops by almost two orders of magnitude compared to the zero-field value, as discussed in a previous work [13]. For this field value, the magnetization vectors of all the Ni nanowires are pointing along the applied field direction.…”

“…By applying positive magnetic field, the first peak in resistance is somewhat shifted from its theoretical position (+0.165 T instead of +0.235 T). This can be explained by invoking the magnetization reversal process of the single-domain nanowires [13]. Indeed, in the low-field unsaturated regime some nanowires are magnetized antiparallel to the external field, see figure 4(b).…”

“…Subsequently, a mechanical polishing using 20 nm silica particles was performed to remove the Au cathode and to get the extremities of all the nanowires at the surface of the template. From atomic force microscopy (AFM) characterization, a steep surface profile was observed and it was found that the nanowire tips overhang the alumina surface by about 20 nm [13]. This rough surface is due to the different polishing rate between the alumina template and the metallic nanowires embedded into it.…”

“…The samples were further investigated using magnetotransport and SQUID magnetometry measurements. More details on the low-temperature experimental setup can be found elsewhere [12,13].…”

Vortex matching effects in Nb thin films due to Ni nanopillars embedded in anodic aluminum oxide substrates

“…These features, i.e. the increase of the critical temperature at certain values of the external magnetic field, can be ascribed to matching effects between the vortices and the lattice of ferromagnetic pins [19]. Sample AON-Nb32 presents, probably due to the slightly smaller Nb thickness, even more pronounced anomalies in the H c2⊥ (T) curve as shown in figure 3(b) on normalized axes.…”