Abstract
Perpendicular magnetic anisotropy ferromagnetic/superconducting (FM/SC) bilayers with a labyrinth domain structure are used to study nucleation of superconductivity on a fractal network, tunable through magnetic history. As clusters of reversed domains appear in the FM layer, the SC film shows a percolative behavior that depends on two independent processes: the arrangement of initial reversed domains and the fractal geometry of expanding clusters. For a full labyrinth structure, the behavior of the upper critical field is typical of confined superconductivity on a fractal network.
Highlights
IntroductionSC wire networks (SWN) [34] and granular superconductors [3, 35]
We show that nucleation of superconductivity is controlled both by the distribution of clusters of reversed domains and their fractal geometry resulting in a double level percolation process
A NdCo5(t nm) amorphous layer is grown by cosputtering [36, 37] with thickness (t) in the 40–80 nm range on a Si substrate covered by a 10 nm thick Al buffer layer and, a 5 nm thick Al capping layer is grown on top
Summary
SC wire networks (SWN) [34] and granular superconductors [3, 35] This labyrinthine structure would allow one to tune a fractal SC behavior through the magnetic film history. We show that nucleation of superconductivity is controlled both by the distribution of clusters of reversed domains and their fractal geometry resulting in a double level percolation process. Once the percolation process is finished and the labyrinthine domain configuration extends homogenously through the sample, the upper critical field Hc2 shows the characteristic temperature dependence of confined superconductivity on a fractal network. Al/NdCo/Al sample is taken out of the chamber so that the Al capping layer becomes oxidized and, a Nb film with thickness dNb = 50 nm is grown on top by sputtering to get the complete FM/SC bilayer structure
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