We used low temperature scanning probe microscopy techniques to investigate superconducting vortex confinement in planar Superconductor/Ferromagnet (S/F) heterostructures realized by Nb/Py thin films. In these systems, new physical phenomena can be observed due to the interaction between superconducting vortices in Nb layer and the periodic, stripe-like, Py magnetic domains.
In general, the arrangement of superconducting vortices in bulk superconductors is only set by their reciprocal repulsion, which leads to a hexagonal vortex lattice formation, also called “Abrikosov lattice”. Nevertheless, when a confinement potential is imposed to the superconductor, the vortex distribution may result strongly affected. In particular, in presence of a ferromagnetic template underneath the superconductor, the nucleation of vortex chains and vortex clusters have been reported.
In this framework, we studied vortex clusters pinned at the bifurcations of stripe-like magnetic domains by using magnetic force microscopy (MFM) and scanning tunneling microscopy and spectroscopy (STM/STS). Magnetic imaging of such dislocations, above and below the S critical temperature Ts, is crucial to get insight into both their magnetic topology and their confinement power for superconducting vortices. Indeed, low temperature MFM experiments allow the imaging of superconducting vortices being sensitive to the magnetic field decay from the normal vortex core, on the scale of the London penetration depth λ. On the other hand, once the magnetic structure is known, low temperature STM/STS, sensitive to the amplitude of the superconducting order parameter rather than the magnetic profile, allows detailed measurements of the electronic density of states outside and inside the vortex core, with a sub-nanometric spatial resolution.
We find that intrinsic bifurcations of the stripe-like magnetic pattern can act as confinement sites for superconducting vortices. Such dislocations, can be particularly effective for vortex pinning because of the local enhancement of the magnetic out-of-plane stray field as well as because of their peculiar magnetic topology.