title: Observation of superconducting vortex clusters in S/F hybrids.
abstract: While Abrikosov vortices repel each other and form a uniform vortex lattice in bulk type-II superconductors, strong confinement potential profoundly affects their spatial distribution eventually leading to vortex cluster formation. The confinement could be induced by the geometric boundaries in mesoscopic-size superconductors or by the spatial modulation of the magnetic field in superconductor/ferromagnet (S/F) hybrids. Here we study the vortex confinement in S/F thin film heterostructures and we observe that vortex clusters appear near magnetization inhomogeneities in the ferromagnet, called bifurcations. We use magnetic force microscopy to image magnetic bifurcations and superconducting vortices, while high resolution scanning tunneling microscopy is used to obtain detailed information of the local electronic density of states outside and inside the vortex cluster. We find an intervortex spacing at the bifurcation shorter than the one predicted for the same superconductor in a uniform magnetic field equal to the thermodynamical upper critical field Hc2. This result is due to a local enhanced stray field and a competition between vortex-vortex repulsion and Lorentz force. Our findings suggest that special magnetic topologies could result in S/F hybrids that support superconductivity even when locally the vortex density exceeds the thermodynamic critical threshold value beyond which the superconductivity is destroyed.
authors.affiliation: "E.R. Caianiello" Physics Department, University of Salerno, Fisciano (SA), 84084, Italy.|Physics Department, Temple University, Philadelphia, PA 19122, United States.
authors.forename: C
authors.initials: C
authors.lastname: Di Giorgio
authors.affiliation: "E.R. Caianiello" Physics Department, University of Salerno, Fisciano (SA), 84084, Italy.|CNR-SPIN Salerno, Fisciano (SA), 84084, Italy.
authors.forename: F
authors.initials: F
authors.lastname: Bobba
authors.affiliation: "E.R. Caianiello" Physics Department, University of Salerno, Fisciano (SA), 84084, Italy.|CNR-SPIN Salerno, Fisciano (SA), 84084, Italy.
authors.forename: A M
authors.initials: AM
authors.lastname: Cucolo
authors.affiliation: "E.R. Caianiello" Physics Department, University of Salerno, Fisciano (SA), 84084, Italy.
authors.forename: A
authors.initials: A
authors.lastname: Scarfato
authors.affiliation: Physics Department, Temple University, Philadelphia, PA 19122, United States.
authors.forename: S A
authors.initials: SA
authors.lastname: Moore
authors.affiliation: Physics Department, Drexel University, Philadelphia, PA 19104, United States.
authors.forename: G
authors.initials: G
authors.lastname: Karapetrov
authors.affiliation: "E.R. Caianiello" Physics Department, University of Salerno, Fisciano (SA), 84084, Italy.
authors.forename: D
authors.initials: D
authors.lastname: D'Agostino
authors.affiliation: Materials Science Division, Argonne National Laboratory, Argonne, IL 60439 United States.|National University of Science and Technology (MISiS), Moscow, 119049, Russia.
authors.forename: V
authors.initials: V
authors.lastname: Novosad
authors.affiliation: Materials Science Division, Argonne National Laboratory, Argonne, IL 60439 United States.
authors.forename: V
authors.initials: V
authors.lastname: Yefremenko
authors.affiliation: Physics Department, Temple University, Philadelphia, PA 19122, United States.
authors.forename: M
authors.initials: M
authors.lastname: Iavarone