Observation of superconducting vortex clusters in S/F hybrids.

S. A. Moore,M. Iavarone,D. D’Agostino,A. Scarfato,A. M. Cucolo,V. Yefremenko,V. Novosad,C. Di Giorgio,G. Karapetrov,F. Bobba

doi:10.1038/srep38557

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.

Highlights

The vortex confinement in peculiar superconductor/ferromagnet nanostructure formed by a superconducting microsquare with a magnetic disk on top has been theoretically addressed by Chen et al.[47]
Low temperature magnetic force microscopy (MFM) has been a useful tool to image mesoscopic vortex configurations: vortex clusters pinned by a periodic array of magnetic dots in S/F structures[48,49] as well as vortex-antivortex pairs and vortex chains in magnetically coupled planar S/F bilayers[50,51,52,53]
In this work we study the superconducting vortex confinement in planar S/F thin film heterostructures caused by intrinsic topological defects in the magnetic template called bifurcations, by low temperature magnetic force microscopy and scanning tunneling microscopy and spectroscopy

Summary

Introduction

The vortex confinement in peculiar superconductor/ferromagnet nanostructure formed by a superconducting microsquare with a magnetic disk on top has been theoretically addressed by Chen et al.[47]. On the other hand, scanning tunneling microscopy and spectroscopy (STM/STS) has been successfully used by Karapetrov et al.[54] to study vortex chain formation in S/F planar heterostructures and by Cren et al.[55] to study the geometrical confinement effects on the stabilization of superdense multivortex and giant vortex phases in mesoscopic superconductors. In this work we study the superconducting vortex confinement in planar S/F thin film heterostructures caused by intrinsic topological defects in the magnetic template called bifurcations, by low temperature magnetic force microscopy and scanning tunneling microscopy and spectroscopy. At the same time we use STM/STS to get high resolution spatial maps of the electronic DOS of the superconductor just above the bifurcation sites in the ferromagnet

Methods

Results

Conclusion