Observation of superconducting vortex clusters in S/F hybrids

Area of Science:

• Condensed Matter Physics
• Materials Science
• Superconductivity

Background:

• Abrikosov vortices typically form a lattice in bulk superconductors due to repulsion.
• Confinement potentials, from geometric boundaries or magnetic field modulation, can induce vortex clustering.
• Superconductor/ferromagnet (S/F) heterostructures offer a platform to study magnetic field-induced confinement.

Purpose of the Study:

• To investigate vortex confinement and spatial distribution in superconductor/ferromagnet thin film heterostructures.
• To understand the role of magnetic inhomogeneities (bifurcations) in the ferromagnet on vortex behavior.
• To analyze the local electronic density of states within and around vortex clusters.

Main Methods:

• Magnetic force microscopy (MFM) for imaging magnetic bifurcations and superconducting vortices.
• High-resolution scanning tunneling microscopy (STM) for probing local electronic density of states.
• Analysis of intervortex spacing in relation to magnetic field conditions and material properties.

Main Results:

• Vortex clusters were observed to form near magnetization inhomogeneities (bifurcations) in the ferromagnet.
• Intervortex spacing within clusters was found to be shorter than predicted for uniform fields at the upper critical field (Hc2).
• This reduced spacing is attributed to local stray fields and the interplay between vortex-vortex repulsion and Lorentz forces.

Conclusions:

• Magnetic topologies in S/F hybrids can lead to vortex clustering near bifurcations.
• These specific magnetic configurations can sustain superconductivity even at local vortex densities exceeding the thermodynamic critical threshold.
• Findings suggest potential for novel superconducting devices utilizing engineered magnetic structures.