Experimental Observation of Vortex Rings in a Bulk Magnet

TL;DR

This study reports the first direct imaging of stable magnetic vortex rings in a bulk ferromagnet, revealing their structure, stability, and potential for future applications in three-dimensional magnetic solitons.

Contribution

It provides the first experimental observation of stable magnetic vortex rings in a bulk magnet using X-ray nanotomography, confirming their existence and stability.

Findings

Magnetic vortex rings are stable and static in bulk ferromagnets.

Vortex rings are composed of vortex-antivortex pairs forming closed loops.

Stability is attributed to dipolar interactions.

Abstract

Vortex rings are remarkably stable structures occurring in numerous systems: for example in turbulent gases, where they are at the origin of weather phenomena [1]; in fluids with implications for biology [2]; in electromagnetic discharges [3]; and in plasmas [4]. While vortex rings have also been predicted to exist in ferromagnets [5], they have not yet been observed. Using X-ray magnetic nanotomography [6], we imaged three-dimensional structures forming closed loops in a bulk micromagnet, each composed of a vortex-antivortex pair. Based on the magnetic vorticity, a quantity analogous to hydrodynamic vorticity, we identify these configurations as magnetic vortex rings. While such structures have been predicted to exist as transient states in exchange ferromagnets [5], the vortex rings we observe exist as stable, static configurations, whose stability we attribute to the dipolar interaction. In addition, we observe stable vortex loops intersected by magnetic singularities [7], at which the magnetisation within the vortex and antivortex cores reverses. We gain insight into the stability of these states through field and thermal equilibration protocols. These measurements pave the way for the observation of complex three-dimensional solitons in bulk magnets, as well as for the development of applications based on three-dimensional magnetic structures.