Experimental observation of vortex rings in a bulk magnet

Abstract

Vortex rings are remarkably stable structures that occur in a large variety of systems, such as in turbulent gases (where they are at the origin of weather phenomena)1, fluids (with implications for biology)2, electromagnetic discharges3 and plasmas4. Although vortex rings have also been predicted to exist in ferromagnets5, they have not yet been observed. Using X-ray magnetic nanotomography6, we imaged three-dimensional structures forming closed vortex loops in a bulk micromagnet. The cross-section of these loops consists of a vortex–antivortex pair and, on the basis of magnetic vorticity (a quantity analogous to hydrodynamic vorticity), we identify these configurations as magnetic vortex rings. Although such structures have been predicted to exist as transient states in exchange ferromagnets5, the vortex rings we observe exist as static configurations, and we attribute their stability to the dipolar interaction. In addition, we observe stable vortex loops intersected by point singularities7 at which the magnetization within the vortex and antivortex cores reverses. We gain insight into the stability of these states through field and thermal equilibration protocols. The observation of stable magnetic vortex rings opens up possibilities for further studies of complex three-dimensional solitons in bulk magnets, enabling the development of applications based on three-dimensional magnetic structures.