  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.
