  Eu$_5$In$_2$Sb$_6$ is a member of a family of orthorhombic nonsymmorphic
rare-earth intermetallics that combines large localized magnetic moments and
itinerant exchange with a low carrier density and perpendicular glide planes.
This may result in special topological crystalline (wallpaper fermion) or axion
insulating phases. Recent studies of Eu$_5$In$_2$Sb$_6$ single crystals have
revealed colossal negative magnetoresistance and multiple magnetic phase
transitions. Here, we clarify this ordering process using neutron scattering,
resonant elastic X-ray scattering, muon spin-rotation, and magnetometry. The
nonsymmorphic and multisite character of Eu$_5$In$_2$Sb$_6$ results in coplanar
noncollinear magnetic structure with an Ising-like net magnetization along the
$a$ axis. A reordering transition, attributable to competing ferro- and
antiferromagnetic couplings, manifests as the onset of a second commensurate
Fourier component. In the absence of spatially resolved probes, the
experimental evidence for this low-temperature state can be interpreted either
as an unusual double-$q$ structure or in a phase separation scenario. The net
magnetization produces variable anisotropic hysteretic effects which also
couple to charge transport. The implied potential for functional domain physics
and topological transport suggests that this structural family may be a
promising platform to implement concepts of topological antiferromagnetic
spintronics.
