  Frustrated itinerant ferromagnets, with non-collinear static spin structures,
are an exciting class of material as their spin chirality can introduce a Berry
phase in the electronic scattering and lead to exotic electronic phenomena such
as the anomalous Hall effect (AHE).
  This study presents a reexamination of the magnetic properties of
Fe$_3$Sn$_2$, a metallic ferromagnet, based on the 2-dimensional kagome bilayer
structure. Previously thought of as a conventional ferromagnet, we show using a
combination of SQUID measurements, symmetry analysis and powder neutron
diffraction, that Fe$_3$Sn$_2$ is a frustrated ferromagnet with a
temperature-dependent non-collinear spin structure. The complexity of the
magnetic interactions is further evidenced by a re-entrant spin glass
transition ($T_f\simeq80$\,K) at temperatures far below the main ferromagnetic
transition ($T\mathrm{_C}$ = 640\,K).
  Fe$_3$Sn$_2$ therefore provides a rare example of a frustrated itinerant
ferromagnet. Further, as well as being of great fundamental interest our
studies highlight the potential of Fe$_3$Sn$_2$ for practical application in
spintronics technology, as the AHE arising from the ferromagnetism in this
material is expected to be enhanced by the coupling between the conduction
electrons and the non-trivial magnetic structure over an exceptionally wide
temperature range.
