  Magnetic materials with noncoplanar magnetic structures can show unusual
physical properties driven by nontrivial topology. Topologically-active states
are often multi-q structures, which are challenging to stabilize in models and
to identify in materials. Here, we use inelastic neutron-scattering experiments
to show that the insulating double perovskites Ba2YRuO6 and Ba2LuRuO6 host a
noncoplanar 3-q structure on the face-centered cubic lattice. Quantitative
analysis of our neutron-scattering data reveals that these 3-q states are
stabilized by biquadratic interactions. Our study identifies double perovskites
as a highly promising class of materials to realize topological magnetism,
elucidates the stabilization mechanism of the 3-q state in these materials, and
establishes neutron spectroscopy on powder samples as a valuable technique to
distinguish multi-q from single-q states, facilitating the discovery of
topologically-nontrivial magnetic materials.
