Cubic double perovskites host noncoplanar spin textures

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 Ba_2YRuO_6 and Ba_2LuRuO_6 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.