  Non-collinear, multi-q spin textures can give rise to exotic, topologically
protected spin structures such as skyrmions, but the reason for their formation
over simple single-q structures is not well understood. While lattice
frustration and the Dzyaloshinskii-Moriya interaction are known to produce
non-collinear spin textures, the role of electron itinerancy in multi-q
formation is much less studied. Here we investigated the non-collinear, helical
spin structures in epitaxial films of the perovskite oxides SrFeO3 and CaFeO3
using magnetotransport and resonant soft x-ray magnetic diffraction. Metallic
SrFeO3 exhibits features in its magnetoresistance that are consistent with its
recently proposed multi-q structure. Additionally, the magnetic Bragg peak of
SrFeO3 measured at the Fe L edge resonance energy asymmetrically broadens with
decreasing temperature in its multi-q state. In contrast, insulating CaFeO3 has
a symmetric scattering peak with an intensity 10x weaker than SrFeO3. Enhanced
magnetic scattering at O K edge prepeak energies demonstrates the role of a
negative charge transfer energy and the resulting oxygen ligand holes in the
magnetic ordering of these ferrates. By measuring magnetic diffraction of
CaFeO3/SrFeO3 superlattices with thick CaFeO3 layers, we find that the CaFeO3
helical ordering is coherent across 1 unit cell-thick SrFeO3 layers but not 6
unit cell-thick layers. We conclude that insulating CaFeO3 supports only a
simple single-q helical structure in contrast to metallic SrFeO3 that hosts
multi-q structures. Our results provide important insight into the role of
electron itinerancy in the formation of multi-q spin structures.
