Itinerancy-dependent noncollinear spin textures in SrFeO3, CaFeO3 , and CaFeO3/SrFeO3

Noncollinear, 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 noncollinear spin textures, the role of electron itinerancy in multi-$q$ formation is much less studied. Here we investigated the noncollinear, helical spin structures in epitaxial films of the perovskite oxides ${\mathrm{SrFeO}}_{3}$ and ${\mathrm{CaFeO}}_{3}$ using magnetotransport and resonant soft x-ray magnetic diffraction. Metallic ${\mathrm{SrFeO}}_{3}$ exhibits features in its magnetoresistance that are consistent with its recently proposed multi-$q$ structure. Additionally, the magnetic Bragg peak of ${\mathrm{SrFeO}}_{3}$ measured at the Fe $L$-edge resonance energy asymmetrically broadens with decreasing temperature in its multi-$q$ state. In contrast, insulating ${\mathrm{CaFeO}}_{3}$ has a symmetric scattering peak with an intensity 10 times weaker than ${\mathrm{SrFeO}}_{3}$. 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 the magnetic diffraction of ${\mathrm{CaFeO}}_{3}/{\mathrm{SrFeO}}_{3}$ superlattices with thick ${\mathrm{CaFeO}}_{3}$ layers, we find that the ${\mathrm{CaFeO}}_{3}$ helical ordering is coherent across 1-unit-cell-thick ${\mathrm{SrFeO}}_{3}$ layers but not 6-unit-cell-thick layers. We conclude that insulating ${\mathrm{CaFeO}}_{3}$ supports only a simple single-$q$ helical structure, in contrast to metallic ${\mathrm{SrFeO}}_{3}$ that hosts multi-$q$ structures. Our results provide important insight into the role of electron itinerancy in the formation of multi-$q$ spin structures.