Biaxial Strain Control of Helimagnetism via Chemical Expansion in Thin Film SrFeO3

We demonstrate control of helimagnetic order in biaxially strained SrFeO3 thin films using neutron diffraction and resonant soft x-ray scattering. SrFeO3, a negative charge-transfer oxide, exhibits a complex magnetic phase diagram that includes multi-q spin structures. Tensile epitaxial strain produces a pronounced shortening of the helimagnetic ordering length and a tilting of the magnetic ordering vector. We interpret this behavior in terms of chemical expansion: lattice dilation under tensile strain lowers the energetic cost of oxygen vacancies, leading to an expanded unit cell that modifies Fe-O hybridization and enhances superexchange relative to double exchange. These results reveal how epitaxial strain can indirectly tune helimagnetism through defect-driven chemical expansion, highlighting the strong coupling between lattice, chemistry, and magnetic order in transition-metal oxides. Our findings establish chemical expansion as an effective mechanism for engineering complex magnetic textures in oxide thin films, with implications for spintronic, magnonic, and quantum information applications.

💡 Research Summary

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In this work the authors investigate how biaxial epitaxial strain influences the helimagnetic order of SrFeO₃ thin films, employing neutron diffraction, resonant soft‑x‑ray scattering (RSXS), and density‑functional theory (DFT). SrFeO₃ is a negative‑charge‑transfer perovskite in which Fe⁴⁺ ions adopt a high‑spin d⁴ configuration that is strongly hybridized with oxygen ligand holes, giving the material a d⁵L‑like electronic structure. This hybridization places the system near a delicate balance between superexchange (SE) and double‑exchange (DE) interactions, which is thought to stabilize a helical spin texture with an incommensurate propagation vector close to the