[Paper Review] Biaxial Strain Control of Helimagnetism via Chemical Expansion in Thin Film SrFeO3

Jennifer Fowlie, Jiarui Li+16 more|arXiv (Cornell University)|Feb 10, 2026

Multiferroics and related materials0 citations

TL;DR

The paper demonstrates that biaxial tensile strain in SrFeO3 thin films shortens the helimagnetic ordering length via chemical expansion from oxygen vacancies, linking lattice strain, defect chemistry, and magnetic order.

ABSTRACT

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.

Motivation & Objective

• Investigate how biaxial strain in SrFeO3 thin films affects helimagnetic order.
• Determine whether strain-induced changes are mediated by defect-driven chemical expansion rather than direct lattice effects.
• Elucidate the interplay between lattice, oxygen stoichiometry, and magnetic exchange mechanisms (superexchange vs double exchange).
• Assess robustness of strain effects across samples grown by PLD and MBE under different oxidizing environments.

Proposed method

• Grow SrFeO3 thin films on various substrates (LaAlO3, LSAT, SrTiO3) using PLD and MBE.
• Characterize structural strain and unit cell parameters via x-ray diffraction and reciprocal space mapping.
• Probe magnetic order with neutron diffraction and resonant soft x-ray scattering at Fe L3 edge.
• Perform first-principles DFT calculations (PBEsol+U, JH, simulated oxygen deficiency) to relate strain, vacancy formation, and magnetic energy landscapes.
• Use background charge approach in DFT to mimic oxygen deficiency and analyze Fe–O hybridization and exchange tendencies.

Figure 1: a)-c) Reciprocal space maps around the (-1 0 3) peak of the substrate: (a) 9 nm PLD-grown SrFeO 3 /SrTiO 3 , (b) 10 nm PLD-grown SrFeO 3 /LSAT and (c) 40 nm PLD-grown SrFeO 3 /LSAT. d) (0 0 2) peak $\theta$ -2 $\theta$ measurements of 15 nm MBE-grown SrFeO 3 /LaAlO 3 (black), 10 nm PLD-gro

Experimental results

Research questions

• RQ1 How does biaxial strain influence the real-space helical ordering length of SrFeO3 in thin films?
• RQ2 Is the strain-driven change in helimagnetism governed by direct lattice effects or by strain-induced changes in oxygen vacancy formation and chemical expansion?
• RQ3 How do superexchange and double exchange contributions shift under tensile vs compressive strain in SrFeO3?
• RQ4 Do films grown by PLD and MBE under different oxidizing conditions show consistent strain-induced magnetic changes?

Key findings

• Tensile biaxial strain shortens the proper screw helimagnetic ordering length by about 10% when the unit cell body diagonal expands by roughly 1%.
• Neutron and RSXS data reveal the helimagnetic vector tilts away from [111] and that a double-q/quadruple-q-like ground state can persist in thin films, consistent with multi-q ordering domains.
• DFT shows that direct strain effects on pdσ hopping are insufficient to explain the trend; oxygen vacancy–driven chemical expansion lowers vacancy formation energy under tensile strain, increasing Fe–O covalency and promoting superexchange over double exchange.
• Oxygen vacancies (x up to ~0.1) in SrFeO3-x shorten the helimagnetic length by enhancing p–d hybridization and SE, a trend reproduced by increasing effective electron density from strain due to vacancy chemistry.
• The transition temperature (paramagnetic to helimagnetic) remains near 120 K across strain, indicating a strain-tunable length scale rather than a shift in ordering temperature.
• Robustness: the strain-magnetic-length trend holds across PLD- and MBE-grown samples and across different oxidizing post-treatments (ozone/oxygen plasma) pointing to a defect-chemistry mediated mechanism.

Figure 2: $d$ -spacing neutron diffraction data around the magnetic Bragg peak of SrFeO 3 . (a) At 1.5 K and (b) at 100 K. Both panels show integrated intensity within three regions of reciprocal space that are equivalent in cubic symmetry; (0.13, 0.13, 0.11), (0.11 0.13 0.13) and (0.13 0.11 0.13).