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

TL;DR

This work tackles the problem of how biaxial strain influences helimagnetism in SrFeO3 thin films, showing that strain-induced chemical expansion via oxygen vacancies can indirectly tune magnetic order. The authors combine neutron diffraction, resonant soft x-ray scattering, and first-principles calculations to separate lattice, electronic, and defect effects under strain. They find that tensile strain shortens the helimagnetic length and tilts the propagation vector, an effect well explained by vacancy-facilitated chemical expansion that enhances superexchange relative to double exchange. The study establishes chemical expansion as a practical mechanism to engineer complex magnetic textures in oxide thin films, with implications for spintronics, magnonics, and reconfigurable oxide electronics, and suggests avenues for electrostatic control through oxygen mobility.

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.

Figures (4)

• 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-grown SrFeO$_3$/LSAT (light blue) and 9 nm PLD-grown SrFeO$_3$/SrTiO$_3$ (red) used for synchrotron x-ray measurements and 40 nm PLD-grown SrFeO$_3$/LSAT used for neutron scattering (dark blue).
• 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).
• Figure 3: Resonant soft x-ray scattering data recorded on the Fe L$_3$ edge in linear horizontal ($\pi$) polarization. (a) $\theta$-2$\theta$ scans across the magnetic Bragg peak of PLD-grown SrFeO$_3$/LSAT. The 20 K (black) and 105 K (red) data are shown. The more intense peak corresponds to the proper screw order, while the less intense peak corresponds to a spin cycloid. Shaded regions are for illustrative purposes and are not numerical fits. Insets sketch the different spiral structures where the incommensurate ordering vector is shown in blue and iron atoms in the body center positions are indicated by yellow spheres. (b) The proper screw ordering length extracted from the $\theta$-2$\theta$ scans as a function of temperature. PLD-grown samples are represented by filled shapes while MBE-grown samples are represented by empty shapes. Error bars (often smaller than the plot marker) represent one standard deviation from the center of a Gaussian lineshape. (c) The base temperature ($\approx$ 20 K) ordering length of the proper screw structure as a function of the [111] body diagonal length of the chemical unit cell for the two series of sample. A survey of bulk values are represented by the gray plot markers Reehuis2012Long2012Ishiwata2020. For consistency, body diagonal length is calculated from room temperature lattice parameters -- SrFeO$_3$ exhibits only a weak thermal contraction of $\approx -0.25 \%$ from room temperature to 100 K Takeda1972. Horizontal error bars represent a $\pm$ 0.005 Å tolerance on the unit cell diagonal while vertical error bars are smaller than the plot markers. The dashed lines are guides to the eye.
• Figure 4: Strain dependence of the [111] helimagnetic ordering length from DFT for (a,b) pristine SrFeO$_3$ and (c,d) oxygen deficient bulk SrFeO$_{3-x}$.(a, c) display the total energy difference between the helimagnetic state (propagating along [111]) and the ferromagnetic state while (b, d) display the ordering length as a function of strain and $x$ respectively.