Designing heterostructures to control oxygen stoichiometry in helimagnetic perovskite strontium ferrite

Abstract

A large challenge in determining the physics of helimagnetic SrFeO3 is in stabilizing the stoichiometric chemical phase over long enough time scales to conduct extensive measurements. Degradation in SrFeO3 manifests mainly as a crossover from metallic to insulating behavior. Using a combination of electronic transport and density functional theory, we show that this degradation is dominated by oxygen loss, possibly on the order of one percent. We further demonstrate that high quality SrFeO3 thin films can be stabilized long-term by combining a nanoscale band insulator capping layer with an ex situ ozone anneal. We show that this produces a nearly-pristine cation sublattice and preserves metallicity for at least several weeks. These results establish a reliable pathway for producing chemically stable SrFeO3 thin films, enabling reproducible studies of its unusual helimagnetism.

Figures (4)

• Figure 1: (a) $\theta-2\theta$ x-ray diffraction scans of a 30 u.c. (11 nm) SrFeO$_x$ thin film with a 4 u.c. (1.6 nm) capping layer of SrTiO$_3$ before (blue) and after (red) ozone anneal. (b) $\theta-2\theta$ scans before and after annealing for a 100 u.c. (38 nm) sample with a 3 u.c. (1.2 nm) capping layer of SrTiO$_3$. Asterisks indicate the peaks characteristic of the brownmillerite phase. (c) Photograph of two SrFeO$_x$ samples before and after ozone-annealing. (d) X-ray absorption in total electron yield around the Fe L$_{2,3}$ edges of a fully oxidized SrFeO$_3$ film.
• Figure 2: (a) Low temperature close-up of the resistive transitions of SrFeO$_3$, labeled in accordance with Ishiwata et alIshiwata2011. (b) Resistivity as a function of temperature showing degradation of the metallicity of bare SrFeO$_3$ over several hours. Metallicity is returned after a repeat ozone anneal. (c) With a thin SrTiO$_3$ capping layer, the metallicity is retained over many hours. The four resistivity versus temperature curves, with the same color coding as in panel (b), are almost indistinguishable. (d) Log-log plot of the time dependence, over several weeks, of the 4 K resistivity of SrFeO$_3$ with different thicknesses of capping layer. Dashed lines are guides to the eye. The asterisk marks one data point that was recorded at 21 K due to the resistivity exceeding the measurement limit at lower temperatures.
• Figure 3: Computed band structure of SrFeO$_x$ with x = 3.0 (a), x = 2.95 (b) and x = 2.9 (c). Red and blue bands denote spin up and spin down respectively. Panels on the right display a close-up view around the Fermi level at the A-point.
• Figure 4: (a) Low magnification high angle annular dark field (HAADF) image of an 11 nm thick film of as-grown SrFeO$_{2.5}$ with a 1.6 nm SrTiO$_3$ cap. (b) Low magnification HAADF image of an 11 nm as-grown SrFeO$_{2.5}$ film with no capping layer. One horizontal fault in the cation sublattice can be identified and is marked with red arrows. (c) Low magnification HAADF image of an 11 nm thick film of SrFeO$_{2.5}$ with a 1.6 nm SrTiO$_3$ capping layer after having been ozone annealed. (d) Low magnification ADF image of the same film as (b) after having been ozone annealed. The cation fault is indicated by red arrows. Both ozone annealed samples in (c) and (d) were initially SrFeO$_3$ and reduced unintentionally by the ion beam or electron beam to SrFeO$_{2.5}$. In all HAADF images the chemical interfaces are marked by yellow bars. (e) On the left a high magnification image of the same sample as in (a) with the electron energy loss spectroscopy (EELS) signal of strontium (middle) and iron (right) taken from the same region. (f) Energy-dispersive x-ray spectroscopy (EDX) maps in a small region of the same sample as in (a) and (e). The combined EDX signals of strontium and iron is shown in the lower right panel.