Long-range ferromagnetic order in ${\mathrm{LaCoO}}_{3-\delta }$ epitaxial films due to the interplay of epitaxial strain and oxygen vacancy ordering

We demonstrate that a combination of electronic structure modification and oxygen vacancy ordering can stabilize a long-range ferromagnetic ground state in epitaxial ${\mathrm{LaCoO}}_3$ thin films. Highest saturation magnetization values are found in the thin films in tension on ${\mathrm{SrTiO}}_3$ and $(\mathrm{La},\mathrm{Sr})(\mathrm{Al},\mathrm{Ta}){\mathrm{O}}_3$ substrates and the lowest values are found in thin films in compression on ${\mathrm{LaAlO}}_3$. Electron microscopy reveals oxygen vacancy ordering to varying degrees in all samples, although samples with the highest magnetization are the most defective. Element-specific x-ray absorption techniques reveal the presence of high spin ${\mathrm{Co}}^{2+}$ and ${\mathrm{Co}}^{3+}$ as well as low spin ${\mathrm{Co}}^{3+}$ in different proportions depending on the strain state. The interactions among the high spin Co ions and the oxygen vacancy superstructure are correlated with the stabilization of the long-range ferromagnetic order.

Figure 1

$\theta -2\theta$ measurements around the 002 peak showing the evolution of the OOP lattice parameter as a function of thickness on (a) STO, (b) LSAT, and (c) LAO substrates, respectively.

Figure 2

Reciprocal space maps showing 013 peak reflections for the $\sim 15$ nm [(a)–(c)] and $\sim 95$ nm [(d)–(f)] LCO films grown on STO [(a)–(d)], LSAT [(b),(e)], and LAO [(c),(f)].

Figure 3

Scanning transmission electron microscopy high angle annular dark field images showing periodic dark contrast planes parallel and perpendicular to the substrate-film interface for $\sim 15$-nm-thick films of ${\mathrm{LaCoO}}_3$ on (a) ${\mathrm{LaAlO}}_3$, (b)(La,Sr)(Al,Ta)${\mathrm{O}}_3$, and (c) ${\mathrm{SrTiO}}_3$. Co-$L$ edge [(g)–(i)] and O-$K$ edge [(d)–(f)] EELS spectra from line scans taken along the bright (red) and dark (black) contrast planes in dark field imaging are shown below the corresponding images.

Figure 4

Scanning transmission electron microscopy images taken in bright field imaging for 75-nm-thick films on (a) LAO, (b) LSAT, and (c) STO.

Figure 5

(a) XA spectra for films grown 15 nm (red) and 70 nm (blue) thick on Nb-doped STO. Arrows indicate relevant features of the spectrum. (b) A set of reference spectra for HS ${\mathrm{Co}}^{2+}$ [25], HS ${\mathrm{Co}}^{3+}$ [26], and LS ${\mathrm{Co}}^{3+}$ [26] are provided to analyze the film spectra.

Figure 6

(a) XA and (c) XLD for films grown on LAO (red), LSAT (green), and STO (blue). The figure on the right (b) is an enlarged region of the Co-$L_3$ edge showing the differences in spectral features.

Figure 7

Magnetization vs field measurements at 5 K for thin (dark) and thick (light) LCO films grown on (a) STO, (b) LSAT, and (c) LAO substrates showing a range of saturated moments. The magnetization vs temperature measurements under $\text{H}=5$ mT (field cooled) for (d) strained and (e) relaxed films show a Curie temperature below $\sim 85$ K for all films except strained films on LAO.

Figure 8

Magnetism trends as a function of increasing film thickness and hence relaxation showing that films on (a) STO exhibit a decreasing saturated moment and films on (b) LAO exhibit an increasing remanent moment.

Figure 9

XA (top panel) and corresponding XMCD (bottom panel) spectra for 8 and 75 nm films grown on Nb-doped STO measured at 25 K using circularly polarized x rays in an alternating 1.5 T magnetic field applied ${30}^{\circ }$ from the in-plane direction.

Figure 10

Schematic electron configurations associated with growth on STO and LSAT substrates showing (a) bulk LS ${\mathrm{Co}}^{3+}$, (b) tetragonally distorted HS ${\mathrm{Co}}^{3+}$, (c) tetragonally distorted LS ${\mathrm{Co}}^{3+}$, and (d) tetragonally distorted, oxygen-deficient HS ${\mathrm{Co}}^{2+}$. Also shown are the configurations for LCO on LAO substrates showing (e) tetragonally distorted LS ${\mathrm{Co}}^{3+}$, (f) tetragonally distorted HS ${\mathrm{Co}}^{3+}$, and (g) tetragonally distorted, oxygen-deficient HS ${\mathrm{Co}}^{2+}$.