Nanoscale domains of ordered oxygen-vacancies in LaCoO₃ films

Highlights: • Nanoscale domains with both horizontally and vertically dark stripes are observed. • The domain sizes and directions depend on the strain state and lattice mismatch. • The modulated dark stripes stem from oxygen-vacancy ordering. • Ferromagnetism is correlated with the oxygen deficiency, strain state and domains. - Abstract: High-quality LaCoO₃ (LCO) thin films were epitaxially grown on SrTiO₃ (STO), (La,Sr)(Al,Ta)O₃ (LSAT) and LaAlO₃ (LAO) substrates using pulsed laser deposition technique. Nanoscale domains with either horizontally or vertically modulated dark stripes are observed in the LCO films on STO and LSAT substrates, whereas only horizontally-modulated domains are found in the films on LAO substrate. The domain sizes and directions strongly depend on the strain state of the films due to lattice mismatch between film and substrate. The lattice strains in the films are mainly relaxed through change of the domain directions. The oxygen deficiency of the films has been revealed by X-ray photoelectron spectroscopy. As confirmed by high-angle annular dark field (HAADF) observations and high-resolution transmission electron microscopy (HRTEM) image simulations, the modulated dark stripes stem from oxygen-vacancy ordering, which causes local lattice expansion. The saturation magnetization increases with the tensile strain in the films. The ferromagnetic character is correlated with the oxygen deficiency, strain state and vertically modulated domains.