Understanding the Effects of Tensile Strain on the Structure and Magnetism of Stoichiometric LaCoO₃ Films

Despite numerous reports of an insulating ferromagnetic state in epitaxial LaCoO₃ thin films, no consensus has been reached on the details of ferromagnetism in these films. To better understand the origins of magnetic order in such films, stoichiometric LaCoO₃ films have been deposited on SrTiO₃(001) and LaAlO₃(001) substrates using off-axis sputtering. This technique allows growth to occur in conditions that minimize deviations from the ideal stoichiometry. SQUID magnetometry shows that ferromagnetism is stabilized only in films grown under tensile strain on SrTiO₃. The magnetic properties of these films (T _C ≈ 70 K, M _sat ≈ 0.3 μ_B/Co, and H _C ≈ 5 kOe) are essentially independent of thickness, consistent with nearly uniform magnetization. At room temperature, strain induced by the SrTiO₃ substrate breaks the rhombohedral symmetry of the bulk structure, leading to a ^- a ^- c ⁰ octahedral tilting and an anisotropic distortion of the Co-centered octahedra. Low-temperature (T = 36 K) X-ray absorption spectroscopy reveals that tensile strain inherent to the SrTiO₃ substrate stabilizes a substantial fraction of high- or intermediate-spin Co³⁺ ions, facilitating magnetic order, whereas films grown on LaAlO₃ are made up nearly entirely of low-spin Co³⁺ ions.