Understanding the Effects of Tensile Strain on the Structure and Magnetism of Stoichiometric LaCoO ₃ 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 (TC ≈ 70 K, Msat ≈ 0.3 μB/Co, and HC ≈ 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–c0 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.