Spin State Disproportionation in Insulating Ferromagnetic LaCoO₃ Epitaxial Thin Films

The origin of insulating ferromagnetism in epitaxial LaCoO₃ films under tensile strain remains elusive despite extensive research efforts are devoted. Surprisingly, the spin state of its Co ions, the main parameter of its ferromagnetism, is still to be determined. Here, the spin state in epitaxial LaCoO₃ thin films is systematically investigated to clarify the mechanism of strain-induced ferromagnetism using element-specific X-ray absorption spectroscopy and dichroism. Combining with the configuration interaction cluster calculations, it is unambiguously demonstrated that Co³⁺ in LaCoO₃ films under compressive strain (on LaAlO₃ substrate) is practically a low-spin state, whereas Co³⁺ in LaCoO₃ films under tensile strain (on SrTiO₃ substrate) have mixed high-spin and low-spin states with a ratio close to 1:3. From the identification of this spin state ratio, it is inferred that the dark strips observed by high-resolution scanning transmission electron microscopy indicate the position of Co³⁺ high-spin state, i.e., an observation of a spin state disproportionation in tensile-strained LaCoO₃ films. This consequently explains the nature of ferromagnetism in LaCoO₃ films. The study highlights the importance of spin state degrees of freedom, along with thin-film strain engineering, in creating new physical properties that do not exist in bulk materials.