Spin State Disproportionation in Insulating Ferromagnetic LaCoO Epitaxial Thin Films.

The origin of insulating ferromagnetism in epitaxial LaCoO 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 LaCoO 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 Co in LaCoO films under compressive strain (on LaAlO substrate) is practically a low-spin state, whereas Co in LaCoO films under tensile strain (on SrTiO 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 Co high-spin state, i.e., an observation of a spin state disproportionation in tensile-strained LaCoO films. This consequently explains the nature of ferromagnetism in LaCoO 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.