  The origin of insulating ferromagnetism in epitaxial LaCoO3 films under
tensile strain remains elusive despite extensive research efforts have been
devoted. Surprisingly, the spin state of its Co ions, the main parameter of its
ferromagnetism, is still to be determined. Here, we have systematically
investigated the spin state in epitaxial LaCoO3 thin films to clarify the
mechanism of strain induced ferromagnetism using element-specific x-ray
absorption spectroscopy and dichroism. Combining with the configuration
interaction cluster calculations, we unambiguously demonstrate that Co3+ in
LaCoO3 films under compressive strain (on LaAlO3 substrate) are 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, we infer 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.
