We present 90 meV resolved Co 2p3d resonant inelastic x-ray scattering linear dichroism spectra of strained LaCoO3 films and a LaCoO3 single crystal. A polarization-dependent low-energy excitation is observed at ∼0.2eV on the tensile-strained LaCoO3/SrTiO3 film, while it is not observed in either bulk LaCoO3 or the compressive-strained LaCoO3/LaAlO
3 film. Guided by cluster calculations, we are able to distinguish the spin-state manifolds close to their transition point of Co3+ ions in
LaCoO3 systems. Through a polarization analysis, we show that the spin state can easily flip from a low-spin 1A1g state in an octahedral symmetry to the high-spin 5B2g or 5Eg states with a small tetragonal distortion. A mixture of spin states suggests that the high-spin
Co3+ plays an important role in long-range ferromagnetic order on both tensile- and compressive-strained LaCoO3 films.
