Optical signatures of strain-induced ferromagnetism in a LaCoO3 thin film

Using spectroscopic ellipsometry, we studied the optical conductivity of LaCoO3 with various degrees of strain. The optical response of the compressively strained LaCoO3 film is qualitatively similar to the one of the unstrained LaCoO3 polycrystalline sample and exhibits a redistribution of the spectral weight between '0.2 and 6 eV, which is most likely related to the thermal excitation of the high-spin (HS) states. The optical response of the ferromagnetic (FM) tensile strained film exhibits clear signatures of the FM state. Below the Curie temperature Tc = 82 K, a spectral weight transfer sets on from high energies (between 3.3 and 5.6 eV) to low energies (between 0.2 and 3.3 eV). The temperature dependence of the low-energy spectral weight can be understood in the framework of the HS biexciton model of Sotnikov et al. [SciPost Phys. 8, 082 (2020)] as corresponding to the increase of the concentration of the HS states that are stabilized below Tc. The magnitude of the redistribution of the spectral weight due to the formation of the FM state is sizable and corresponds to 0.009 e per Co ion. We discuss it in terms of the effective kinetic energy of Co 3d bands.