Exchange interaction between the high spin Co3+ states in LaCoO3

The formation of the exchange interaction between HS Co3+ ions, which are excited in LaCoO3, is studied within the multielectron approach. Two main contributions appear to be antiferromagnetic (AFM) and ferromagnetic (FM). When the ground state is LS, the total interaction is AFM. The crossover to the HS state may result in the FM ordering. The mean-field magnetic phase diagrams on the plane spin gap-temperature have been calculated without and with spin-orbital interaction in the HS term. Without spin-orbital interaction the reentrant magnetic order is possible. The spin-orbital coupling removes the reentrant phase transition and stabilizes the LS state. For known from experimental data values of the spin gap and exchange interaction, the ideal stoichiometric LaCoO3 is very close to the LS–HS crossover and magnetic ordering border. The violations of local coordination and symmetry of the Co3+-oxygen complexes that take place in the intergrain boundaries, at the surface of single crystals, and in the thin films on the strained substrate, may result in the formation of the HS state and FM order for such materials.