Charge transfer, orbital polarization, and magnetism of (LaCoO₃)_n+(LaTiO₃)_n (n=1,2) superlattice

The perovskite LaCoO₃ has earned much attention because of the many relevant spin states of the Co³⁺ cation (ranging from S=0 to S=2 and their combinations). When LaCoO₃ forms a superlattice with other transition metal oxides, the combination of charge transfer, quantum confinement, strain and structural distortions can lead to interesting changes of its properties. We have studied systematically the electronic and magnetic properties of LaCoO₃+LaTiO₃ (LCO+LTO) superlattices using density functional theory + U calculations. We find that one electron (formal charge) is transferred from Ti to Co, resulting in Co²⁺ (d⁷) in the LCO+LTO superlattice. There are two relevant spin states of the Co²⁺: (i) Low-spin (LS) S=0.5, and (ii) high-spin (HS) S=1.5; and mixtures of HS and LS across the different Co²⁺ cations can give other effective spin states such as S=1. Unlike Co³⁺ in LaCoO₃ or Co²⁺ in CoO, both the HS and LS in the superlattice have strong orbital polarization. We will describe the energetic stability of HS versus LS and its U dependence, the magnetic ordering of the HS and LS ground states, how antiferromagnetic and ferromagnetic superexchange interactions underlie the computed orderings.