Strain-driven spin-state transition and superexchange interaction in LaCoO ₃: Ab initio study

Using spin density functional theory with the Hubbard correction, we investigate the magnetic structure of strained LaCoO ₃. We show that beyond biaxial tensile strain of 2.5%, local magnetic moments originating from the high spin state of Co3 ⁺ emerge in a low spin Co3 ⁺ matrix. In contrast, we find that compressive strain is not able to stabilize a magnetic state due to geometric constraints. LaCoO ₃ accommodates tensile strain via spin-state disproportionation, resulting in an unusual sublattice structure. In tensile-strained LaCoO ₃, the first nearest-neighbor (n.n.) exchange coupling is ferromagnetic (FM), while the second n.n. interaction is stronger and antiferromagnetic (AFM). This unusual feature of the exchange parameters is qualitatively verified with a model superexchange calculation. Due to the competition between the FM and the AFM couplings in the system, we find that the most probable magnetic structure of tensile-strained LaCoO ₃ is a canted-spin structure, which may explain the relatively small observed magnetic moment of 0.7μ _B/Co3 ⁺.