Exchange coupling in CaMnO₃ and LaMnO₃: configuration interaction and the coupling mechanism

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The equilibrium structure and exchange constants of CaMnO₃ and LaMnO₃ have been investigated using total energy unrestricted Hartree-Fock (UHF) and localised orbital configuration interaction (CI) calculations on the bulk compounds and Mn₂O₁₁¹⁴⁻ and Mn₂O₁₁¹⁶⁻ clusters. The predicted structure and exchange constants for CaMnO₃ are in reasonable agreement with estimates based on its Néel temperature. A series of calculations on LaMnO₃ in the cubic perovskite structure shows that a Hamiltonian with independent orbital ordering and exchange terms accounts for the total energies of cubic LaMnO₃ with various spin and orbital orderings. Computed exchange constants depend on orbital ordering. UHF calculations tend to underestimate exchange constants in LaMnO₃, but have the correct sign when compared with values obtained by neutron scattering; exchange constants obtained from CI calculations are in good agreement with neutron scattering data provided the Madelung potential of the cluster is appropriate. Cluster CI calculations reveal a strong dependence of exchange constants on Mn d eg orbital populations in both compounds. CI wave functions are analysed in order to determine which exchange processes are important in exchange coupling in CaMnO₃ and LaMnO₃.