Magnetic frustration in α-NaMnO2 and CuMnO2

The magnetic and electronic properties of α-NaMnO2 and delafossite CuMnO2 are investigated using a full-potential linearized augmented plane wave method. For the monoclinic structure at room temperature, CuMnO2 behaves like a frustrated spin-lattice. For the triclinic structure at low temperature, the obtained magnetic configurations of the lowest energy for both α-NaMnO2 and CuMnO2 are consistent with experiments. However, the exchange constants are all positive. This reveals that the magnetic frustration is only partially relieved at low temperature. It is also found that the Mn3+ ions are in a high-spin state. Taking into account the on-site Coulomb correlations, our results show that α-NaMnO2 and CuMnO2 are charge-transfer insulators.