Magnetism and spin waves in KCoO₂

KCoO2 has been reported in two different crystal structures, P4/nmm with square pyramidal and I-42d with tetrahedral coordination of the Co³⁺ atom with d⁶ configuration. These unusual coordinations  lead to interesting magnetic properties. The magnetic moment formation and exchange interactions are studied using first-principles calculations. The band structures are calculated using the quasiparticle self-consistent GW method and the exchange interactions are extracted from the transverse spin susceptibility using a linear response approach in the rigid spin approximation. For P4/nmm, a magnetic moment of  4 μ_B is obtained and explained by the d-orbitals contributions to the bands. The exchange interactions are found to lead to a doubling of the cell with antiferromagnetic ordering with a large antiferromagnetic (AFM) super-exchange coupling between the Co along the [100] direction and a smaller AFM coupling between the Co with opposite apical O orientation in the [110] direction. In the I-42d structure, the FM configuration exhibits a high spin state with ~4 μ_B while the AFM configuration exhibits a low spin with ~2 μ_B moments. Because the opposite spin atoms in this structure are related by a rotation operation, this system is altermagnetic and the bands along certain low symmetry directions show spin splitting. The spin wave dispersions will be presented for both structures.