Magnetism of spinel cobaltite ACo₂O₄ (A = Zn and Li)

Cubic spinel cobaltites ZnCo₂O₄ and LiCo₂O₄ are expected to be geometrically-frustrated magnets, where magnetic Co ions form the sublattice of corner sharing tetrahedra (pyrochlore lattice). For ZnCo₂O₄, the magnetism is expected to be dominated by Co³⁺ (3d ⁶), which has orbital degentracy in the high-spin state. For LiCo₂O_4, the magnetism is expected to be dominated by Co^3.5+ (3d ^5.5), which has orbital and charge degeneracies. We synthesized plycrystals of ZnCo₂O₄ and LiCo₂O₄ to investigate the structural and magnetic properties. ZnCo₂O₄ exhibits the absence of magnetic transition at temperatures down to ~ 2 K, although the magnetic susceptibility measurements in this compound reveal the antiferromagnetic Weiss temperature q_W ~ -130 K. LiCo₂O_4, on the other hand, exhibits an antiferromagnetic transition at T_N ~ 30 K with the antiferromagnetic Weiss temperature q_W ~ -110 K. The experimental results suggest that, while ZnCo₂O₄ is a highly-frustrated magnet, the antiferromagnetic transition in LiCo₂O₄ is driven by the charge degree of freedom.