Magnetic oxygen in transition metal oxides: A case study of Ba2CoO4

Transition metal oxides (TMOs) exhibit exotic magnetic properties, often difficult to understand in conventional wisdom. Magnetic insulator Ba2CoO4 has mystified the community, because the CoO4 tetrahedron appears to be completely isolated with the nearest Co atoms far apart (~5 Å), making it difficult to account for long-range magnetic ordering seen experimentally using only Co. By first-principles calculations in bulk Ba2CoO4, we illustrate for the first time that the magnetic moment on oxygen atoms are the origin of the unexpected long-range magnetic ordering and low magnetic dimensionality. We find that the magnetic moment is not only localized on Co atoms, as assumed in all conventional data analysis, but also distributed on its tetrahedrally-coordinated O atoms, making CoO4 the magnetic building block instead of Co alone. Having oxygen contribute to the magnetic moment may be identified as a universal property of magnetic TMOs, which will require a fresh look at conventional models of magnetism in TMOs.