We find using LSDA+U band structure calculations that the novel one-dimensional cobaltate Ca3Co2O6 is not a ferromagnetic half-metal but a Mott insulator. Both the octahedral and the trigonal Co ions are formally trivalent, with the octahedral being in the low-spin and the trigonal in the high-spin state. The inclusion of the spin-orbit coupling leads to the occupation of the minority-spin d2 orbital for the unusually coordinated trigonal Co, producing a giant orbital moment (1.57 μB). It also results in an anomalously large magnetocrystalline anisotropy (of order 70 meV), elucidating why the magnetism is highly Ising-like. The role of the oxygen holes, carrying an induced magnetic moment of 0.13 μB per oxygen, for the exchange interactions is discussed.
@article{arxiv.cond-mat/0504490,
title = {Nature of magnetism in Ca$_3$Co$_2$O$_6$},
author = {Hua Wu and M. W. Haverkort and Z. Hu and D. I. Khomskii and L. H. Tjeng},
journal= {arXiv preprint arXiv:cond-mat/0504490},
year = {2007}
}