Investigating the electronic structures and local symmetry of Co ions in BaCoO₃ by X-ray magnetic circular dichroism
Yi-Ying Chin1,2*, Z. Hu3, H.-J. Lin2, S. Agrestini3, J. Weinen3, C. Martin4, S. Hebert4, A. Maignan4, A. Tanaka5, J. C. Cezar6, N. B. Brookes6, Y.-F. Liao2, K.-D. Tsuei2, C. T. Chen2, D. I. Khomskii7, L. H. Tjeng3
1Department of Physics, National Chung Cheng University, Chiayi, Taiwan
2National Synchrotron Radiation Research Center, Hsinchu, Taiwan
3Max Planck Institute for Chemical Physics of Solids, Dresden, Germany
4Laboratoire CRISMAT, CNRS-ENSICAEN, Caen, France
5Department of Quantum Matter, Hiroshima University, Higashi-Hiroshima, Japan
6European Synchrotron Radiation Facility, Grenoble, France
7Institute of Physics II, University of Cologne, Cologne, Germany
* Presenter:Yi-Ying Chin,
For an ionic 3d⁵ configuration, one would suppose a high-spin ground state since the intra-atomic exchange coupling between five 3d electrons should be large enough to overcome the crystal field splitting, while SrCoO₃ has both ferromagnetic and metallic behaviors due to the negative charge-transfer energy resulting in an intermediate-spin ground state of Co. However, according to the resistivity experiments, BaCoO₃ is a small-gap semiconductor, different from SrCoO₃. Hence, the electronic structure of BaCoO₃ is expected to deviate from that of SrCoO₃. The difference might come from their crystal structures. BaCoO₃ has a highly one-dimensional crystal structure and is composed of c-axis chains forming two-dimension triangular lattice. Neutron powder diffraction and μSR experiments indicated an intra-chain ferromagnetic coupling and an antiferromagnetic coupling between chains. The high temperature magnetic susceptibility data showed that Co in BaCoO₃ is in a low-spin ground state, but the effective moment is larger than the spin-only value of a low-spin 3d⁵.
To determine experimentally the electronic structure of the Co ions in BaCoO₃, we performed X-ray photoemission spectroscopy and X-ray magnetic circular dichroism experiments. The semiconductor behavior of BaCoO₃ was confirmed by the valence band spectrum. Moreover, our quantitative analysis demonstrated that the Co ions are in the low-spin 3d⁶L state and the orbital moment of Co is finite, but much less than one, implying the lower local symmetry of the Co ions in BaCoO₃.

Keywords: Low-spin State, X-ray Magnetic Circular Dichroism, Negative Charge Transfer, Magnetic Anisotropy, Orbital Moment