Abstract
Ferromagnetic insulating behavior has recently been demonstrated in cobalt-substituted at room temperature [Posadas et al., Phys. Rev. B 87, 144422 (2013)]. Experimentally it was found that a well-defined hysteresis loop only occurs at high Co concentrations of 30%–40%. X-ray photoelectron spectroscopy also indicated that Co substitutes for Ti with Co being in high-spin +2 oxidation state. In this work we employ density-functional theory to explain the experimentally observed properties of cobalt-substituted . We examine in detail the role of oxygen vacancy (OV) defects and their formation of defect complexes with the Co ions as the origin of the ferromagnetic insulating behavior. Our first-principles thermodynamic calculations indicate that OV defects are much more likely to occur next to Co atoms where their formation energies could be reduced by as much as 1.28 eV compared to that in bulk . We also find that Co in these Co-OV complexes occurs in the high-spin state in agreement with core level spectroscopy, and identify a linear arrangement of the Co-OV defect complexes to be the most energetically favorable structure. These defect complexes are also shown to interact ferromagnetically and that their magnetic interaction is found to be short ranged, consistent with the relatively high Co concentrations needed experimentally for ferromagnetism to be observed in cobalt-substituted .
- Received 14 April 2014
- Revised 31 July 2014
DOI:https://doi.org/10.1103/PhysRevB.90.125130
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