Anion-Vacancy-Induced Magneto−Crystalline Anisotropy in Fluorine-Doped Hexagonal Cobaltites

摘要

The two cobalt hexagonal perovskites 6H-Ba6Co6F0.93O16 and 10H-Ba5Co5F0.77O12.88 werenprepared, and their structures were examined by X-ray and neutron diffraction and by 19F solid state NMRnspectroscopy. The magnetic and transport properties of these compounds were probed by magneticnsusceptibility and electrical resistivity measurements, and their electronic structures by density functionalnand tight-binding calculations. The [BaOF1-x] layers of these compounds create corner-sharing tetrahedralnCo2O7 dimers at the interface between their face-sharing octahedral oligomers. Our density functionalncalculations leads to an unambiguous charge distribution model, which assigns high-spin Co3+ ions for thentetrahedral sites and low-spin Co3+/Co4+ ions for the octahedral sites, and this model should be valid fornthe parent BaCoO3-δ and the related oxychlorides and oxybromides as well. The F- vacancies in then[BaOF1-x] layers cause a strong distortion in the tetrahedral dimer Co2O7, which in turn affects the spinnorientation of the high-spin Co3+ ions of the CoO4 tetrahedra, i.e., parallel to the c-direction innBa6Co6F1-xO16-δ but perpendicular to the c-direction in Ba5Co5F1-xO13-δ. This difference in the spinnorientations is related to the d-states of the distorted CoO4 tetrahedra with high-spin Co3+ (d6) ion on thenbasis of tight binding calculations and spin-orbit coupling as perturbation.