Creating a low-symmetry insulating, ferroelectric, and antiferromagnetic material from a high-symmetrical metallic ferromagnet via defect engineering: The case of LaBaC o2 O5 + δ compounds

First-principles calculations are performed to investigate the properties of defect-free LaBaCo2O6 materials as well as LaBaCo2O5 films possessing ordered oxygen vacancy structures. These calculations predict that the defect-free LaBaCo2O6 crystallizes in a tetragonal state with an axial ratio smaller than unity, and is metallic and ferromagnetic as consistent with the literature. In contrast, the LaBaCo2O5 film adopts a low-symmetry monoclinic state having an axial ratio larger than 1, is insulating and even ferroelectric, as well as exhibits a G-type antiferromagnetic ordering. Our calculations further reveal the origin of these defect-induced changes in properties. Moreover, further experiments we conducted demonstrate the possibility to grow highly epitaxial LaBaCo2O5 films with ordered oxygen vacancies, via reduction treatments, and confirm some of our predictions. These findings can open a new avenue for the design and synthesis of multiferroics via defect engineering.