Multifunctional oxides - influence of defects on the ferroic properties

Transition metal oxides exhibit many physical phenomena, among them ferroic properties such as ferroelasticity, ferroelectricity and ferromagnetism, or their combination in multiferroics. The stoichiometry of transition metal oxides depends on the oxygen partial pressure and changes conductivity and ferroic properties. Ternary/quaternary oxides are discussed, which correlate local defect-induced structure changes with changes of the elastic, polarization and magnetic properties. The microscopic interactions are determined by density functional theory as basis for more large-scale simulations with effective Hamiltonians. Oxygen vacancies in SrTiO3 accumulate in an external electric field and reduce the hardness. For Sr/O excess SrO(SrTiO3)n phases with additional SrO planes occur, which change the X-Ray reflectivity. Ion-irradiation triggers additional point defects which can form stable aggregates. In YMn2O5 several antiferromagnetic phases coexist with ferroelectricity; YFeMnO5 exhibits only one commensurable ferrimagnetic phase. Based on spin-polarized DFT calculations a Heisenberg model yields the coupling constants of the two compounds and relates them to crystal-field interactions.