Search for novel multifunctional materials: a comprehensive neutron and synchrotron diffraction study on cobaltates and spinels

摘要

Multiferroics exhibit tremendous potential in technological applications, of which the interplaybetween the ferro-/antiferromagnetic, ferroelectricity, and ferroelasticity can be manipulatedby external parameters such as electric or magnetic fields and even can be created byproper control of internal parameters, such as oxygen-content, cation doping, and internalpressure. Invaluable insight into the multiferroicity can be provided by investigating the roleof such parameters in the crystallographic and magnetic structures of transition metal oxides.In this thesis, a comprehensive studies of contribution of oxygen-deficiencies and cation dopingto the various phases are presented, which utilize the neutron and synchrotron powderdiffraction.The multivalent nature of cobalt ions in SrCoO3d causes an oxygen-content dependentphase diagram. It is vital to determine the precise crystallographic and magnetic structure ofoxygen-vacancy ordered SrCoO3d , which provides prerequisite to reveal the mechanism ofits multiferroicity in the corresponding thin film samples. Using the neutron and synchrotronpowder diffraction techniques, the correct space group and precise magnetic structures aredetermined for the different oxygen-vacancy ordered phases: the brownmillerite SrCoO2:5,the tetragonal SrCoO2:875, and the cubic SrCoO3.Ferroelectricity can be induced by the canted spin configuration, which exists widely inthe frustrated spin systems. With such an expectation, Zn-substituted CuFe2O4 was studiedand a comprehensive phase diagram was built for Cu1xZnxFe2O4. Spin canting couldlead to a spin spiral phase which could possibly induce a multiferroic state as observed inCuFeO2. The purpose of this project was to further investigate the spin canting in ZndopedCuFe2O4 and elucidate the possibility of multiferroicity in this system. Furthermore,pure ZnFe2O4 is already an interesting highly frustrated spin system. Magnetite is the oldestknown magnet. As the first known multiferroics, the ferroelectricity in Fe3O4 is drivenby the Verwey transition. However, the microscopic origin of the Verwey transition in Fe3O4is still in debate. In order to obtain a deeper insight into the mechanism of the charge ordering,Cu-doped Fe3O4 was investigated using the high-resolution neutron and synchrotrondiffraction. The main emphasis was the stability of the Verwey transition with charge carrierdoping.