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

Abstract

Multiferroics exhibit tremendous potential in technological applications, of which the interplay between the ferro-/antiferromagnetic, ferroelectricity, and ferroelasticity can be manipulated by external parameters such as electric or magnetic fields and even can be created by proper control of internal parameters, such as oxygen-content, cation doping, and internal pressure. Invaluable insight into the multiferroicity can be provided by investigating the role of 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 doping to the various phases are presented, which utilize the neutron and synchrotron powder diffraction.

The multivalent nature of cobalt ions in SrCoO3􀀀d causes an oxygen-content dependent phase diagram. It is vital to determine the precise crystallographic and magnetic structure of oxygen-vacancy ordered SrCoO3􀀀d , which provides prerequisite to reveal the mechanism of its multiferroicity in the corresponding thin film samples. Using the neutron and synchrotron powder diffraction techniques, the correct space group and precise magnetic structures are determined 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 in the frustrated spin systems. With such an expectation, Zn-substituted CuFe2O4 was studied and a comprehensive phase diagram was built for Cu1􀀀xZnxFe2O4. Spin canting could lead to a spin spiral phase which could possibly induce a multiferroic state as observed in CuFeO2. The purpose of this project was to further investigate the spin canting in Zndoped CuFe2O4 and elucidate the possibility of multiferroicity in this system. Furthermore, pure ZnFe2O4 is already an interesting highly frustrated spin system. Magnetite is the oldest known magnet. As the first known multiferroics, the ferroelectricity in Fe3O4 is driven by the Verwey transition. However, the microscopic origin of the Verwey transition in Fe3O4 is 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 synchrotron diffraction. The main emphasis was the stability of the Verwey transition with charge carrier doping.