Investigation of Spin and Charge Order in Ferrite Spinels by Synchrotron X-ray and Neutron Diffraction

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Copyright: Kareri, Yousef
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Abstract
Strongly correlated electron systems such as transition metal oxides host a complex interplay between electron spin, charge and orbital degrees of freedom. Hence, the electronic interactions taking place in these materials challenge our understanding of their underlying mechanisms and offer an immense opportunity for fundamental and applied research. The competing interactions can induce exotic phenomena such as charge-ordered phases, metal-to-insulator transitions, magnetic phase transitions, colossal magnetoresistance, multiferroicity and even high-temperature superconductivity. Among these materials, we highlight Fe3O4 as the oldest known magnet, which undergoes a metal-to-insulator transition: the Verwey transition at 120K. In 1939, Verwey explained this transition by proposing an ordering of the charges of the Fe2+ and Fe3+ ions. However, due to the complexity of the structure of magnetite its crystallographic details and therefore, the precise charge ordering pattern remains debated. The novel approach in this thesis is to trace the evolution of the Verwey transition in Fe3O4 upon Cu-doping through high-resolution neutron and synchrotron X-Rays diffraction experiments. The other end member, CuFe2O4 possesses a tetragonal crystal structure with a perfectly collinear ferrimagnetic order and a TC of 790 K. Its 2-dimensional layered counterpart CuFe2O4 has attracted considerable attention due to its magnetic spin cycloidal structure and its multiferroic properties. We did, therefore, raise the question: can we induce a spin canting and finally a cycloidal spin structure which could be the basis for a multiferroic ground state by Zn-doping in CuFe2O4. The end member of this doping series, ZnFe2O4 displays a 3-dimensional geometrically frustrated magnetic structure with short-range antiferromagnetic order. Although, ZnFe2O4 can be considered as an antiferromagnet with an N'eel temperature of 10K, its magnetic interactions are more complex since spin frustration occurs between the antiferromagnetically coupled third-neighbour spins rather than between the ferromagnetically coupled first-neighbour spins. In this thesis, high-resolution synchrotron X-ray and neutron powder diffraction experiment in combination with magnetisation measurements were performed on powder and single crystal samples of Cu-doped Fe3O4, Zn-doped CuFe2O4 and pure ZnFe2O4 in order to understand the underlying mechanisms in this class of spinel materials.
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Author(s)
Kareri, Yousef
Supervisor(s)
Ulrich, Clemens
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Publication Year
2020
Resource Type
Thesis
Degree Type
PhD Doctorate
UNSW Faculty
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