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

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 occu