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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

The discovery of emergent phenomena arising from the interplay between magnetism, ferroelectricity and structure in multiferroic materials provides a myriad of potential capabilities in future solid state technologies. Despite extensive investigations in this field, many unsolved issues surround magnetoelectric coupling in several multiferroic systems, and in many cases there are competing approaches that attempt to describe their underlying mechanisms. Collectively, the projects in this thesis represent a two-pronged approach that utilises the complementary techniques of Raman light scattering and neutron diffraction as a means to investigate the contentious role of crystal structure in magnetoelectric coupling for the type-II multiferroic RMnO3 (R = Tb, Dy) and RMn2O5 systems (R = Tb, Ho, Y), and furthermore in potentially-multiferroic ErFeO3.The isotopic substitution of oxygen-18 significantly modifies the lattice dynamics of transition-metal oxide systems. In many materials, this can result in isotope-induced shifts in magnetic, ferroelectric, or superconducting phase transition temperatures that provide new understanding into the underlying physical processes that define their properties. Remarkably, oxygen-isotope substitution in RMnO3 (R = Tb, Dy) systems does not alter the onset of the multiferroic regime, indicating that magnetoelectric coupling is primarily a magnetically-driven phenomenon within these compounds. The combination of Raman scattering and neutron diffraction techniques allowed several key insights to be obtained. For both RMnO3 and RMn2O5 systems, Raman spectroscopy revealed that magnetically-driven ferroelectricity imposes significant effects that alter mode energies and lifetimes which indicate displacement-induced ferroelectricity. Furthermore, we observe remarkable structural behaviours in the form of altered bond lengths or faint Bragg reflections that indicate the existence of symmetry breaking. Our Raman study on ErFeO3 presents eviden

High quality epitaxial SrCoOx (oxygen deficient SrCoO3) thin films were grown on (110) DyScO3 substrates by pulsed laser deposition.The disappearance of half order peaks in X-ray diffraction as well as the XAS at the O K-edge indicates an oxygen content of x approximate to 2.8 in the thin films. Magnetization measurements reveal that the specific substrate strain suppresses the ferromagnetism found in the corresponding bulk material and the emergence of an antiferromagnetic-type spin correlation as predicted by theoretical calculations. Our work demonstrates that the magnetism can be tuned by in-plane strain in SrCoOx thin films.

High quality epitaxial SrCoOx (oxygen deficient SrCoO3) thin films were grown on (110) DyScO3 substrates by pulsed laser deposition.The disappearance of half order peaks in X-ray diffraction as well as the XAS at the O K-edge indicates an oxygen content of x approximate to 2.8 in the thin films. Magnetization measurements reveal that the specific substrate strain suppresses the ferromagnetism found in the corresponding bulk material and the emergence of an antiferromagnetic-type spin correlation as predicted by theoretical calculations. Our work demonstrates that the magnetism can be tuned by in-plane strain in SrCoOx thin films.

High quality epitaxial SrCoOx (oxygen deficient SrCoO3) thin films were grown on (110) DyScO3 substrates by pulsed laser deposition.The disappearance of half order peaks in X-ray diffraction as well as the XAS at the O K-edge indicates an oxygen content of x approximate to 2.8 in the thin films. Magnetization measurements reveal that the specific substrate strain suppresses the ferromagnetism found in the corresponding bulk material and the emergence of an antiferromagnetic-type spin correlation as predicted by theoretical calculations. Our work demonstrates that the magnetism can be tuned by in-plane strain in SrCoOx thin films.

High quality epitaxial SrCoOx (oxygen deficient SrCoO3) thin films were grown on (110) DyScO3 substrates by pulsed laser deposition.The disappearance of half order peaks in X-ray diffraction as well as the XAS at the O K-edge indicates an oxygen content of x approximate to 2.8 in the thin films. Magnetization measurements reveal that the specific substrate strain suppresses the ferromagnetism found in the corresponding bulk material and the emergence of an antiferromagnetic-type spin correlation as predicted by theoretical calculations. Our work demonstrates that the magnetism can be tuned by in-plane strain in SrCoOx thin films.

The complex interplay between ferromagnetism and ferroelectricity found in multiferroic systems represents an impressive opportunity for research and technologies based upon the manipulation of both spin and charge degrees of freedom, but also signifies a considerable challenge to reveal the underlying mechanisms. Together, the projects presented in this thesis represent an array of innovative approaches utilising neutron scattering and complementary techniques to investigate the effect of internal and external influences upon the strong electron correlations in multiferroics with exciting potential for future spintronic applications.In thin film multilayers, where the pertinent physics occurs within only a few unit cells from the interface, it is vital to probe the structural, magnetic and chemical properties with element and depth sensitivity. Using state-of-the-art polarised neutron reflectivity and x-ray magnetic resonant reflectivity, a clear link between modified regions of magnetism and stoichiometry is demonstrated to form at the atomically sharp interface of half metallic ferromagnet La0.67Sr0.33MnO3 / room temperature multiferroic BiFeO3.On the other side, even small influences can play a major role in exhibited ground state, enabling new insight to be gained into intricate electron interactions. Neutron diffraction was used as a powerful tool to explore these properties. Through unique epitaxial constraints of a (110)-oriented SrTiO3 substrate and an intermediate layer of SrRuO3, thin film BiFeO3 is demonstrated to grow as a single domain system and retain the incommensurate spin cycloid found in bulk, opening an exciting new avenue of research in thin film heterostructures. Combined neutron diffraction and theoretical modelling show that the introduction of chemical pressure through cation substitution in doped BiFeO3 significantly improves magnetic properties due to modified superexchange interactions. Finally, using in-situ applied magnetic fields with

Depth-sensitive magnetic, structural, and chemical characterization is important in the understanding and optimization of physical phenomena emerging at the interfaces of transition metal oxide heterostructures. In a simultaneous approach we have used polarized neutron and resonant x-ray reflectometry to determine the magnetic profile across atomically sharp interfaces of ferromagnetic La0.67Sr0.33MnO3/multiferroic BiFeO3 bilayers with subnanometer resolution. In particular, the x-ray resonant magnetic reflectivity measurements at the Fe and Mn resonance edges allowed us to determine the element-specific depth profile of the ferromagnetic moments in both the La0.67Sr0.33MnO3 and BiFeO3 layers. Our measurements indicate a magnetically diluted interface layer within the La0.67Sr0.33MnO3 layer, in contrast to previous observations on inversely deposited layers [P. Yu, Phys. Rev. Lett. 105, 027201 (2010)PRLTAO0031-900710.1103/PhysRevLett.105.027201]. Additional resonant x-ray reflection measurements indicate a region of altered Mn and O content at the interface, with a thickness matching that of the magnetic diluted layer, as the origin of the reduction of the magnetic moment. 2014 American Physical Society.

Depth-sensitive magnetic, structural, and chemical characterization is important in the understanding and optimization of physical phenomena emerging at the interfaces of transition metal oxide heterostructures. In a simultaneous approach we have used polarized neutron and resonant x-ray reflectometry to determine the magnetic profile across atomically sharp interfaces of ferromagnetic La0.67Sr0.33MnO3/multiferroic BiFeO3 bilayers with subnanometer resolution. In particular, the x-ray resonant magnetic reflectivity measurements at the Fe and Mn resonance edges allowed us to determine the element-specific depth profile of the ferromagnetic moments in both the La0.67Sr0.33MnO3 and BiFeO3 layers. Our measurements indicate a magnetically diluted interface layer within the La0.67Sr0.33MnO3 layer, in contrast to previous observations on inversely deposited layers [P. Yu, Phys. Rev. Lett. 105, 027201 (2010)PRLTAO0031-900710.1103/PhysRevLett.105.027201]. Additional resonant x-ray reflection measurements indicate a region of altered Mn and O content at the interface, with a thickness matching that of the magnetic diluted layer, as the origin of the reduction of the magnetic moment. 2014 American Physical Society.

Depth-sensitive magnetic, structural, and chemical characterization is important in the understanding and optimization of physical phenomena emerging at the interfaces of transition metal oxide heterostructures. In a simultaneous approach we have used polarized neutron and resonant x-ray reflectometry to determine the magnetic profile across atomically sharp interfaces of ferromagnetic La0.67Sr0.33MnO3/multiferroic BiFeO3 bilayers with subnanometer resolution. In particular, the x-ray resonant magnetic reflectivity measurements at the Fe and Mn resonance edges allowed us to determine the element-specific depth profile of the ferromagnetic moments in both the La0.67Sr0.33MnO3 and BiFeO3 layers. Our measurements indicate a magnetically diluted interface layer within the La0.67Sr0.33MnO3 layer, in contrast to previous observations on inversely deposited layers [P. Yu, Phys. Rev. Lett. 105, 027201 (2010)PRLTAO0031-900710.1103/PhysRevLett.105.027201]. Additional resonant x-ray reflection measurements indicate a region of altered Mn and O content at the interface, with a thickness matching that of the magnetic diluted layer, as the origin of the reduction of the magnetic moment. 2014 American Physical Society.

Depth-sensitive magnetic, structural, and chemical characterization is important in the understanding and optimization of physical phenomena emerging at the interfaces of transition metal oxide heterostructures. In a simultaneous approach we have used polarized neutron and resonant x-ray reflectometry to determine the magnetic profile across atomically sharp interfaces of ferromagnetic La0.67Sr0.33MnO3/multiferroic BiFeO3 bilayers with subnanometer resolution. In particular, the x-ray resonant magnetic reflectivity measurements at the Fe and Mn resonance edges allowed us to determine the element-specific depth profile of the ferromagnetic moments in both the La0.67Sr0.33MnO3 and BiFeO3 layers. Our measurements indicate a magnetically diluted interface layer within the La0.67Sr0.33MnO3 layer, in contrast to previous observations on inversely deposited layers [P. Yu, Phys. Rev. Lett. 105, 027201 (2010)PRLTAO0031-900710.1103/PhysRevLett.105.027201]. Additional resonant x-ray reflection measurements indicate a region of altered Mn and O content at the interface, with a thickness matching that of the magnetic diluted layer, as the origin of the reduction of the magnetic moment. 2014 American Physical Society.

Multiferroic materials that have a unique coupling of magnetic and electronic degrees of freedom are possible candidates for the next generation of data storage and data transfer technology. For this thesis we performed inelastic neutron scattering on an oxygen isotopically substituted sample DyMnO3. This was done in order to better understand the structural and magnetic interaction in these materials, and in an attempt to improve the magnetic signal for better performance in applications.A comparison between DyMnO3, and also multiferroic TbMnO3 to the isostructural antiferromagnetic series RVO3 R = [Tb, Ce, Dy, Nd] is carried out in order to better understand the complex correlations in the multiferroic materials. Inelastic neutron scattering experiments were performed on TbVO3, CeVO3 and DyMnO3 and results from previously performed experiments on DyVO3, NdVO3 and TbMnO3 were compared to the crystal field excitation calculations. Crystal field excitation calculations used the point charge ionic model, and were found to predict the experimental crystal field excitations, with a deviation of 0.5 meV. In TbVO3 and CeVO3 the interaction between the molecular field and the crystal field excitations is confirmed. Moreover, the point charge ionic model was found to accurately predict the properties of the crystal field excitations for the strongly correlated electron systems: RMnO3 multiferroics and the RVO3 antiferromagnetic vanadates.

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