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Abstract
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 complementary neutron and bulk techniques, first evidence of potential multiferroicity is shown to arise in the spinel FeCr2S4 in the orbitally ordered state.