With the overriding goal of developing functional multiferroic systems with technological potential, this thesis focuses on the role of orbital and charge ordering in coupling magnetism and ferroelectricity in synthetic quadruple perovskites. Using x-ray diffraction as the primary characterisation tool, modulations to crystal ordering have been interpreted in terms of orbital occupation and charge variation. Expanding on previous magnetic structure studies and polarisation measurements, structural analysis of CaMn<sub>7</sub>O<sub>12</sub> has led to the experimental realisation of a new mechanism for multiferroicity, resulting from a "magneto-orbital helix". Motivated by the idea of tuning multiferroic properties through varying manganese valence, the doped system CaCu<sub>x</sub>Mn<sub>7-x</sub>O<sub>12</sub> has been studied. Structural models considering the possibility of domain formation and multiple coexisting modulations have been tested against x-ray diffraction data. Finally, motivated by theoretical predictions of ferroelectric phases and multiferroicity in doped, simple, manganite perovskites, a structural model for the low temperature phase of NaMn<sub>7</sub>O<sub>12</sub> has been developed, based upon theoretical predictions for orbital ordering and the experimentally determined magnetic structure. This model has been tested against previously measured neutron diffraction data. The importance of understanding crystal formation and domain structures when applying theoretical models has been highlighted, and has prompted the consideration of future work involving viewing and manipulating twin formation.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:680430 |
| Date | January 2015 |
| Creators | Perks, Natasha J. |
| Contributors | Radaelli, Paolo G. |
| Publisher | University of Oxford |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
| Source | http://ora.ox.ac.uk/objects/uuid:c9287592-bcf4-40e0-ba2b-88f87e3b7450 |
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