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In the quest for new functional materials and heterostructures to lower the energy consumption of small-scale devices, continue Moore’s law1, and go beyond CMOS, ferroic and multiferroic materials hold great promise2,3. In contrast to semiconductor devices, these ferroic materials are capable of using a different order parameter (i.e. electric polarization or magnetic moment) to function as an on/off switch as well as having other unique capabilities. This reduces the energy limits faced by modern current based devices as they are scaled to ultra-small dimensions. Most ferroelectrics which have been incorporated into multiferroic devices to date are proper ferroelectrics in the perovskite phase. In contrast, this work explores hexagonal improper ferroelectrics and compatible oxide magnetic materials to expand the field of multiferroic devices and seek novel functionality. Molecular-beam epitaxy is used to fabricate thin films of these ferroic materials. This dissertation includes a thorough look at the thickness scaling of the improper ferroelectric h-LuFeO3. This is enabled by the development of a single crystal epitaxial iridium electrode that enables lower ferroelectric switching voltages than have been demonstrated before in literature, as well as the ability to scale h-LuFeO3 down to only one unit cell thick and still observe structural ferroelectricity. Also, we note that h-LuFeO3 does not follow the well-established Janovec-Kay-Dunn scaling law4,5 as do proper ferroelectrics, but rather a modified version with a different power dependence which has not been seen before in literature. Studying h-LuFeO3 and the development of the iridium electrode enabled the next section of this dissertation which is incorporating ferroelectric h-LuFeO3 with a magnetic material to develop epitaxial room temperature multiferroic superlattices. The oxide ferrimagnet CoFe2O4 is found to be a compatible companion in these heterostructures and the structure and properties are explored. Lastly, this works expands the phase space of these hexagonal ferroic materials by fabricating a new member of the ferrimagnetic RFe2O4 structure (R lanthanide rare earth) with R3 _m space group symmetry. The unique capabilities of epitaxial stabilization are harnessed to incorporate dysprosium into the structure to make DyFe2O4 which is not a stable phase in bulk form and has never been fabricated before. The structure of this new phase is analyzed by x-ray diffraction. 1 Moore, G. E. (McGraw-Hill New York, NY, USA:, 1965). 2 Manipatruni, S., Nikonov, D. E. & Young, I. A. Beyond CMOS computing with spin and polarization. Nature Physics 14, 338-343 (2018). 3 Bibes, M. & Barthélémy, A. Towards a magnetoelectric memory. Nature materials 7, 425-426 (2008). 4 Janovec, V. On the theory of the coercive field of single-domain crystals of BaTiO 3. Cechoslovackij fiziceskij zurnal 8, 3-15 (1958). 5 Kay, H. & Dunn, J. Thickness dependence of the nucleation field of triglycine sulphate. Philosophical Magazine 7, 2027-2034 (1962).

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148 pages


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Epitaxy; Multiferroics; Oxide; Thin Films


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Schlom, Darrell

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Fennie, Craig J.
van Dover, R. B.

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Materials Science and Engineering

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Ph. D., Materials Science and Engineering

Degree Level

Doctor of Philosophy

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Attribution-NonCommercial-ShareAlike 4.0 International


dissertation or thesis

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