Interfacial control of ferroic order in oxide heterostructures
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Date
2022
Publication Type
Doctoral Thesis
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Abstract
Oxide electronics have emerged as an alternative to replace the current silicon-based technology. Owing to a rich elemental composition compared to that of doped silicon, transition metal oxides can host a wide range of physical phenomena. This is especially true when oxides are integrated into ultrathin epitaxial heterostructures, in which additional properties arise from the created interfaces. Their crystal structure is, furthermore, compatible with long-range order. In particular, ferromagnetic and ferroelectric systems have gathered considerable attention due to their characteristic non-volatile response to applied external fields. While ferroic oxides are indisputable candidates for low-energy-consuming applications, there are still a few setbacks left to overcome in order to integrate them into competitive device schemes.
With the work performed during the course of this thesis, we strive to provide solutions for existing limitations to the implementation of ferroic states in nanoscale devices. We place emphasis on interfacial effects in epitaxial heterostructures and their influence on ferroic order. Making use of a non-conventional approach of epitaxially combining layers with different ferroic anisotropies, we uncover novel fundamental concepts likely to benefit the ever-evolving field of oxide electronics.
We identify the in-plane-polarized Aurivillius compounds as promising candidates for tuning interfacial electrostatics and achieving interfacial polar continuity in epitaxial hybrid heterostructures with ferroelectric perovskite oxides. The stabilized coalescent layer-by-layer growth mode ensures the single-crystallinity of these layered ferroelectrics, while the sub-unit-cell thickness control of the films enables detailed investigations of their polar state. For instance, the thickness-dependent in-plane-polarized domain configuration in the resulting epitaxial Aurivillius films prompts us to propose new means for ferroic domain and functional domain-wall engineering via structural defect ordering.
Moving ahead with the integration of Aurivillius films into perovskite-based heterostructures, we show that the Aurivillius compounds utilized as in-plane-polarized buffer layers can overcome the notorious limitation associated with the critical thickness for ferroelectricity in canonical out-of-plane-polarized perovskite ferroelectrics. We additionally demonstrate that buffers of the Aurivillius phase can be instrumental for domain and domain-wall engineering in the room-temperature multiferroic BiFeO3. In particular, we observe a uniform chirality stabilized in Néel-like domain walls in BiFeO3 grown on our in-plane-polarized Bi5FeTi3O15 Aurivillius layer. This likely constitutes one of the first experimental signatures of the electric counterpart to the Dzyaloshinskii-Moriya interaction in magnetically ordered compounds.
Lastly, we explore magnetoelectric phase control in heterostructures combining both ferroelectric and ferromagnetic order. In a proof-of-concept multiferroic heterostructure, we mimic magnetoelectric domain walls by inserting ultrathin ferromagnetic La1-xSrxMnO3 in between two ferroelectric layers. We show that its magnetization and conductivity can be controlled by changing polarization directions in the adjacent ferroelectric layers only. This opens up new possibilities for voltage-based tuning of magnetization and conductivity at the nanoscale.
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Contributors
Examiner: Fiebig, Manfred
Examiner: Trassin, Morgan
Examiner : Koster, Gertjan
Examiner : Bibes, Manuel
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ETH Zurich
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Subject
Ferroelectricity; ferromagnetism; Oxide electronics; thin films; epitaxy; Non-volatile memory
Organisational unit
03918 - Fiebig, Manfred / Fiebig, Manfred
Notes
Funding
188414 - Multifunctional oxide electronics using natural ferroelectric superlattices (SNF)
Related publications and datasets
Is supplemented by: http://hdl.handle.net/20.500.11850/587196