Probing domains and topological defects in ferroics
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Date
2023
Publication Type
Doctoral Thesis
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Abstract
The physical and chemical functionality of solids is defined, in many aspects, by spatial inhomogeneity, with prominent examples in doped semiconductors or magnetic data storage. In materials with ferroic order, inhomogeneity manifests in the form of domains, that is, regions with different electric, magnetic, or structural long-range order separated by boundaries called domain walls. Magnetoelectric multiferroics are a particularly promising class of ordered materials because of their coexisting electric and magnetic orders rendering them candidates for low-energy or multi-state storage devices. In particular, the interplay between electric and magnetic order within domains and at domain walls endows magnetoelectric multiferroics with unique functional degrees of freedom. However, our understanding of these phenomena still resides at a fundamental level.
This doctoral thesis unveils the electric and magnetic interplay on the domain level at the microscopic scale and discusses a control route to tailor the average domain size on a sub-micron length scale. In addition, novel methodologies to probe domain populations below the optical resolution limit are explored. As a model compound, hexagonal manganites are investigated. In these compounds, electric and magnetic orders arise independently, and therefore the interplay between the orders is expected to be weak or non-mandatory. This is in contrast to multiferroic materials where the electric order is directly induced by the magnetic order, and the interplay is therefore inherently pronounced, leading to one-to-one rigidly coupled magnetoelectric domains. Importantly, however, the independence of electric and magnetic orders in the hexagonal manganites can leave the freedom for a richer landscape of domains and domain walls. It is shown in this doctoral thesis that hexagonal manganites indeed show a rich variety of domain walls. More specifically, three types of magnetic(-electric) domain walls are identified, along with magnetic vortices that form between the different domain wall intersections. This rich landscape of domain walls and vortices has its origins --- contrary to general belief --- in a pronounced bulk magnetoelectric coupling phenomenon present in these materials.
Moving forward, another inherent aspect that determines the functionality of ordered materials is the average domain size which, in principle, defines how small an information ``bit'' can be made. This doctoral thesis explores chemical substitution as a route to tailor the domain size in hexagonal manganites from the micron- down to the angstrom range while preserving the inherent bulk magnetoelectric coupling mechanism. The investigation of domains in the sub-micrometer range requires imaging techniques that overcome the limitations of optical resolution. To circumvent this limitation, an experimental proof-of-concept is presented. More specifically, by combining pixel-by-pixel analysis and numerical simulations, we expand the capabilities of far-field optical second-harmonic generation (SHG) to probe nanodomain populations. In addition, combined SHG and scanning probe techniques (SPM) enable studies on the level of individual domain walls.
The research results presented in this doctoral thesis, hence, provide insights that expand the field of multiferroics from a fundamental but also from a methodical point of view. These findings are relevant not only for bulk multiferroic systems but could be extended to the emergent field of thin-film multiferroics, for example to explore magnetoelectric coupling effects in hexagonal manganites thin films or to tailor magnetoelectric domains during synthesis. These results highlight the importance of microscopic inhomogeneity, specifically domains and domain walls, and its potential to promote their magnetoelectric functionality in these materials.
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Examiner: Fiebig, Manfred
Examiner : Kreisel, Jens
Examiner: Lottermoser, Thomas
Examiner : Makarov, Denys
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ETH Zurich
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Subject
Multiferroics; non-linear optics; magnetoelectric coupling; magnetoelectric materials; domains; domain walls; topological defects; magnetoelectric domains; hexagonal manganites; chromium oxide; second-harmonic microscopy; antiferromagnetism; EquipSent (non-profit); Enable Education Everywhere
Organisational unit
03918 - Fiebig, Manfred / Fiebig, Manfred