Benjamin Winchester

Ferroelastic switching for nanoscale non-volatile magnetoelectric devices

Multiferroics, where (anti-) ferromagnetic, ferroelectric and ferroelastic order parameters coexist, enable manipulation of magnetic ordering by an electric field through switching of the electric polarization. It has been shown that realization of magnetoelectric coupling in a single-phase multiferroic such as BiFeO(3) requires ferroelastic (71 degrees, 109 degrees) rather than ferroelectric (180 degrees) domain switching. However, the control of such ferroelastic switching in a single-phase system has been a significant challenge as elastic interactions tend to destabilize small switched volumes, resulting in subsequent ferroelastic back-switching at zero electric field, and thus the disappearance of non-volatile information storage. Guided by our phase-field simulations, here we report an approach to stabilize ferroelastic switching by eliminating the stress-induced instability responsible for back-switching using isolated monodomain BiFeO(3) islands. This work demonstrates a critical step to control and use non-volatile magnetoelectric coupling at the nanoscale. Beyond magnetoelectric coupling, it provides a framework for exploring a route to control multiple order parameters coupled to ferroelastic order in other low-symmetry materials.

Domain Dynamics During Ferroelectric Switching

The role of defects and interfaces on switching in ferroelectric materials is observed with high-resolution microscopy. The utility of ferroelectric materials stems from the ability to nucleate and move polarized domains using an electric field. To understand the mechanisms of polarization switching, structural characterization at the nanoscale is required. We used aberration-corrected transmission electron microscopy to follow the kinetics and dynamics of ferroelectric switching at millisecond temporal and subangstrom spatial resolution in an epitaxial bilayer of an antiferromagnetic ferroelectric (BiFeO3) on a ferromagnetic electrode (La0.7Sr0.3MnO3). We observed localized nucleation events at the electrode interface, domain wall pinning on point defects, and the formation of ferroelectric domains localized to the ferroelectric and ferromagnetic interface. These results show how defects and interfaces impede full ferroelectric switching of a thin film.

Origin of suppressed polarization in BiFeO3 films

We have studied the origin of suppressed remanent polarization in 4-variant BiFeO3 by correlating microscopic observations of ferroelectric/ferroelastic domain structures and ferroelectric measurements of (001) epitaxial BiFeO3 thin films with 2- and 4-ferroelastic domain variants. Piezoelectric force microscopy revealed that domain wall pinning was the cause of the reduced polarization observed in 4-variant BiFeO3. Using repetitive switching, the unswitched domains were completely switched and the remanent polarization reached a value comparable to 2-variant BiFeO3. These results demonstrate that control of ferroelastic domains in rhombohedral systems is necessary in order to obtain high performance and reliable ferroelectric and magnetoelectric devices.

Phase-field simulation of domain structures in epitaxial BiFeO3 films on vicinal substrates

The ferroelectric domain structures of epitaxial BiFeO3 thin films on miscut substrates were studied using a phase-field model. The effects of substrate vicinality towards (100) are considered by assuming charge-compensated surface and film/substrate interface. The predicted domain structures show remarkable agreement with existing experimental observations, including domain wall orientations and local topological domain configurations. The roles of elastic, electric, and gradient energies on the domain structures were analyzed. It is shown that the substrate strain anisotropy due to the miscut largely determines the domain variant selection and domain configurations.

Electroelastic fields in artificially created vortex cores in epitaxial BiFeO3 thin films

We employ phase-field modeling to explore the elastic properties of artificially created 1-D domain walls in (001)p-oriented BiFeO3 thin films, composed of a junction of the four polarization variants, all with the same out-of-plane polarization. It was found that these junctions exhibit peculiarly high electroelastic fields induced by the neighboring ferroelastic/ferroelectric domains. The vortex core exhibits a volume expansion, while the anti-vortex core is more compressive. Possible ways to control the electroelastic field, such as varying material constant and applying transverse electric field, are also discussed.

In-Situ Cross-Sectional Switching of Multiferroic BiFeO3 Thin Films

The ferroelectric BiFeO3 has garnered much attention as a single-phase multiferroic, possessing coupled ferroelectric and antiferromagnetic ordering. Many applications of such ferroelectric materials rely on the repeatable switching between ferroelectric states under an applied field. BiFeO3 is a particularly complicated case as it possesses a large number of polarization states (eight), and only a subset of switching between them results in a reorientation of the antiferromagnetic ordering. Magneto-electric heterostructures which rely on the exchange interaction between the antiferromagnetic BiFeO3 and a ferromagnetic material [1, 2] at the interface therefore require deterministic control of ferroelectric switching. Specifically, a change of the antiferromagnetic order occurs only if there is a change of the polarization axis, that is ferroelastic 71° or 109° switching [3]. In this work we use in-situ TEM to study such switching in a (001) oriented BiFeO3 thin film.

2-D Mapping of Ferroelectric Domains by Transmission Electron Microscopy

One of the key advantageous of ferroelectric materials is the vanishingly small critical size required for ferroelectricity as well as their narrow domain walls giving the potential for very high density data storage or use in confined geometries such as superlattices. Even at large length scales ferroelectric switching is dominated by nanoscale defects that modify the local energy density. For the common displacive ferroelectric materials, the determination of the local polarization can be accomplished by a direct interpretation of atom positions obtained by transmission electron microscopy (TEM) [1]. In this work we employ such methods to study the multiferroic BiFeO3 (BFO), a displacive perovskite ferroelectric. In particular we use the technique to study polarization spatial variation at the thin film interface and characterize polarization changes resulting from local ferroelectric switching.