Colin Heikes

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.

Breaking of Valley Degeneracy by Magnetic Field in Monolayer MoSe 2

Using polarization-resolved photoluminescence spectroscopy, we investigate the breaking of valley degeneracy by an out-of-plane magnetic field in back-gated monolayer MoSe2 devices. We observe a linear splitting of -0.22  meV/T between luminescence peak energies in σ+ and σ- emission for both neutral and charged excitons. The optical selection rules of monolayer MoSe2 couple the photon handedness to the exciton valley degree of freedom; so this splitting demonstrates valley degeneracy breaking. In addition, we find that the luminescence handedness can be controlled with a magnetic field to a degree that depends on the back-gate voltage. An applied magnetic field, therefore, provides effective strategies for control over the valley degree of freedom.

Giant Ferroelectric Polarization in Ultrathin Ferroelectrics via Boundary‐Condition Engineering

Tailoring and enhancing the functional properties of materials at reduced dimension is critical for continuous advancement of modern electronic devices. Here, the discovery of local surface induced giant spontaneous polarization in ultrathin BiFeO3 ferroelectric films is reported. Using aberration-corrected scanning transmission electron microscopy, it is found that the spontaneous polarization in a 2 nm-thick ultrathin BiFeO3 film is abnormally increased up to ≈90-100 µC cm-2 in the out-of-plane direction and a peculiar rumpled nanodomain structure with very large variation in c/a ratios, which is analogous to morphotropic phase boundaries (MPBs), is formed. By a combination of density functional theory and phase-field calculations, it is shown that it is the unique single atomic Bi2 O3-x layer at the surface that leads to the enhanced polarization and appearance of the MPB-like nanodomain structure. This finding clearly demonstrates a novel route to the enhanced functional properties in the material system with reduced dimension via engineering the surface boundary conditions.

Size Effect on Spontaneous Flux-closure Domains in BiFeO 3 Thin Films

1. Department of Materials Science and Engineering, University of Michigan, Ann Arbor, MI 48109 2. Department of Chemical Engineering and Materials Science, University of California Irvine, Irvine, CA 92697 3. Department of Materials Science and Engineering, Penn State University, University Park, PA 16802 4. Department of Materials Science and Engineering, Cornell University, Ithaca, NY 14853 5 Department of Physics and Astronomy, University of California Irvine, Irvine, California 92697.

Publisher Correction: Anisotropic polarization-induced conductance at a ferroelectric–insulator interface

In the version of this Letter originally published, the right-hand arrow in Fig. 3b was incorrectly labelled; see correction note for details. Also, ref. 29 was incorrectly included in the reference list; it has now been removed.

Anisotropic polarization-induced conductance at a ferroelectric–insulator interface

Coupling between different degrees of freedom, that is, charge, spin, orbital and lattice, is responsible for emergent phenomena in complex oxide heterostrutures1,2. One example is the formation of a two-dimensional electron gas (2DEG) at the polar/non-polar LaAlO3/SrTiO3 (LAO/STO)3–7 interface. This is caused by the polar discontinuity and counteracts the electrostatic potential build-up across the LAO film3. The ferroelectric polarization at a ferroelectric/insulator interface can also give rise to a polar discontinuity8–10. Depending on the polarization orientation, either electrons or holes are transferred to the interface, to form either a 2DEG or two-dimensional hole gas (2DHG)11–13. While recent first-principles modelling predicts the formation of 2DEGs at the ferroelectric/insulator interfaces9,10,12–14, experimental evidence of a ferroelectrically induced interfacial 2DEG remains elusive. Here, we report the emergence of strongly anisotropic polarization-induced conductivity at a ferroelectric/insulator interface, which shows a strong dependence on the polarization orientation. By probing the local conductance and ferroelectric polarization over a cross-section of a BiFeO3–TbScO3 (BFO/TSO) (001) heterostructure, we demonstrate that this interface is conducting along the 109° domain stripes in BFO, whereas it is insulating in the direction perpendicular to these domain stripes. Electron energy-loss spectroscopy and theoretical modelling suggest that the anisotropy of the interfacial conduction is caused by an alternating polarization associated with the ferroelectric domains, producing either electron or hole doping of the BFO/TSO interface.The striped polarization domains in a BiFeO3/TbScO3 heterostructure induce alternating p- and n-type doping at the interface, giving rise to strongly anisotropic in-plane conductance.

Defect-assisted Reorganization of Ferroelectric Domain Walls Revealed by Aberration-corrected Electron Microscopy

Domain walls (DWs) in ferroelectrics are quasi-2D functional units possessing peculiar properties that are absent in the bulk materials. The ability to produce ordered patterns of DWs is a prerequisite both for the fundamental study of DW properties and the design of novel nanodevices based on DW functionalites. In recent years, extensive efforts have been made towards fabricating periodic domain and DW structures in ferroelectric thin films, mainly through modifying elastic and electrostatic boundary conditions at the film interfaces. One of the major limitations of such a method, however, has been that once the choice of substrate is set further modifications to control or alter domain patterns during material synthesis becomes difficult, reducing the parameter space for creating more complex structures with ordered DW patterns and thus imposing resitrictions on the functionalities of the system. Here, with a combination of aberration-corrected scanning transmission electron microscopy (STEM) and electron energy loss spectroscopy (EELS), we show that atomically-thin charged defects that are deliberately introduced during the film growth can be used as nano-building-blocks for tailoring and reorganizing DW patterns. Such defect engineering can produce novel periodic mixed-type DW structures that are inaccessible by conventional boundary-condition-tuning methods

Control of Domain Structures in Multiferroic Thin Films through Defect Engineering

Domain walls (DWs) have become an essential component in nanodevices based on ferroic thin films. The domain configuration and DW stability, however, are strongly dependent on the boundary conditions of thin films, which make it difficult to create complex ordered patterns of DWs. Here, it is shown that novel domain structures, that are otherwise unfavorable under the natural boundary conditions, can be realized by utilizing engineered nanosized structural defects as building blocks for reconfiguring DW patterns. It is directly observed that an array of charged defects, which are located within a monolayer thickness, can be intentionally introduced by slightly changing substrate temperature during the growth of multiferroic BiFeO3 thin films. These defects are strongly coupled to the domain structures in the pretemperature-change portion of the BiFeO3 film and can effectively change the configuration of newly grown domains due to the interaction between the polarization and the defects. Thus, two types of domain patterns are integrated into a single film without breaking the DW periodicity. The potential use of these defects for building complex patterns of conductive DWs is also demonstrated.

Interaction between Ferroelectric Polarization and Defects in BiFeO3 Thin Films

Nanoscale impurity defects, with structures different from host materials, are known to commonly exist in functional complex oxides as a result of slight stoichiometry fluctuations that occur during material growth. Local perturbations induced by these defects, such as charge, strain, and atomic interaction, could have a profound effect on the physical properties of oxide nanomaterials. A direct correlation of the defects to the material functionalities, however, are often hampered by the lack of a fundamental understanding of the microscopic mechanisms underlying the coupling between the defects and the host lattice. Here, with a combination of atomic-scale STEM and in situ TEM, we perform a systematic study of atomic-scale polarization structures and microscopic domain-switching processes in the prototypical multiferroic BiFeO3 thin films to explore the interaction between ferroelectric polarization and defects.

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.