Controlling double vortex states in low-dimensional dipolar systems

Ferroelectric Texture of Individual Barium Titanate Nanocrystals

Ferroelectric materials display exotic polarization textures at the nanoscale that could be used to improve the energetic efficiency of electronic components. The vast majority of studies were conducted in two dimensions on thin films that can be further nanostructured, but very few studies address the situation of individual isolated nanocrystals (NCs) synthesized in solution, while such structures could have other fields of applications. In this work, we experimentally and theoretically studied the polarization texture of ferroelectric barium titanate (BaTiO3, BTO) NCs attached to a conductive substrate and surrounded by air. We synthesized NCs of well-defined quasicubic shape and 160 nm average size that conserve the tetragonal structure of BTO at room temperature. We then investigated the inverse piezoelectric properties of such pristine individual NCs by vector piezoresponse force microscopy (PFM), taking particular care to suppress electrostatic artifacts. In all of the NCs studied, we could not detect any vertical PFM signal, and the maps of the lateral response all displayed larger displacement amplitude on the edges with deformations converging toward the center. Using field phase simulations dedicated to ferroelectric nanostructures, we were able to predict the equilibrium polarization texture. These simulations revealed that the NC core is composed of 180° up and down domains defining the polar axis that rotate by 90° in the two facets orthogonal to this axis, eventually lying within these planes forming a layer of about 10 nm thickness mainly composed of 180° domains along an edge. From this polarization distribution, we predicted the lateral PFM response, which was revealed to be in very good qualitative agreement with the experimental observations. This work positions PFM as a relevant tool to evaluate the potential of complex ferroelectric nanostructures to be used as sensors.

Theoretical Understanding of Polar Topological Phase Transitions in Functional Oxide Heterostructures: A Review

The exotic topological phase is attracting considerable attention in condensed matter physics and materials science over the past few decades due to intriguing physical insights. As a combination of "topology" and "ferroelectricity," the ferroelectric (polar) topological structures are a fertile playground for emergent phenomena and functionalities with various potential applications. Herein, the review starts with the universal concept of the polar topological phase and goes on to briefly discuss the important role of computational tools such as phase-field simulations in designing polar topological phases in oxide heterostructures. In particular, the history of the development of phase-field simulations for ferroelectric oxide heterostructures is highlighted. Then, the current research progress of polar topological phases and their emergent phenomena in ferroelectric functional oxide heterostructures is reviewed from a theoretical perspective, including the topological polar structures, the establishment of phase diagrams, their switching kinetics and interconnections, phonon dynamics, and various macroscopic properties. Finally, this review offers a perspective on the future directions for the discovery of novel topological phases in other ferroelectric systems and device design for next-generation electronic device applications.

Micromagnetic Simulation of Round Ferromagnetic Nanodots with Varying Roughness and Symmetry

Magnetic nanodots are of high interest for basic research due to their broad spectrum of possible magnetic states and magnetization reversal processes. Besides, they are of technological interest since they can be applied in magnetic data storage, especially if vortex states occur in closed dots or open rings. While producing such nanorings and nanodots from diverse magnetic materials by lithographic techniques is quite common nowadays, these production technologies are naturally prone to small deviations of the borders of these nanoparticles. Here we investigate the influence of well-defined angular-dependent roughness of the edges, created by building the nanoparticles from small cubes, on the resulting hysteresis loops and magnetization reversal processes in five different round nanodots with varying open areas, from a thin ring to a closed nanodot. By varying the orientation of the external magnetic field, the impact of the angle-dependent roughness can be estimated. Especially for the thinnest ring, significant dependence of the transverse magnetization component on the field orientation can be found.

Characteristics and controllability of vortices in ferromagnetics, ferroelectrics, and multiferroics

Topological defects in condensed matter are attracting e significant attention due to their important role in phase transition and their fascinating characteristics. Among the various types of matter, ferroics which possess a switchable physical characteristic and form domain structure are ideal systems to form topological defects. In particular, a special class of topological defects-vortices-have been found to commonly exist in ferroics. They often manifest themselves as singular regions where domains merge in large systems, or stabilize as novel order states instead of forming domain structures in small enough systems. Understanding the characteristics and controllability of vortices in ferroics can provide us with deeper insight into the phase transition of condensed matter and also exciting opportunities in designing novel functional devices such as nano-memories, sensors, and transducers based on topological defects. In this review, we summarize the recent experimental and theoretical progress in ferroic vortices, with emphasis on those spin/dipole vortices formed in nanoscale ferromagnetics and ferroelectrics, and those structural domain vortices formed in multiferroic hexagonal manganites. We begin with an overview of this field. The fundamental concepts of ferroic vortices, followed by the theoretical simulation and experimental methods to explore ferroic vortices, are then introduced. The various characteristics of vortices (e.g. formation mechanisms, static/dynamic features, and electronic properties) and their controllability (e.g. by size, geometry, external thermal, electrical, magnetic, or mechanical fields) in ferromagnetics, ferroelectrics, and multiferroics are discussed in detail in individual sections. Finally, we conclude this review with an outlook on this rapidly developing field.

Morphogenesis mechanisms in the solvothermal synthesis of BaTiO3 from titanate nanorods and nanotubes

A rich variety of single crystalline BaTiO3 (BT) nanostructures have been synthesized by two different routes using titanate nanorods and nanotubes as precursors. Free standing, mixed or agglomerated nanotori, solid or hollow nanospheres and nanocubes were obtained. A careful analysis of the shape evolution of the resulting BT nano-objects obtained with both types of precursors and different parameters (precursor composition and shape, temperature, Ba/Ti molar ratio) allowed an improved understanding of the nanostructure formation. The morphogenesis models at play such as Ostwald ripening and the Kirkendall effect have been identified. Other mechanisms hereafter called the self and merging rebuilding processes and a tentative Turing-reaction-diffusion-model are proposed to explain the formation of these obtained nanoparticles.

Vortex domain structure in ferroelectric nanoplatelets and control of its transformation by mechanical load

Vortex domain patterns in low-dimensional ferroelectrics and multiferroics have been extensively studied with the aim of developing nanoscale functional devices. However, control of the vortex domain structure has not been investigated systematically. Taking into account effects of inhomogeneous electromechanical fields, ambient temperature, surface and size, we demonstrate significant influence of mechanical load on the vortex domain structure in ferroelectric nanoplatelets. Our analysis shows that the size and number of dipole vortices can be controlled by mechanical load, and yields rich temperature-stress (T-S) phase diagrams. Simulations also reveal that transformations between “vortex states” induced by the mechanical load are possible, which is totally different from the conventional way controlled on the vortex domain by the electric field. These results are relevant to application of vortex domain structures in ferroelectric nanodevices, and suggest a novel route to applications including memories, mechanical sensors and transducers.