Domain Walls In Multiferroics Research Articles

Ferrorotational domain walls revealed by electric quadrupole second harmonic generation microscopy

Domain walls are ubiquitous in materials that undergo phase transitions driven by spontaneous symmetry breaking. Domain walls in ferroics and multiferroics have received tremendous attention recently due to their emergent properties distinct from their domain counterparts, for example, their high mobility and controllability, as well as their potential applications in nanoelectronics. However, it is extremely challenging to detect, visualize and study the ferro-rotational (FR) domain walls because the FR order, in contrast to ferromagnetism (FM) and ferroelectricity (FE), is invariant under both the spatial-inversion and the time-reversal operations and thus hardly couple with conventional experimental probes. Here, an FR candidate $\mathrm{NiTiO_{3}}$ is investigated by ultrasensitive electric quadrupole (EQ) second harmonic generation rotational anisotropy (SHG RA) to probe the point symmetries of the two degenerate FR domain states, showing their relation by the vertical mirror operations that are broken below the FR critical temperature. We then visualize the real-space FR domains by scanning EQ SHG microscopy, and further resolve the FR domain walls by revealing a suppressed SHG intensity at domain walls. By taking local EQ SHG RA measurements, we show the restoration of the mirror symmetry at FR domain walls and prove their unconventional nonpolar nature. Our findings not only provide a comprehensive insight into FR domain walls, but also demonstrate a unique and powerful tool for future studies on domain walls of unconventional ferroics, both of which pave the way towards future manipulations and applications of FR domain walls.

Interconversion of multiferroic domains and domain walls

Systems with long-range order like ferromagnetism or ferroelectricity exhibit uniform, yet differently oriented three-dimensional regions called domains that are separated by two-dimensional topological defects termed domain walls. A change of the ordered state across a domain wall can lead to local non-bulk physical properties such as enhanced conductance or the promotion of unusual phases. Although highly desirable, controlled transfer of these properties between the bulk and the spatially confined walls is usually not possible. Here, we demonstrate this crossover by confining multiferroic Dy0.7Tb0.3FeO3 domains into multiferroic domain walls at an identified location within a non-multiferroic environment. This process is fully reversible; an applied magnetic or electric field controls the transformation. Aside from expanding the concept of multiferroic order, such interconversion can be key to addressing antiferromagnetic domain structures and topological singularities.

Manipulation of Magnetic Domain Walls by Ferroelectric Switching: Dynamic Magnetoelectricity at the Nanoscale.

Controlling magnetism using voltage is highly desired for applications, but remains challenging due to a fundamental contradiction between polarity and magnetism. Here, we propose a mechanism to manipulate magnetic domain walls in ferrimagnetic or ferromagnetic multiferroics using the electric field. Different from those studies based on static domain-level couplings, here the magnetoelectric coupling relies on the collaborative spin dynamics around domain walls. Accompanying the reversal of spin chirality driven by polarization switching, a "rolling-downhill"-like motion of the domain wall is achieved in nanoscale, which tunes the magnetization locally. Our mechanism opens an alternative route to the pursuit of practical and fast converse magnetoelectric functions via spin dynamics.

Dynamics of Composite Domain Walls in Multiferroics in Magnetic Field and Their Instability

We study theoretically the dynamics of composite domain walls (DW) in multiferroic material GdFeO3 driven by magnetic field H. Two antiferromagnetic orders of Fe and Gd spins and Gd-ion displacement interact with each other in this system with coupling c at low temperatures. We have numerically simulated the corresponding time-dependent Ginzburg–Landau equations in which magnetic field H couples to Fe-spin order parameter. We vary H and c systematically and calculate velocity v and the internal structure in a stationary state for magnetic DW and magneto-electric DW. DW mobility v/H increases with H and decreases with c for both DWs, but their characteristics differ between the two. We have also analytically studied the smooth characteristics of magneto-electric DW by a perturbation theory. Another finding is a splitting instability at a large H. A magneto-electric DW splits into a pair of magnetic DW and electric DW when c is large, while a magnetic DW splits when c is small. The internal structure of composite DW deforms with increasing c and H, and modulations in different order parameters separate in space. Their relative distances show a noticeable enhancement with approaching the splitting instability.

Domains and domain walls in multiferroics

Abstract Multiferroics are materials combining several ferroic orders, such as ferroelectricity, ferro- (or antiferro-) magnetism, ferroelasticity and ferrotoroidicity. They are of interest both from a fundamental perspective, as they have multiple (coupled) non-linear functional responses providing a veritable myriad of correlated phenomena, and because of the opportunity to apply these functionalities for new device applications. One application is, for instance, in non-volatile memory, which has led to special attention being devoted to ferroelectric and magnetic multiferroics. The vision is to combine the low writing power of ferroelectric information with the easy, non-volatile reading of magnetic information to give a “best of both worlds” computer memory. For this to be realised, the two ferroic orders need to be intimately linked via the magnetoelectric effect. The magnetoelectric coupling – the way polarization and magnetization interact – is manifested by the formation and interactions of domains and domain walls, and so to understand how to engineer future devices one must first understand the interactions of domains and domain walls. In this article, we provide a short introduction to the domain formation in ferroelectrics and ferromagnets, as well as different microscopy techniques that enable the visualization of such domains. We then review the recent research on multiferroic domains and domain walls, including their manipulation and intriguing properties, such as enhanced conductivity and anomalous magnetic order. Finally, we discuss future perspectives concerning the field of multiferroic domain walls and emergent topological structures such as ferroelectric vortices and skyrmions.

Electric and antiferromagnetic chiral textures at multiferroic domain walls.

Chirality, a foundational concept throughout science, may arise at ferromagnetic domain walls1 and in related objects such as skyrmions2. However, chiral textures should also exist in other types of ferroic materials, such as antiferromagnets, for which theory predicts that they should move faster for lower power3, and ferroelectrics, where they should be extremely small and possess unusual topologies4,5. Here, we report the concomitant observation of antiferromagnetic and electric chiral textures at domain walls in the room-temperature ferroelectric antiferromagnet BiFeO3. Combining reciprocal and real-space characterization techniques, we reveal the presence of periodic chiral antiferromagnetic objects along the domain walls as well as a priori energetically unfavourable chiral ferroelectric domain walls. We discuss the mechanisms underlying their formation and their relevance for electrically controlled topological oxide electronics and spintronics.

Creating multiferroic and conductive domain walls in common ferroelastic compounds

Domain walls in ferroelectrics and ferroelastics often present peculiar functional properties, offering an intriguing route toward the design of nano-devices. Here we use first-principles simulations to illustrate an approach for engineering such walls, working with representative ferroelastic perovskites LaGaO3 and CaTiO3 (insulating, non-magnetic, non-polar). We show that a wide range of substitutional dopants can be used to create long-range-ordered structures confined within the walls of these compounds, yielding functional interfaces with tailor-made properties. We thus identify clear-cut strategies to produce metallic walls within an insulating matrix. Further, we find ways to create magnetic walls that also display ferroelectric order (proper or improper), thus providing an original route to obtain magnetoelectric multiferroics. Given the recent developments on the preparation of high-density domain structures in perovskite films, our results suggest a definite path toward new functional nano-materials.

Hydrodynamics of domain walls in ferroelectrics and multiferroics: Impact on memory devices

The standard “Kittel Law” for the thickness and shape of ferroelectric, ferroelastic, or ferromagnet domains assumes mechanical equilibrium. The present paper shows that such domains may be highly nonequilibrium, with unusual thicknesses and shapes. In lead germanate and multiferroic lead zirconate titanate iron tantalate domain wall instabilities resemble hydrodynamics (Richtmyer–Meshkov and Helfrich–Hurault, respectively).

Magnetoelectric memory effect in the Y-type hexaferrite BaSrZnMgFe12O22**Project supported by the National Natural Science Foundation of China (Grant Nos. 11534015 and 51371193).

We report on the magnetic and magnetoelectric properties of the Y-type hexaferrite BaSrZnMgFe12O22, which undergoes transitions from a collinear ferrimagnetic phase to a proper screw phase at 310 K and to a longitudinal conical phase at 45 K. Magnetic and electric measurements revealed that the magnetic structure with spiral spin order can be modified by applying a magnetic field, resulting in magnetically controllable electric polarization.It was observed that BaSrZnMgFe12O22 exhibits an anomalous magnetoelectric memory effect: the ferroelectric state can be partially recovered from the paraelectric phase with collinear spin structure by reducing magnetic field at 20 K. We ascribe this memory effect to the pinning of multiferroic domain walls, where spin chirality and structure are preserved even in the nonpolar collinear spin state.

Ferroelectric and Magnetic Domain Walls in High Temperature Multiferroic Films and Heterostructures

In the last decade, considerable attention has been focused on the search of new multiferroic materials and the ways of improvement of their magnetoelectric properties. In this short review, we survey the progress in study of multiferroics focusing the high temperature multiferroic bismuth ferrite and rare earth iron garnets. We discuss the recent results of investigation of domain walls in multiferroics, concentrating the most important magnetoelectric manifestations (electric polarization and magnetization), and the pinning effect appearing as clamping of ferroelectric and magnetic domain walls.

Direct Observation of Magnetic Field Induced Ferroelectric Domain Evolution in Self-Assembled Quasi (0-3) BiFeO3–CoFe2O4 Thin Films

Strain-mediated magnetoelectric (ME) coupling effect is expected in self-assembly heterostructures engineered by ferroelectric and ferromagnetic materials, contributing to the enhanced overall magnetoelectric effect. Microstructures as well as the connectivity configuration are considered to play a significant role in achieving efficient magnetoelectric properties. Different from the conventional (1-3) and (2-2) type composite films, we fabricate BiFeO3-CoFe2O4 (BFO-CFO) composite thin films with a novel quasi (0-3) type connectivity via a dual-target pulsed laser deposition process. The self-assembly growth mechanism has been studied, which demonstrates that the perovskite (BFO) matrix segments the connectivity of spinel (CFO) resulting in a quasi (0-3) composite. Direct observation of ferroelectric domain wall motion under external magnetic fields proves a strong magnetoelectric coupling effect in these (0-3) thin films. Our preliminary findings reveal the promising application potential of this new structure as multiferroic domain wall devices.

Multiferroic materials and magnetoelectric physics: symmetry, entanglement, excitation, and topology

Multiferroics are those materials with more than one ferroic order, and magnetoelectricity refers to the mutual coupling between magnetism and electricity. The discipline of multiferroicity has never been so highly active as that in the first decade of the twenty-first century, and it has become one of the hottest disciplines of condensed matter physics and materials science. A series of milestones and steady progress in the past decade have enabled our understanding of multiferroic physics substantially comprehensive and profound, which is further pushing forward the research frontier of this exciting area. The availability of more multiferroic materials and improved magnetoelectric performance are approaching to make the applications within reach. While seminal review articles covering the major progress before 2010 are available, an updated review addressing the new achievements since that time becomes imperative. In this review, following a concise outline of the basic knowledge of multiferroicity and magnetoelectricity, we summarize the important research activities on multiferroics, especially magnetoelectricity and related physics in the last six years. We consider not only single-phase multiferroics but also multiferroic heterostructures. We address the physical mechanisms regarding magnetoelectric coupling so that the backbone of this divergent discipline can be highlighted. A series of issues on lattice symmetry, magnetic ordering, ferroelectricity generation, electromagnon excitations, multiferroic domain structure and domain wall dynamics, and interfacial coupling in multiferroic heterostructures, will be revisited in an updated framework of physics. In addition, several emergent phenomena and related physics, including magnetic skyrmions and generic topological structures associated with magnetoelectricity will be discussed.

Interplay between elasticity, ferroelectricity and magnetism at the domain walls of bismuth ferrite

Multiferroic domain walls have recently been proposed as the active element in devices related to spintronics, data storage, and magnetic logic. Among multiferroics, BiFeO3 is by far the most studied material. Its domain walls have shown rich behaviours that include conductivity in an otherwise insulating crystal, and magnetotransport properties that are markedly different from those of the bulk. In this article we summarize the experimental evidence and the current models used to understand the interplay between elastic, electric, and magnetic properties of the domain walls. Starting from the considera- tion of antiferromagnetic domain structures on a background of ferroelectric domains, and emphasizing pinning effects, we proceed to discuss the microscopic behavior of ferroelectricity and magnetism at the walls. We describe how domain organization in BiFeO3 is caused by structural transformations, and scrutinize modelling works pinpointing their characteristic features. Finally, we summarize the recent progress and list open questions for future study on BiFeO3 domain structures. (© 2016 WILEY-VCH Verlag GmbH &Co. KGaA, Weinheim)

Domains and domain walls in multiferroics

Multiferroics are gathering solid-state matter in which several types of orders are simultaneously allowed, as ferroelectricity, ferromagnetism (or antiferromagnetism), ferroelasticity, or ferrotoroidicity. Among all, the ferroelectric/ferromagnetic couple is the most intensively studied because of potential applications in novel low-power magnetoelectric devices. Switching of one order thanks to the other necessarily proceeds via the nucleation and growth of coupled domains. This review is an introduction to the basics of ferroelectric/ferromagnetic domain formation and to the recent microscopy techniques devoted to domains imaging, providing new insights into the archetypal multiferroic domain morphologies. Some relevant examples are also given to illustrate some of the unexpected properties of domain walls, as well as the way these domain walls can be manipulated altogether thanks to various types of magnetoelectric coupling. Les multiferroïques rassemblent les solides qui permettent la cohabitation de plusieurs ordres ferroïques tels que la ferroélectricité, le ferromagnétisme (ou l'antiferromagnétisme), la ferroélasticité ou la ferrotoroïdicité. Parmi ces ordres, le couple ferroélectrique/ferromagnétique est celui qui permet d'envisager le plus directement la réalisation de dispositifs originaux dans lesquels, par exemple, l'aimantation peut être renversée par l'application d'un champ électrique, par simple application d'une tension et avec un faible coût énergétique. La manipulation d'un ordre par un autre doit opérer par nucléation et croissance de domaines nécessairement couplés. Cette revue introduit les mécanismes gouvernant la formation de ces domaines et les techniques les plus récentes pour leur observation à l'échelle microscopique, donnant ainsi accès aux morphologies les plus typiques de domaines multiferroïques. Quelques exemples parmi les plus significatifs permettront d'illustrer les propriétés spécifiques liées aux parois de ces domaines, ainsi que la façon dont ces parois peuvent être manipulées les unes par l'intermédiaire des autres grâce à différents types de couplage magnétoélectrique.

Complex domain walls inBiFeO3

We present an investigation of the ferroelectric and magnetic domain walls in multiferroics ${\mathrm{BiFeO}}_{3}$ combining first-principles structural data with a phenomenological modeling of the spin structure. We show that the structural distortions at the ferroelectric walls may lead to peculiar spin arrangements with nontrivial topologies. Further, we find that the magnetic walls depend strongly on the nature of the structural discontinuity, as the three possible cases considered---which correspond, respectively, to angles of ${71}^{\ensuremath{\circ}},{109}^{\ensuremath{\circ}}$, and ${180}^{\ensuremath{\circ}}$ between the polarizations of the neighboring domains---present distinctive features.

Controllable electrical conduction at complex oxide interfaces

Abstractmagnified imageOxide interfaces have sparked considerable interest due to their fascinating physical properties and promising possibilities for next‐generation nanoelectronics. Three intriguing oxide interfaces, hetero‐, homo‐, and tubular interfaces, will be discussed in detail in this review. First, the development and fundamental properties of classical heterointerfaces will be elaborated. Using LaAlO3/SrTiO3 as a case study, we present the most generic approach to control the two‐dimensional electron conduction at the heterointerface. Secondly, several key issues based on interesting observations on ferroic homointerfaces will be addressed. Multiple controls of fascinating functionalities based on multiferroic BiFeO3 domain walls will be presented as a model system. Finally, as the most challenging task, we review a new type of complex oxide interfaces, the tubular interface. Further insights of recently emerged tubular interfaces in BiFeO3/CoFe2O4 system will be explored, which serve as one of the focal roles in developing new functional interfaces and potentially new type of nanostructures. (© 2014 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Electric field control of multiferroic domain wall motion

The dynamics of a multiferroic domain wall in which an electric field can couple to the magnetization via inhomogeneous magnetoelectric interaction is investigated by the collective-coordinate framework. We show how the electric field is capable of delaying the onset of the Walker breakdown of the domain wall motion, leading to a significant enhancement of the maximum wall velocity. Moreover, we show that in the stationary regime the chirality of the domain wall can be efficiently reversed when the electric field is applied along the direction of the magnetic field. These characteristics suggest that the multiferroic domain wall may provide a new prospective means to design faster and low-power-consumption domain wall devices.

A phenomenological model for ferroelectric domain walls and its implications for BiFeO3–PbTiO3multiferroic compounds

Multiferroics are an important family of materials with special properties suitable for applications in advanced technological devices. In most of the cases, the ferroelectric ordering and domain wall formation determine and control their operation and functionality. However, the physical mechanisms by which these domain walls are formed are still not yet completely clarified. Also, in the last few years, few advances are found in the mechanisms used to explain the domain walls and their influence on the materials properties. In this work, the domain walls in ferroelectric multiferroics were investigated by using high-resolution transmission electron microscopy and image simulations. The ferroelectric switching was also observed by piezoresponse force microscopy. A three-dimensional atomic-level framework of 90° ferroelectric domain walls was proposed and the structural and ferroelectric features at the domain walls, such as length, width, and angle between domains, were determined. From these studies, it was found that ferroelectric and structural features of multiferroic BiFeO3–PbTiO3 compounds, such as domain-orientation, electrical conductivity, magnetic ordering, and brittleness due to strains at the domain walls, can be controlled by particular atomic substitutions at the A site of the perovskite structure.

Phase coexistence and magnetically tuneable polarization in cycloidal multiferroics

In some magnetic ferroelectrics the polarization P direction can be selected by a magnetic field H. Here we show that in a (Y${}_{1\ensuremath{-}x}$Sm${}_{x}$)MnO${}_{3}$ single crystal, both the direction and the magnitude of P can be controlled solely by H. We argue that this remarkable dual control arises from both the phase coexistence of $bc$-cycloidal regions within an $ab$-cycloidal matrix and the annihilation of multiferroic domain walls by $H$ cycling. We show that phase coexistence occurs even within the high-temperature spin-collinear phase and argue that this could be a general property arising from the strong frustration of magnetic interactions in these oxides.

Induced motion of domain walls in multiferroics with quadratic interaction

We theoretically study the dynamics of 180-degree domain wall of the ab-type in magnetic materials with quadratic magnetoelectric interaction in external alternating magnetic and electric fields. The features of the oscillatory and translational motions of the domain walls and stripe structures depending on the parameters of external fields and characteristics of the multiferroics are discussed. The possibility of the domain walls drift in a purely electric field is established.