Perspective: Emergent topologies in oxide superlattices
TL;DR: In this paper, the authors review and explore future directions in such ferroic superlattices wherein recent studies have revealed complex emergent polarization topologies, novel states of matter, and intriguing properties that arise from our ability to manipulate materials with epitaxial strain, interfacial coupling and interactions, size effects, and more.
Abstract: The ability to synthesize high-quality, complex-oxide heterostructures has created a veritable playground in which to explore emergent phenomena and exotic phases which arise from the interplay of spin, charge, orbital, and lattice degrees of freedom. Of particular interest is the creation of artificial heterostructures and superlattices built from two or more materials. Through such approaches, it is possible to observe new phases and phenomena that are not present in the parent materials alone. This is especially true in ferroelectric materials where the appropriate choice of superlattice constituents can lead to structures with complex phase diagrams and rich physics. In this article, we review and explore future directions in such ferroic superlattices wherein recent studies have revealed complex emergent polarization topologies, novel states of matter, and intriguing properties that arise from our ability to manipulate materials with epitaxial strain, interfacial coupling and interactions, size effects, and more. We focus our attention on recent work in (PbTiO3)n/(SrTiO3)n superlattices wherein exotic polar-vortex structures have been observed. We review the history of these observations and highlights of recent studies and conclude with an overview and prospectus of how the field may evolve in the coming years.
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TL;DR: Progress over the last decade in the study of topological structures in ferroic thin films and heterostructures is explored, including the observation of topology structures and control of their structures and emergent physical phenomena through epitaxial strain, layer thickness, electric, magnetic fields, etc.
Abstract: Topological spin/polarization structures in ferroic materials continue to draw great attention as a result of their fascinating physical behaviors and promising applications in the field of high-density nonvolatile memories as well as future energy-efficient nanoelectronic and spintronic devices. Such developments have been made, in part, based on recent advances in theoretical calculations, the synthesis of high-quality thin films, and the characterization of their emergent phenomena and exotic phases. Herein, progress over the last decade in the study of topological structures in ferroic thin films and heterostructures is explored, including the observation of topological structures and control of their structures and emergent physical phenomena through epitaxial strain, layer thickness, electric, magnetic fields, etc. First, the evolution of topological spin structures (e.g., magnetic skyrmions) and associated functionalities (e.g., topological Hall effect) in magnetic thin films and heterostructures is discussed. Then, the exotic polar topologies (e.g., domain walls, closure domains, polar vortices, bubble domains, and polar skyrmions) and their emergent physical properties in ferroelectric oxide films and heterostructures are explored. Finally, a brief overview and prospectus of how the field may evolve in the coming years is provided.
87 citations
Journal Article•
TL;DR: In this article, the magnetic force microscopy imaging of permalloy (Ni(80)Fe(20)) 0.3 to 1 micrometer in diameter and 50 nanometers thick circular magnetic dot has been used to detect a magnetic vortex.
Abstract: Spin structures of nanoscale magnetic dots are the subject of increasing scientific effort, as the confinement of spins imposed by the geometrical restrictions makes these structures comparable to some internal characteristic length scales of the magnet. For a vortex (a ferromagnetic dot with a curling magnetic structure), a spot of perpendicular magnetization has been theoretically predicted to exist at the center of the vortex. Experimental evidence for this magnetization spot is provided by magnetic force microscopy imaging of circular dots of permalloy (Ni(80)Fe(20)) 0.3 to 1 micrometer in diameter and 50 nanometers thick.
56 citations
TL;DR: In this article, a review of the recent progress in ferroelectric-oxide superlattices in which researchers can control structure and physical properties through the application of epitaxial strain, layer thickness, temperature, electric field, etc.
Abstract: Topological structures in ferroic materials have drawn great interest in recent years due to the richness of the underlying physics and the potential for applications in next generation electronics. Recent advances in atomically precise thin-film materials synthesis and characterization of structural/physical phenomena at unprecedented length/energy/time scales have enabled us to study exotic phases and their associated physics [Rosler et al., Nature 442, 797 (2006); S. Das, Nature 568, 368 (2019); Yadav et al., Nature 530, 198 (2016); and Stoica et al., Nat. Mater. 18, 377 (2019)]. It is appropriate that, in the second century of ferroelectrics, some dramatic discoveries are propelling the field into directions heretofore unimaginable. In this review, we explore the recent progress in ferroelectric-oxide superlattices in which researchers can control structure and physical properties through the application of epitaxial strain, layer thickness, temperature, electric field, etc. We provide a discussion of exotic topological structures (e.g., closure domains, vortices, polar skyrmions, and other exotic phases) and associated functionalities in ferroelectric/paraelectric superlattices. We conclude with a brief overview of and prospects for how the field may evolve in the coming years.
34 citations
Journal Article•
TL;DR: A synchrotron x-ray study of lead titanate as a function of temperature and film thickness for films as thin as a single unit cell implies that no thickness limit is imposed on practical devices by an intrinsic ferroelectric size effect.
Abstract: Understanding the suppression of ferroelectricity in perovskite thin films is a fundamental issue that has remained unresolved for decades. We report a synchrotron x-ray study of lead titanate as a function of temperature and film thickness for films as thin as a single unit cell. At room temperature, the ferroelectric phase is stable for thicknesses down to 3 unit cells (1.2 nanometers). Our results imply that no thickness limit is imposed on practical devices by an intrinsic ferroelectric size effect.
30 citations
TL;DR: The transition from a highly homogeneous polarization state to the formation of rotational nanodomains has been achieved by controlling the superlattice period while maintaining compressive clamping of the superLattice to the cubic SrTiO3 substrate.
Abstract: Ferroelectrics are characterized by domain structures as are other ferroics. At the nanoscale though, ferroelectrics may exhibit non-trivial or exotic polarization configurations under proper electrostatic and elastic conditions. These polar states may possess emerging properties not present in the bulk compounds and are promising for technological applications. Here, the observation of rotational polarization topologies at the nanoscale by means of aberration-corrected scanning transmission electron microscopy is reported in BaTiO3/SrTiO3 superlattices grown on cubic SrTiO3(001). The transition from a highly homogeneous polarization state to the formation of rotational nanodomains has been achieved by controlling the superlattice period while maintaining compressive clamping of the superlattice to the cubic SrTiO3 substrate. The nanodomains revealed in BaTiO3 prove that its nominal tetragonal structure also allows rotational polar textures.
16 citations
References
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TL;DR: The racetrack memory described in this review comprises an array of magnetic nanowires arranged horizontally or vertically on a silicon chip and is an example of the move toward innately three-dimensional microelectronic devices.
Abstract: Recent developments in the controlled movement of domain walls in magnetic nanowires by short pulses of spin-polarized current give promise of a nonvolatile memory device with the high performance and reliability of conventional solid-state memory but at the low cost of conventional magnetic disk drive storage. The racetrack memory described in this review comprises an array of magnetic nanowires arranged horizontally or vertically on a silicon chip. Individual spintronic reading and writing nanodevices are used to modify or read a train of ∼10 to 100 domain walls, which store a series of data bits in each nanowire. This racetrack memory is an example of the move toward innately three-dimensional microelectronic devices.
4,052 citations
TL;DR: A model interface is examined between two insulating perovskite oxides—LaAlO3 and SrTiO3—in which the termination layer at the interface is controlled on an atomic scale, presenting a broad opportunity to tailor low-dimensional charge states by atomically engineered oxide heteroepitaxy.
Abstract: Polarity discontinuities at the interfaces between different crystalline materials (heterointerfaces) can lead to nontrivial local atomic and electronic structure, owing to the presence of dangling bonds and incomplete atomic coordinations. These discontinuities often arise in naturally layered oxide structures, such as the superconducting copper oxides and ferroelectric titanates, as well as in artificial thin film oxide heterostructures such as manganite tunnel junctions. If polarity discontinuities can be atomically controlled, unusual charge states that are inaccessible in bulk materials could be realized. Here we have examined a model interface between two insulating perovskite oxides--LaAlO3 and SrTiO3--in which we control the termination layer at the interface on an atomic scale. In the simple ionic limit, this interface presents an extra half electron or hole per two-dimensional unit cell, depending on the structure of the interface. The hole-doped interface is found to be insulating, whereas the electron-doped interface is conducting, with extremely high carrier mobility exceeding 10,000 cm2 V(-1) s(-1). At low temperature, dramatic magnetoresistance oscillations periodic with the inverse magnetic field are observed, indicating quantum transport. These results present a broad opportunity to tailor low-dimensional charge states by atomically engineered oxide heteroepitaxy.
3,977 citations
TL;DR: The discovery of ferroelectricity in a perovskite manganite, TbMnO3, where the effect of spin frustration causes sinusoidal antiferromagnetic ordering and gigantic magnetoelectric and magnetocapacitance effects are found.
Abstract: The magnetoelectric effect--the induction of magnetization by means of an electric field and induction of polarization by means of a magnetic field--was first presumed to exist by Pierre Curie, and subsequently attracted a great deal of interest in the 1960s and 1970s (refs 2-4). More recently, related studies on magnetic ferroelectrics have signalled a revival of interest in this phenomenon. From a technological point of view, the mutual control of electric and magnetic properties is an attractive possibility, but the number of candidate materials is limited and the effects are typically too small to be useful in applications. Here we report the discovery of ferroelectricity in a perovskite manganite, TbMnO3, where the effect of spin frustration causes sinusoidal antiferromagnetic ordering. The modulated magnetic structure is accompanied by a magnetoelastically induced lattice modulation, and with the emergence of a spontaneous polarization. In the magnetic ferroelectric TbMnO3, we found gigantic magnetoelectric and magnetocapacitance effects, which can be attributed to switching of the electric polarization induced by magnetic fields. Frustrated spin systems therefore provide a new area to search for magnetoelectric media.
3,769 citations
TL;DR: This study experimentally establishes magnetic materials lacking inversion symmetry as an arena for new forms of crystalline order composed of topologically stable spin states in the chiral itinerant-electron magnet MnSi.
Abstract: Skyrmions represent topologically stable field configurations with particle-like properties. We used neutron scattering to observe the spontaneous formation of a two-dimensional lattice of skyrmion lines, a type of magnetic vortex, in the chiral itinerant-electron magnet MnSi. The skyrmion lattice stabilizes at the border between paramagnetism and long-range helimagnetic order perpendicular to a small applied magnetic field regardless of the direction of the magnetic field relative to the atomic lattice. Our study experimentally establishes magnetic materials lacking inversion symmetry as an arena for new forms of crystalline order composed of topologically stable spin states.
3,651 citations
TL;DR: Real-space imaging of a two-dimensional skyrmion lattice in a thin film of Fe0.5Co 0.5Si using Lorentz transmission electron microscopy reveals a controlled nanometre-scale spin topology, which may be useful in observing unconventional magneto-transport effects.
Abstract: Crystal order is not restricted to the periodic atomic array, but can also be found in electronic systems such as the Wigner crystal or in the form of orbital order, stripe order and magnetic order. In the case of magnetic order, spins align parallel to each other in ferromagnets and antiparallel in antiferromagnets. In other, less conventional, cases, spins can sometimes form highly nontrivial structures called spin textures. Among them is the unusual, topologically stable skyrmion spin texture, in which the spins point in all the directions wrapping a sphere. The skyrmion configuration in a magnetic solid is anticipated to produce unconventional spin-electronic phenomena such as the topological Hall effect. The crystallization of skyrmions as driven by thermal fluctuations has recently been confirmed in a narrow region of the temperature/magnetic field (T-B) phase diagram in neutron scattering studies of the three-dimensional helical magnets MnSi (ref. 17) and Fe(1-x)Co(x)Si (ref. 22). Here we report real-space imaging of a two-dimensional skyrmion lattice in a thin film of Fe(0.5)Co(0.5)Si using Lorentz transmission electron microscopy. With a magnetic field of 50-70 mT applied normal to the film, we observe skyrmions in the form of a hexagonal arrangement of swirling spin textures, with a lattice spacing of 90 nm. The related T-B phase diagram is found to be in good agreement with Monte Carlo simulations. In this two-dimensional case, the skyrmion crystal seems very stable and appears over a wide range of the phase diagram, including near zero temperature. Such a controlled nanometre-scale spin topology in a thin film may be useful in observing unconventional magneto-transport effects.
2,683 citations