We use molecular dynamics simulations to understand the mechanisms of polarization switching in ferroelectric BaTiO$_3$ achieved with external electric field. For tetragonal and orthorhombic ferroelectric phases, we determine the switching paths, and show that polarization rotation through intermediate monoclinic phases (a) facilitates switching at low fields (b) is responsible for a sharp anisotropy in polarization switching. We develop understanding of this through determination of detailed electric field-temperature phase diagrams, that are fundamental to technological applications based on electromechanical and switching response of ferroelectrics.
The elastic response of BaTiO$_3$ single crystals during electric field cycling at room temperature has been studied using in-situ Resonant Ultrasound Spectroscopy (RUS), which allows monitoring of both the elastic and anelastic changes caused by ferroelectric polarization switching. We find that the first ferroelectric switching of a virgin single crystal is dominated by ferroelastic 90° switching. In subsequent ferroelectric switching, ferroelastic switching is reduced by domain pinning and by the predominance of 180° ferroelectric domains, as confirmed by polarized light microscopy. RUS und…
Ferroelectric switching at edge dislocations in BaTiO$_3$ modelled at the atomic scale
Interdisciplinary Centre for Advanced Materials Simulation (ICAMS), Ruhr-University Bochum（波鸿鲁尔大学先进材料模拟跨学科中心）；Center for Interface-Dominated High Performance Materials (ZGH), Ruhr-University Bochum（波鸿鲁尔大学界面主导高性能材料中心）；Faculty of Physics and Astronomy, Ruhr-University Bochum（波鸿鲁尔大学物理与天文学系）；Univ. Lille, CNRS, INRAE, Centrale Lille, UMR 8207 - UMET - Unité Matériaux et Transformations（里尔大学、法国国家科学研究中心、法国国家农业食品与环境研究院、里尔中央理工学院、UMR 8207 材料与转化单元）
Switching kinetics in ultrathin epitaxial BaTiO3 films confirms the existence of the homogeneous switching (without domains) in ultrathin ferroelectric films. We suppose that the homogeneous switching in ultrathin films is a common phenomenon for all ferroelectric materials.
Previous studies of Barkhausen noise in PZT have been limited to the energy spectrum (slew rate response voltages versus time), showing agreement with avalanche models; in barium titanate other exponents have been measured acoustically, but only at ambient temperatures. In the present study we report the Omori exponent (-0.95$\pm$0.03) for aftershocks in PZT and extend the barium titanate studies to a wider range of temperature.
Negative capacitance in BaTiO$_3$/SrTiO$_3$ superlattices is investigated by Monte Carlo simulations in an atomistic effective Hamiltonian model, using fluctuation formulas for responses to the local macroscopic field that incorporates depolarizing fields. We show epitaxial strain can tune the negative capacitance of the BaTiO$_3$ ferroelectric layer and the overall capacitance of the system over a broad temperature range. In addition, we predict and explain an original switching of the negative capacitance from the BaTiO$_3$ layer to the SrTiO$_3$ layer at low temperatures for intermediate st…
We have proposed the polarization rotation theory to understand the underlying physics of the large piezoelectric effect in ferroelectric crystals by referring to the coherent rotation model in ferromagnetism. When both the electric field energy and crystalline anisotropy energy are taken into account, the polarization in BaTiO3 crystal can be rotated toward the field direction. The numerical results have indicated the highly anisotropic polarization rotations and the field-induced-phases with different symmetries in the crystal. This theory is helpful for understanding the polarization revers…
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 transiti…
Over the years attempts have been made to compensate for the inherent weaknesses in the bulk state of the multiferroic BiFeO3, such as high leakage current and the absence of ferromagnetic correlation, and exploit its magnetoelectric potential by forming solid solutions with other perovskites. Studies in the recent few years have shown that alloying of BiFeO3 with BaTiO3, both with and without additives, can induce both ferroelectric and ferromagnetic switching. While the coexistence of both the ferroic orders is encouraging from the view point of technological applications, the origin of ferr…
We investigate ferroelectric- and resistive switching behavior in 18-nm-thick epitaxial BaTiO$_3$ (BTO) films in a model electrolyte-ferroelectric-semiconductor (EFS) configuration. The system is explored for its potential as a ferroelectric microelectrode in bioelectronics. The BTO films are grown by pulsed laser deposition (PLD) on semiconducting Nb-doped (0.5 wt\%) SrTiO$_{3}$ (Nb:STO) single crystal substrates. The ferroelectric properties of the bare BTO films are demonstrated by piezoresponse force microscopy (PFM) measurements. Cyclic voltammetry (CV) measurements in EFS configuration, …
Magnetoelectric coupling permits a magnetic order parameter to be addressed electrically or vice versa, and could find use in data storage, field sensors and actuators. Coupling constants for single phase materials such as chromium dioxide, boracites and manganites are typically as low as 10^{-12} - 10^{-9} s/m, e.g. because the polarisations and magnetisations are small. Two phase multiferroics with strain mediated coupling, such as laminates, composites and epitaxial nanostructures, are more promising because each phase may be independently optimised. The resulting magnetoelectric switching …
We have investigated the crystal structure and the ferroelectric properties of BaTiO3 thin films with YBa2Cu3O7-d as the bottom and Au as the top electrode. Epitaxial heterostructures of YBa2Cu3O7-d and BaTiO3 were prepared by dc and rf sputtering, respectively. The crystal structure of the films was characterised by x-ray diffraction. The ferroelectric behaviour of the BaTiO3 films was confirmed by hysteresis loop measurements using a Sawyer Tower circuit. We obtain a coercive field of 30 kV/cm and a remanent polarisation of 1.25 \muC/cm. At sub-switching fields the capacitance of the films o…
We carry out first-principles calculations for CaTiO$_3$/BaTiO$_3$ superlattices with epitaxial strain corresponding to growth on a SrTiO$_3$ substrate, and consider octahedral rotations as well as ferroelectric distortions. The calculations are done as a function of electric displacement field, and both a macroscopic and a local electrostatic analysis are carried out. We find that strong octahedral rotations occur for TiO$_6$ octahedra sandwiched between CaO layers on both sides, but are strongly suppressed if either neighboring layer is a BaO layer. Due to the resulting enhancement of the fe…
Polarization induced spin switching of atoms in magnetic materials opens for possibilities to design and develop advanced spintronic devices, in particular, storage devices where the magnetic state can be controlled by an electric field. We employ density-functional theory calculations to study the magnetic properties of a perovskite strontium cobalt oxyfluoride Sr$_2$CoO$_3$F (SCOF) in a hybrid perovskite heterostructure, where SCOF is sandwiched between two ferroelectic BaTiO$_3$ (BTO) layers. Our calculations show that the spin state of the central Co atom in SCOF can be controlled by alter…
We present a first-principles study of electronic and magnetic properties of thin Co films on a BaTiO$_3$(001) single crystal. The crystalline structure of 1, 2, and 3 monolayer thick Co films was determined and served as input for calculations of the electronic and magnetic properties of the films. The estimation of exchange constants indicates that the Co films are ferromagnetic with a high critical temperature, which depends on the film thickness and the interface geometry. In addition, we calculated x-ray absorption spectra, related magnetic circular dichroism (XMCD) and linear dichroism (…
Functional devices with ultrathin ferroelectric layers have been attracted as a promising candidate for next-generation memory and logic device applications. Using the ultrathin ferroelectric layers, particularly approaching the two-dimensional limit, however, it is still challenging to control ferroelectric switching and to observe ferroelectricity by spectroscopic tools. In particular, conventional methods such as electrical measurements and piezoelectric response force microscopy are very limited due to leakage currents and the smallness of the ferroelectric signals. Here, we show that the …
Ferroelectric polar displacements have recently been observed in conducting electron-doped BaTiO3. The co-existence of a ferroelectric phase and conductivity opens the door to new functionalities which may provide a unique route for novel device applications. Using first-principles methods and electrostatic modeling we explore the effect that the switchable polarization of electron-doped BaTiO3 (n-BaTiO3) has on the electronic properties of the SrRuO3/n-BaTiO3 (001) interface. Ferroelectric polarization controls the accumulation or depletion of electron charge at the interface, and the associa…
For decades it has been a well-known fact that among the few ferroelectric compounds in the perovskite family namely BaTiO3, KNbO3, PbTiO3 Na1/2Bi1/2TiO3 the dielectric and piezoelectric properties of BaTiO3 is considerably higher than the others in polycrystalline form at room temperature. Further, similar to ferroelectric alloys exhibiting morphotropic phase boundary, single crystals of BaTiO3 exhibits anomalously large piezoelectric response when poled away from the direction of spontaneous polarization at room temperature. These anomalous features in BaTiO3 remained unexplained so far from…
Properties of (BaTiO$_3$)$_1$/(BaZrO$_3$)$_n$ ferroelectric superlattices (SLs) with $n = {}$1--7 grown in the [001] direction are calculated from first principles within the density functional theory. It is revealed that the quasi-two-dimensional ferroelectricity occurs in these SLs in the barium titanate layers with a thickness of one unit cell; the polarization is oriented in the layer plane and weakly interacts with the polarization in neighboring layers. The ferroelectric ordering energy and the height of the barrier separating different orientational states of polarization in these SLs a…
We propose a tunneling heterostructure by replacing one of the metal electrodes in a metal/ferroelectric/metal ferroelectric tunnel junction with a heavily doped semiconductor. In this metal/ferroelectric/semiconductor tunnel diode, both the height and the width of the tunneling barrier can be electrically modulated due to the ferroelectric field effect, leading to a colossal tunneling electroresistance. This idea is implemented in Pt/BaTiO3/Nb:SrTiO3 heterostructures, in which an ON/OFF conductance ratio above 10$^4$ can be readily achieved at room temperature. The colossal tunneling electror…
Proximity to phase transitions (PTs) is frequently responsible for the largest dielectric susceptibilities in ferroelectrics. The impracticality of using temperature as a control parameter to reach those large responses has motivated the design of solid solutions with phase boundaries between different polar phases at temperatures (typically room temperature) significantly lower than the paraelectric-ferroelectric critical temperature. The flat energy landscapes close to these PTs give rise to polarization rotation under external stimuli, being responsible for the best piezoelectrics so far an…
The dynamical processes associated with electric field manipulation of the polarization in a ferroelectric remain largely unknown but fundamentally determine the speed and functionality of ferroelectric materials and devices. Here we apply sub-picosecond duration, single-cycle terahertz pulses as an ultrafast electric field bias to prototypical BaTiO$_3$ ferroelectric thin films with the atomic-scale response probed by femtosecond x-ray scattering techniques. We show that electric fields applied perpendicular to the ferroelectric polarization drive large amplitude displacements of the titanium…
Electric-field control of spin-dependent properties has become one of the most attractive phenomena in modern materials research due the promise of new device functionalities. One of the paradigms in this approach is to electrically toggle the spin polarization of carriers injected into a semiconductor using ferroelectric polarization as a control parameter. Using first-principles density functional calculations, we explore the effect of ferroelectric polarization of electron-doped BaTiO3 (n-BaTiO3) on the spin-polarized transmission across the SrRuO3/n-BaTiO3 (001) interface. Our study reveal…
Using first-principles calculations, we estimated the impact of large applied electric E fields on the structural, dielectric, and ferroelectric properties of typical ferroelectrics. At large fields, the structural parameters change significantly, decreasing the strain between the different structural phases. This effect favours a polarization rotation model for ferroelectric switching in which the electronic polarization rotates between the directions of tetragonal, rhombohedral and orthorhombic phases. We estimate coercive fields E_c ~31 MV/m and ~52 MV/m at zero temperature for bulk ferroel…
We study the electrocaloric effect in the classic ferroelectric BaTiO3 through a series of phase transitions driven by applied electric field and temperature. We find both negative and positive electrocaloric effects, with the negative electrocaloric effect, where temperature decreases with applied field, in monoclinic phases. Macroscopic polarization rotation is evident through the monoclinic and orthorhombic phases under applied field, and is responsible for the negative electrocaloric effect.
In ferroelectricity, atomic-scale dipole moments interact collectively to produce strong electro-mechanical coupling and switchable macroscopic polarization. Hence, the functionality of ferroelectrics emerges at a solid-solid phase transformation that is accompanied by a sudden disappearance of an inversion symmetry. Much effort has been put to understand the ferroelectric transition at the polarization length scale. Nevertheless, the dipole-moment origin of ferroelectricity has remained elusive. Here, we used variable-temperature high-resolution transmission electron microscopy to reveal the …
Hybrid Barium Titanate Waveguide Designs For Efficient Nonlinear Frequency Conversion
Fraunhofer Institute for Applied Optics and Precision Engineering IOF（弗劳恩霍夫应用光学和精密工程研究所）；Institute of Applied Physics, Abbe Center of Photonics, Friedrich Schiller University Jena（耶拿弗里德里希·席勒大学阿贝光子学中心应用物理研究所）；Max Planck School of Photonics（马克斯·普朗克光子学校）
Typical ferroelectrics possess a large spontaneous polarization Ps but simultaneously a large remnant polarization Pr as well, resulting in an inferior energy storage density.A mechanism that can reduce the Pr while maintain the Ps is demanded to enhance the energy storage property of ferroelectrics.In the present study, it is shown that after acceptor doping and aging treatment, the domain switching in ferroelectrics becomes reversible, giving rise to a pinched double hysteresis loop. The pinched loop with a large Ps and a small Pr thus results in an enhanced energy storage density. The physi…
Optimizing ferroelectrics for contemporary high-frequency applications asks for the fundamental understanding of ferroelectric switching and domain wall (DW) motion in ultrafast field pulses while the microscopic understanding of the latter is so far incomplete. To close this gap in knowledge, ab initio-based molecular dynamics simulations are utilized to analyze the dynamics of 180$^\grad# DWs in the prototypical ferroelectric material BaTiO 3 . How ultrafast field application initially excites the dipoles in the system and how they relax to their steady state via transient negative capacitan…
Interface engineering in perovskite oxide superlattices has developed into a flourishing field, enabling not only further tuning of the exceptional properties, but also giving access to emergent physical phenomena. Here, we reveal a new mechanism for enhancing the electric polarization by the interface-induced oxygen octahedral tilts in BaTiO3/CaTiO3 superlattices. By combining a novel genetic algorithm with density functional theory (DFT), we predict that the true ground states in 1:1 and 2:2 BaTiO3/CaTiO3 superlattices grown on SrTiO3 adopt Pc symmetry with a large proper electric polarizati…
Understanding the crystal field splitting and orbital polarization in non-centrosymmetric systems such as ferroelectric materials is fundamentally important. In this study, taking BaTiO$_3$ (BTO) as a representative material we investigate titanium crystal field splitting and orbital polarization in non-centrosymmetric TiO$_6$ octahedra with resonant X-ray linear dichroism at Ti $L_{2,3}$-edge. The high-quality BaTiO$_3$ thin films were deposited on DyScO$_3$ (110) single crystal substrates in a layer-by-layer way by pulsed laser deposition. The reflection high-energy electron diffraction (RHE…
We propose energy band engineering to enhance tunneling electroresistance (TER) in ferroelectric tunnel junctions (FTJs). We predict that an ultrathin dielectric layer with a smaller band gap, embedded into a ferroelectric barrier layer, acts as a switch controlling high and low conductance states of an FTJ depending on polarization orientation. Using first-principles modeling based on density functional theory, we investigate this phenomenon for a prototypical SrRuO3/BaTiO3/SrRuO3 FTJ with a BaSnO3 monolayer embedded in the BaTiO3 barrier. We show that in such a composite-barrier FTJ, ferroel…
In elastically coupled multiferroic heterostructures that exhibit full domain correlations between ferroelectric and ferromagnetic sub-systems, magnetic domain walls are firmly pinned on top of ferroelectric domain boundaries. In this work we investigate the influence of pinned magnetic domain walls on the magnetization reversal process in a Co40Fe40B20 wedge film that is coupled to a ferroelectric BaTiO3 substrate via interface strain transfer. We show that the magnetic field direction can be used to select between two distinct magnetization reversal mechanisms, namely (1) double switching ev…
To enhance the efficiency of next-generation ferroelectric (FE) electronic devices, new techniques for controlling ferroelectric polarization switching are required. While most prior studies have attempted to induce polarization switching via the excitation of phonons, these experimental techniques required intricate and expensive terahertz sources and have not been completely successful. Here, we propose a new mechanism for rapidly and efficiently switching the FE polarization via laser-tuning of the underlying dynamical potential energy surface. Using time-dependent density functional calcul…
The control of magnetization via the application of an electric field, known as magnetoelectric coupling, is among the most fascinating and active research areas today. In addition to fundamental scientific interest, magnetoelectric effects may lead to new device concepts for data storage and processing. There are several known mechanisms for magnetoelectric coupling that include intrinsic effects in single-phase materials, strain induced coupling in two-phase composites, and electronically-driven effects at interfaces. Here we explore a different type of magnetoelectric effect at a ferromagne…
Above-bandgap femtosecond optical excitation of a ferroelectric/dielectric BaTiO3/CaTiO3 superlattice leads to structural responses that are a consequence of the screening of the strong electrostatic coupling between the component layers. Time-resolved x-ray free-electron laser diffraction shows that the structural response to optical excitation includes a net lattice expansion of the superlattice consistent with depolarization-field screening driven by the photoexcited charge carriers. The depolarization-field-screening-driven expansion is separate from a photoacoustic pulse launched from the…
Recent results in electric-field control of magnetism have paved the way for the design of alternative magnetic and spintronic devices with enhanced functionalities and low power consumption. Among the diversity of reported magnetoelectric effects, the possibility of switching on and off long-range ferromagnetic ordering close to room temperature stands out. Its binary character opens up the avenue for its implementation in magnetoelectric data storage devices. Here we show the possibility to locally switch on superferromagnetism in a wedge-shaped polycrystalline Fe thin film deposited on top …
