Ferroelectric Transition Research Articles

Antiferromagnetic spin canting and magnetoelectric multipoles in h−YMnO3

Hexagonal ${\mathrm{YMnO}}_{3}$ is a prototype antiferromagnet which exhibits multiferroic behavior with the ferroelectric and magnetic transitions occurring at different temperatures. We observe an out-of-plane canting of the ${\mathrm{Mn}}^{3+}$ magnetic moments using resonant x-ray diffraction (RXD) in a single crystal of this material. These canted moments result in the symmetry-forbidden (0,0,1) magnetic Bragg reflection, which is observed at the Mn ${L}_{2,3}$ absorption edges. We also observe an unexpected difference in the RXD spectral shapes at different temperatures. Using ab initio calculations, we explore the possibility that this behavior arises due to the interference between scattering from the canted magnetic moments and parity-odd atomic multipoles on the ${\mathrm{Mn}}^{3+}$ ions.

Prominent Size Effects without a Depolarization Field Observed in Ultrathin Ferroelectric Oxide Membranes.

The increasing miniaturization of electronics requires a better understanding of material properties at the nanoscale. Many studies have shown that there is a ferroelectric size limit in oxides, below which the ferroelectricity will be strongly suppressed due to the depolarization field, and whether such a limit still exists in the absence of the depolarization field remains unclear. Here, by applying uniaxial strain, we obtain pure in-plane polarized ferroelectricity in ultrathin SrTiO_{3} membranes, providing a clean system with high tunability to explore ferroelectric size effects especially the thickness-dependent ferroelectric instability with no depolarization field. Surprisingly, the domain size, ferroelectric transition temperature, and critical strain for room-temperature ferroelectricity all exhibit significant thickness dependence. These results indicate that the stability of ferroelectricity is suppressed (enhanced) by increasing the surface or bulk ratio (strain), which can be explained by considering the thickness-dependent dipole-dipole interactions within the transverse Ising model. Our study provides new insights into ferroelectric size effects and sheds light on the applications of ferroelectric thin films in nanoelectronics.

Ferroelectricity and superconductivity in strained EuxSr1−xTiO3 films

The superconducting transition of ${\mathrm{SrTiO}}_{3}$ can be influenced by tuning its ferroelectric transition, but the underlying reasons remain poorly understood. Here, we investigate compressively strained, Sm-doped films of ${\mathrm{Eu}}_{x}{\mathrm{Sr}}_{1\text{--}x}{\mathrm{TiO}}_{3}$ that were grown by molecular beam epitaxy to determine the effect of alloying with Eu on both superconductivity and ferroelectricity, both of which are present in strained ${\mathrm{SrTiO}}_{3}$ films. Remarkably, superconductivity survives up to $x=0.14$. Films at the lowest alloy concentration studied here, $x=0.09$, exhibit no suppression of their superconducting transition temperature, but there is a strong reduction of the upper critical field $({H}_{c2})$, compared to nonalloyed, strained ${\mathrm{SrTiO}}_{3}$ films. In addition, these films lack the sharp ferroelectric transition that appears in films without Eu in second harmonic generation measurements. We postulate that Eu alloying causes a crossover from a globally ordered ferroelectric state to one with only short-range polar order. We discuss the connection between the loss of global polar order and the change in the superconducting properties.

Impact of temperature on a ferroelectric interfaced negative capacitance double gate junctionless accumulation mode field effect transistor-compact model

Ferroelectric interfaced negative capacitance field effect transistors are gaining popularity for low power applications; however, as temperature is a constant influencing factor, further study is required to comprehend how these devices are influenced. Through a proposed compact model, this paper analytically investigates the influence of temperature on a ferroelectric interfaced negative capacitance double gate junctionless accumulation mode field effect transistor. This device integrates the benefits of negative capacitance with the junctionless accumulation mode structure. An extensive comparison of the proposed device is made with the existing structure to evaluate the benefits offered by the ferroelectric layer at different temperatures. The Landau–Khalatnikov equation and Pao–Sah integral are employed to obtain the surface potential and drain current model with temperature variation. Various key parameters of the device have been analysed extensively by varying the temperature from 200 to 500 K. It has been found that internal voltage amplification declines as temperature rises, but the sub-threshold swing increases from 46 to 72 mV decade −1 with an increase in temperature. Additionally, with a progressive rise in temperature, the loss of gain and degradation of gate capacitance are observed.

Phase Transition and Dynamics of Defects in the Molecular Piezoelectric TMCM-MnCl3 and the Effect of Partial Substitutions of Mn

We present dielectric and anelastic spectroscopy measurements of the molecular piezoelectric TMCM-MnCl3 and TMCM-Mn0.95M0.05Cl3 (M = Cu, Fe, Ni; TMCM = trimethylchlorometylammonium), whose powders were pressed into discs and bars and deposited as films on Si by Matrix-Assisted Pulsed Laser Evaporation (MAPLE). As in other molecular ferroelectrics, the dielectric permittivity ϵ′ drops at the structural transition temperature TC, below which the number of directions that the polar TMCM molecules visit is reduced, with the formation of ferroelectric domains. Concomitantly, the Young’s modulus E starts increasing and the elastic energy loss has a step-like increase, attributable to the motion of the domain walls. Both the dielectric and elastic anomalies indicate the improper character of the ferroelectric transition, where the ordering of the molecular orientations is not driven by the cooperative interaction of their electric dipoles. Below room temperature, at least two thermally activated relaxation processes appear both in the dielectric and anelastic spectra, whose real and imaginary parts measured at several frequencies can be fit with the Havriliak–Negami formula. The microscopic parameters so-obtained indicate that they are due to point defects, and it is argued that they are Cl vacancies and their complexes with TMCM vacancies. The considerable width of these relaxation maxima is explained by the geometry of the hexagonal perovskite structure. The partial substitution of Mn with 5% Ni has little effect on the anelastic and dielectric spectra, while Cu and, especially, Fe cause a large enhancement of the losses attributed to domain wall relaxation, with substantial contributions also above TC. The condensation of water from the humidity in the powders compacted by cold pressing was observed and discussed. The piezoelectric activity of the films was assessed by PFM.

Temperature‐Dependent Anharmonic Phonons in Quantum Paraelectric KTaO3 by First Principles and Machine‐Learned Force Fields

AbstractUnderstanding collective phenomena in quantum materials from first principles is a promising route toward engineering materials properties and designing new functionalities. This work examines the quantum paraelectric state, an elusive state of matter characterized by the smooth saturation of the ferroelectric instability at low temperature due to quantum fluctuations associated with anharmonic phonon effects. The temperature‐dependent evolution of the soft ferroelectric phonon mode in the quantum paraelectric KTaO3 in the range 0–300 K is modeled by combining density functional theory (DFT) calculations with the stochastic self‐consistent harmonic approximation assisted by an on‐the‐fly machine‐learned force field. The calculated data show that including anharmonic terms is essential to stabilize the spurious imaginary ferroelectric phonon predicted by DFT in the harmonic approximation, in agreement with experiments. Augmenting the DFT workflow with machine‐learned force fields allows for efficient stochastic sampling of the configuration space using large supercells in a wide temperature range, inaccessible to conventional ab initio protocols. This work proposes a robust computational workflow capable of accounting for collective behaviors involving different degrees of freedom and occurring at large time/length scales, paving the way for precise modeling and control of quantum effects in materials.

Ferroelastic and 90∘ ferroelectric domains in Bi2WO6 single crystals

High-quality single crystals of [Formula: see text] are grown using a flux method. With different flux growth recipes, we aim to control the crystallization temperature to be lower and higher than the ferroelectric transition temperature, resulting in mono-domain and multi-domain [Formula: see text] crystals, respectively. Abundant ferroelastic orthorhombic twin domains are observed in the multi-domain crystals under an optical microscope. PFM studies unveil the 90[Formula: see text] polarization change across those ferroelastic domain walls, as well as the absence of 180[Formula: see text] ferroelectric domains in the as-grown multi-domain crystals, indicating a high energy cost of 180[Formula: see text] ferroelectric domains. Moreover, a 45[Formula: see text] tilt of the 90[Formula: see text] ferroelectric domain walls is discovered, and this tilt creates a new type of charged 90[Formula: see text] ferroelectric walls, which have not been observed in other bulk ferroelectrics.

Lattice dynamics and phase transitions peculiarities of Ba[formula omitted]Sr[formula omitted]TiO[formula omitted] solid solutions with high Sr content

In this study, the lattice dynamics and phase transition peculiarities of BaxSr1−xTiO3 solid solution samples with x =1,0.5 and 0.3 are investigated by the Raman spectroscopy in a wide temperature range. The compositional and temperature dependences of soft mode frequencies were obtained. In contrast to ceramics with x =1 and 0.5, the soft E(TO) mode frequency in a solid solution with x =0.3 exhibits temperature dependence similar to the classical behavior of the soft mode for displacement-type transitions. From the analysis of the activation energy of titanium ion jumps, it was found that with increasing strontium concentration, the mechanism of these jumps changes from thermal over the potential barrier to tunneling under the barrier. This, in turn, leads to a significant increase in the lifetime of the rhombohedral distorted state of the crystal lattice and suppression of the ferroelectric transition by the Slater mechanism.

Electronic Origin of Laser-Induced Ferroelectricity in SrTiO3.

Although ultrafast control of the nonthermally driven ferroelectric transition of paraelectric SrTiO3 was achieved under laser excitation, the underlying mechanism and dynamics of the photoinduced phase transition remain ambiguous. Here, the determinant formation mechanism of ultrafast ferroelectricity in SrTiO3 is traced by nonadiabatic dynamics simulations. That is, the selective excitation of multiple phonons, induced by photoexcited electrons through the strong correlation between electronic excitation and lattice distortion, results in the breaking of the crystal central symmetry and the onset of ferroelectricity. The accompanying population transition between 3dz2 and 3dx2-y2 orbitals excites multiple phonon branches, including the two high-energy longitudinal optical modes, so as to drive the titanium ion away from the center of the oxygen octahedron and generate a metastable ferroelectric phase. Our findings reveal a cooperative electronic and ionic driving mechanism for the laser-induced ferroelectricity that provides new schemes for the optical control of ultrafast quantum states.

Coupled ferroelectricity and superconductivity in bilayer Td-MoTe2.

Achieving electrostatic control of quantum phases is at the frontier of condensed matter research. Recent investigations have revealed superconductivity tunable by electrostatic doping in twisted graphene heterostructures and in two-dimensional semimetals such as WTe2 (refs. 1-5). Some of these systems have a polar crystal structure that gives rise to ferroelectricity, in which the interlayer polarization exhibits bistability driven by external electric fields6-8. Here we show that bilayer Td-MoTe2 simultaneously exhibits ferroelectric switching and superconductivity. Notably, a field-driven, first-order superconductor-to-normal transition is observed at its ferroelectric transition. Bilayer Td-MoTe2 also has a maximum in its superconducting transition temperature (Tc) as a function of carrier density and temperature, allowing independent control of the superconducting state as a function of both doping and polarization. We find that the maximum Tc is concomitant with compensated electron and hole carrier densities and vanishes when one of the Fermi pockets disappears with doping. We argue that this unusual polarization-sensitive two-dimensional superconductor is driven by an interband pairing interaction associated with nearly nested electron and hole Fermi pockets.

Can the ferroelectric soft mode trigger an antiferromagnetic phase transition?

Type-II multiferroics, where spin interactions induce a ferroelectric polarization, are interesting for new device functionalities due to large magnetoelectric coupling. We report on a new type of multiferroicity in the quadruple-perovskite BiMn3Cr4O12, where an antiferromagnetic phase is induced by the structural change at the ferroelectric phase transition. The displacive nature of the ferroelectric phase transition at 125 K, with a crossover to an order-disorder mechanism, is evidenced by a polar soft phonon in the THz range and a central mode. Dielectric and pyroelectric studies show that the ferroelectric critical temperature corresponds to the previously reported Néel temperature of the Cr3+ spins. An increase in ferroelectric polarization is observed below 48 K, coinciding with the Néel temperature of the Mn3+ spins. This increase in polarization is attributed to an enhanced magnetoelectric coupling, as no change in the crystal symmetry below 48 K is detected from infrared and Raman spectra.

Effect of Sm and Fe co-doping on structural, dielectric and magnetic properties of YCrO3 perovskite

In this report, impact of co-doping of samarium (Sm3+) and iron (Fe3+) on structural, dielectric and magnetic properties of YCrO3 compound has been investigated. Here, YCrO3 and Y1-xSmxCr1-yFeyO3 (x = 0.3, y = 0.1) compounds are synthesized successfully through sol–gel auto combustion route. The synthesized samples are characterized through X-ray diffraction (XRD), surface electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDX), impedance analyzer and vibrating sample magnetometer (VSM) techniques. Rietveld refinement of XRD analysis reveals that both the compounds are crystallized in distorted orthorhombic structure belongs to Pnma space group. It is observed that with co-doping of both Sm3+ and Fe3+, lattice unit cell volume increases. SEM micrograph shows that the grains are slightly elongated as compared to the undoped sample. The temperature variation of real part of dielectric permittivity (εr) and tangent loss (tan δ) are analyzed over a frequency range 40 KHz − 500 KHz at a temperature range 300 K − 523 K. The temperature dependent dielectric permittivity reveals that both εr and tan δ decrease with co-doping. Moreover, both the compounds show relaxor like ferroelectric transition. Room temperature field dependent magnetization analysis shows that maximum magnetization value increases with co-doping. The result reveals that the enhanced properties with co-doping can be useful for practical device applications.

Anisotropic Rashba coupling to polar modes in KTaO3

Motivated by the discovery of superconductivity in KTaO3-based heterostructures, we study a pairing mechanism based on spin-orbit assisted coupling between the conduction electrons and the ferroelectric (FE) modes present in the material. We use ab initio frozen-phonon computations to show a linear-in-momentum Rashba-like coupling with a strong angular dependence in momentum for the lower manifold, deviating from the conventional isotropic Rashba model. This implies the Rashba-like interaction with the polar modes has substantial L = 3 cubic harmonic corrections, which we quantify for each electronic band. The strong anisotropy of the Rashba interaction is captured by a microscopic toy model for the electrons. We find its origin to be the angular dependence in electronic momentum imposed by the kinetic term on the degenerate manifold. A comparison between the toy model and ab initio results indicates that additional symmetry allowed terms beyond odd-parity spin-conserving inter-orbital hopping processes are needed to describe the Rashba-like polar interaction between the electrons and the soft FE mode.

Thermophysical properties and phase transitions of ferroelectric PbFe-=SUB=-0.5-=/SUB=-Nb-=SUB=-0.5-=/SUB=-O-=SUB=-3-=/SUB=- at high temperatures

The thermophysical properties of the ferroelectric PbFe0.5Nb0.5O3 in the temperature range 300-800 K are studied. Anomalies in the heat capacity and thermal diffusion are found in the region of the diffuse ferroelectric transition Tc~380 K, the Burns temperature Td~670 K, and the intermediate temperature T^*~470 K. The dominant phonon heat transfer mechanisms in a ferroelectric with a nanopolar structure. Keywords: heat capacity, thermal diffusion, thermal conductivity, ferroelectric, phase transitions, relaxor.

Теплофизические свойства и фазовые переходы сегнетоэлектрика PbFe-=SUB=-0.5-=/SUB=-Nb-=SUB=-0.5-=/SUB=-O-=SUB=-3-=/SUB=- в области высоких температур

The thermophysical properties of the ferroelectric PbFe0.5Nb0.5O3 in the temperature range 300–800 K are studied. Anomalies in the heat capacity and thermal diffusion are found in the region of the diffuse ferroelectric transition Tc~380 K, the Burns temperature Td~670 K, and the intermediate temperature T^*~470 K. The dominant phonon heat transfer mechanisms in a ferroelectric with a nanopolar structure.

Electric-field-induced non-ergodic relaxor to ferroelectric transition in BiFeO3-xSrTiO3 ceramics.

While BiFeO3-based solid solutions show great promise for applications in energy conversion and storage, realizing this promise necessitates understanding the structure-property relationship in particular pertaining to the relaxor-like characteristics often exhibited by solid solutions with polar-to-non-polar morphotropic phase boundaries. To this end, we investigated the role of the compositionally-driven relaxor state in (100 - x)BiFeO3-xSrTiO3 [BFO-xSTO], via in situ synchrotron X-ray diffraction under bipolar electric-field cycling. The electric-field induced changes to the crystal structure, phase fraction and domain textures were monitored via the {111}pc, {200}pc, and 1/2{311}pc Bragg peaks. The dynamics of the intensities and positions of the (111) and (111̄) reflections reveal an initial non-ergodic regime followed by long-range ferroelectric ordering after extended poling cycles. The increased degree of random multi-site occupation in BFO-42STO compared to BFO-35STO is correlated with an increase of the critical electric field needed to induce the non-ergodic-to-ferroelectric transition, and a decrease in the degree of domain reorientation. Although both compositions show an irreversible transition to a long-range ferroelectric state, our results suggest that the weaker ferroelectric response in BFO-42STO is related to an increase in ergodicity. This, in turn, serves to guide the development of BFO-based systems into promising platform for further property engineering towards specific capacitor applications.

Effect of magnetic phase coexistence on spin-phonon coupling and magnetoelectric effect in polycrystalline Sm0.5Y0.5Fe0.58Mn0.42O3

The various physical properties with their intercorrelations in polycrystalline samples of Sm0.5Y0.5Fe0.58Mn0.42O3 [SYFM (58–42)] were investigated. The dc magnetization measurements revealed a weak ferromagnetic (WFM) transition at TÑ361 K that is followed by an incomplete spin reorientation (SR) transition at TSR1∼ 348 K. A first order magnetic transition (FOMT) around TSR2 ∼ 292 K completes the spin reorientation transition and the material enters into a nearly collinear antiferromagnetic (AFM) state. Robust magnetodielectric (MD) is found to be present in material at room temperature. True ferroelectric transition with TFE = 108 K and having a value of saturation polarization (∼0.06 μC/cm2 at 15 K) have been found in the specimen. This spin phonon coupling (SPC) stabilizes the ferroelectric state and responsible for magnetoelectric coupling (ME). We argue that wave vector (q) dependence of the spin-pair correlation affects the electronic and magnetodielectric properties in a similar way.

Giant flexoelectric coefficients at critical ferroelectric transition

Flexoelectric materials, which could generate polarization in response to a strain gradient, could be utilized in a wide range of devices such as actuators and sensors. However, the flexoelectric coefficients of most dielectrics are too small to be utilized. One of the most promising flexoelectric materials is ferroelectrics whose flexoelectric coefficients are orders of magnitude higher than those of ordinary dielectrics. Despite of intensive research in the past twenty years, the record-high flexoelectric coefficient (μ1122=100μC/m) of Ba0.67Sr0.33TiO3 ferroelectric has not been surpassed. Here we show that by tuning the first-order paraelectric-ferroelectric phase transition of BaTiO3 to near-critical through doping BaZrO3, a drastic enhancement of flexoelectric coefficient (μ1122=140μC/m) occurs, which is ascribed to the enhancement of both intrinsic and extrinsic flexoelectricity resulting from the strong lattice instability at the critical transition. This study demonstrates a general design method to achieve high flexoelectric coefficients in a wide range of ferroelectric systems through searching for critical ferroelectric transition points.

Hexagonal RFeO3 (R = Dy, Er, and Lu) Films Grown on Glass Substrates with Both Magnetic and Ferroelectric Orders

Hexagonal rare-earth ferrites (h-RFeO3) are promising multiferroic materials owing to the spontaneous magnetization, high ferroelectric transition temperature, and fascinating features provided by strong magnetoelectric coupling. However, h-RFeO3 is metastable, and its synthetic methods are limited, particularly when ionic size of R is large. One of useful synthesis methods is epitaxial thin-film growth on single-crystal substrates, because the metastable phase is stabilized by the lattice match between h-RFeO3 and the substrate. Compared to single-crystal substrates, glass substrates are more cost-effective. However, films on glass tend to consist of a thermodynamically stable phase owing to the amorphous nature of glass substrates. Herein, we report that a h-RFeO3 pure phase can be obtained as thin film even on glass substrates. The metastable phase was formed owing to the nonequilibrium synthesis process of pulsed laser deposition. The obtained h-RFeO3 thin films on glass with R = Dy, Er, and Lu exhibited weak ferromagnetism with Néel temperatures of 120–140 K. Furthermore, a clear magnetic phase transition from weak ferromagnetic to antiferromagnetic phase was observed in the h-ErFeO3 films. Additionally, piezoelectric force microscopy measurements exhibited the ferroelectricity of the film at room temperature. These results provide a convenient synthesis method of a pure phase multiferroic h-RFeO3 system.

Terahertz time-domain spectroscopic ellipsometry of metallic strontium titanate

The coexistence of the polar instability and metallic conduction has attracted much attention. Ferroelectric soft optic modes and carrier electrons were investigated in A- and B-site heterovalent doped strontium titanate SrTiO3 crystals by the terahertz time-domain spectroscopic ellipsometry. The observed real and imaginary parts of a complex dielectric constant were fitted by the damped harmonic oscillator model for the ferroelectric soft optic mode and the Drude-Smith model for carrier electrons. The observed soft mode frequency increases as the La and Nb content increases. It is found that the square of the ferroelectric soft mode frequency linearly changes to the carrier electron concentration. This fact indicates that the carrier electrons suppress the ferroelectric instability and the ferroelectric soft mode frequency increases as the carrier concentration increases.