Thickness‐dependent domain wall reorientation in 70/30 lead magnesium niobate‐ lead titanate thin films

Continued reduction in length scales associated with many ferroelectric film‐based technologies is contingent on retaining the functional properties as the film thickness is reduced. Epitaxial and polycrystalline lead magnesium niobate‐lead titanate (70PMN‐30PT) thin films were studied over the thickness range of 100‐350 nm for the relative contributions to property thickness dependence from interfacial and grain‐boundary low permittivity layers. Epitaxial PMN‐PT films were grown on SrRuO₃/(001)SrTiO₃, while polycrystalline films with {001}‐Lotgering factors >0.96 were grown on Pt/TiO₂/SiO₂/Si substrates via chemical solution deposition. Both film types exhibited similar relative permittivities of ~300 at high fields at all measured thicknesses with highly crystalline electrode/dielectric interfaces. These results, with the DC‐biased and temperature‐dependent dielectric characterization, suggest irreversible domain wall mobility is the major contributor to the overall dielectric response and its thickness dependence. In epitaxial films, the irreversible Rayleigh coefficients reduced 85% upon decreasing thickness from 350 to 100 nm. The temperature at which a peak in the relative permittivity is observed was the only measured small signal quantity which was more thickness‐dependent in polycrystalline than epitaxial films. This is attributed to the relaxor nature present in the films, potentially stabilized by defect concentrations, and/or chemical inhomogeneity. Finally, the effective interfacial layers are found to contribute to the measured thickness dependence in the longitudinal piezoelectric coefficient.     

Direct visualization of magnetic‐field‐induced magnetoelectric switching in multiferroic aurivillius phase thin films

Multiferroic materials displaying coupled ferroelectric and ferromagnetic order parameters could provide a means for data storage whereby bits could be written electrically and read magnetically, or vice versa. Thin films of Aurivillius phase Bi₆Ti_2.8Fe_1.52Mn_0.68O₁₈, previously prepared by a chemical solution deposition (CSD) technique, are multiferroics demonstrating magnetoelectric coupling at room temperature. Here, we demonstrate the growth of a similar composition, Bi₆Ti_2.99Fe_1.46Mn_0.55O₁₈, via the liquid injection chemical vapor deposition technique. High‐resolution magnetic measurements reveal a considerably higher in‐plane ferromagnetic signature than CSD grown films (M_S=24.25 emu/g (215 emu/cm³), M_R=9.916 emu/g (81.5 emu/cm³), H_C=170 Oe). A statistical analysis of the results from a thorough microstructural examination of the samples, allows us to conclude that the ferromagnetic signature can be attributed to the Aurivillius phase, with a confidence level of 99.95%. In addition, we report the direct piezoresponse force microscopy visualization of ferroelectric switching while going through a full in‐plane magnetic field cycle, where increased volumes (8.6% to 14% compared with 4% to 7% for the CSD‐grown films) of the film engage in magnetoelectric coupling and demonstrate both irreversible and reversible magnetoelectric domain switching.     

Submicron crystalline buildup and size‐dependent energy harvesting characteristic in PZN–PZT ternary ferroelectrics

Piezoelectric energy harvester converts low‐frequency vibrational energy in the environment into electrical energy, enabling the purpose of self‐supplying power for low‐energy consumption devices. The key to miniaturizing energy harvester is the buildup of the submicron‐grained ceramic with a high transduction coefficient (d×g), which is still a big challenge from a technical point of view. In this work, the popular ternary system of Pb(Zn_1/3Nb_2/3)O₃–Pb(Zr_0.5Ti_0.5)O₃ (PZN–PZT) has been selected as objective compound, and the submicron‐grained ceramics were prepared by a combination of high‐energy ball milling and pressureless sintering technology. The results revealed that nanocrystalline PZN–PZT powders can be synthesized by one step mechanochemical route without the calcination stage. Using these nanopowders as precursors, dense ceramics with different grain size have been prepared through tailoring the sintering temperature. The study of size‐dependent energy harvesting characteristic evidenced an optimum transduction coefficient of 7980×10^?15 m²/N was obtained for 950°C sintered specimen, which has uniform microstructure with mean grain size of 0.33 μm. In the mode of the cantilever‐type energy harvester constructed by this material, the output power at low frequency of 89 Hz was as high as 69 μW at an acceleration of 10 m/s², showing the suitability for piezoelectric generators harvesting environmental vibrational energy.     

Composition‐ and Pressure‐Induced Relaxor Ferroelectrics: First‐Principles Calculations and Landau‐Devonshire Theory

We report calculations with first‐principles density‐functional theory and Landau–Devonshire theory that provide an atomic‐scale mechanism for the composition‐ and pressure‐induced relaxor ferroelectrics. A multiphase with coexisted cubic structures (MPCCS) is found to correspond to any of the composition‐ and pressure‐induced relaxor ferroelectrics. On the other hand, a normal ferroelectric without relaxor behavior is structurally characterized by a single phase. Furthermore, the presence of the MPCCS in a composition‐ and pressure‐induced relaxor increases the degrees of freedom of relaxors and no energy barriers are involved for the rotations of the polarization direction, leading to high electromechanical coefficients.     

Ergodicity of fine-grained canonical relaxor ferroelectric (Bi0.5Na0.5)1-xBaxTiO3 films

The drastic reduction in dimensions in thin films, together with the low crystallization temperatures used, normally results in a large reduction in the grain size. It has been reported that relaxor ferroelectric states are stabilized at room temperature for fine-grained ceramics and films that behave as normal ferroelectrics for large grains. In this work, the effects of the grain size reduction on the relaxor characteristics are analyzed for a composition that is already a canonical relaxor with a nonergodic state at room temperature: (Bi_0.5Na_0.5)_1-xBa_xTiO₃ (BNBT). The comparison of the local polar ordering within BNBT grains studied with piezoresponse force microscopy on large-grained ceramics and fine-grained thin films shows that the development of stable long-range ferroelectric order with the application of an electric field is hampered due to the small grain size of the grains. The ergodic character of the high-temperature phase is thus stabilized at room temperature, following a similar mechanism as the one discussed for other noncanonical relaxors.     

Preparation and Characterization of Sol—Gel-Derived Lead Magnesium Niobium Titanate Thin Films with Pure Perovskite Phase and Lead Oxide Cover Coat

A sol–gel-derived Pb(Mg_1/3Nb_2/3)_0.7Ti_0.3O₃ (PMNT) thin film was prepared using spin coating and a PbO cover coat technique. The amount of lead excess in the precursor solution had significant effects on the phase development and microstructure of the PMNT film. The PbO cover coat proved to be effective on suppressing the formation of pyrochlore phases. PMNT thin films with a pure perovskite structure were obtained by adding 30 mol% excess lead in the precursor solution and coating the PbO layer on the top of the film. The remnant polarization ( P _r), the dielectric constant (ɛ_r), and the dissipation factor (tan δ) of these thin films, which had a thickness of 150 nm, were determined to be 9 μC/cm², 1370, and 0.031, respectively.     

Large Piezoelectricity and Ferroelectricity in Mn‐Doped (Bi0.5Na0.5)TiO3‐BaTiO3 Thin Film Prepared by Pulsed Laser Deposition

Mn‐doped (Bi_0.5Na_0.5)_0.94Ba_0.06TiO₃ (MnBNBT) thin films were prepared on SrRuO₃ (SRO)‐coated (001) SrTiO₃ (STO) single crystal substrates by pulsed laser deposition under different processing conditions. Structural characterization (i.e., XRD and TEM) confirms the epitaxial growth of STO/SRO/MnBNBT heterostructures. Through the judicious control of deposition temperature, the defect level within the films can be finely tuned. The MnBNBT thin film deposited at the optimized temperature exhibits superior ferroelectric and piezoelectric responses with remanent polarization P_r of 33.0 μC/cm² and piezoelectric coefficient d₃₃ of 120.0 ± 20 pm/V.     

In-Plane Polarized 0.7Pb(Mg1/3Nb2/3)O3–0.3PbTiO3 Thin Films

Ferroelectric 0.7Pb(Mg_1/3Nb_2/3)O₃–0.3PbTiO₃ (PMN-PT) thin films were deposited on ZrO₂/SiO₂/silicon substrates using a chemical-solution-deposition method. Using a thin PZT film as a seed layer for the PMN-PT films, phase-pure perovskite PMN-PT could be obtained via rapid thermal annealing at 750°C for 60 s. The electrical properties of in-plane polarized thin films were characterized using interdigitated electrode arrays on the film surface. Ferroelectric hysteresis loops are observed with much larger remanent polarizations (∼24 μC/cm²) than for through-the-thickness polarized PMN-PT thin films (10–12 μC/cm²) deposited on Pt/Ti/Si substrates. For a finger spacing of 20 μm, the piezoelectric voltage sensitivity of in–plane polarized PMN-PT thin films was ∼20 times higher than that of through-the-thickness polarized PMN-PT thin films.     

TEM Characterization of Single- and Multilayer Triol-Based Sol–Gel PZT (53/47) Thin Films

Integrated lead zirconate titanate thin films deposited on Pt/Ti/SiO₂/Si substrates using a novel triol-based route were characterized using X-ray diffraction and transmission electron microscopy. Crack-free single-layer PZT films of up to 200 nm thick were prepared by triol-based sol–gel processing onto Pt/Ti/SiO₂/Si substrates. Films ∼75 nm thick exhibited a microstructure free of pores and second phase. As film thickness increased, film texture changed from {100} to {111} perovskite. Essentially, single-phase multilayer films could be prepared by the deposition and pyrolysis of several 75 nm layers, followed by a single crystallization step. The influence of heat-treatment schedule on the microstructure and orientation of the multilayer films is discussed. Comparison has been made between multilayer films prepared using the triol-based sol and an inverted mixing order/acetic acid-based sol.     

Nanocrystalline buildup,relaxor behavior,and polarization characteristic in PZT–PNZN quaternary ferroelectrics

The Pb(Zr_0.47Ti_0.53)O₃–Pb[(Ni_0.6Zn_0.4)_1/3Nb_2/3]O₃ (PZT–PNZN) quaternary ferroelectrics presenting the high piezoelectric coefficients are the active element in a range of piezoelectric devices. However, the buildup of nanoceramics with stable perovskite structure is still a big challenge, which prohibits the miniaturization of piezoelectric devices. In this work, the high dense PZT–PNZN nanoceramics with average grain size of 130 nm were prepared by a combination of high‐energy ball milling (HEBM) and spark plasma sintering (SPS). The domain configuration and relaxor behavior in this nanoceramic are reported for the first time. Although the noncubic crystal structure and dielectric anomaly suggest a ferroelectric behavior, only very slim polarization–electric (P–E) hysteresis loops are observed. The suppression of macroscopic ferroelectric polarization mainly originates from the decrease in crystal tetragonality and the enhanced clamping effect on domain wall motion. The piezoelectric properties, d₃₃=65 pC/N and k_p=17%, raise the possibility of PZT–PNZN nanoceramics for application in multilayer piezoelectric devices.     

Greatly enhanced photocurrent in inorganic perovskite [KNbO3]0.9[BaNi0.5Nb0.5O3‐σ]0.1 ferroelectric thin‐film solar cell

Inorganic perovskite [KNbO₃]_0.9[BaNi_0.5Nb_0.5O_3‐σ]_0.1 (KBNNO) ferroelectric thin films with narrow band gap (1.83 eV) and high room‐temperature remnant polarization (Pr = 0.54 μC/cm²) was grown successfully on the Pt(111)/Ti/SiO₂/Si(100) substrates by pulsed laser deposition. Ferroelectric solar cells with a basic structure of ITO/KBNNO/Pt were further prepared based on these thin films, which exhibited obvious external‐poling dependent photovoltaic effects. When the devices were negatively poled, the short‐circuit current and open‐circuit voltage were both significantly higher than those of the devices poled positively. This is attributed to enhanced charge separation under the depolarization field induced by the negative poling, which is superimposed with the built‐in field induced by the Schottky barriers at the interfaces between KBNNO and the two electrodes. When a poling voltage of ‐1 V was applied, the device showed a short‐circuit current as high as 27.3 μA/cm², which was by two orders of magnitude larger than that of the KBNNO thick‐film (20 μm) devices reported previously. This work may inspire further exploration for lead‐free inorganic perovskite ferroelectric photovoltaic devices.     

Thermal stability and electric‐field‐induced strain behaviors for PIN‐PSN‐PT piezoelectric ceramics

Pb (In_1/2Nb_1/2) O₃‐Pb (Sc_1/2Nb_1/2) O₃‐PbTiO₃ (PIN‐PSN‐PT) ternary ceramics with compositions near morphotropic phase boundary (MPB) were fabricated by solid‐state‐sintering process. Dielectric and piezoelectric properties of xPIN‐yPSN‐zPT (x = 0.19, 0.23 and z = 0.365, 0.385) ceramics were investigated as a function of temperature, showing high T_r‐t and T_c on the order of 160 ~ 200°C and 280 ~ 290°C, respectively. The xPIN‐yPSN‐0.365PT (x = 0.19 and 0.23) ceramics do not depolarize at the temperature up to 200°C, showing a better thermal stability when compared to the state‐of‐the‐art relaxor‐PbTiO₃ systems. A slight variation (<9%) of k_p, k_t, and k₃₃ was observed in the temperature range of 25°C‐160°C for xPIN‐yPSN‐0.385PT (x = 0.19 and 0.23) ceramics. Rayleigh analysis was employed to quantify the contribution of domain wall motion to piezoelectric response, where the domain wall contribution was found to increase with composition approaching MPB for PIN‐PSN‐PT system.     

Low Temperature Ionic Conductivity of an Acceptor‐Doped Perovskite: II. Impedance of Single‐Crystal BaTiO3

Low temperature conductivity mechanisms were identified in acceptor‐doped BaTiO₃ single crystals equilibrated and quenched from high temperature under different oxygen partial pressures. A range of acceptor ionization states were quenched into samples doped with manganese or iron. Using an appropriate equivalent circuit to interpret impedance spectroscopy data, room temperature conductivity mechanisms in the single crystal samples were identified, and the permittivity/temperature dependence was also shown to be self‐consistent with the nature of a first‐order ferroelectric phase transition. The primary, low temperature, conduction mechanism in acceptor‐doped BaTiO₃ was determined to be dominated by the migration of oxygen vacancies. The activation energy for oxygen vacancy migration was experimentally determined to have a value of nearly 0.7 eV. This activation energy represents an intrinsic value for vacancy hopping and confirms our previous work that revealed minimal interaction between acceptor dopants and oxygen vacancies in BaTiO₃ in contrast to the well‐documented evidence of defect association in SrTiO₃.     

Microstructure and electric properties of (Na0.85K0.15)0.5Bi0.5TiO3 composited films with alternative TiO2 layers

In this work, in order to investigate the effect of TiO₂ layer on the microstructure and piezoelectric properties of (Na_0.85K_0.15)_0.5Bi_0.5TiO₃ (NKBT) thin films, TiO₂ layer was inserted at the interface between the NKBT thin film and substrate and on both sides of the NKBT, i.e., at the interface and on the top of the NKBT thin film. NKBT composited films with alternative TiO₂ layer were deposited on Pt/Ti/SiO₂/Si substrate by aqueous sol‐gel method. X‐ray diffraction observation found that the degree of (100) preferred orientation strengthened with TiO₂ layers added, especially on both sides of NKBT thin film. The TiO₂/NKBT/TiO₂ composited film with both TiO₂ layer of 40 nm thickness exhibited a remnant polarization value P_r of 22.6 μc/cm³ and effective piezoelectric coefficient of approximate 77 pm/V, which are much larger than that of the single‐layered NKBT thin film with P_r value of 13.7 μc/cm³ and of 56 pm/V, respectively. According to the investigation of the temperature‐dependent ferroelectric property, it was found that the P_r gradually increased, and in the meantime the coercive voltage gradually moved to higher voltage with testing temperature varied from 20 to ?150°C. Besides, applied voltage dependence of leakage current density measurement indicated that the TiO₂ layer would effectively lower the leakage current of the films, and the TiO₂/NKBT/TiO₂ composited film both TiO₂ layer of 40 nm exhibited the lowest leakage current.     

Structure‐property relationships in BiScO3–PbTiO3–PbMg1/3Nb2/3O3 ceramics near the morphotropic phase boundary

The phase structure, dielectric, ferroelectric, and piezoelectric properties of (1?2x)BiScO₃–xPbTiO₃–xPbMg_1/3Nb_2/3O₃ ceramics (x = 0.30‐0.46) were studied. It was found that an increase in x leads to a structural phase transition between the rhombohedral and tetragonal phase via an intermediate monoclinic phase and to a crossover from the nonergodic relaxor state to the ferroelectric one. It was proposed that at x > 0.42 the phase transition changes from second to first order. The assumption about the existence of a tricritical point on the phase diagram at x ≈ 0.42 with the enhanced dielectric response has been made. The observed structure‐property relationships of the studied solid solutions are discussed. It is shown that the solid solutions with x = 0.42 are characterized by the high piezoelectric parameters (d₃₃ = 509 pC/N, d₃₁ = ?178 pC/N, d_h = 153 pC/N), which makes possible their applications in sonar equipment.     

Ferroic ordering and charge‐spin‐lattice order coupling in Gd‐doped Fe3O4 nanoparticles relaxor multiferroic system

We investigated the effect of gadolinium doping (1‐5 at.%) on the magnetic and dielectric properties of Fe₃O₄ nanoparticles, synthesized by the chemical co‐precipitation technique, primarily to understand the onset of multifunctional properties such as ferroelectricity and magnetodielectric coupling. The substitution of larger Gd³⁺ ions at smaller Fe³⁺ octahedral sites in inverse spinel Fe₃O₄ has significantly influenced the morphology, average crystallite size, and more importantly, the magneto‐crystalline anisotropy and saturation magnetization. The magneto‐crystalline anisotropy and the saturation magnetization decreases substantially, however, significant increase in the average crystallite size is observed upon Gd doping. Furthermore, temperature‐dependent dielectric studies suggest that these nanoparticle systems exhibit relaxor ferroelectric behavior, with much pronounced ferroelectric polarization moment recorded for 5 at.% Gd doped Fe₃O₄ as compared to its undoped counterpart.     

High energy storage efficiency and thermal stability of A-site-deficient and 110-textured BaTiO3–BiScO3 thin films

The development of thin film dielectrics having both high energy density and energy conversion efficiency, as well as good thermal stability, is necessary for practical application in high-temperature power electronics. In addition, there is a demand for the development of new Pb-free high-energy density dielectric materials due to environmental concerns. In this regard, thin films of weakly coupled relaxors based on solid solutions of BaTiO₃–BiMeO₃ have shown good promise, because they exhibit a remarkably large polarization over a wide temperature range. Nevertheless, the performance of Pb-free thin films has lagged behind that of their Pb-based counterparts in terms of thermal stability and energy conversion efficiency. Toward this end, most recent studies on BaTiO₃–BiMeO₃ systems have focused on the optimization of material composition, while relatively less attention has been paid to other aspects such as defect chemistry and crystallographic texture. In this study, we examine the effects of A-site vacancy and crystallographic texture on the energy storage performance of BaTiO₃–BiScO₃ thin films synthesized using pulsed laser deposition (PLD). It is shown that a high energy storage density (W_r) of ~28.8 J/cm³ and a high efficiency of η >90% are achieved through a combination of moderate A-site vacancy concentration and (110) crystallographic texture. Furthermore, W_r remains nearly temperature independent while a high efficiency of η >80% is maintained for temperatures up to 200°C, which constitutes one of the best performances for Pb-free ferroelectric films for high-temperature capacitor applications.     

Electric‐field induced phase transition and fatigue behaviors of (Bi0.5+x/2Na0.5‐x/2)0.94Ba0.06Ti1‐xFexO3 ferroelectrics

The dielectric, piezoelectric properties, and fatigue behaviors of stoichiometric (Bi_0.5+x/2Na_0.5‐x/2)_0.94Ba_0.06Ti_1‐xFe_xO₃ (BNBT‐xFe) ferroelectrics are investigated. Fe substitution leads to the downward shift of the ferroelectric‐relaxor transition temperature (T_F‐R) and increase in strain. Meanwhile, fatigue behaviors of the modified ceramics are significantly enhanced. Ex situ X‐ray diffraction and transmission electron microscopy reveal microscopic mechanism for polarization fatigue on different compositions. The fatigue‐free behavior of ferroelectric BNBT‐0.03Fe is not only attributed to a mechanism involving the formation of defect dipoles, which reduces the pinning effect of migratory oxygen vacancies on domain walls, but is also connected to the decrease in easily suppressed field‐induced ferroelectric tetragonal phase. While for ergodic relaxor BNBT‐0.09Fe, the absence of domain wall contributes to the good fatigue resistance behavior. Interestingly, electric cycling results in an increased fraction of relaxor phase, accompanying by the increase in the total strain and decrease in remnant polarizations.     

Structural and Magnetoelectric Behavior of Lanthanum Manganite‐Based Multiferroic Heterostructures

The effect of fabrication method on the structure of (100 ? x) wt% BaTiO₃ + x wt% La_0.7Ba_0.3MnO₃ (BT + BLM) and (100 ? x) wt% Na, Bi, Sr‐doped PZT + x wt% La_0.65Pb_0.35MnO₃ (PZTNB‐1 + PLM) magnetoelectric ceramics was studied. Profound interdiffusion of two interacting phases occurs in nearly all cases. The BT + BLM and PZTNB‐1 + PLM ceramics exhibit low piezoelectric parameters even with small manganite contents (10–20 wt%). The increased content of the magnetostrictive phase complicates the polarization process due to the high conductivity of La_0.7Ba_0.3MnO₃ and La_0.65Pb_0.35MnO₃. Doping of BaTiO₃ and PZTNB‐1 with small additions of manganite components affects piezoelectric properties, thereby lowering efficiency of the resulting material.     

Observation of oxygen pyramid tilting induced polarization rotation in strained BiFeO3 thin film

Oxygen octahedral tilting has been recognized to strongly interact with spin, charge, orbital, and lattice degrees of freedom in perovskite oxides. Here, we observe a strain-driven stripe-like morphology of two supertetragonal (monoclinic Cc and Cm ) phases in the strained BiFeO₃/LaAlO₃ thin films. The two supertetragonal phases have a similar giant axial ratio but differences in oxygen pyramid tilting mode. Especially, the competition between polar instability and oxygen pyramid tilting is identified using atomically resolved scanning transmission electron microscopy, leading to the polarization rotation across the phase boundary. In addition, microtwins are observed in the Cc phase. Our findings provide new insights of the coupling between ferroelectric polarization and oxygen pyramid tilting in oxide thin films and will help to design novel phase morphology with desirable ferroelectric polarization and properties for new applications in perovskite oxides.