Room-temperature multiferroicity in NiFe2O4 and its magnetoelectric coupling intensified through defect engineering

The well-known ferromagnetic oxide, NiFe₂O₄, was studied as a potential candidate for room-temperature Type II magnetoelectrics. A spin canting as one of the essential requirements for Type II multiferroics was induced by breaking the stoichiometry, that is, intentionally subtracting Fe ions. We observed that Fe ions were first subtracted exclusively from the tetrahedral sites, leading to an increase in the magnetoelectric coupling owing to an increasing degree of spin canting. The enhancement in the magnetoelectric coupling culminated beyond the subtraction level of ~30 at.%, at which Fe ions started to be removed from the octahedral sites. Alongside, we observed that the subtraction of Fe ions gives rise to a ferroelectricity due to the formation of defect complexes that establish an internal bias field.     

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.     

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.     

Structural origin of room temperature poling enhanced piezoelectricity in modified Pb(Mg1/3Nb2/3)O3‐30%PbTiO3 crystals

The piezoelectric properties of [110]‐oriented Mn‐doped Pb(Mg_1/3Nb_2/3)O₃‐30%PbTiO₃ single crystals was found to be enhanced by poling at room temperature, relative to traditional poling on field‐cooling (FC). High‐resolution x‐ray diffraction data revealed a phase transition sequence of cubic→tetragonal→ orthorhombic (O)→monoclinic B‐type (M_B) on field‐cooling with a phase coexistence of single domain O and M_B at 300 K in the FC state; whereas poling at room temperature revealed a M_B single phase. Accordingly, the structural origin of the piezoelectricity enhancement on poling at room temperature is attributed to a pure M_B phase. It is probable, in the FC state, that field‐cooling results in defect dipole migration, which then acts to stabilize regions of the O phase on cooling into the M_B one.     

Ferroelectric and magnetoelectric origins of multiferroic SmCrO3

We investigated the magnetic and ferroelectric properties of orthochromite SmCrO₃ by using density functional theory simulations. The atom coordinates of both Pbnm and Pna2₁ were calculated, and Pna2₁ was found to be the ground state structure. The spontaneous polarization in Pna2₁ structure is sensitive to delicate structure change which is induced by different antiferromagnetic order, and its direction is along the c axis. The spin structure of Pbnm was analyzed and it was found to not support ferroelectricity. The results revealed the origin of ferroelectricity of multiferroic SmCrO₃ and explained the magnetoelectric effect.     

Intrinsic origin of enhanced piezoelectricity in alkali niobate-based lead-free ceramics

An orientational average method is used to calculate the piezoelectric coefficients of KNbO₃ and K_0.5Na_0.5NbO₃ (KN and KNN) piezoceramics, which is the first time that an ab initio method has been used to directly compare the piezoelectric coefficients with the experimental results of the orthorhombic alkali niobate-based lead-free piezoceramics. It is found that the longitudinal piezoelectric coefficient of orthorhombic KN and KNN piezoceramics is mainly contributed by d₃₃ and d₁₅ of their single crystal. The approximately 70% increase in the calculated is confirmed in pure orthorhombic KNN over KN piezoceramics. The increased response of the internal coordinates of Na and O atoms to macroscopic strain is considered the main origin of the enhanced piezoelectricity. The underlying mechanism is that the smaller atomic size of Na provides more room for itself and the adjacent O atoms in the perovskite structure, which leads to the increased local structural instability. In this case, the Na and its nearby O atoms are easier to move when an external stress is applied on the KNN and then induce the substantial change in polarization. This work provides a microscopic insight to understand the enhanced piezoelectricity in KNN piezoceramics.     

Modulation of magnetoelectric coupling through systematically engineered spin canting in nickel–zinc ferrite

A nickel–zinc ferrite system, which is one of the well-known versatile soft-ferromagnetic oxides, was investigated in terms of magnetoelectric (ME) coupling at room temperature. Herein, we demonstrated that spin canting is manipulated through a composition-induced structural transition from an inverse to a normal spinel structure, leading to modulation in the ME coupling. The ME coefficient was maximized at 60 at.% Zn substitution with a value of 0.1 mV/(Oe·cm), denoting ∼70% enhancement compared to that of the pure nickel ferrite. It was revealed that the interspin angle is enhanced along the octahedral site at up to ∼60 at.% Zn substitution, consistent with the composition level at the culmination of the ME coupling, evidenced by X-ray diffraction profiles and magnetic hysteresis loops combined with density functional theory calculations. Given that this approach is based on a tractable fabrication method, this study is expected to be widely used in modulation of the ME coupling in spinel-structured oxides.     

Remarkable piezoelectricity and stable high‐temperature dielectric properties of quenched BiFeO3–BaTiO3 ceramics

Effects of quenching process on dielectric, ferroelectric, and piezoelectric properties of 0.71BiFeO₃?0.29BaTiO₃ ceramics with Mn modification (BF–BT?xmol%Mn) were investigated. The dielectric, ferroelectric, and piezoelectric properties of BF–BT?xmol%Mn were improved by quenching, especially to the BF–BT?0.3 mol%Mn ceramics. The dielectric loss tanδ of quenched BF–BT?0.3 mol%Mn ceramics was only 0.28 at 500°C, which was half of the slow cooling one. Meanwhile, the remnant polarization P_r of quenched BF–BT?0.3 mol%Mn ceramics increased to 21 μC/cm². It was notable that the piezoelectric constant d₃₃ of quenched BF–BT?0.3 mol%Mn ceramics reached up to 191 pC/N, while the T_C was 530°C, showing excellent compatible properties. The BF–BT?xmol%Mn system ceramics showed to obey the Rayleigh law within suitable field regions. The Rayleigh law results indicated that the extrinsic contributions to the dielectric and piezoelectric responses of quenched BF–BT?xmol%Mn ceramics were larger than the unquenched ceramics. These results presented that the quenched BF–BT?xmol%Mn ceramics were promising candidates for high‐temperature piezoelectric devices.     

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.     

Phase transition and piezoelectricity of BaZrO3-modified (K,Na)NbO3 lead-free piezoelectric thin films

(K,Na)NbO₃ (KNN) is a promising lead-free ferroelectric/piezoelectric system, to which incorporating BaZrO₃ can greatly enhance its piezoelectricity, but the mechanism is not clear. This work was conducted to investigate the phase transition in the BaZrO₃-modifed KNN system and its contribution to piezoelectricity enhancement, using thin films with a fixed orientation and high compositional homogeneity fabricated by a sol-gel method. Two ferroelectric-to-ferroelectric phase transitions are revealed, which correspond to monoclinic M_C- M_A phase transition at higher temperature and rhombohedral-monoclinic M_C phase transition at lower temperature. It is difficult to distinguish these phases in KNN-based bulk materials, but their differences are clear when conducting high-resolution X-ray reciprocal space mapping (RSM) on the present thin films. Piezoresponse force microscopy experiments also revealed an interesting finding that local piezoelectricity of monoclinic phases was higher than that of rhombohedral ones in KNN-based thin films. This work could shed insights on the fundamental understandings for the effect of the chemical doping, and offer guidance for property optimization in the KNN-based lead-free piezoelectrics.     

Improved Performance of YIG (Y3Fe5O12) Films Grown on Pt‐Buffered Si Substrates by Chemical Solution Deposition Technique

High‐quality Yttrium iron garnet (YIG) films with crack‐free surface and improved magnetic performance were grown on platinum (Pt)‐buffered Si substrates by chemical solution deposition technique. The saturation magnetization of obtained YIG films can reach 124 emu/cm³, which was the 88% theoretical value of YIG single crystal. The effects of annealing condition were also discussed. When annealed at 750°C for 1 h, YIG films showed a very small coercive field of 12 Oe and the peak‐to‐peak ferromagnetic resonance linewidth can be as low as 95 Oe at 9.10 GHz. The results demonstrated that YIG films prepared on Pt‐buffered Si substrates can be beneficial to the application of YIG films to integrated devices.     

Ferroelectric and magnetic properties in (1−x)BiFeO3–x(0.5CaTiO3–0.5SmFeO3) ceramics

Multiferroic ceramics were prepared and characterized in (1?x)BiFeO₃–x(0.5CaTiO₃–0.5SmFeO₃) system by a standard solid‐state reaction process. The structure evolution was investigated by X‐ray diffraction and Raman spectrum analyses. The refinement results confirmed the different phase assemblages with varying amounts of polar rhombohedral R3c and nonpolar orthorhombic Pbnm as a function of the substitution content. In the compositions range of 0.2≤x≤0.5, polar R3c and nonpolar Pbnm coexisted, which was referred to polar‐to‐nonpolar morphotropic phase boundary (MPB). According to the dielectric and DSC analysis results, the ceramics with x≤0.2 changed to diffused ferroelectric, and the ferroelectric properties were enhanced significantly. Two dielectric relaxations were detected in the temperature range of 200‐300 K and 500‐700 K, respectively. The high‐temperature dielectric relaxation was attributed to the grain‐boundary effects. While the low temperature dielectric relaxation obtained in the samples with x=0.3‐0.5 was related to the charge transfer between Fe²⁺ and Fe³⁺. The magnetic hysteresis loops measured at different temperature indicated the enhanced magnetic properties in the present ceramics, which could be attributed to the suppressed cycloidal spin magnetic structure by Ti ions. In addition, the rare‐earth Sm spin moments might also affect the magnetic properties at relatively lower temperature.     

Phase coexistence and large piezoelectricity in BaTiO3‐CaSnO3 lead‐free ceramics

Ferroelectric phase coexistence was constructed in (1?x)BaTiO₃‐xCaSnO₃ lead‐free ceramics, and its relationship with the piezoelectricity of the materials was investigated to ascertain potential factors for strong piezoelectric response. It is found that the addition of CaSnO₃ caused a series of phase transitions in the (1?x)BaTiO₃‐xCaSnO₃ ceramics, and a ferroelectric coexistence of rhombohedral, orthorhombic, and tetragonal phases is formed at x = 0.08, where the ceramics exhibit the lowest energy barrier and consequently facilitate the polarization rotation and extension, resulting in the optimal piezoelectricity of d₃₃ and k_p values of 550 pC/N and 0.60, respectively. Our study provides an intuitive insight to understand the origin of high piezoelectricity in the ceramics with the coexistence of multiferroelectric phases.     

Thermally stable high strain and piezoelectric characteristics of (Li,Na, K)(Nb,Sb)O3-CaZrO3 ceramics for piezo actuators

The 0.97(Na_0.5K_0.5)(Nb_1−xSb_x)O₃-0.03CaZrO₃ ceramic with x = 0.09 exhibits a high d₃₃ of 518 pC/N and a strain of 0.13% at 4.0 kV/mm owing to its orthorhombic-pseudocubic polymorphic phase boundary (PPB) structure. However, these values decreased considerably above 90°C owing to its low Curie temperature (T_C), indicating that its thermal stability is not sufficient for practical applications. Li₂O was added to the specimen with x = 0.11 to improve its thermal stability of the strain and d₃₃ by increasing the T_C without degrading the actual d₃₃ and strain values. The 0.97(Li_0.04Na_0.46K_0.5)(Nb_0.89Sb_0.11)O₃-0.03CaZrO₃ ceramic, having an orthorhombic-tetragonal PPB structure, exhibits a d₃₃ of 502 pC/N and a strain of 0.16%. This large strain was maintained up to 150°C and the d₃₃ slightly decreased to 475 pC/N at 130°C. Therefore, this lead-free ceramic displays excellent piezoelectric characteristics with improved thermal stability, indicating that it can be applied to piezoelectric actuators.     

Pressure-driven phase transition and energy conversion in ferroelectrics: Principles,materials, and applications

The pressure-driven explosive energy-conversion (EEC) effect of ferroelectric (FE) materials has been extensively studied in scientific research and high-tech applications owing to its high pulse-power output capability. The fundamental principle of this effect is pressure-driven phase transition and depolarization in FE materials, accompanied by discharging behavior from the charge release upon pressure loading. Pb(Zr,Ti)O₃ has been an excellent example of a materials exhibiting these properties. However, recent investigations have been focused on developing other lead-based or lead-free materials with a higher energy-storage ability and better temperature stability. In this article, we review the recent progress achieved in the past decades on different types of lead-based and lead-free ceramics, single crystals, and multilayer films, based on their unique pressure-driven phase transition and energy-conversion properties. Their pulse power discharging performance under actual shock-wave compression is also summarized, followed by a detailed discussion of the failure mechanism under shock-wave compression. Finally, several issues and perspectives are proposed for future investigation in this area. All these not only assist in the design of new materials for high-performance EEC but are also helpful for the practical application of these promising materials in pulse-power technologies.     

On Structure,Optical Properties and Photodegradated Ability of Aurivillius‐Type Bi3TiNbO9 Nanoparticles

Bi₃TiNbO₉ nanoparticles with an acceptor dopant of Ni²⁺ ion were prepared by the conventional Pechini sol–gel synthesis. The X‐ray polycrystalline diffraction measurements (XRD) and the Rietveld refinements of Bi₃TiNbO₉ samples were completed. The surface property of Bi₃TiNbO₉ nanoparticles was investigated by transmission electron microscope, scanning electron microscope), and N₂ adsorption–desorption isotherms. Bi₃TiNbO₉ nanoparticles showed an optical band gap with energy of 3.1 eV in the UV region. While the Ni²⁺‐doping could greatly reduce the band energy of Bi₃TiNbO₉:xNi²⁺ nanoparticles to 2.79 eV (x = 0.05) and 2.61 eV (x = 0.1). This indicates that the Ni‐doped samples could be excited by UV–visible light. The photocatalytic abilities were tested by the photodegradation on methylene blue solution (MB) and phenol solutions excited by visible light. Accordingly, the photocatalytic activity was improved by the Ni‐doping in B‐sites in this Aurivillius‐type structure. The results concluded that Bi₃TiNbO₉:Ni²⁺ would be a possible candidate as a visible light‐driven photocatalyst. The effective photocatalysis was discussed on the structure characteristic and experiment such as polarized Aurivillius (Bi₂O₂)²⁺ layers, luminescence, and decay lifetimes, etc.     

Sintering behavior,structural phase transition,and microwave dielectric properties of La1‐xZnxTiNbO6‐x/2 ceramics

La_1‐xZn_xTiNbO_6‐x/2 (LZTN‐x) ceramics were prepared via a conventional solid‐state reaction route. The phase, microstructure, sintering behavior, and microwave dielectric properties have been systematically studied. The substitution of a small amount of Zn²⁺ for La³⁺ was found to effectively promote the sintering process of LTN ceramics. The corresponding sintering mechanism was believed to result from the formation of the lattice distortion and oxygen vacancies by means of comparative studies on La‐deficient LTN ceramics and 0.5 mol% ZnO added LTN ceramics (LTN+0.005ZnO). The resultant microwave dielectric properties of LTN ceramics were closely correlated with the sample density, compositions, and especially with the phase structure at room temperature which depended on the orthorhombic‐monoclinic phase transition temperature and the sintering temperature. A single orthorhombic LZTN‐0.03 ceramic sintered at 1200°C was achieved with good microwave dielectric properties of ε_r~63, Q×f~9600 GHz (@4.77 GHz) and τ_f ~105 ppm/°C. By comparison, a relatively high Q × f~80995 GHz (@7.40 GHz) together with ε_r~23, and τ_f ~?56 ppm/°C was obtained in monoclinic LTN+0.005ZnO ceramics sintered at 1350°C.     

Polymorphic Phase Transformation of Lead Monoxide and Its Influence on Lead Zirconate Titanate Formation

The polymorphic phase transformation of PbO from massicot to litharge phase is easily produced by mechanical force. The transformation is relatively quick by wet ball-milling. The additions of TiO₂ and ZrO₂ tend to retard it. A second-order exponential relation is observed between the fractional conversion and the milling time for pure PbO, PbO-TiO₂, and PbO-ZrO₂-TiO₂ systems with a rutile constituent. This exponential relation is changed into a first-order logarithmic relation when anatase instead of rutile is used. Although the PbO _ss phase is observed irrespective of the phase of the PbO in the raw materials, this PbO _ss peak is more obvious when massicot PbO or anatase TiO₂ is used. The formation of PbO _ss phase is also dependent on the reactivity of the ZrO₂, and its presence can affect the formation and piezoelectric properties of PZT ceramics.     

Structural and Electrical Characteristics of (1−x)Pb(Lu1/2Nb1/2)O3–xPbTiO3 Ceramics with Low PbTiO3

A solid solution of (1?x)Pb(Lu_1/2Nb_1/2)O₃–xPbTiO₃ with composition of 0.01 ≤ x ≤ 0.08 have been prepared successfully. XRD analysis indicates the crystal structure adopts an orthorhombic (O) phase in 0.01 ≤ x ≤ 0.06 interval and becomes the coexistence of O and rhombohedral (R) phase at x = 0.07, then turns into R phase mostly at x = 0.08. In addition, two sets of superlattice reflections due to B‐site ordering and antiparallel cation displacement are distinguished by XRD and the superstructures which arise from antiparallel cation displacement disappear gradually with the increasing x. The grain size increases gradually with the increasing x, and then becomes the bimodal microstructure at x ≥ 0.06 due to the coexistence of O and R phase. The dielectric spectra exhibit Curie temperature decreases from 248°C to 147°C with increasing x from 0.01 to 0.08. As 0.01 ≤ x ≤ 0.04, the samples display typical double hysteresis loops, suggesting antiferroelectric nature, then turn into ferroelectric gradually at x = 0.05. Finally, it exhibit typical ferroelectric hysteresis loops in 0.06 ≤ x ≤ 0.08 interval.     

Structure,electrical, and thermal expansion properties of PZnTe–PZT ternary system piezoelectric ceramics

xPb(Zn_0.5Te_0.5)O₃–(1?x)Pb(Zr_0.5Ti_0.5)O₃ (PZnTe–PZT) ceramics were prepared by the solid‐state reaction method. The phase structure, microstructure, ferroelectric and dielectric properties and thermal expansion properties were systematically investigated. X‐ray diffraction analysis showed the morphotropic phase boundary (MPB) existed at the composition of x = 0.08, which was the coexistence of the rhombohedral phase and the tetragonal phase. The grain size of ceramics decreased rapidly from 10‐20 μm to 1‐3 μm when the PZnTe was added in. The PZnTe–PZT ceramics at the MPB composition showed the largest high field effective piezoelectric coefficient and the lowest strain hysteresis H. The dielectric permittivity and phase transition temperature exhibited strongly compositional dependence. A good linear relation was shown in T_m temperature vs x content and a DPT behavior was found in xPZnTe–(1?x)PZT (x = 0.02‐0.08). The thermal expansion properties showed a low thermal expansion coefficient in the low temperature while a high thermal expansion coefficient in the high temperature. Besides, the thermal expansion curve also showed the characteristic of DPT in PZnTe–PZT ceramics.