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.     

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.     

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.     

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.     

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.     

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.     

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.     

Studies on dielectric relaxation in ceramic multiferroic Gd1−xYxMnO3

Dielectric study over a broadband was carried out from 10 to 70 K on ceramic Gd_1?xY_xMnO₃ (x=0.2, 0.3 and 0.4). For all the compositions, a prominent sharp peak about ~18 K was observed in the temperature dependence of both ε′(T) and ε″(T) at all frequencies, indicating a long‐range ferroelectric (FE) transition. Using Cole‐Cole fit to the permittivity data, the relaxation time τ and the dielectric strength ?ε were estimated. Temperature variation of τ(T) in the Arrhenius representation is found to be nonlinear (non‐Debyean relaxation), with increasing barrier‐activation energy over successive temperature‐windows. Interestingly, for all the compositions, we witness a jump in τ(T) about the ferroelectric transition temperature, concurred by a broad‐maximum in ?ε(T),signifying the critical slow down of relaxations near long‐range FE‐correlations.     

Scaling Effects in Perovskite Ferroelectrics: Fundamental Limits and Process‐Structure‐Property Relations

Ferroelectric materials are well‐suited for a variety of applications because they can offer a combination of high performance and scaled integration. Examples of note include piezoelectrics to transform between electrical and mechanical energies, capacitors used to store charge, electro‐optic devices, and nonvolatile memory storage. Accordingly, they are widely used as sensors, actuators, energy storage, and memory components, ultrasonic devices, and in consumer electronics products. Because these functional properties arise from a noncentrosymmetric crystal structure with spontaneous strain and a permanent electric dipole, the properties depend upon physical and electrical boundary conditions, and consequently, physical dimension. The change in properties with decreasing physical dimension is commonly referred to as a size effect. In thin films, size effects are widely observed, whereas in bulk ceramics, changes in properties from the values of large‐grained specimens is most notable in samples with grain sizes below several micrometers. It is important to note that ferroelectricity typically persists to length scales of about 10 nm, but below this point is often absent. Despite the stability of ferroelectricity for dimensions greater than ~10 nm, the dielectric and piezoelectric coefficients of scaled ferroelectrics are suppressed relative to their bulk counterparts, in some cases by changes up to 80%. The loss of extrinsic contributions (domain and phase boundary motion) to the electromechanical response accounts for much of this suppression. In this article, the current understanding of the underlying mechanisms for this behavior in perovskite ferroelectrics is reviewed. We focus on the intrinsic limits of ferroelectric response, the roles of electrical and mechanical boundary conditions, grain size and thickness effects, and extraneous effects related to processing. In many cases, multiple mechanisms combine to produce the observed scaling effects.     

Additively patterned ferroelectric thin films with vertical sidewalls

The functional properties of electroceramic thin films can be degraded by subtractive patterning techniques used for microelectromechanical (MEMS) applications. This work explores an alternative deposition technique, where lead zirconate titanate (PZT) liquid precursors are printed onto substrates in a desired geometry from stamp wells (rather than stamp protrusions). Printing from wells significantly increased sidewall angles (from ~1 to >35 degrees) relative to printing solutions from stamp protrusions. Arrays of PZT features were printed, characterized, and compared to continuous PZT thin films of similar thickness. Three‐hundred‐nanometer‐thick printed PZT features exhibit a permittivity of 730 and a loss tangent of 0.022. The features showed remanent polarizations of 26 μC/cm², and coercive fields of 95 kV/cm. The piezoelectric response of the features produced an e_31,f of ?5.2 C/m². This technique was also used to print directly atop prepatterned substrates. Optimization of printing parameters yielded patterned films with 90° sidewalls. Lateral feature sizes ranged from hundreds of micrometers down to one micrometer. In addition, several device designs were prepatterned onto silicon on insulator (SOI) wafers (Si/SiO₂/Si with thicknesses of 0.35/1/500 μm). The top patterned silicon was released from the underlying material, and PZT was directly printed and crystallized on the free‐standing structures.     

Ceramic processing and multiferroic properties of the perovskite YMnO3-BiFeO3 binary system

The perovskite (1−x)YMnO₃−xBiFeO₃ binary system is very promising because of its multiferroic end members. Nanocrystalline phases have been recently obtained by mechanosynthesis across the system, and the perovskite structural evolution has been described. Two continuous solid solutions with orthorhombic Pnma and rhombohedral R3c symmetries were found, which coexist within a broad compositional interval of 0.5 ≤ x ≤ 0.9. This might be a polar-nonpolar morphotropic phase boundary region, at which strong phase-change magnetoelectric responses can be expected. A major issue is phase decomposition at moderate temperatures that highly complicates ceramic processing. This is required for carrying out a sound electrical characterization and also for their use in devices. We present here the application of Spark Plasma Sintering to the ceramic processing of YMnO₃-BiFeO₃ phases. This advanced technique, when combined with nanocrystalline powders, allowed densifying phases at reduced processing temperatures and times, so that perovskite decomposition was avoided. Electrical measurements were accomplished, and the response was shown to be mostly dominated by conduction. Nonetheless, the intrinsic dielectric permittivity was obtained, and a distinctive enhancement in the phase coexistence region was revealed. Besides, Rayleigh-type behavior characteristic of ferroelectrics was also demonstrated for all rhombohedral phases. Magnetic characterization was performed in this region, and antiferromagnetism was shown.     

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.     

Strain effects on domain structures in ferroelectric thin films from phase‐field simulations

Strain and applied external electric fields are known to influence domain evolution and associated ferroelectric responses in ferroelectric thin films. Here, phase‐field simulations are used to predict equilibrium domain structures and polarization‐field (P‐E) hysteresis loops of lead zirconate titanate (PZT) thin films under a series of mismatch strains, ranging from strongly tensile to strongly compressive. In particular, the evolution of domains and the P‐E curves under different applied strains reveal the mesoscale mechanism, the appearance of in‐plane polarization during domain switching, that is responsible for a relatively small coercive field and remnant polarization. A Landau energy distribution is analyzed to better understand the domain evolution under various strain conditions. The results provide guidance for choice of mismatched strains to yield the desired P‐E hysteresis loops and the domain structures.     

Preparation and Spontaneous Polarization–Magnetization of a New Ceramic Ferroelectric–Ferromagnetic Composite

Ceramic composites with the composition of x PMZNT·(1 – x )NiCuZn have been prepared using a standard ceramic technique, in which x varies as 0, 0.1, 0.2, 0.4, 0.6, 0.9, and 1.0. PMZNT is the abbreviated form of 0.92Pb(Mg_1/3Nb_2/3)O₃·0.04Pb(Zn_1/3Nb_2/3)O₃·0.04PbTiO₃ (PMN-PZN-PT). NiCuZn is the abbreviated form of Ni_0.2Cu_0.2Zn_0.6Fe₂O₄. The presence of ferroelectric PMZNT phase and ferromagnetic NiCuZn ferrite phase has been confirmed using X-ray diffractometry. Ferroelectric hysteresis loops and magnetic hysteresis loops have been observed and studied. In polarization–electric-field curves, the remnant polarization and coercive fields display little asymmetric characterization because of the existence of the internal electric field. When the amount of NiCuZn ferrite phase increases, the coercive field increases. Meanwhile, the saturation magnetization decreases and the coercivity of the composites increases with the increase of phase fraction of PMZNT, because the interaction between magnetic grains (or magnetic connectivity) is weakened by the existence of nonmagnetic PMZNT phase distributed in the magnetic phases. Under an applied magnetic and electric field, the magnetization and polarization of the composites can be easily tuned. The sintered composites possess high density and fine-grained microstructure. The average grain size of NiCuZn ferrite grains is slightly larger than that of the PMZNT grains.     

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.     

Composition control of lead-free piezoelectric BNT thin ceramic films deposited by ex situ sputtering

(Bi_0.5Na_0.5)TiO₃ thin film growth by ex situ sputtering has been investigated and reported in this paper. An original approach, based on the growth process, was used in order to precisely control the film composition, which has never been reported in BNT growth. The bismuth content in the films and so the composition of amorphous sputtered films was controlled by a slight heating of the substrate during the growth (150–240°C). Then, films were crystallized, obviously without any change in composition, by a post-annealing treatment. More precisely, without substrate heating and using a stoichiometric target, the film presents an excess of Bi but when it is deposited at 200°C the film becomes stoichiometric. It was shown that the sticking coefficient of Bi is particularly sensitive even at low substrate temperatures, whereas Na and Ti sticking coefficients are not impacted. Followed by a post-annealing in air at 650°C, the composition of the amorphous BNT films deposited at 200°C remains stoichiometric and the film exhibits a high (100) preferred orientation in a pure perovskite phase and a dense microstructure. The evaluation of the electrical properties as a function of the Bi content in the film, adjusted by the deposition temperature, shows a strong impact on the ferroelectric properties where the best performances were obtained with the stoichiometric BNT film deposited at 200°C.     

Residual stress and electrical properties of Pb(Zr0.52,Ti0.48)O3 thin films RF sputtered directly on Cu foils

Polycrystalline Pb(Zr_0.52Ti_0.48)O₃ (PZT) thin films between 250 and 1000 nm thick were deposited on Cu foils via RF magnetron sputtering. Samples were crystallized ex situ between 550°C and 750°C in a low oxygen partial pressure atmosphere, pO₂, in order to avoid oxidation of the substrate. These were compared to films made on more common Pt/TiO_x/SiO₂/Si substrates also crystallized under low pO₂ conditions. The mismatch of the coefficients of thermal expansion for Cu and PZT caused large compressive residual stresses to develop in the films, whereas films on Pt‐Si experienced more moderate tensile stresses. Stress was measured using the sin²ψ method. In addition to mechanical implications, i.e., film cracking and delamination, the effect of residual stress on electrical properties is discussed. Dielectric constants of PZT were lower on Cu than on Pt/TiO_x/SiO₂/Si. This could be due either to a dead layer effect or to the residual stress imposed by the substrate. The remanent polarizations for films on Cu were between 18 and 41 μC/cm², while coercive fields were between 37 and 54kV/cm. Rayleigh analysis was used to describe the role of defects affecting domain wall mobility, as they act as pinning centers and decrease the extrinsic polarization response.     

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.     

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.     

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.