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The strong coupling between magnetism and ferroelectricity was found in rare-earth manganites, where the electric polarization could be induced by special magnetic ordering. There is no theoretical model that would allow us to study the static and dynamic properties of electric polarization in strongly correlated magnetic dielectrics. In the presented research, we have taken the main step toward the construction of such a fundamental model and made a direct connection between the microscopic Katsura–Nagaosa–Balatsky theory and Mostovoy’s phenomenological model for magnetically induced polarization. A novel description of the ferroelectricity of spin origin is proposed within the framework of the many-particle quantum hydrodynamics method. It is applied to the study of cells of magnetic ions, where the electric dipole moment is proportional to the vector product of spins. Our approach is based on the many-particle Pauli equation, where the influence of an external magnetic field is considered. We define the electric dipole moment operator of the ion cell and introduce the macroscopic polarization as the quantum mechanical average of that operator. We formulate a model for the description of nonequilibrium polarization and derive a new polarization evolution equation. The polarization switching in ferroelectric magnets with the spiral spin-density-wave state is considered, and we demonstrate that the proposed model yields known results and can predict novel effects. The dynamic magnetoelectric effect can be investigated by employing this novel equation to study the evolution of polarization.

We report on the dielectric permittivities in 1 kHz to 4 MHz frequency region of the ferroelectric liquid crystal (FLC) material CB500 having a TGBA phase. Molecular and collective relaxation processes have been studied in bulk and in confined environment. In the bulk sample, a Goldstone mode (GM) and a molecular mode were observed in the SmC* phase and the softmode (SM) was observed in the TGBA phase. The Goldstone mode was also present in the FLC sample confined in 0.2 μm Anopore membranes. The relaxation frequency of this mode was shifted to the higher-frequency region in comparison to that observed in the bulk sample. In the Anopore confined FLC sample, another relaxation process (AP) was observed, which possibly arises due to some collective movement of the molecules attached to the cavity walls. The relaxation frequency of this process is almost independent of temperature and this process is only observed in the SmC* phase.

We calculate the quantum phase diagram of an extended Falicov-Kimball model in the intermediate coupling regime using a constrained path quantum Monte Carlo technique. The mixed-valence regime is dominated by a Bose-Einstein condensation of excitons with a built-in electric polarization.

Within the framework of Pertsev's thermodynamic theory, the equilibrium polarization states and the physical properties of single-domain Pb(Zr_0.35Ti_0.65)O₃ (PZT) thin films epitaxially grown on dissimilar cubic substrates are investigated. The "misfit strain-temperature" phase diagrams are obtained for the films. The presence of stability range of the rhombohedral phase is the characteristic feature of these phase diagrams, which separates the stability ranges of the tetragonal phase and the orthorhombic phase. The coexistence of the rhombohedral phase and the tetragonal phase in PZT film can be explained by Pertsev's theory with the gradient strain case with a suitable stress form.

The purpose of this paper is to study the intrinsic mechanical contributions of poled piezoelectric ceramics under the three ferroelectric phases [tetragonal (4mm), rhombohedral (3m) and orthorhombic (mm2)]. The intrinsic elastic coefficients and spontaneous strains of saturated and unsaturated poling piezoelectric ceramics are analyzed and calculated by probability density functions of orientation (PDFOs) and Reuss or Voigt average, and the analytical results of the intrinsic mechanical properties of piezoelectric ceramics with the poling degree are obtained. In addition, this paper also calculates the Young’s modulus and shear modulus of barium titanate (BaTiO₃) piezoelectric ceramics by PDFO which are based on the Reuss, Voigt and Hill averages. Comparing them with the published theoretical calculation data shows that the results calculated by the finite element method under the saturated and unsaturated poling states are within the range of the results calculated by us using the three average theories through PDFO, and it shows the accuracy of calculation by PDFO.

Ceramics with the composition Pb_1-xK_2x-3yM_yNb₂O₆ (PKMN) with x = 0.29, y = 0.145 and M = Gd³⁺, Y³⁺ were synthesized by the solid-state reaction route between the corresponding oxides and carbonates. The crystal structure was confirmed by X-ray diffraction (XRD). The temperature dependence of dielectric properties were measured from 35 to 595°C. Well-developed P–E (polarization–electric field) hysteresis loops were observed in the materials. Determining the piezoelectric constants, K_p = 20%, K_t = 49%, d₃₃ = 110, and quality factor, Q_m = 33, reveals that the material Y³⁺-modified PKN can be useful for transducer applications.

A gel was formed when an aqueous solution of BaCl₂, NbF₅ and citric acid in stoichiometric ratio is heated on a water bath at 100°C. This gel on decomposition at 700°C yielded the nano phase of BaNb₂O₆ as confirmed by X-ray diffraction study. The crystal structure is orthorhombic with unit cell parameter a = 12.2136 Å, b = 10.2413 Å, c = 7.8831 Å. The estimated grain size is 88 nm. Dielectric studies reveal the dielectric relaxation mechanism. Conducting charges and free charges both contribute to the dielectric relaxation in this material. Electrical properties of the material were studied using impedance spectroscopy technique. Detailed analysis of impedance spectrum suggested that the electrical properties are strongly temperature-dependent. The relaxation is polydispersive and conduction is mainly through grains. AC conduction activation energies are estimated from Arrheneus plots and conduction mechanism is discussed.

Magnetism and transport are two key functional ingredients in modern electronic devices. In oxide heterostructures, ferroelectricity can provide a new route to control these two properties via electrical operations, which is scientifically interesting and technologically important. In this brief review, we will introduce recent progresses on this fast developing research field. Several subtopics will be covered. First, the ferroelectric polarization tuning of interfacial magnetism will be introduced, which includes the tuning of magnetization, easy axis, magnetic phases, as well as exchange bias. Second, the ferroelectric polarization tuning of transverse and tunneling transport will be reviewed.

Herein, we report the fabrication of UV-active NaNbO₃/Al₂O₃ (30 wt.%) ferroelectric heterojunction photocatalyst with enhanced photoelectrochemical activity to Methylene Blue (MB). Pure NaNbO₃ nanorods (NRs) were synthesized by a facile hydrothermal process and NaNbO₃/Al₂O₃ composite was prepared by an ultrasonic vibration method. The heterostructures showed a remarkable increase in photocurrent density and the MB degradation ratio reached 88% in the initial 15 min and eventually reached 98.6% in 75 min. The high photocatalytic efficiency is attributed to the ferroelectric polarization of NaNbO₃ with the forming of heterojunction electric fields assisted by Al₂O₃. These properties demonstrate that NaNbO₃/Al₂O₃ ferroelectric heterojunction photocatalyst shows promising photoactivity and provides an effective method to improve the photocatalytic properties of other ferroelectrics.

Polycrystalline BFO thin films of similar thickness but subjected to different annealing durations (2 hr and 4 hr) were deposited on SRO/Pt/TiO₂/SiO₂/Si substrates via RF sputtering. Phase identification by using X-ray diffractions confirms the pure BFO phase of the thin films fabricated. Polarization–Electric Field (P–E) loop study measured at different frequencies shows that a longer annealing duration led to the formation of space charges in the BFO thin films and thus caused a poor ferroelectric property observed in the hysteresis loop. The results of leakage current measurement for the two BFO films are consistent with the ferroelectric measurement, where a higher leakage current is demonstrated by the BFO film annealed for 4 hr.

Epitaxial PbZr_0.52Ti_0.48O₃ (PZT) films with perfectly c-axis oriented tetragonal phase were deposited on SrTiO₃ (STO) substrates using a SrRuO₃ (SRO) buffer layer by pulsed laser deposition (PLD) method. Ferroelectric behavior of PZT on STO substrates along with an improved remnant polarization (2P_r) of 118 μC/cm² and a low coercive field (E_c) of 193 kV/cm at 15 V were observed at room temperature indicating that the SRO/STO substrates with small lattice misfit can make some contributions to enhance PZT film's ferroelectric properties. Moreover, the capacitance characteristics of the PZT thin films were detected, a large dielectric constant of 1476, the charging and discharging characteristics determine the large dielectric strength of the PZT thin film capacitors.

Based on density-functional calculations, we have studied possible ferroelectric switching path in monodomain single crystal of rhombohedral BiFeO₃, a prototypical multiferroic compound. By carefully studying the behaviors of FeO₆ corner-sharing double-tetrahedrons, we find abrupt changes in total energy and oxygen atomic positions, and therefore polarizations, occur in the ferroelectric switching path of rhombohedral BiFeO₃. Detailed analyses suggest that such behavior might be caused by the frustrated magnetic ordering in the paraelectric phase of rhombohedral BiFeO₃, where three O atoms and the Bi atom are in the same plane perpendicular to the polarization direction. This is supported by the fact that the ferroelectric switching for paramagnetic BiFeO₃ is smooth and has a much lower energy barrier than that of antiferromagnetic BiFeO₃.

Bi_0.85La_0.15FeO₃ (BLFO) nanotubes with an average diameter of about 200 nm and wall thickness of about 20 nm are fabricated by sol–gel alumina template technique, and their room-temperature multiferroic properties are studied. Piezoelectricity and ferroelectricity in the BLFO nanotubes are revealed by piezoresponse force microscopy study of individual nanotube. Doping the BiFeO₃ nanotubes with La reduces the crystallization temperature and results in a reduced lose of Bi and therefore improved multiferroic properties of the nanotubes. Enhanced weak ferromagnetism is also observed, and it is attributed to the nano-crystalline structure of the nanotubes.

The explicit expression of Helmholtz free energy has been obtained from the equation of state from effective field approach. From the Helmholtz free energy, four characteristic temperatures describing a first-order ferroelectric phase transitions have been determined. The physical meaning of coefficients in Landau-type free energy has been revealed by comparison with the expanding Helmholtz function. Temperature dependence of polarization under different bias, and hysteresis loops at different temperatures are presented and discussed. These results provide the basic understandings of the static properties of first-order ferroelectric phase transitions.

Specific heat of barium titanate single crystals of different quality has been precisely measured with special attention to the temperature region above the ferroelectric phase transition. It was assumed that excess specific heat in the paraelectric phase has a fluctuation nature and the experimental data were analyzed in the framework of Levanyuk's theory for multiaxial ferroelectrics. Within this approach the correlation parameter δ is estimated to be 0.66 × 10^-16 cm² for Remeika-type crystals and 0.45 × 10^-16 cm² for TSSG (top-seeded solution growth) crystals. These values are in good accordance with earlier estimations.

The recently proposed Equivalent Dipole Model for describing the electromechanical properties of ionic solids in terms of 3 ions and 2 bonds has been applied to PZT ceramics and lead-free single crystal piezoelectric materials, providing analysis in terms of an effective ionic charge and the asymmetry of the interatomic force constants. For PZT it is shown that, as a function of composition across the morphotropic phase boundary, the dominant bond compliance peaks at 52% ZrO₂. The stiffer of the two bonds shows little composition dependence with no anomaly at the phase boundary. The effective charge has a maximum value at 50% ZrO₂, decreasing across the phase boundary region, but becoming constant in the rhombohedral phase. The single crystals confirm that both the asymmetry in the force constants and the magnitude of effective charge are equally important in determining the values of the piezoelectric charge coefficient and the electromechanical coupling coefficient. Both are apparently temperature dependent, increasing markedly on approaching the Curie temperature.

The sequence of ground states for SrTiO₃ film subjected to epitaxial strain as well as to mechanical stress along the [001] and [110] axes is calculated from first principles within the density functional theory. Under the fixed-strain boundary conditions, an increase in the lattice parameter of a substrate results in the $I4cm\to I4∕mcm\to Ima2\to Cm\to Fmm2\to Ima2$(II) sequence of ground states. Under the fixed-stress boundary conditions, the phase sequence is different and depends on how the stress is applied. It is revealed that the simultaneous presence of competing ferroelectric and antiferrodistortive instabilities in SrTiO₃ gives rise to the appearance of metastable phases, whose number increases dramatically under the fixed-stress conditions. In the metastable phases, the octahedral rotation patterns are shown to differ substantially from those in the ground state. It is suggested that in systems with competing instabilities, each polar phase has its optimal octahedral rotation pattern which stabilizes this phase and creates a potential barrier preventing this phase to be transformed into other structures.

The polycrystalline samples of 0.8BiSm_xFe${}_{1-x}$O₃–0.2PbTiO₃ ($x=0.05,0.10,0.15$ and 0.20) were prepared by using the conventional solid-state reaction technique and sintered at high temperature (850${}^{\circ }$C). X-ray diffraction (XRD) confirms the distorted rhombohedral crystal structure for all the composites at room temperature. The surface morphology was checked by field-emission scanning electron microscope (FESEM) technique and homogeneous mixing of the components was confirmed by energy-dispersive analysis of X-ray (EDAX). The detailed study of dielectric properties of the composites reveals an increasing nature of dielectric constant (${\epsilon }_r$) and loss tangent (tan$\delta$) with the increase of temperature due to thermal activation. The Arrhenius plots of temperature dependence of AC conductivity yield the activation energy within the material at high-temperature range. The ferroelectric study shows that the remnant polarization decreases with the increase of Samarium (Sm) concentration.

The temperature dependence of the specific heat of the single crystal Pb(In${}_{0.5}$Nb${}_{0.5}$)O₃-Pb(Mg${}_{1∕3}$Nb${}_{2∕3}$)O₃-PbTiO₃ (PIN-PMN-PT) was obtained at 80–500K by means of ac-calorimetry, and comparison with the calorimetric data for PMN was made, allowing one to determine absolute values of the specific heat of PIN-PMN-PT. For the high-temperature region ($T>200$K), the background heat capacity was calculated, the anomalies of the heat capacity for both compounds were determined, and the hump of the temperature dependence of specific heat PIN-PMN-PT was detected for the first time.

$(1-y)$(BiFe${}_{1-x}$GdxO₃)–y(PbZrO₃) composites $(y=0.5)$, having four different Gd concentrations ($x=0.05$, 0.1, 0.15, and 0.2), were synthesized and their structural, dielectric, and ferroelectric properties have been studied using different characterization techniques. In addition, to investigate the effect of ion implantation on the microstructure and dielectric properties, these composites were exposed to 2MeV He${}^{+}$-ions. Modifications of the structure, surface morphology and electrical properties of the samples before and after ion exposure were demonstrated using powder X-ray diffraction (XRD), scanning electron microscopy (SEM) technique, and LCR meter. The compositional analysis was carried out using energy dispersive X-ray spectrometry (EDS). XRD results demonstrated a decrease in the intensity profile of the dominant peak by a factor of 6 showing a degradation of the crystallinity. Willliamson–Hall (WH) plots reveal reduction in the grain size after irradiation along with an increase in strain and dislocation density. A decrease in the dielectric constant and loss has been recorded after ion beam exposure with reduction in ac conductivity value. The contribution of grain and grain boundary effect in conduction mechanism has been addressed using Nyquist plots. All the samples demonstrate a lossy ferroelectric loop which shows a clear modification upon irradiation. The role of structural defects modifying the physical properties of the composite materials is discussed in this work.