Magnetization reversal induced by Mn substitution in spinel chromite NiCr2O4

Polycrystalline Ni(Cr_1?xMn_x)₂O₄ (0.1 ≤ x ≤ 0.325) ceramic samples were studied through different protocols of dc magnetization measurements. The samples exhibit 2 kinds of magnetic compensation effects below the ferrimagnetic transition temperature T_C. Remarkable magnetization reversal is observed between the 2 compensation temperatures T_comp1 and T_comp2, which is regarded as arising from the negative exchange coupling between the 2 magnetic sublattices at different crystallographic sites. The magnetization is reversed at T_SR due to spin‐reorientation caused by magnetostructural coupling. The spin‐reorientation is supported by Mn substitution and T_SR is increased to 96 K when x reaches 0.325. However, it is suppressed due to the strong ionic site preference and thus the magnetization is slightly increased in the negative direction of the magnetic field. Near the 2 compensation temperatures, tunable magnetic switching effects can be obtained just by changing the magnitude of the applied magnetic field. Moreover, both normal and inverse magnetocaloric effects were also demonstrated.     

Low‐Temperature Sintering and Microwave Dielectric Properties of CaMoO4‐Based Temperature Stable LTCC Material

The CaMoO₄–xY₂O₃–xLi₂O ceramics were prepared by the solid‐state reaction method. The sintering behavior, phase evolution, microstructure, and microwave dielectric properties were investigated. CaMoO₄ solid solution was obtained when x = 0.030, and two‐phase system including tetragonal CaMoO₄ phase and cubic Y₂O₃ phase formed when 0.066 ≤ x ≤ 1.417. A temperature stable CaMoO₄‐based microwave dielectric ceramic with ultralow sintering temperature (775°C) was obtained in the CaMoO₄–xY₂O₃–xLi₂O system when x = 0.306, which showed good microwave dielectric properties with a low permittivity of 9.5, a high Q_f value of 63 240 GHz, and a near‐zero temperature coefficient of resonant frequency of +7.2 ppm/°C.     

Enhanced Multiferroic and Magnetocapacitive Properties of (1 − x)Ba0.7Ca0.3TiO3–xBiFeO3 Ceramics

The structures, Curie temperature, dielectric relaxor behaviors, ferroelectricity, ferromagnetism, and magnetocapacitance of the (1?x)Ba_0.70Ca_0.30TiO₃–xBiFeO₃ [(1?x)BCT–xBF, x = 0–0.90] solid solutions have been systematically investigated. The ceramics have coexisted tetragonal (T) and orthorhombic (O) phases when x ≤ 0.06, coexisted pseudocubic (PC) and O phases when x = 0.065, coexisted cubic and O phases when 0.07 ≤ x ≤ 0.12, PC phase when 0.21 ≤ x ≤ 0.42, coexisted T and rhombohedral (R) phases when 0.52 ≤ x ≤ 0.70, and R phase when x ≥ 0.75. Significantly, composition‐dependent microstructures and Curie temperature are observed, the average grain size increases from 1.9 μm for x = 0, reaches 12.0 μm for x = 0.67, and then decreases to 1.3 μm for x = 0.90. At room temperature, the ceramics with x = 0.42–0.70 show piezoelectric properties and multiferroic behaviors, characterized by the polarization‐electric field, polarization current intensity–electric field, and magnetization–magnetic field curves, the composition with x = 0.67 has maximum polarization, remnant polarization, maximum magnetization, and remnant magnetization of 15.0 μC/cm², 9.1 μC/cm², 0.33 emu/g, and 0.14 emu/g, respectively. In addition, the magnetocapacitance is evidenced by the increased relative dielectric constant with increasing the applied magnetic field (H). With ΔH = 8 kOe, the composition with x = 0.67 shows the largest values of (ε_r(H) ? ε_r(0))/ε_r(0) = 2.96% at room temperature. The structure–property relationship is discussed intensively.     

Dielectric and ferroelectric properties of lanthanum‐modified lead zirconate stannate titanate (42/40/18) ceramics

The effect of lanthanum (La) content on the phase transformation of Pb_1?3x/2La_x(Zr_0.42Sn_0.40Ti_0.18)O₃ (PLZST 100x/42/40/18, 0 ≤ x ≤ 0.06) ceramics was investigated by the dielectric and ferroelectric properties. The base composition PLZST 0/42/40/18 located in the ferroelectric (FE) rhombohedral phase region. As x increased, the compositions showed successively FE and antiferroelectric (AFE) state at room temperature, and their peak temperatures (T_max) decreased gradually in line as T_max = 162.21‐1507x. Evidence was presented that there were two dielectric anomalies in PLZST 2/42/40/18, which were corresponding to the FE‐AFE and AFE‐paraelectric (PE) phase transformations, respectively. With increasing the dc bias fields, the two phases merged into one. PLZST 3/42/40/18 showed AFE characteristics with the first loop outside of the second loop and there was only one dielectric inflection. The critical lanthanum content occurred at x = 0.03 from the dielectric temperature spectra and hysteresis loops. Furthermore increase in La above 0.03, these compositions showed typical antiferroelectric behaviors with double hysteresis loops. The stored energy properties of the three compositions (PLZST 4/42/40/18, 5/42/40/18 and 6/42/40/18) displayed different temperature dependencies from room temperature to 140°C (over their respective T_max). Comparing the above results with previous investigations on PLZSTs, some questions were discussed.     

Novel High‐Temperature Antiferroelectric‐Based Dielectric NaNbO3–NaTaO3 Solid Solutions Processed in Low Oxygen Partial Pressures

The (1?x)NaNbO₃–(x)NaTaO₃ solid solution was investigated for x ≤ 0.4 in terms of new high‐temperature and high‐permittivity dielectric system that is suitable for base metal inner electrode capacitor applications. The addition of Ta significantly enhanced the resistivity of the dielectric, resulting in superior resistivity than the dielectrics‐formulated BaTiO₃ systems that dominate the multilayer ceramic capacitor dielectric devices. The voltage dependence of the permittivity was also superior to BaTiO₃‐based materials, providing higher capacitance at higher temperatures. A transmission electron microscopy study illustrated that the grains had so‐called core‐shell structure. According to the electron diffraction analysis, the core region had an inhomogeneous structure between antiferroelectric and ferroelectric phases, and shell region had an incommensurate ferroelectric‐like structure. The core and shell region had Nb‐ and Ta‐rich composition, respectively, and their interface was compositionally sharp, implying that shell region was formed via a liquid phase during the sintering process with an incongruent Ta dissolution reprecipitation. We anticipate that these or similar materials based on the alkali‐niobate perovskites can be further enhanced to provide capacitor solutions from 150°C to 250°C, which is an important range for a number of new AC–DC invertor and engine control units.     

Dielectric,Ferroelectric, and Piezoelectric Properties of BiFeO3–BaTiO3 Ceramics

Perovskite solid solution ceramics of (1 ? x)BiFeO₃–xBaTiO₃ (1 ? x)BF–xBT, 0.2 ≤ x ≤ 0.45) with high electrical resistivity were prepared by solid‐state reaction method. Actual ferroelectric hysteresis loops and temperature dependence of dielectric constant of the ceramics were obtained. Ceramics of 0.7BF–0.3BT with small rhombohedral distortion show highest remnant polarization (P_r = 26.0 μC/cm²) and piezoelectric coefficient (d₃₃ = 134 pC/N). Compositions with pseudo‐cubic symmetry (intermediate phases) show relaxor‐like dielectric anomaly. The values of P_r and d₃₃ decrease with increasing BT content, from 24.8 μC/cm² and 104 pC/N for 0.65BF–0.35BT to 8.2 μC/cm² and 5 pC/N for 0.55BF–0.45BT.     

Piezoelectric properties of (Na0.5K0.5)(Nb1‐xSbx)O3‐SrTiO3 ceramics with tetragonal‐pseudocubic PPB structure

CuO‐added 0.96(Na_0.5K_0.5)(Nb_1‐xSb_x)O₃‐0.04SrTiO₃ ceramics sintered at the low temperature of 960°C for 10 hours showed dense microstructures and high relative densities. The specimens with 0.0 ≤  x ≤ 0.04 had orthorhombic‐tetragonal polymorphic phase boundary (PPB) structure. Tetragonal‐pseudocubic PPB structure was observed in specimens with 0.05 ≤  x ≤ 0.07, while the specimen with x = 0.08 has a pseudocubic structure. The structural variation in the specimens is explained by the decreases in the orthorhombic‐tetragonal transition temperature and Curie temperature with the addition of Sb⁵⁺ ions. The specimens with 0.05 ≤  x ≤ 0.07, which have tetragonal‐pseudocubic PPB structure, had large electric field‐induced strains of 0.14%‐0.016%. Moreover, these specimens also showed increased d₃₃ values between 280 pC/N and 358 pC/N. In particular, the specimen with x = 0.055 showed particularly enhanced piezoelectric properties: d₃₃ of 358 pC/N, k_p of 0.45, and the electric field‐induced strain of 0.16% at 4.5 kV/mm.     

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.     

Dielectric and Raman Spectroscopic Studies of Na0.5Bi0.5TiO3–BaSnO3 Ferroelectric System

A series of lead‐free perovskite solid solutions of (1 ? x) Na_0.5Bi_0.5TiO₃(NBT)—x BaSnO₃(BSN), for 0.0 ≤ x ≤ 0.15 have been synthesized using a high‐temperature solid‐state reaction route. The phase transition behaviors are studied using dielectric and Raman spectroscopic techniques. The ferroelectric to relaxor phase transition temperature (T_FR) and the temperature corresponding to maximum dielectric permittivity (T_m) are estimated from the temperature‐dependent dielectric data. Dielectric studies show diffuse phase transition around ~335°C in pure NBT and this transition temperature decreases with increase in x. The disappearance of x‐dependence of A₁ mode frequency at ~134 cm^?1 for x ≥ 0.1 is consistent with rhombohedral‐orthorhombic transition. In situ temperature dependence Raman spectroscopic studies show disappearance and discontinuous changes in the phonon mode frequencies across rhombohedral (x < 0.1)/orthorhombic (x ≥ 0.1) to tetragonal transition.     

Low‐Temperature Sintering Li2MoO4/Ni0.5Zn0.5Fe2O4 Magneto‐Dielectric Composites for High‐Frequency Application

The coexistence of Li₂MoO₄ (LMO) and Ni_0.5Zn_0.5Fe₂O₄ (NZO) has been proven and their low‐temperature‐sintered magneto‐dielectric composites (1?x)LMO–xNZO (volume fraction factor x = 0.1, 0.3, 0.5, 0.7) were prepared by the conventional solid‐state reaction method and were sintered below 700°C. It is found that the optimal sample (x = 0.5) has good and relatively stable magneto‐dielectric performance in the frequency range from 10 MHz to 1 GHz with permittivity between 7.14 and 6.84, dielectric loss tangent between 0.09 and 0.02, and permeability between 5.23 and 3.30, magnetic loss tangent between 0.06 and 0.65, respectively. Furthermore, the verified chemical compatibility with silver indicates that the LMO–NZO ceramics are potential for low‐temperature cofired ceramic application and their multifunctional magneto‐dielectric properties also make them for potential applications in electronic devices.     

Crystal structure,dielectric properties,and optical bandgap control in KNbO3–BiMnO3 ceramics

(1 − x)KNbO₃–xBiMnO₃ (0 ≤ x ≤ 0.25) ceramics were prepared by the solid-state reaction method. An X-ray diffraction analysis combined with Raman spectroscopy showed the co-solubility of Bi and Mn in the orthorhombic structure to be less than 5% BiMnO₃. Orthorhombic and pseudocubic symmetries coexist in the 0.05 ≤ x ≤ 0.15 region, coinciding with a bimodal grain size distribution. This coexistence of crystal symmetries is further corroborated by several anomalies in the dielectric behavior, which can be ascribed to structural phase transitions. For x ≥ 0.20, only one dielectric anomaly is detected around 100°C, which is commensurate with in situ Raman spectroscopy analysis. This work also shows that Bi/Mn co-doping can be employed to tailor the bandgap of KNbO_3, which narrows continuously with increasing x, resulting in ∼1-eV narrowing for single-phase x = 0.25. This may offer the possibility to employ this ferroic material in photoresponsive technologies.     

Synthesis and Thermal,Structural, Dielectric,Magnetic and Magnetoelectric Studies of BiFeO3‐MgFe2O4 Nanocomposites

Spinel–perovskite magnetoelectric (ME) nanocomposites xMgFe₂O₄–(1?x)BiFeO₃, x = 0.1, 0.2, 0.3, and 0.4 were synthesized by sol‐gel method and characterized by differential thermal analysis, X‐ray diffraction analysis, dielectric and magnetic measurements. The samples were calcined at various temperatures and then the effect of annealing temperature on structural and magnetic properties was studied. From transmission electron microscopy, the average crystal size was found to be 30–50 nm. The magnetic behavior is found to be dependent on annealing temperature and magnesium ferrite content. The dielectric behavior with frequency and temperature has been modified with the induction of magnesium ferrite. The relative change of dielectric constant with magnetic field was observed in the nanocomposites. This relative change of magnetic field‐induced dielectric constant can also be expressed by Δε ~ γM² (where γ is magnetoelectric coupling coefficient).     

Structural and Electrical Properties of Ni‐Substituted Mg‐Mn Nanoferrite by Coprecipitation Route

Ni²⁺ ions doped on Mg_0.40Mn_0.60‐xNi_xFe₂O₄ compositions with 0.00 ≤ x ≤ 0.60 have been synthesized by coprecipitation method and taken for the present work to study the dielectric properties and impedance characterization using the XRD and electrical measurements. The X‐ray diffraction and FT‐IR revealed that the ferrite has single‐phase cubic spinel structure. The calculated particle size from XRD data verified using SEM as well as AFM. These photographs show that the ferrites have crystalline size in the range of 20–50 nm. It was observed that the particle size decreased and Ni concentration increased. The dielectric constant and dielectric loss decreased with increase in nonmagnetic Ni²⁺ ions. Electrical properties indicate that synthesized nanoferrite particles have high resistivity.     

Effect of dysprosium substitution on crystal structure and physical properties of multiferroic BiFeO3 ceramics

The polycrystalline samples of multiferroic Bi_1−xDy_xFeO₃ (x = 0, 0.1, and 0.2) were prepared by a modified solid state reaction method and characterized by X-ray diffraction, scanning electron microscopy, differential thermal analysis, dielectric and magnetic measurements. It was shown that the introduction of the Dy³⁺ ions stabilizes the perovskite structure and improves phase purity. The coexistence of the rhombohedral and orthorhombic phases was found to exist within the investigated concentration range 0.1 ≤ x ≤ 0.2. The changes and anomalies observed in dielectric response over a wide frequency range were correlated with the structural evolution and the development in microstructure. The SQUID measurements of the field-dependent magnetization at different temperatures demonstrated Dy doping to be a very effective method for inducing a weakly ferromagnetic state in the ferroelectric R3c phase of BiFeO₃ in the absence of an external magnetic field.     

Order–Disorder Phase Transition and Magneto‐Dielectric Properties of (1−x)LiFe5O8–xLi2ZnTi3O8 Spinel‐Structured Solid Solution Ceramics

In this study, the spinel solid solution ceramics (1?x)LiFe₅O₈–xLi₂ZnTi₃O₈ (0 ≤ x ≤ 1) were prepared via the solid‐state reaction method. The phase evolution, sintering behaviors, microstructures, magneto‐dielectric properties, and microwave dielectric properties were systematically investigated. The XRD and SEM analysis indicated that the LiFe₅O₈ phase and the Li₂ZnTi₃O₈ phase were almost fully soluble in each other at any proportion. Meanwhile, the evidence of ionic substitution has been directly observed at the atomic scale by means of scanning transmission electron microscopy, which is further confirmed by the Raman spectroscopy. Evidence shows that the magnetic and dielectric properties are quite sensitive to the compositions. The optimal results with remarkable magneto‐dielectric properties of μ′ = 38.2, tanδ_μ = 0.25, ε′ = 19.6, tanδ_ε = 8 × 10^?3 at 1 MHz, and ε′ = 19.1, Q × f = 10 400 GHz at about 7 GHz have been obtained in 0.25LiFe₅O₈–0.75Li₂ZnTi₃O₈ sample. The design of complex spinel solid solution can generate novel magneto‐dielectric single‐phase ceramics combining both high permeability and good dielectric properties, which provides a way in developing multifunctional materials for applications in electronic devices.     

Dielectric Relaxations and Phase Transition in Laser Crystals Gd2SiO5 and Yb‐Doped Gd2SiO5

The crystals Gd₂SiO₅ and Yb‐doped Gd₂SiO₅ were grown by Czochralski method successfully. Dielectric properties of the two crystals were investigated in the temperature range from 100 to 1100 K and the frequency range from 20 Hz to 10 MHz. Our results revealed that these crystals show intrinsic dielectric behavior in the temperature below room temperature, whereas in above room temperature, a relaxor‐like dielectric anomaly and a metal–insulator transition were observed. It was found that the relaxor‐like anomaly contains two dielectric relaxations. Impedance analysis showed that the low‐ and high‐temperature relaxations are dipolar‐ and Maxwell–Wagner‐type relaxation, respectively. The transition was found to be strongly related to oxygen vacancies, which leads to positive temperature coefficient of resistance and an abnormal dielectric behavior contrary to the conventional thermally activated behavior at temperatures near the transition temperature.     

Low‐Temperature Relaxations Associated with Mixed‐Valent Structure in Sr2TiMnO6

The dielectric properties of Sr₂TiMnO₆ ceramic samples were investigated as functions of temperature (100 K ≤ T ≤ 320 K) and frequency (100 Hz ≤ f ≤ 10 MHz). Two thermally activated dielectric relaxations were observed. The sample was confirmed to possess multivalent states of Mn and Ti ions and the coexistence of electron holes and electrons. Our results revealed that both relaxations are bulk effect related to localized carriers hopping inside grains. It was suggested that the low‐temperature relaxation (LTR) can be related to dipolar effect due to electron holes, and the high‐temperature relaxation (HTR) was associated with the electrons hopping between Ti³⁺ and Ti⁴⁺ ions.     

Thermally Stable BaTiO3‐Bi(Mg2/3Nb1/3)O3 Solid Solution with High Relative Permittivity in a Broad Temperature Usage Range

A combined X‐ray diffraction (XRD), Raman spectra, X‐ray photoelectron spectroscopy, Scanning electron microscopy, and dielectric characterization of (1–x)BaTiO₃?xBi(Mg_2/3Nb_1/3)O₃ ceramic system were investigated for compositions of 0 ≤ x ≤ 0.2. Single‐phase perovskite‐type XRD patterns were observed for all compositions. A systematically structural change from tetragonal to pseudocubic symmetry occurred at 0.04 < x < 0.06, which agrees well with the analysis of Raman spectra. Dielectric measurements indicated that the crossover from a classic ferroelectric to relaxor ferroelectric occurred at x ≥ 0.04. Compared with other compositions, the temperature independence of relative permittivity at T > T_m significantly ameliorated at x = 0.1: near‐stable temperature coefficient of higher relative permittivity (~6800 ± 15%) and the corresponding loss tanδ ≤ 0.09 over a more broader temperature range of 25°C–240°C (1 kHz), which indicates that this ceramic is a promising dielectric material for elevated temperature dielectrics.     

Temperature stability and electrical properties in La‐doped KNN‐based ceramics

To improve the temperature stability and electrical properties of KNN‐based ceramics, we simultaneously consider the phase boundary and the addition of rare earth element (La), 0.96K_0.5Na_0.5Nb_0.96Sb_0.04O₃‐0.04(Bi_1‐xLa_x)_0.5Na_0.5ZrO₃ (0 ≤ x ≤ 1.0) ceramics. More specifically, we investigate how the phase boundary and the addition of La³⁺ affect the phase structure, electrical properties, and temperature stability of the ceramic. We show that increasing the La³⁺ content leads to a change in phase structure, from a rhombohedral‐tetragonal (R‐T) phase coexistence to a cubic phase. More importantly, we show that the appropriate addition of La³⁺ (x = 0.2) can simultaneously improve the unipolar strain (from 0.127% to 0.147%) and the temperature stability (i.e., the unipolar strain of 0.147% remains unchanged when T is increased from 25 to 80°C). In addition, we find that the ceramics with x = 0.2 exhibit a large piezoelectric constant (d₃₃) of ~430 pC/N, a high Curie temperature (T_C) of ~240°C and a fatigue‐free behavior (after 10⁶ electric cycles). The enhanced electrical properties mostly originate from the easy domain switching, whereas the improved temperature stability can be attributed to the R‐T phase boundary and the appropriate addition of La³⁺.     

Effects of A1/A2‐Sites Occupancy upon Ferroelectric Transition in (SrxBa1−x)Nb2O6 Tungsten Bronze Ceramics

The dielectric and ferroelectric characteristics of (Sr_xBa_1?x)Nb₂O₆ unfilled tungsten bronze ceramics have been investigated together with the structure. The dielectric and ferroelectric characteristics of the present ceramics vary significantly with x, and the A1/A2‐sites occupancy has been determined as the primary parameter governing this variation tendency. Ba ions tend to occupy A2‐sites, Sr ions tend to occupy A1‐sites, and one A1‐site is empty. When the ratio of Sr/Ba is close to 1:4 (where four A2‐sites are just occupied by Ba ions, and one A1‐site is occupied by Sr ion while another A1‐site is empty), the normal ferroelectric transition is observed with one anomaly in the tanδ–T curve (x = 0.25). When the ratio of Sr/Ba is far away from 1:4, the typical relaxor behavior is indicated together with three anomalies in the tanδ–T curve (x = 0.75). The incommensurate oxygen octahedral tilting and A‐site random distribution are considered to be the structure origins for the relaxor ferroelectricity and low temperature dielectric relaxations.