Dielectric,multiferroic and magnetodielectric properties of (1-x)BaTiO3-xSr2CoMoO6 solid solution

The magnetoelectric composites of (1-x)BaTiO₃-xSr₂CoMoO₆ (x=0.02, 0.04, 0.06, 0.08, 0.10 and 0.12) are synthesized successfully via solid state reaction. The dielectric, multiferroic and magnetodielectric properties were discussed in details. The XRD data confirm the single perovskite phase among the composites. In addition, as the content of Sr₂CoMoO₆ increase, the diffraction peaks shift to higher angle which indicates the gradually decreased lattice parameters. The observed dielectric relaxation phenomenon could be attributed to Maxwell-Wagner (MW) model. The weak ferromagnetism was observed for x=0.02, 0.04 with the remnant magnetization (M_r) 0.025 memu/g, 0.065 memu/g and coercivity (H_c) 4 Oe, 10 Oe, respectively despite the anti-ferromagnetic nature of Sr₂CoMoO₆. Furthermore, the M − H hysteresis loops of (1-x)BaTiO₃-xSr₂CoMoO₆ change obviously with polarization state, which is related to the ordering degree of the Co and Mo sites. The reason for observed magnetodielectric effect is the influence of magnetoresistance (MR) on Maxwell-Wagner (MW) effect.     

Room temperature magnetoelectric coupling study in multiferroic Bi4NdTi3Fe0.7Ni0.3O15 prepared by a multicalcination procedure

Single-phase Bi₄NdTi₃Fe_0.7Ni_0.3O₁₅ polycrystalline samples were synthesized following a multicalcination procedure. The sample exhibited multiferroic property at room temperature, which was demonstrated by the ferroelectric (2P_r=8.52 μC/cm², 2E_c=89 kV/cm at applied electric field 110 kV/cm) and magnetic (2M_r=388 m emu/g, 2H_c=689 Oe at applied magnetic field 1.04 T) hysteresis loops. More importantly, magnetoelectric coupling effect is observed from measurements of electrical properties not only under small but also under large electric signal when an external magnetic field is applied. The present results suggest a new candidate for a room temperature multiferroic material with magnetoelectric coupling effect.     

Magnetoelectricity in CoFe2O4 nanocrystal-P(VDF-HFP) thin films

Transition metal ferrites such as CoFe₂O₄, possessing a large magnetostriction coefficient and high Curie temperature (T_c > 600 K), are excellent candidates for creating magnetic order at the nanoscale and provide a pathway to the fabrication of uniform particle-matrix films with optimized potential for magnetoelectric coupling. Here, a series of 0–3 type nanocomposite thin films composed of ferrimagnetic cobalt ferrite nanocrystals (8 to 18 nm) and a ferroelectric/piezoelectric polymer poly(vinylidene fluoride-co-hexafluoropropene), P(VDF-HFP), were prepared by multiple spin coating and cast coating over a thickness range of 200 nm to 1.6 μm. We describe the synthesis and structural characterization of the nanocrystals and composite films by XRD, TEM, HRTEM, STEM, and SEM, as well as dielectric and magnetic properties, in order to identify evidence of cooperative interactions between the two phases. The CoFe₂O₄ polymer nanocomposite thin films exhibit composition-dependent effective permittivity, loss tangent, and specific saturation magnetization (M_s). An enhancement of the effective permittivity and saturation magnetization of the CoFe₂O₄-P(VDF-HFP) films was observed and directly compared with CoFe₂O₄-polyvinylpyrrolidone, a non-ferroelectric polymer-based nanocomposite prepared by the same method. The comparison provided evidence for the observation of a magnetoelectric effect in the case of CoFe₂O₄-P(VDF-HFP), attributed to a magnetostrictive/piezoelectric interaction. An enhancement of M_s up to +20.7% was observed at room temperature in the case of the 10 wt.% CoFe₂O₄-P(VDF-HFP) sample.     

Dielectric properties and magnetoelectric coupling in polar magnet (Fe,Zn)2Mo3O8

A complete understanding of magnetoelectric (ME) coupling in polar magnet Fe₂Mo₃O₈ (FMO) remains elusive. Here, we report the magnetodielectric (MD) and ME effects of (Fe_1-xZn_x)₂Mo₃O₈ (x = 0, 0.05) single crystals. The observation of MD effect in FMO shows a sharp peak once T is below T_N due to the metamagnetic transition, which is converted into an obvious step upon cooling. Meanwhile, the change of magnetization M modulated by electric field E is striking at the dielectric constant peak/step. In (Fe_0.95Zn_0.05)₂Mo₃O₈, however, the sharp peak at just below T_N is intact, while the step at much lower T_N is highly damaged. The ME response to 5% Zn²⁺ dopants at the two T-ranges shows different behavior. Combining with spin wave theory, it is possible to speculate that the inverse Dzyaloshinskii-Moriya interaction might play a dominant role in the ME nature at just below T_N, which is different from that at much lower T_N.     

Sintering behavior and microwave dielectric properties of a new low-permittivity ceramic system Ca(Mg1−xAlx)(Si1−x/2Alx/2)2O6

The diopside ceramics with a formula of Ca(Mg_1−xAl_x)(Si_1−x/2Al_x/2)₂O₆ (x=0.01–0.3) were synthesized via a traditional solid-state reaction method, and their solid solubility, sintering behavior and microwave dielectric properties were investigated. The results revealed that the solubility limit of Al₂O₃ in Ca(Mg_1−xAl_x)(Si_1−x/2Al_x/2)₂O₆, which is defined as x, was between 0.15 and 0.2, and a second phase of CaAl₂SiO₆ presented when the x value reached 0.2. Appropriate Al³⁺ substitution for Mg²⁺ and Si⁴⁺ could promote the sintering process and lower the densification temperature, and a broadened densification temperature range of 1250–1300 °C was obtained for the compositions of x=0.08–0.15. With the increase of the x value, the dielectric constant (ε_r) increased roughly linearly, and the temperature coefficient of frequency (τ_f) showed a rising trend. The Q×f values increased from 57,322 GHz to 59,772 GHz as the x value increased from 0.01 to 0.08, and then they were saturated in the range of x=0.08–0.2. Further increase of the x value (x≥0.25) deteriorated the microwave dielectric properties. Good microwave dielectric properties of ε_r=7.89, Q×f=59,772 GHz and τ_f=−42.12 ppm/°C were obtained for the ceramics with the composition of x=0.08 sintered at 1275 °C.     

0-3 type magnetoelectric 0.94Na0.5Bi0.5TiO3-0.06BaTiO3:CoFe2O4 composite ceramics with a deferred thermal depolarization

Developing Na_0.5Bi_0.5TiO₃-based magnetoelectric (ME) coupling composites with higher depolarization temperature is highly valuable for the environment-friendly smart electronic devices. We have developed a new kind of 0-3 type 0.94Na_0.5Bi_0.5TiO₃-0.06BaTiO₃:xCoFe₂O₄ (NBTBT:xCFO, x = 0, 0.1, 0.2, 0.3) composite ceramics with a deferred depolarization temperature, together with an additional strong ME coupling of 9.2 mV/cm·Oe for the NBTBT:0.2CFO. The basic structure, ferroelectric/ferromagnetic properties, and the depolarization temperature of the NBTBT:xCFO composite ceramics were investigated. It was found that an enhancement of depolarization temperature (>25 °C) was obtained in these 0-3 type composites relative to the pure NBTBT ones (115 °C vs 90 °C). The mechanism of the enhanced depolarization temperature of the composites is discussed. The present results demonstrate that NBTBT:xCFO composites have great potential for ME devices.     

Compositional dependence of phase structure and electrical properties in (K0.50Na0.50)0.97Bi0.01(Nb1−xZrx)O3 lead-free ceramics

(K_0.50Na_0.50)_0.97Bi_0.01(Nb_1-xZr_x)O₃ (KNBNZ) lead-free ceramics were prepared by the conventional solid-state sintering process. Their phase structure is dependent on the Zr content in the investigated range, and the ceramics endure a phase transition from pseudocubic to orthorhombic with increasing Zr content. Improved piezoelectric properties have been observed when the poling temperature is located at ~100 °C because of the coexistence of orthorhombic and tetragonal phases. Their dielectric and piezoelectric properties were enhanced by doping Zr, the ceramic with x=0.02 showing optimal electrical properties, i.e., d₃₃~161 pC/N, k_p~0.41, Q_m~81, T_c~370 °C, and T_o−t~130 °C. These results show that the KNBNZ ceramic is a promising lead-free piezoelectric material.     

Preparation and magnetoelectric properties of NiFe2O4–PZT composites obtained in-situ by gel-combustion method

Magnetoelectric composites of xNiFe₂O₄–(1 ? x)Pb(Zr,Ti)O₃ with x = 2, 5, 10, 20, 30% were prepared by citrate–nitrate combustion using PZT-based template powders. In order to ensure a better connectivity of dissimilar phases, we have used chemical methods for preparation in situ composites, followed by adequate sintering procedure. The structural, microstructural and functional properties of di-phase magnetoelectric composites of NiFe₂O₄–PZT are reported. The XRD analysis is demonstrating the synthesis of pure ferrite phase directly on the ferroelectric templates. An excellent mixing was obtained in the composite powders, as proved by a detailed SEM analysis.The magnetic and dielectric behaviors of the ceramic composites vary with the ratio of the two phases. The dielectric behavior is greatly influenced by the magnetic phase. The magnetoelectric (ME) coefficient was measured as a function of applied DC magnetic field. The maximum ME coefficient (dE/dH) varies from 0.0011 mV/(cm Oe) to 0.5 mV/(cm Oe) with increasing of NF addition.     

Effect of donor dopants cerium and tungsten on the dielectric and electrical properties of high Curie point ferroelectric strontium niobate

The perovskite-like layered structure (PLS) A₂B₂O₇ compound Sr₂Nb₂O₇ was doped with donor dopants CeO₂ and WO₃ to explore their doping effect on its A and B site, respectively. The doped ceramics were prepared by Spark Plasma Sintering. For Ce doping on the A site, single phase was maintained up to 5 mol% Ce (x=0.05 in (Sr_1−xCe_x)₂Nb₂O₇). For W doping on the B site, single phase was maintained at 2.5 mol% W (x=0.025 in Sr₂(W_xNb_1−x)₂O₇). The cerium and tungsten doping both inhibited grain growth and changed the grain morphology, leading to less anisotropic grains. The Curie point T_c was obtained by measuring the temperature dependence of the dielectric constant and it was found to reduce for both Ce and W doped SNO. The W doped ceramics showed a diffuse ferroelectric phase transition at the Curie point. The DC resistivity of tungsten and cerium doped SNO increased compared to undoped SNO at temperatures below 700 °C. These results showed that both Ce and W had a strong influence on the dielectric and electrical properties of the Sr₂Nb₂O₇ ceramics.     

The effect of Bi2O3 compensation during thermal treatment on the crystalline and electrical characteristics of bismuth titanate thin films

Bismuth titanate thin films are deposited on ITO/glass substrates by rf magnetron sputtering at room temperature using a Bi₄Ti₃O₁₂ ceramic target. The deposited Bi₄Ti₃O₁₂ films are annealed in a conventional furnace in ambient air for 10 min at temperatures ranging from 550 to 640 °C. One specimen is annealed in a crucible containing additional Bi₂O₃ compensation powder, while the other specimen is annealed in ambient air. XRD analysis shows that the crystal phases of films annealed with Bi₂O₃ powder are better than those of films annealed without Bi₂O₃ powder. Furthermore, the EDS results reveal that the bismuth weight percentage of the former is higher than that of the latter. SIMS analysis shows that the bismuth decreases near the surface of Bi₄Ti₃O₁₂ film annealed without Bi₂O₃ powder, but reveals a stable distribution throughout the film annealed with Bi₂O₃ powder. These results imply that bismuth is readily evaporated during the thermal treatment process, particularly from the region near the film surface. Finally, the dielectric and polarization properties of the thin films annealed with Bi₂O₃ powder are found to be superior to those of the films annealed in ambient air.     

Preparation and characterization of novel spinel Li4Ti5O12−xBrx anode materials

Br-doped Li₄Ti₅O₁₂ in the form of Li₄Ti₅O_12−xBr_x (0 ≤ x ≤ 0.3) compounds were successfully synthesized via solid state reaction. The structure and electrochemical properties of the spinel Li₄Ti₅O_12−xBr_x (0 ≤ x ≤ 0.3) materials were investigated. The Li₄Ti₅O_12−xBr_x (x = 0.2) presents the best discharge capacity among all the samples, and shows better reversibility and higher cyclic stability compared with pristine Li₄Ti₅O₁₂, especially at high current rates. When the discharge rate was 0.5 C, the Li₄Ti₅O_12−xBr_x (x = 0.2) sample presented the excellent discharge capacity of 172 mAh g⁻¹, which was very close to its theoretical capacity (175 mAh g⁻¹), while that of the pristine Li₄Ti₅O₁₂ was 123.2 mAh g⁻¹ only.     

Temperature dependent magnetic properties of CoFe2O4/CTAB nanocomposite synthesized by sol–gel auto-combustion technique

A CoFe₂O₄/cetyl trimethylammonium bromide (CTAB) nanocomposite has been fabricated by a sol–gel auto-combustion method. Characterization of the material revealed the composition of the crystalline phase as CoFe₂O₄ while FT-IR confirmed the presence of CTAB on the nanoparticles. From X-ray line profile fitting, average crystallite size was estimated to be 22±6 nm. SEM analysis showed a porous sheet-like morphology with internal nanosize grains of about 30 nm. The room temperature coercive field (H_c) of the CoFe₂O₄/CTAB nanocomposite was found to be 1045 Oe which is close to the previously reported room temperature values for bulk CoFe₂O₄. The H_c was observed to decrease almost linearly with the square root of the temperature (√T) according to Kneller's law. From the linear fit of H_c versus √T, the zero-temperature coercivity (H_c0) and superparamagnetic blocking temperature (T_B) of the CoFe₂O₄/CTAB nanocomposite were found to be ∼9.1 kOe and ∼425 K, respectively. The remanence magnetization (M_r), the reduced remanent magnetization (M_r/M_s), and the effective magnetic anisotropy (K_eff) decrease with increasing temperature. The M_r/M_s value of 0.6 at 10 K higher than the theoretical value of 0.5 for non-interacting single domain particles with the easy axis randomly oriented suggests the CoFe₂O₄/CTAB nanocomposite to have cubic magnetocrystalline anisotropy according to the Stoner–Wohlfarth model.     

The Direct and the Converse Magnetoelectric Effect in Multiferroic Cobalt Ferrite–Barium Titanate Ceramic Composites

We report on a systematic study of the magnetoelectric effect in cobalt ferrite (CoFe₂O₄)—barium titanate (BaTiO₃) ceramic composites with (0‐3) connectivity. Both the converse magnetoelectric coefficient, α_C, and the direct voltage magnetoelectric coefficient, α^E, were measured in dependence on composition and electric and magnetic bias fields. The strongest ME effect was observed in the composition (1?x) CoFe₂O₄–xBaTiO₃ with x = 0.5 yielding values α_C = 25 psm^?1 and α^E = 3.2 mV/(cm·Oe). We show that the proper conversion between these two coefficients demands knowledge about the dielectric permittivity of the sample. For low BaTiO₃ content the dielectric coefficient of the composite yields a better correspondence, whereas for high BaTiO₃ content the sample's average dielectric coefficient yields a better match. The influence of mutual orientation of polarization and magnetization on the ME effect is addressed. We found that for measurements performed parallel to the polarization direction (longitudinal effect), the ME coefficient is approximately twice as large and of opposite sign in comparison to the measurements perpendicular to the polarization direction (transverse effect). This difference has been rationalized in terms of the different contributions of the material coefficient tensor components to the ME effect, the demagnetizing factor, and losses. The obtained results provide a better understanding of peculiarities of the ME effect in bulk ceramic composites.     

Study of crystal structure,dielectric, magnetic and magnetoelecrtic properties of xCoFe2O4-(1-x)Na0.5Bi0.5TiO3 composites

Multiferroic composites of spinel ferrite and ferroelectric xCoFe₂O₄ – (1-x)Na_0.5Bi_0.5TiO₃ (with x = 0.10,0.30,0.50) were efficiently prepared by standard solid state reaction mechanism. X-ray diffractometer was used to analyze crystal structure of the prepared composites. The observed XRD patterns of the composites comprise peaks of both the phases i.e. ferrite and ferroelectric, with no sign of secondary peaks. Rietveld refinement of XRD data further confirms the coexistence of these two phases with cubic (Fd3m) and rhombohedral (R3c) symmetry corresponding to ferrite and ferroelectric phase respectively. The 3-dimensional overview of crystal structure of pure CoFe₂O₄ and Na_0.5Bi_0.5TiO₃ and of composite 0.50CoFe₂O₄?0.50Na_0.5Bi_0.5TiO₃ is generated by using refined parameters. The dielectric constant (ε´) and dielectric loss (tanδ) values were recorded as a function of frequency ranging from 100?Hz to 7?MHz and at different temperatures. Both ε´ and tanδ follow dispersion pattern at lower frequencies while show frequency independent behavior at higher frequencies. The magnetic evaluation carried by analyzing M-H hysteresis loop reveals the ferrimagnetic characteristics of these composites. The highest value of magnetic moment is 1.12μ_B observed for composite 0.50CoFe₂O₄ – 0.50Na_0.5Bi_0.5TiO_3. Magnetoelectric (ME) voltage coefficient (α) was also demonstrated to observe the interaction between ferrite and ferroelectric phases. The highest value of α (72.72μV/Oe cm) is obtained for low ferrite composition 0.10CoFe₂O₄ – 0.90Na_0.5Bi_0.5TiO₃, which suggests the dependence of magnetoelectric response on the resistivity of the composites.     

Room temperature multiferroic properties of Ni-doped Aurivillus phase Bi5Ti3FeO15

Single-phase Aurivillius Bi₅Ti₃Fe_0.5Ni_0.5O₁₅ (BTFN) ceramics were synthesized by the solid-state reaction method. The substitution of Ni for half Fe ions does not introduce magnetic impurity phase but increases magnetic moment more than two orders. The ferroelectric and magnetic Curie temperatures are determined to be 1100 K and 726 K. The room-temperature multiferroic behavior of the BTFN ceramics were demonstrated by the ferroelectric (2P_r=8.5 μC/cm², 2E_c=74 kV/cm) and ferromagnetic (2M_r=27.86 m emu/g, 2H_c=553 Oe) measurements. The ferromagnetism can be ascribed to the aggregation of magnetic ions at the inner octahedra by Ni doping and the spin canting of magnetic-ion-based sublattices via the Dzyaloshinskii-Moriya interaction. The present work suggests the possibility of doped Bi₅Ti₃FeO₁₅ as a potential room-temperature multiferroic.     

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).     

Sol–gel auto combustion synthesis of CoFe2O4/1-methyl-2-pyrrolidone nanocomposite: Its magnetic characterization

A magnetic nanocomposite was generated by the sol–gel auto-combustion method in the presence of 1-methyl-2-pyrrolidone, a functional solvent. The temperature-dependent magnetic properties of the CoFe₂O₄ nanoparticles have been extensively studied in the temperature range of 10–400 K and magnetic fields up to 80 kOe. Zero field cooled (ZFC) and field cooled (FC) curves indicate that the blocking temperature (T_B) of the CoFe₂O₄ nanoparticles is above 400 K. It was found from M–H curves that the low temperature saturation magnetization values are higher than bulk value of CoFe₂O₄. The saturation magnetization (M_s), remanence magnetization (M_r), reduced remanent magnetization (M_r/M_s) and coercive field (H_c) values decrease with increasing temperature. The M_r/M_s value of 0.75 at 10 K indicates that the CoFe₂O₄ nanoparticles used in this work have, as expected, cubic magnetocrystalline anisotropy according to the Stoner–Wohlfarth model. T^1/2 dependence of the coercive field was observed in the temperature range of 10–400 K according to Kneller's law. The extrapolated T_B and the zero-temperature coercive field values calculated according to Kneller's law are almost 427 K and 13.2 kOe, respectively. The room temperature H_c value is higher than that of the previously reported room temperature bulk values. The effective magnetic anisotropy constant (K_eff) was calculated as about 0.23×10⁶ erg/cm³ which is lower than that of the bulk value obtained due to disordered surface spins.     

Studies on electrical properties of La0.8Sr0.2Ga0.8Mg0.2O2.80 (LSGM) and LSGM-SrSn1−xFexO3 (x = 0.8; 0.9) composites and their chemical reactivity

We report the electrical conductivity properties of solid-state synthesized perovskite-like La_0.8Sr_0.2Ga_0.8Mg_0.2O_2.80 (LSGM) and LSGM-SrSn_1−xFe_xO₃ (x = 0.8; 0.9) composites. LSGM exhibits both bulk and grain-boundary contribution in the ac impedance plots. The grain-boundary conductivity (σ_gb) is slightly (≤half-order of magnitude) higher than that of the bulk oxide ion conductivity (σ_bulk). Powder XRD study reveals that no chemical reaction occurs between LSGM and SrSn_1−xFe_xO₃ (1:1 wt.%) at 1000 °C (48 h) and forms a single-phase perovskite-like compound at 1300 °C (48 h) in air, while in hydrogen atmosphere, at 800 °C for 48 h, a growth of LaSrGaO₄ and LaSrGa₃O₇ impurity phases and formation of metallic Fe was observed. The LSGM-SrSn_1−xFe_xO₃ (x = 0.8; 0.9) composites show a single or part of semicircle in air at low-temperature regime. The electrical conductivity of the composites were found to be much higher compared to pure LSGM and lower about an order of magnitude than those of pure Sn-doped SrFeO₃ perovskite.     

Waste Form of Silver Iodide (AgI) with Low-Temperature Sintering Glasses

This study focuses on the fabrication and characterization of waste forms of silver iodide (AgI) using low-temperature sintering glasses. We investigated the glass composition of bismuth-phosphate-zinc oxide, which enables low-temperature sintering to fabricate waste forms of AgI. We confirm that novel waste forms were fabricated with Bi₂O₃-P₂O₅-M_xO_y glasses, where M_xO_y is a metal oxide, such as ZnO, CaO (or CaCO₃), MgO, or Na₂O (or Na₂CO₃), at sintering temperatures of 600–650°C. The samples were characterized using X-ray diffraction (XRD), thermogravimetric analysis (TGA), scanning electron microscopy coupled with energy-dispersive spectroscopy (SEM-EDS), inductively coupled plasma-atomic emission/absorption spectrometry (ICP-AES/AAS), and ion chromatography (IC) used for product consistency testing (PCT). Our findings indicate that chemically durable waste forms of AgI were formed and that the loss of iodine during heat treatment was effectively suppressed by a low sintering temperature.     

Effect of CoFe2O4 mole percentage on multiferroic and magnetoelectric properties of Na0.5Bi0.5TiO3/CoFe2O4 particulate composites

Na_0.5Bi_0.5TiO₃ (NBT), CoFe₂O₄ (CFO) as well as particulate composites containing different mole percentages of NBT and CFO were synthesized by the solid-state sintering route and characterized for their ferroelectric and ferrimagnetic hysteresis loops, magnetostriction and magnetoelectric (ME) output. The mole% of CFO was found to influence the ferroelectric and ferrimagnetic hysteresis loops as well as magnetostriction and piezomagnetic coefficients which in turn had a significant effect on the magnetoelectric voltage coefficient. The highest magnetoelectric voltage coefficient (α) of 0.5 mV/cm/Oe was recorded in (65) NBT–(35) CFO composite.