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The multiferroic fluids has an obvious magnetodielectric effects, and presents large magnetoelectric coupling coefficient of 89.8 V (cm Oe)−1.

To further the exploration of perovskite nanowires, TiO2@CH3NH3PbI3 (TiO2@MAPbI3) core–shell nanowire arrays were successfully prepared via immersion and spin-coating methods. Because the shell thickness has a significant influence on the carrier transport capacity of nanowire arrays, different shell thicknesses were obtained by changing the precursor concentration. Subsequently, the relationship between the precursor concentration and shell thicknesses and the resulting properties of the nanowire arrays were investigated. The X-ray diffraction results showed that the prepared nanowire arrays consisted of only MAPbI3, TiO2, and fluorine-doped tin oxide phases, with no impurities. From the scanning electron microscopy and energy dispersive X-ray spectroscopy results, the MAPbI3 shell material was successfully coated onto the core layer of the TiO2 nanowire arrays. In addition, the average size of the core–shell nanowire arrays and the shell thickness were obtained using scanning electron microscopy and related software analyses. The results showed that the shell thickness was the largest (40 nm) when the precursor concentration was the lowest (0.025 mol L−1). Ultraviolet–visible spectroscopy showed that when the precursor concentration was 0.025 mol L−1 and the shell thickness was the largest, the nanowire array exhibited the highest absorbance and the smallest band gap, which is conducive to generating more carriers and improving its photovoltaic performance; the J–V curve showed the highest photoelectric conversion efficiency at this concentration and shell thickness. Therefore, it can be inferred that the shell thickness may affect the optical and photovoltaic properties. The relationship between the precursor concentration and thickness as well as the influence of this relationship on the properties of core–shell nanowire arrays should be further explored, to establish a foundation for the use of perovskite nanowires in the photovoltaic field.

Multiferroic materials attracted much attention because of magnetoelectric (ME) coupling effect. Herein, a typical single-phase multiferroic BiFeO3 (BFO) was co-doped at A and B sites by solid-state method, and Bi0.78La0.08Sm0.14Fe0.85Ti0.15O3 (BLSFTO) ceramics were prepared at different sintering temperatures (940, 960, 980 and, 1000?C). The effects of sintering temperature on the microstructure, morphology, dielectric, ferroelectric and magnetic properties were systematically studied. The remanent polarization (Pr) and coercive field (Ec) of the BLSFTO ceramics show a non-linear change with sintering temperature. The remanent polarization reaches maximum (0.0560 ?C/cm2) in the sample sintered at 1000?C, whereas the maximum Ec of 1.32 kV/cm was obtained in the specimen sintered at 940?C. The remanent magnetization (Mr) increases with the increase of sintering temperature, while change of the saturation magnetization with sintering temperature is negligible. When BLSFTO specimen is sintered at 1000?C, Mr reaches the maximal value of 0.1344 emu/g.

The Ca3Ti2O7 materials exhibit hybrid improper ferroelectricity due to the rotation and tilt coupling of TiO6 octahedron. However, the lower remnant polarization of Ca3Ti2O7 ceramics obtained by conventional solid-state method limits its practical application. Herein, the single-phase Ca3Ti2O7 ceramics with enhanced ferroelectricity were synthesized using an acetic acid sol–gel method and two-stage sintering. The sintering process does not significantly change the distortion amplitude of TiO6 octahedron in Ca3Ti2O7 ceramics but has a greater impact on the grain size and the concentration of oxygen vacancy. The synergistic effects of grain size and oxygen vacancy have been achieved in the Ca3Ti2O7 ceramics prepared by the optimal sintering process (1400 °C/12 h − 1550 °C/18 h), which leads to its enhanced hybrid ferroelectricity (Pr = 1.34 μC/cm2 and EC = 120 kV/cm). These findings demonstrate that the sol–gel method is an effective approach to obtain the Ca3Ti2O7 materials with enhanced hybrid improper ferroelectricity.

To enhance the piezoelectric responses, Ba0.85Ca0.15Zr0.1Ti0.9O3 (BCZT) ceramics were fabricated by a sol–gel method in various sintering atmospheres. The effects of oxygen partial pressure on the microstructure and electrical properties of BCZT ceramics were systematically investigated. The samples sintered in air, O2 and N2 are the coexistence of the orthorhombic (O)-tetragonal (T) phase at room temperature. The phase transition temperature of the O–T phase of BCZT samples sintered in air, O2 and N2 is gradually closer to room temperature. The results of EDS, XPS and the activation energy confirm the significant effect of oxygen sintering atmosphere. BCZT samples sintered in oxygen atmosphere show excellent ferroelectricity (2Pr = 31.62 μC/cm2, 2EC = 3.46 kV/cm) and significantly enhanced piezoelectricity (d33 = 604.3 pC/N, d 33 * = 647.5 pm/V and kp = 58%), resulting from the minimum oxygen vacancy, the phase transition temperature of O–T closer to room temperature and dense microstructure. These results demonstrate that adjusting oxygen partial pressure in sintering is an effective way to enhance the piezoelectric responses of BCZT ceramics.

Magnetoelectric composite materials have attracted more and more attention because of their coupling of ferroelectricity and ferromagnetism. It is a hotspot to realize the combination of ferromagnetic phase and ferroelectric phase. In this work, we used a new strategy to prepare CoFe2O4/BaTiO3 composite ceramics: firstly, porous ferromagnetic CoFe2O4 phase was prepared by annealing of MOFs (metal organic frameworks) precursor Fe3[Co(CN)6]2. And then, the ferroelectric BaTiO3 phase in-situ grew in the pores of CoFe2O4 by a hydrothermal method. In the end, the CoFe2O4/BaTiO3 composite ceramics sintered at different temperatures have been synthesized. The effects of sintering temperature on the structure, dielectric and ferroelectric properties have also been studied. Because the crystallinity and density increase with the increase of sintering temperature, the composite ceramic sintered at 1200 °C shows the best dielectric properties. It is found that sintering temperature has little effect on the ferroelectric and magnetic properties of ceramics. Taking the CoFe2O4/BaTiO3 composite ceramic sintered at 1200 °C as an example, derived from the interaction between the ferromagnetic CoFe2O4 phase and ferroelectric BaTiO3 phase, the applied magnetic field lead to the reduction of Pr and Ec.

Ba0.85Ca0.15Zr0.9Ti0.1O3 (BCZT) ceramics with various grain sizes were fabricated through solid-state method followed by different sintering temperatures. The influences of grain size and phase structure on the electric properties and energy storage performance were systematically investigated. The remnant polarization and quasi-static piezoelectric constant of BCZT samples increase gradually as grain size increases, and the samples with the grain size of 44.37 µm exhibit excellent ferroelectricity and piezoelectricity (2Pr = 25.4 µC/cm2, d33 = 513 pC/N). Although the energy storage density of BCZT samples with the grain size of 8.28–44.37 µm is relative lower, all the ceramic samples have higher energy storage efficiency (82–87.4%). There is the maximum energy storage density in phase transition temperature of tetragonal–cubic phase for all BCZT ceramics and the maximum energy storage density first increases and then decreases with the increase of grain size. But the energy storage density of BCZT ceramics near room temperature decreases with increasing of grain size. The smaller grain size is beneficial to improve the room temperature energy storage density of BCZT samples and is disadvantageous to obtaining superior ferroelectric and piezoelectric properties.

CoFe2O4 particles with two different sizes were prepared using hydrothermal method by controlling the technological parameters. On that basis, core–shell structure CoFe2O4@BaTiO3 (CFO@BTO) particles were prepared by sol–gel process. The surfactant treated composite particles were then distributed into a highly-insulating heptane to form CFO@BTO composite fluids. Influences of CFO particle size on the magnetodielectric and magnetoelectric coupling effect were comparatively investigated. XRD, TEM and SAED results indicate that the size of CFO particles are ~ 12 and ~ 18 nm, respectively, while the prepared CFO@BTO particles show bi-phase and core–shell composite structure, and the size of CFO@BTO particles are ~ 28 and ~ 35 nm, respectively. The composite fluids show quasi super paramagnetic behavior at room temperature due to the size effect and Brovnian motion. When the size of CFO is 18 nm, the CFO@BTO composite fluids shows larger remnant magnetization and coercive field. Strong dependence of dielectric properties on external magnetic field has been observed, of which the composite fluids show negative magnetodielectric effect, and the value of permittivity was decreased by 8% at 500 Oe. When size of CFO is 12 nm, the corresponding specimen with shows more obvious magnetic response, and it has larger remnant polarization. The values of both coercive electric field and remnant polarization of the two kinds of CFO@BTO composite fluids have been enhanced distinctly under the action of external magnetic field. When the particle size of CFO is smaller, the magnetolectric coupling coefficient αE of CFO@BTO composite fluid is larger, and this value decreases with increasing external magnetic field. When the particle sizes of CFO are 12 and 18 nm, the maximal values of αE are about 7 and 10 V/cm·Oe, respectively. These values are several orders of magnitude larger than that of CFO/BTO and other magnetoelectric composite ceramics. The strong coupling effect can be explained based on the formation, movement and variation of chain-like structure in the presence of external field. These results may supply useful information for enhancing coupling effect and might play important roles in practical applications in novel magnetoelectric devices.

Multiferroic composite ceramics xNi0.15Cu0.25Zn0.6Fe2O4-(1-x) Ba0.85Ca0.15Zr0.1Ti0.9O3 (x = 0, 0.2, 0.3, 0.4, 0.5, 0.6, 0.9 and 1) were prepared by combining chemical co-precipitation method with sol–gel method; the microstructure, dielectric, ferroelectric, magnetic and multiferroic properties were comparatively investigated. XRD results show that all the specimens have obvious bi-phases structure. The grains with larger size are magnetic phase Ni0.15Cu0.25Zn0.6Fe2O4(NCZF), while that of smaller size can be attributed to ferroelectric phase Ba0.85Ca0.15Zr0.1Ti0.9O3(BCZT). The dielectric constant of the sample with x = 0.6 is the largest at low frequency but the specimen x = 0.2 has the highest value of dielectric constant in high-frequency region. When x = 0.9, the ceramic has the largest loss, while it presents the lowest loss value when x = 0.3. The height of the peak decreases with increase in the frequency, and the position of the peak moves to higher temperature range with the decrease in x. The dielectric loss increases sharply with temperature, especially when x = 0.9, when the temperature is higher than 300 °C, the dielectric loss reaches hundreds of times under low frequency. Al samples present apparent ferroelectric hysteresis loops, but further characterization shows that the hysteresis may be attributed to leakage current. The remnant polarization(Pr) and coercive field(Ec) do not monotonically change with x, Pr is 0.1576 μC/cm2 at 2 kHz when x = 0.6. The magnetization monotonically changes with the x, indicating that there is a strong interfacial interaction between the two phases. Under the action of an external magnetic field of 1 mT, the magnetoelectric (ME) coupling is the strongest (relative polarization change 37%) when x = 0.2.

Organic–inorganic hybrid perovskite is currently one of the most promising photovoltaic materials. In the present work, CH3NH3PbI3 nanowires were prepared by saturated vapor-assisted crystallization. The effects of annealing time on the size (width or height) and optical properties of CH3NH3PbI3 nanowires were systematically studied. It was found that the width of nanowires first decreases and then increases with the increase in annealing time. Moreover, as the size of the nanowires decreases, the absorbance increases, but the transmittance and the band gap decrease.

The (1 − x) Bi (Fe0.9Al0.05Yb0.05) O3–xBaTiO3 (BFAYO-x BTO) ceramics were synthesized by microwave sintering (MWS) and conventional sintering (CS) methods. The crystal structure, surface morphology, dielectric and ferroelectric properties were investigated. XRD patterns show that BTO tetragonal and BFO rhombohedral phases coexist in both MWS and CS samples, and that the diffraction peaks of MWS samples shift to lower angles due to the larger ionic radius of Ba2+ compared with Bi3+. The lattice constant of BFAYO-x BTO ceramics increases with the increase of BTO content. It was found that the average grain size of the MWS ceramic is much smaller than that of the CS ceramic. Dielectric measurements reveal that the dielectric loss and dielectric constant increase with BTO content increasing. The introduction of BTO into BFAYO-based ceramic contribute to the enhancement of the dielectric relaxation behavior. The P–E hysteresis loops demonstrate that the remnant polarization (Pr) and the coercive field (EC) of the MWS samples are smaller than those of the CS samples. Pr and EC first increase, then decrease and then increase again with the increase of BTO content. The maximum and minimum values of the remnant polarization were obtained at x = 0.25 and 0.3, respectively.

Ni0.5Zn0.5Fe2O4/Ba0.8Sr0.2TiO3 (NZFO/BST) composite ceramics with different additions of Bi2O3 (0%, 1%, 3%, and 5% wt) were synthesized via the solid-phase sintering method. XRD analysis indicates that the composite ceramics show bi-phase structure, without apparent secondly phases can be observed. SEM images show that all the specimens have relative dense structure, the grain size is in the range 1–2 μm. A certain amount of Bi2O3 is beneficial to grain growth, the average grain size of NZFO/BST ceramics increases from 179 to 297 nm when the concentration of Bi2O3 is 0 and 3%, respectively. EDS results indicate that the distribution of NZFO phase and BST phase is not uniform. The sample NZFO/BST-3%Bi2O3 not only has the largest dielectric constant (er) in the whole temperature range (25–500 °C), but also presents the smallest dielectric loss (tanδ) at high temperature region (> 400 °C), and the maximum tanδ value is less than 0.4 for all the samples, indicating excellent dielectric performance. Compared with other samples, the specimen NZFO/BST-3%Bi2O3 has the largest remnant polarization (~ 1.14 μC/cm2). The sample NZFO/BST-3%Bi2O3 has the largest leakage current density, while the ceramic NZFO/BST-5%Bi2O3 presents the opposite behavior, the value of leakage current of the sample NZFO/BST-3%Bi2O3 is about 7 times larger than that of the specimen NZFO/BST-5%Bi2O. All the composite ceramics show soft magnetic behavior, since Bi2O3 is a non-magnetic phase, addition of Bi2O3 will dilute the magnetic moment per unit volume so as to decrease the magnetic properties of the ceramics. The sample NZFO/BST-3%Bi2O3 has the second largest saturation magnetization (Mr) of 11.27 emu/g and the smallest coercive field (HC) ~ 3.54 Oe. All the samples show strong magnetoelectric coupling effect, the highest magnetoelectric coupling coefficient is ~ 47 Vcm−1Oe−1, which was obtained in the sample without addition of Bi2O3. In short, the addition of Bi2O3 not only affects the structure of composite ceramics, but also plays an important role in affecting the magnetoelectric performance.

Ni0.5Zn0.5Fe2O4/Ba0.8Sr0.2TiO3 (NZFO/BST) magnetoelectric composites were synthesized through traditional solid-phase method, and effects of IrO2 addition on the microstructure, surface appearance and magnetoelectric properties were comparatively investigated. XRD patterns demonstrate a spinel structure of NZFO and tetragonal perovskite structure of BST, and the addition of IrO2 is conducive to improving the crystallinity of materials. SEM presents that a certain amount of IrO2 is beneficial to the growth of ceramic grains and the densification of ceramics. Through SEM and EDS analysis, it is found that the smaller grain size is BST, and its size is about 100 nm. Sample NZFO/BST-5% has the largest average grain size (~ 277.51), while sample NZFO/BST-1% has a relative density of 91.23%. The relaxation behavior is more obvious with the addition of a certain content of IrO2, which leads to the increase of dielectric constants. In the high temperature region (> 200 ℃), the sample exhibits a large dielectric loss, particularly, the more the content of IrO2, the more obvious this phenomenon is. The addition of IrO2 causes the larger leakage current of NZFO/BST composites so that samples NZFO/BST-3% and NZFO/BST-5% exhibit poor ferroelectricity, in addition, the magnetic properties of the sample are also weakened, and the NZFO/BST composites exhibited a soft ferromagnetic behavior.

Ba0.7Sr0.3TiO3 ceramics has been synthesized by solid state reaction method. The effects of sintering time on microstructure and electric properties of Ba0.7Sr0.3TiO3 ceramics have been investigate...

BFO thin films were deposited on Pt substrate by RF magnetron sputtering technology. The effects of annealing atmosphere on the phase evolution, microstructure, surface element chemical state, leakage current density, dielectric, ferroelectric and magnetic properties of BFO thin films were systematically investigated. The XRD analyses reveal that O2 annealing atmosphere benefits to prepare single phase BFO thin film compared with air. AFM images demonstrate that BFO thin film annealed in O2 shows smaller grain size and better surface roughness than that annealed in air. The XPS analyses clarify that Fe2+ and Fe3+ ions are co-existed in both obtained BFO thin films, while BFO thin film annealed in O2 shows lower Fe2+ and oxygen vacancy concentration. Comparing with BFO thin film annealed in air, BFO thin film annealed in O2 shows a lower leakage current density of 4.4 × 10−6 A/cm2 with enhanced dielectric, ferroelectric and magnetic properties. Our results provide useful reference for practical application of BFO thin film prepared by RF magnetron sputtering.

Ba0.85Ca0.15Zr0.1Ti0.9O3 (BCZT) ceramics with super large grain size (>50 μm) have been successfully fabricated by sol-gel method. The effects of pH value of precursor solution on microstructure, electric properties and fatigue behavior were systematically studied. BCZT ceramics with super large grain size (35–55 μm) were prepared by BCZT powders with large particle size (136–221 nm) by controlling the pH value. The grain size and densification of BCZT ceramics decrease with the increase of pH value of precursor solution. XRD results indicate that there is the coexistence of rhombohedral and tetragonal phase in BCZT ceramics synthesized by precursor solution with a pH of 3 and are the coexistence of orthorhombic and tetragonal phase in BCZT ceramics synthesized by precursor solution with a pH of 5 and 7. BCZT ceramics shows obvious diffuse ferroelectric-paraelectric phase transition characteristic and the diffuseness behavior enhances as the pH value increases. The remnant polarization and coercive electric field of BCZT ceramics reduce with the decreased grain size caused by the increase of pH value. The excellent piezoelectric properties (d33 = 585.6 pC/N and d33∗ = 898 pm/V) of BCZT ceramics have been obtained by super large grain size and the construction of phase boundary of tetragonal phase and orthorhombic phase. Furthermore, BCZT ceramics with large grain size still has high fatigue resistance to bipolar electric cycling.

Magnetoelectric composites have attracted extensive attentions due to the possibility of room temperature coupling effect between the ferroelectric and magnetic phases. We report the comparative study of structure, dielectric, ferroelectric, magnetic properties as well as coupling effect for (1-x)Mn0.5Zn0.5Fe2O4-xBa0.85Sr0.15Ti0.9Hf0.1O3 ((1-x)MZFO- xBSTHO) (x = 0.2, 0.35, 0.5, 0.65, 0.8) composite ceramics prepared by combining coprecipitation process with sol-gel method. The X-ray diffraction result confirm bi-phase structure of the composites, without apparent chemical reaction generated between MZFO and BSTHO. The samples present relative density surface, the mean grain size is in the range of 1–2 μm, which increases slightly with the decrease of molar ratio. The dielectric constant and loss show non-monotonic variation with molar ratio. Temperature and frequency dependence of relaxation peaks are due to the relaxation polarization induced by interface, space charges and weakly bound ions. The peak of the dielectric constant disappears when x is larger than 0.5, while the loss peaks were still observed for all the samples. The remnant polarization increases with x decreasing while the leakage current decreases because the concentration of high-resistance BSTHO is increased. The sample with x = 0.20 shows the largest saturated magnetization of 54 emu/g while the maximal effective saturated magnetization for MZFO is larger than 80 emu/g, obtained at x = 0.65. The coupling coefficient decreases first and then increases, finally it decreases again with the increase of x. The maximal magnetoelectric coupling coefficient is 1420 μV/(cm.Oe), which is obtained for the sample at x = 0.65. The anomalous magnetic and coupling behaviors can be attributed to the strong interface interaction between MZFO and BSTHO.

We report an anomalous magnetoelectric coupling effect of CoFe2O4–BaTiO3 binary mixed fluids, which were prepared by distributing surfactant treated (oleic acid) magnetic CoFe2O4 and ferroelectric BaTiO3 nanoparticles into highly insulating base fluid (silicone oil). A strong coupling effect in this mixed fluid has been observed, in which both the remnant polarization/magnetization and coercive electric/magnetic field are found to be enhanced visibly in the presence of an external magnetic/electric field. Colossal converse and direct magnetoelectric coupling coefficients are estimated to be αH = 1.22 × 10–4Oe·cm/V and αE = 2.52 × 104 V/(cm·Oe), respectively. Moreover, this coefficient clearly depends on the volume fraction of the two kinds of particles and the applied external magnetic/electric field. Even more amazing, CoFe2O4 also shows a strong magnetoelectric coupling effect with a coefficient as high as 4.2 × 10–5 Oe·cm/V. Further analysis indicates that the aggregated, chain-like structure of the pa...

Magnetoelectric composite ceramics Fe3O4/PbZr0.52Ti0.48O3 with different molar ratios (Fe3O4/PbZr0.52Ti0.48O3 = 1:1, 1:2 and 1:8) were prepared by combining hydrothermal method and sol–gel method, effects of molar ratio on the structure, dielectric and multiferroic properties were investigated. The results indicate that the synthesized composites show bi-phase structure, ruling out the presence of any obvious impurity phases. The grains can be divided into two types, the larger grain is attributed to PbZr0.52Ti0.48O3 while the smaller one can be considered to be Fe3O4. With the molar ratio increasing, the grain shape of PbZr0.52Ti0.48O3 changes from stripe to bulk-like while the shape and size of Fe3O4 is near the same. Both the dielectric constant and loss decrease with frequency, the specimen with the molar ratio of 1:8 shows the largest dielectric constant, while the sample has the lowest dielectric loss when the molar ratio is 1:2. With the increase of molar ratio, the height of the relaxation peak decreases and the peak position shifts to higher temperature. When the molar ratio is 1:8, the relaxation peak disappears due to less interface polarization. The remnant polarization increases with increasing the molar ratio, the maximum value is 1.12 μC/cm2, obtained at 1 kHz when the molar ratio is 1:8. Anomalous magnetic properties are observed, in which the magnetization increases first and then decreases with molar ratio, the largest saturation magnetization is ~ 30 emu/g when the ratio is 1:2 due to the strong interface interaction. The sample with the ratio of 1:1 shows the largest magnetic loss because of the highest content of magnetic phase. The magnetization shows a monotonic variation behavior with the molar ratio, indicating the strong interface interaction between the two phases. The maximal ME coupling coefficient is about 5.12 mV/(cm.Oe) for the sample 1:2 due to the stronger magnetic and ferroelectric properties.

High performance multiferroic composites have attracted great attentions because they are more beneficial to increasing magnetoelectric coupling effect. We report the effects of molar ratio (4:1, 2:1, 1:1, 1:2 and 1:4) on the structural, dielectric and multiferroic properties of Co0.6Cu0.3Zn0.1Fe2O4/Ba0.9Sr0.1Zr0.1Ti0.9O3 (CCZFO/BSZTO) composite ceramics prepared by sol-gel method. The XRD results confirm successful formation of the CCZFO/BSZTO composites without presence of any impurity phase. The mean grain size is about 1.5 mm and it shows double dispersing behavior, the larger grains belong to CCZFO while the smaller ones are ascribed to BSZTO. The maximum dielectric constant and loss appear with the molar ratio is 1:1 due to its more grain boundaries. Two peaks appear in the e∼T curves, the peak corresponding to the lower temperature corresponds to the Curie temperature of BSZTO, while the other one is resulted by the relaxation polarization. The height of the relaxation peak decreases and the position shifts to higher temperature range with the increase of molar ratio. The maximal remanent polarization is 3.76 μC/cm2, obtained at 1 kHz when the molar ratio is 1:1. An enhanced magnetic properties are observed when the molar ratio is 1:1 due to the stronger interface interaction effect between the two phases. Strong direct magnetoelectric coupling coefficient of 1.53 V/(cm.Oe) is obtained at the field of 8000 Oe for the sample 1:4. These results may provide valuable information for improving the multiferroic properties of composite multiferroic materials.

Mn0.5Mg0.5Fe2O4/Ba0.8Sr0.2Ti0.9Zr0.1O3 composite ceramics were prepared by sol-gel method and the effects of molar ratio (3:1, 2:1, 1:1, 1:2, 1:3) between the magnetic and ferroelectric phases on the microstructure, dielectric, ferroelectric and magnetic properties have been comparatively investigated. The results indicate that the obtained composite ceramics show bi-phase structure, without other apparent impurities. The surface is relatively uniform except for some pores, and the grain size of all the samples is near 1 μm. The dielectric constant shows decreasing trend with the molar ratio while the loss presents the opposite behavior. Only one relaxation peak can be observed when the molar ratio is less than 1:1, but two peaks are found when the molar ratio exceeds 1:1. The height of the peak decreases with the frequency and molar ratio while the position shifts to higher temperature range with the molar ratio. The dielectric loss increases sharply with the temperature and it is amounts to several thousand when the temperature is lager than 300 °C in the lower molar ratios. Both the resident polarization and coercive electric field increase with the measured voltage while they decrease with the frequency. The leakage current decreases with the molar ratio due to the lower conductivity of Mn0.5Mg0.5Fe2O4. The magnetization shows a monotonic variation behavior with the molar ratio, indicating the strong interface interaction between the two phases. The maximal ME coupling coefficient is about 800 mV/(cm.Oe) for the sample 3:1 due to the stronger magnetic properties.

BTO/CMZFO composite ceramics with different molar ratios (the molar of BTO to that of CMZFO) (3:1, 2:1, 1:1, 1:2, 1:3) were prepared by conventional solid state reaction method. Effects of molar ratio on the dielectric, magnetic properties and coupling effect were investigated. The results indicate that the ceramics show bi-phase composite structure, without any obvious impurities can be found. The grain size can be categorized into two types, one is BTO with the grain size of 2–3 μm while the other one is CMZFO with the size less than 0.5 μm. The dielectric constant of all the composites is less than 500 in the frequency range from 20 to 2 MHz, and it decreases with decreasing the molar ratio while the dielectric loss shows the opposite behavior. The frequency stability of dielectric constant decreases with the addition of CMZFO due to the increased relaxation polarization. The dielectric constant and loss present non-monotonic dependence on the molar ratio, one or more relaxation peaks can be found. With the reduction of molar ratio, the height of the relaxation peak decreases and the position of the relaxation peak shifts to lower temperature. The remnant polarization (Pr) range is from 2 to 7.5 μC/cm2, it decreases first and then increases with the molar ratio, this non-monotonic transition can be attributed to the combined action of ferroelectric phase and leakage current. The magnetization decreases and then increases with the diminution of molar ratio, the largest remnant magnetization (Mr) of the composites is 31.52 emu/g when the ratio is 1:3. The largest effective magnetization of CMZFO component in the composite is 84.855 emu/g for the specimen with the molar ratio 2:1, which is due to the strongest interface interaction between the two phases compared with other samples. The coupling coefficient increases rapidly and then decreases slowly with the applied magnetic field, and the maximum coupling coefficient is near 1.2 mV/cmOe, which is obtained at about 2 kOe in the sample with the molar ratio 1:3.

Ni1−xZnxFe2O4/BaTiO3 (x = 0.3, 0.4, 0.5, 0.6, and 0.7) magnetoelectric composite ceramics have been prepared by combining the coprecipitation and sol–gel methods, and their structural and multiferroic properties studied and compared. The results indicate that the synthesized composites present biphase and composite structure, with no evident impurities observed. The lattice of the Ni1−xZnxFe2O4 crystal structure is distorted owing to the incorporation of Zn2+ ions. The samples present irregular microstructure and abnormal grain growth, which can be attributed to the heterogeneous distribution of the ferroelectric and magnetic phases during preparation. The chemical composition of the larger grains is Ni1−xZnxFe2O4, while that of the smaller grains is proven to be BaTiO3. The dielectric constant of the ceramics first increases then decreases as the Zn2+ ion content is increased, which is related to the irregular microstructure of the ceramics. Both the frequency dependence of the dielectric loss and the temperature dependence of the dielectric constant present two relaxation peaks for all samples. The dielectric loss peaks are attributed to the slow polarization process, such as turning-direction and space-charge polarization, while the dielectric constant peaks can be ascribed to the ferroelectric phase transition of BaTiO3 and relaxation polarization of the composites. The abnormal magnetization behaviors can be induced by the A–B superexchange interaction caused by the addition of nonmagnetic Zn2+ ions.

Co0.5Ni0.5Fe2O4(CNFO) were prepared by chemical coprecipitation method and the particles were tuned by varying annealing temperature (400, 500, 600, 700 and 800 °C), and on this basis, CNFO/Ba0.85Sr0.15TiO3 (BST) composites were prepared by sol–gel method. Effects of CNFO particle size on the microstructure, dielectric and multiferroic properties were systematically investigated and discussed. The results confirm pure CNFO and BST phases, without any obvious impurity phases. The particle size of CNFO is in the range of 20–50 nm, and all the composites show relatively uniform grain size except some pores presented on the surface. The dielectric constant and loss decrease with the particle size as a whole, while the permittivity shows strong dependence of the measuring frequency due to the relaxation polarization mechanism, of which the relaxation peak decreases and shifts to high temperature range with the frequency. The polarization increases with the testing voltage but decreases with the test frequency increasing, the remnant polarization decreases with the increase of particles size, the highest value is 2.2 μC/cm2, obtained at 100 Hz for the sample with the smallest particle size. The maximal remnant magnetization is 11.2 emu/g, which is also obtained when the particle size is the smallest due to its strongest interface interaction. The ME coefficient decreases with increasing the particles size and the maximum ME coefficient is about 13.2 mV/(cm Oe). These results provide valuable information for improving the magnetoelectric properties of composites by tuning the particles size.