Sol–gel synthesis of Nd-doped BiFeO3 multiferroic and its characterization

Neodymium doped bismuth ferrite (BiFeO₃, BFO) nanoparticles were successfully synthesized by a facile sol–gel route. The influence of annealing temperature, time, Bi content and solvent on the crystal structure of BFO was studied. Results indicated that the optimum processing condition of BFO products was 550–600 °C/1.5 h with excess 3–6% Bi and ethylene glycol as solvent. On the other hand, Nd³⁺ ion was introduced into the BFO system and the effect of Nd³⁺ concentration on the structure, magnetic and dielectric properties of BFO were investigated. It was found that the magnetization of BFO was enhanced significantly with Nd³⁺ substitution, being attributed to the suppression of the spiral cycloidal magnetic structure led by the crystal structure transition. Furthermore, with increasing Nd³⁺ content, the dielectric constant was found to decrease while the dielectric loss was enhanced, which was mainly due to the hoping conduction mechanism with the reduction of oxygen vacancies.     

Magnetoelectric coupling in multiferroic BiFeO3 by co-doping with strontium and nickel

We report the effects of the Sr²⁺ and Ni²⁺ co-doping of BiFeO₃ on the crystal structure and multiferroic properties of Bi_1?xSr_xFe_1-yNi_yO₃ (x?=?0.05, 0.0?≤?y?≤?0.10, and Δy?=?0.05) that is synthesized using assisted high-energy ball milling. The mixtures of Bi₂O₃, Fe₂O₃, SrO and NiO were milled for 5?h, pressed at 900?MPa, and sintered at 800?°C in order to obtain cylindrical test pieces. X-ray diffraction and Rietveld refinement elucidated the effects of Sr²⁺ and Ni²⁺ on the crystal structure. Co-doping with SrNi in suitable proportions stabilizes rhombohedral BiFeO₃. High contents of Ni²⁺ promote the precipitation of secondary phases in the forms of NiFe₂O₄ and Bi₂₅FeO₄₀. The magnetic behavior was examined by means of vibrating sample magnetometry. The results showed a change in the magnetic order from antiferromagnetic for the undoped sample to the ferromagnetic order for the co-doped samples. This change is attributed to the modulations in the magnetic moment due to crystal structure distortions. All samples show high relative permittivity values, which were enhanced by doping with Sr²⁺. Ni²⁺ cations increase the dielectric dissipation factor; this enhancement is related to their interactions with cations of a different oxidation state, such as Fe³⁺, Fe²⁺, Ni²⁺, Bi³⁺ and Sr²⁺ in the crystal structure of BiFeO₃. The magnetoelectric coupling that was evaluated using magnetodielectric measurements was above 4% at 1?kHz for the higher applied magnetic field of 18?kOe.     

Structural and multiferroic properties of Pr and Ti co-doped BiFeO3 ceramics

Bi_0.9Pr_0.1FeO₃ (BPF), BiFe_0.9Ti_0.1O₃ (BFT), Bi_0.9Pr_0.1Fe_0.9Ti_0.1O₃ (BPFT-10), and Bi_0.9Pr_0.1Fe_0.95Ti_0.05O₃ (BPFT-5) ceramics are prepared for a comparison study. X-ray diffraction indicates that all of the samples crystallize in rhombohedral structures with R3c symmetry. The Pr and Ti co-doped samples show an especially low dielectric loss of 0.02–0.04 throughout the entire investigated frequency range. A markedly improved polarization hysteresis loop is successfully achieved for samples BPFT-10 and BPFT-5, and their remnant polarization P_r values are 0.11 and 0.29 μC/cm², respectively. Magnetic measurements indicate that the substitution of Ti⁴⁺ for Fe³⁺ improves the ferromagnetic properties due to the suppression of the spiral spin structure. A remnant magnetization M_r of 0.176 emu/g was observed for BPFT-10 at 5 K.     

Effect of annealing temperature on the multiferroic properties of 0.7BiFeO3–0.3Bi0.5Na0.5TiO3 solid solution prepared by sol–gel method

Room-temperature multiferroic 0.7BiFeO₃–0.3Bi_0.5Na_0.5TiO₃ solid solution ceramics have been prepared by the sol–gel method. We have discussed the annealing temperature dependence of the multiferroic properties. The samples are annealed at 1023, 1123, 1223 and 1323 K for 3 h, respectively. X-ray diffraction patterns identify that all samples are pure. Scanning electron micrographs present the increasing grain size with higher annealing temperature. Magnetic, ferroelectric and dielectric properties are enhanced obviously with the increase in annealing temperature. The coexistence of ferroelectric and ferromagnetic properties is also proved at room temperature. In addition, it reveals that the optimal annealing temperature accompanied with favorable multiferroic properties of 0.7BiFeO₃–0.3Bi_0.5Na_0.5TiO₃ solid solution ceramics is near 1223 K.     

Multiferroic Consequence of Porous (BiFeO_3)_x–(BiCrO_3)_(1-x) Composite Thin Films by Novel Sol–Gel Method

In this work, we have presented a spin-coating method to produce thin films started with pure BiCrO_3(BCO) and ended up with BiFeO_3(BFO) by increasing x values in the(BiFeO_3)_x–(BiCrO_3)_(1-x)composites. All the produced thin films have been crystallized at the annealing temperatures of 400 °C for 0.5 h. The XRD and EDAX spectrums give insight that the two crystal phases related to BCO and BFO stayed together within the thin film matrices. SEM analysis showed that the prepared composite had macroporous morphology with interconnected pores and its width(size) decreased with increasing x values. The strong correlations are observed among the microstructure, dielectric, ferroelectric, ferromagnetic properties and Fe concentration. Among all composites, the composition of 0.75 shows an attractive magnetization, polarization, switching and improved dielectric behaviors at room temperature. Significant increase in the multiferroic characteristics of 0.75 composition is due to arise of lower leakage current by causing reduction in oxygen vacancy density, and enhancement of super-exchange magnetic interaction between Fe~(3+) and Cr~(3+) at BFO/BCO interface layers. Our result shows that the thin layer on Pt(111)/Ti/SiO_2/Si substrate possesses simultaneously improved ferroelectric and ferromagnetic properties which make an inaccessible potential application for nonvolatile ferroelectric memories.     

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.     

Mechanosynthesis of the multiferroic cubic spinel Co2MnO4: Influence of the calcination temperature

Multiferroic materials showing magnetoelectric coupling are required in various technological applications. Many synthetical approaches can be used to improve the magnetic and/or electrical properties, in particular when the materials exhibit cationic valence fluctuations, as in the Co₂MnO₄ cubic spinel. In this compound, Co and Mn ions are in competition at the tetrahedral and octahedral positions, depending on their various oxidation states. The Co₂MnO₄ was prepared following two techniques: by a soft chemical route based on a modified polymer precursor method, and by a mechanoactivation route. Both approaches yield polycrystalline powders, but their crystallites sizes and particles morphologies differ as a function of the calcination conditions. The magnetic characterization (ZFC/FC cycles, ordering temperatures, ferromagnetic coercive fields and saturation magnetizations) showed that the synthesis procedure influenced the physical properties of Co₂MnO₄ mainly through the size of the magnetic domains, which play an important role on the magnetic interactions between the Co/Mn cations.     

Processing and characterization of Sr doped BiFeO3 multiferroic materials by high energetic milling

Multiferroic ceramics based on BiFeO₃ and Sr-doped BiFeO₃ have been processed by high energetic milling and later thermal treatments at reduced temperatures to synthesize the perovskite structure. Single phase materials are obtained at 800 °C, reducing the temperature needed to complete the reaction in solid state method. Ceramics with densities higher than 94% were obtained at 1000 °C, and up to 98% at 1050 °C. All the ceramics were obtained in a single thermal treatment, where synthesis, grain growth and densification take place. The addition of Sr stabilizes the BiFeO₃ perovskite structure, avoiding its decomposition. Ceramics with higher dielectric permittivity and conductivity than non-doped materials are obtained, due to the increase of the amount of oxygen vacancies. It is shown for the first time that the grain boundary conductivity is increased in BiFeO₃ by doping with Sr.     

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.