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.     

Role of (La,Gd) co-doping on the enhanced dielectric and magnetic properties of BiFeO3 ceramics

The effect of the La³⁺ and Gd³⁺ co-doping on the structure, electric and magnetic properties of BiFeO₃ (BFO) ceramics are investigated. For the compositions (x=0 and 0≤y≤0.15) in the perovskite structured La_xGd_yBi_1−(x+y)FeO₃ system, a tiny residual phase of Bi₂Fe₄O₉ is noticed. Such a secondary phase is suppressed with the incorporation of ‘La’ content (x). The magnitude of dielectric constant (ε_r) increases progressively by increasing the ‘La’ content from x=0 to 0.15 with a remarkable decrease of dielectric loss. For x=0.15, the system La_xGd_yBi_1−(x+y)FeO₃ exhibits highest remanent magnetization (M_r) of 0.18 emu/g and coercive magnetic field (H_C) of ~1 T in the presence of external magnetic field of 9 T at 300 K. The origin of enhanced dielectric and magnetic properties of La_xGd_yBi_1−(x+y)FeO₃ and the role of doping elements, La³⁺, Gd³⁺ has been discussed.     

Effect of La-Ni substitution on structural,magnetic and microwave absorption properties of barium ferrite

In this paper, La-Ni substituted barium ferrite nanoparticles were prepared by a co-precipitation method. The morphology, structure, magnetic and microwave absorption properties of samples were accomplished by X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), transmission electron microscope (TEM), vibrating sample magnetometer (VSM) and vector network analysis. From the results evaluation, it can be seen that the magnetoplumbite structure for all of the samples have been formed and the average crystallite sizes of Ba_1−xLa_xFe_12−xNi_xO₁₉ nanoparticles within in the range of 50.9–65.5 nm. Ba_0.9La_0.1Fe_11.9Ni_0.1O₁₉ exhibits a remarkable reflection loss of −13.5 dB at 13.05 GHz with a matching thickness of 1.5 mm. The reflection loss results indicate that Ba_0.9La_0.1Fe_11.9Ni_0.1O₁₉ nanoparticles may be used as a potential for thin microwave absorbers.     

Effect of Nd and Nb co-doping on the structural,magnetic and optical properties of multiferroic BiFeO3 nanoparticles prepared by sol-gel method

BiFeO₃, BiFe_0.95Nb_0.05O₃ and Bi_1−xNd_xFe_0.95Nb_0.05O₃ (x=0.0, 0.05, 0.10, 0.15 and 0.20) nanoparticles are successfully synthesized via a tartaric acid-assisted sol-gel technique for the first time. The effect of Nd and Nb co-doping on the structure, morphology, and magnetic and optical properties is investigated. X-ray diffraction (XRD) and Raman measurements demonstrate that Nd³⁺ ions and Nb⁵⁺ ions co-doping at A and B-sites of BiFeO₃ can result in a structural transformation (from single rhombohedral phase (R3c) to two coexisting phases (R3c and Pbam)). The morphology of the nanoparticles seems to be approximately cubic, and by the co-doping, the size of the nanoparticles decreases from ~132 to ~35 nm. Notable improvement in the remanent magnetization of the sample with x=0.15 is observed with a value of 0.285 emu/g, being 20 times higher than that of the undoped sample (BiFeO₃). A decrease in the optical band gap is also observed in the Nd and Nb co-doped nanoparticles, indicating their favorable potential in photocatalytic applications.     

Structural,electrical, dielectric and magnetic behavior of Gd1−xBixFe1−yZryO3 nanoparticles for advanced technological applications

Gd_1−xBi_xFe_1−yZr_yO₃ nanoparticles were synthesized via micro-emulsion route with different molar concentrations of Bi⁺³ (x) and Zr⁺⁴ (y). The values of x and y were kept in the range 0.00, 0.15, 0.30, 0.45 and 0.60. The characterizations were done by the thermo-gravimetric analysis (TGA), X-ray Diffraction (XRD), Fourier transform infrared spectroscopy (FTIR) and scanning electron microscopy (SEM). The average particle size was ~50 nm. The effect of Bi³⁺ and Zr⁴⁺ contents on electrical, dielectric and magnetic parameters were studied. The DC resistivity measurements showed at certain Bi³⁺ and Zr⁴⁺ contents, more than two fold increase in electrical resistivity from 68×10⁸ Ω cm to 150×10⁸ Ω cm. The magnetic measurements showed the paramagnetic nature of Gd_1−xBi_xFe_1−yZr_yO₃ nanoparticles. The electrical and magnetic properties of these nanoparticles suggested that these materials are potential candidates for the fabrication of telecommunication and switching devices.     

A-site strontium doping effects on structure,magnetic, and photovoltaic properties of (Bi1−xSrx)FeO3−δ multiferroic ceramics

Raman spectroscopy, X-ray diffraction (XRD), magnetization hysteresis loop, synchrotron X-ray absorption spectroscopy, and photovoltaic effects have been measured in (Bi_1−xSr_x)FeO_3−δ (BFO100xSr) ceramics for x=0.0, 0.05, 0.10, and 0.15. Raman spectra and XRD reveal a rhombohedral R3c structure in all compounds. A-site Sr²⁺ doping increases fluctuations in cation-site occupancy and causes broadening in Raman modes. BFO15Sr exhibits a strong ferromagnetic feature due to reduction of FeOFe bond angle evidenced by the extended synchrotron X-ray absorption fine structure. The heterostructure of indium tin oxide (ITO) film/(Bi_1−xSr_x)FeO_3−δ ceramic/Au film exhibit clear photovoltaic (PV) responses under blue illumination of λ=405 nm. The maximal power-conversion efficiency and external quantum efficiency in ITO/BFO5Sr/Au are about 0.004% and 0.2%, respectively. A model based on optically excited charges in the depletion region between ITO and (Bi_1−xSr_x)FeO_3−δ can well describe open-circuit voltage and short-circuit current as a function of illumination intensity.     

Effect of Zr substitution on structural,magnetic, and optical properties of Bi0.9Dy0.1Fe1−xZrxO3 multiferroic ceramics prepared by rapid liquid phase sintering method

Zr substituted Bi_0.9Dy_0.1Fe_1−xZr_xO₃ (x=0.03, 0.06 and 0.10) multiferroic ceramics were synthesized by rapid liquid phase sintering technique to improve its multiferroic properties. Rietveld structural refinement of XRD patterns and Raman spectra revealed a partial structural phase transition from rhombohedral (R3c) to biphasic structure (R3c+P4mm) on codoping. The substitution of larger ionic radii and higher valence Zr⁴⁺ ions at Fe-site leads to decrease in the grain size as a result of charge compensation at Fe site. The weak ferromagnetic behavior were observed in all samples along with maximum M_r value of 0.159 emu/g for x=0.03 concentration, which is also endorsed by second order Raman modes. The distortion in FeO₆ octahedra due to Zr substitution leads to splitting of electronic bands of 3.2 eV into multiplets, which in turn reduced the optical band gap value in the range of 2.06–2.10 eV for all samples.     

Oxygen separation from air using Ba0.95La0.05FeO3−δ membranes fitted with porous La1−xSrxFeO3−δ layers

Selective oxygen separation from air was performed using perovskite-type oxide membranes made of Ba_0.95La_0.05FeO_3−δ. We demonstrated that surface modification of Ba_0.95La_0.05FeO_3−δ membranes with La_1−xSr_xFeO_3−δ catalyst layers led to an increase in oxygen permeation fluxes at 700–930 °C. We studied the effects of oxygen vacancy amounts, surface area, particles size, surface treatment of La_1−xSr_xFeO_3−δ on the oxygen permeability of the membranes fitted with La_1−xSr_xFeO_3−δ catalyst layers. Among the catalyst layers tested, the membranes fitted with La_0.9Sr_0.1FeO_3−δ (x=0.1) showed the highest oxygen permeation flux probably because of its higher porosity and uniform morphology without open voids, which would increase the number of surface reaction sites. The obtained results suggest the feasibility of further upgrading the membrane performance by using surface catalyst layers having a homogeneous morphology and a different composition from that of the mother membrane.     

Effect of Ni2+ substitution on structural,magnetic, dielectric and optical properties of mixed spinel CoFe2O4 nanoparticles

Nanoparticles of Co_1−xNi_xFe₂O₄ with x=0.0, 0.10, 0.20, 0.30, 0.40 and 0.50 were synthesized by co-precipitation method. The structural analysis reveals the formation of single phase cubic spinel structure with a narrow size distribution between 13–17 nm. Transmission electron microscope images are in agreement with size of nanoparticles calculated from XRD. The field emission scanning electron microscope images confirmed the presence of nano-sized grains with porous morphology. The X-ray photoelectron spectroscopy analysis confirmed the presence of Fe²⁺ ions with Fe³⁺. Room temperature magnetic measurements showed the strong influence of Ni²⁺ doping on saturation magnetization and coercivity. The saturation magnetization decreases from 91 emu/gm to 44 emu/gm for x=0.0–0.50 samples. Lower magnetic moment of Ni²⁺ (2 µ_B) ions in comparison to that of Co²⁺ (3 µ_B) ions is responsible for this reduction. Similarly, overall coercivity decreased from 1010 Oe to 832 Oe for x=0.0–0.50 samples and depends on crystallite size. Cation distribution has been proposed from XRD analysis and magnetization data. Electron spin resonance spectra suggested the dominancy of superexchange interactions in Co_1−xNi_xFe₂O₄ samples. The optical analysis indicates that Co_1−xNi_xFe₂O₄ is an indirect band gap material and band gap increases with increasing Ni²⁺ concentration. Dispersion behavior with increasing frequency is observed for both dielectric constant and loss tangent. The conduction process predominantly takes place through grain boundary volume. Grain boundary resistance increases with Ni²⁺ ion concentration.     

Structural,vibrational, electrical and magnetic properties of Bi1−xPrxFeO3

Crystalline solid solution of Bi_1−xPr_xFeO₃ (x=0.05, 0.1, and 0.15) ceramics has been successfully synthesized by a low temperature assisted co-precipitation method. Rietveld-refinement of the X-ray diffraction data reveals rhombohedral structure for Bi_1−xPr_xFeO₃ (x=0.05, 0.10) and triclinic for Bi_1−xPr_xFeO₃ (x=0.15). The crystallite sizes of the Bi_1−xPr_xFeO₃ (x=0.05, 0.1 and 0.15) are found to be approximately 33, 27 and 22 nm respectively calculated using Debye–Scherrer equation. The SEM images of Bi_1−xPr_xFeO₃ (x=0.05, 0.10 and 0.15) ceramics show grains with almost spherical morphology. 4A1 and 7E Raman modes have been observed in the range 100–650 cm⁻¹ and two phonon modes centered around 1150–1450 cm⁻¹ have also been observed corresponding to 2A₄ (LO), 2E₈ (TO) and 2E₉ (TO) modes of Bi_1−xPr_xFeO₃ (x=0.05, 0.1 and 0.15). The changes in Raman modes such as prominent frequency shift, line broadening and intensity have been noticed with the increase of Pr concentration in BiFeO₃ (BFO) suggesting a structural transformation as revealed by the Rietveld refinement. An anomaly in the temperature dependent dielectric studies has been noticed in all the samples at the vicinity of Neel temperature (T_N) indicating a magnetic ordering and an increase in magnetization with increase of Pr concentration is noticed from the room temperature magnetic studies. Further, the leakage current density is found to be reduced with increasing Pr concentration.     

Dielectric and ferroelectric properties of Ho-doped BiFeO3 nanopowders across the structural phase transition

We have studied Ho-doped BiFeO₃ nanopowders (Bi_1−xHo_xFeO₃, x = 0–0.15), prepared via sol-gel method, in order to analyse the effect of substitution-driven structural transition on dielectric and ferroelectric properties of bismuth ferrite. X-ray diffraction and Raman study demonstrated that an increased Ho concentration (x ≥ 0.1) has induced gradual phase transition from rhombohedral to orthorhombic phase. The frequency dependent permittivity of Bi_1−xHo_xFeO₃ nanopowders was analysed within a model which incorporates Debye-like dielectric response and dc and ac conductivity contributions based on universal dielectric response. It was shown that influence of leakage current and grain boundary/interface effects on dielectric and ferroelectric properties was substantially reduced in biphasic Bi_1−xHo_xFeO₃ (x > 0.1) samples. The electrical performance of Bi_0.85Ho_0.15FeO₃ sample, for which orthorhombic phase prevailed, was significantly improved and Bi_0.85Ho_0.15FeO₃ has sustained strong applied electric fields (up to 100 kV/cm) without breakdown. Under strong external fields, the polarization exhibited strong frequency dependence. The low-frequency remnant polarization and coercive field of Bi_0.85Ho_0.15FeO₃ were significantly enhanced. It was proposed that defect dipolar polarization substantially contributed to the intrinsic polarization of Bi_0.85Ho_0.15FeO₃ under strong electric fields at low frequencies.     

Effect of Nb substitution on magnetic,ferroelectric and photocatalytic properties of Bi0.95Sm0.05Fe1-xNbxO3 (0 ≤ x ≤ 0.1) nanoparticles

The single phase Bi_0.95Sm_0.05Fe_1-xNb_xO₃ (0 ≤ x ≤ 0.1) nanoparticles were synthesized by the sol-gel route, and the effect of Nb substitution on their magnetic, ferroelectric and photocatalytic properties were studied. X-ray diffractometry confirms a phase transformation from rhombohedral to orthorhombic with an increase in Nb substitution. The grain size decreases significantly, and the morphology of grains becomes homogeneous with the increase of Nb concentration. The maximum remnant magnetization (0.014 emu/g), coercivity (565 Oe) and polarization (0.592 μC/cm²) are observed in Bi_0.95Sm_0.05Fe_0.9Nb_0.1O₃. It has been observed that the energy band gap has been slightly reduced from 2.14 to 2.03 eV with Nb substitution, indicating an improvement of photocatalytic activity. The methylene blue degradation is used to represent the photocatalytic ability of Bi_0.95Sm_0.05Fe_1-xNb_xO₃ nanoparticles. The highest degradation efficiency (~74%) of methylene blue is obtained in Bi_0.95Sm_0.05Fe_0.93Nb_0.07O₃, which is much higher than that of Bi_0.95Sm_0.05FeO₃ (~51%) and can be attributed to the optimum particle size and the smallest energy band gap.     

Crystal structure transformation and improved dielectric and magnetic properties of La-substituted BiFeO3 multiferroics

Samples of Bi_1−xLa_xFeO₃ with x = 0.1, 0.3, 0.5, and 0.7 have been synthesized by two stage solid state reaction method. Structural characterization was performed using powder x-ray diffraction at room temperature. The crystal structure of perovskite phases are further characterized via Rietveld analysis which revealed a structural transition from R3c symmetry of the parent phase of BiFeO₃ to orthorhombic Pnma symmetry of LaFeO₃. However the intermediate samples with x = 0.3 and 0.5 are bi – phasic (i.e. a combination of rhombohedral R3c and orthorhombic Pbam phases co-exist). Rietveld Refinement presents a good agreement between measured and simulated patterns. The transition from a rhombohedral to orthorhombic unit cell is suggested to be driven by the dilution of the stereochemistry of the lone pair of Bi³⁺ at A- site. M-H hysteresis loops are recorded at room temperature up to a field of 15 kOe. The G-type antiferromagnetic spin structure and the magnetic moment are very sensitive to increasing La concentration at A-site. La substitution transformed antiferromagnetic BiFeO₃ into ferromagnetic which is closely related to the structural phase transition and modification of antiparallel spin structure. Dielectric constant (ε′) and dissipation factor (tan δ) measured in frequency range 1 kHz to 5 MHz showed dispersion behaviour at low frequencies.     

Enhanced multiferroic properties in Pr-doped BiFe0.97Mn0.03O3 films

Bi_1−xPr_xFe_0.97Mn_0.03O₃ (x=0.00, 0.05, 0.10, 0.15, 0.20) thin films were deposited on FTO/glass substrate using chemical solution deposition. The influences of Pr doping on the crystalline structure and multiferroic properties were investigated. In the X-ray diffraction and Raman spectra results, the crystal structures of Bi_1−xPr_xFe_0.97Mn_0.03O₃ films revealed a gradual transformation from the trigonal structure to the tetragonal structure. The leakage current densities of Bi_1−xPr_xFe_0.97Mn_0.03O₃ films are one order of magnitude lower than that of BiFeO₃. Compared with unsaturated polarization-electric field hysteresis loop of BiFeO₃ film, the Pr and Mn co-doped BFO films have significantly improved ferroelectric properties. The improved remnant polarization (Pr=91.3 µC/cm²) and the positive switching current (I=0.028 A) have been observed in Bi_0.85Pr_0.15Fe_0.97Mn_0.03O₃ film. The improved electrical properties are attributed to the structure transformation, increasing grain boundaries, low oxygen vacancies ratio and increasing Fe³⁺ concentration. In addition, the saturation magnetization of Bi_0.85Pr_0.15Fe_0.97Mn_0.03O₃ film is 1.81 emu/cm³, which is approximately three times higher than pure BiFeO₃ (M_s=0.67 emu/cm³).     

Synthesis,structure and oxygen thermally programmed desorption spectra of La1−xCaxAlyFe1−yO3−δ perovskites

Perovskites La_1−xCa_xAl_yFe_1−yO_3−δ (x, y = 0 to 1) were prepared by high-temperature solid-state synthesis based on mixtures of oxides produced by colloidal milling. The XRD analysis showed that perovskites La_0.5Ca_0.5Al_yFe_1−yO_3−δ with a high Fe content (1 − y = 0.8–1.0) were of orthorhombic structure, perovskites with a medium Fe content (1 − y = 0.8–0.5) were of rhombohedral structure, and perovskite with the lowest Fe content (1 − y = 0.2) were of cubic structure. Thermally programmed desorption (TPD) of oxygen revealed that chemical desorption of oxygen in the temperature range from 200 to 1000 °C had proceeded in the two desorption peaks. The low-temperature α-peak (in the 200–550 °C temperature range) was brought about by oxygen liberated from oxygen vacancies; the high-temperature β-peak (in the 550–1000 °C temperature range) corresponded to the reduction of Fe⁴⁺ to Fe³⁺. The chemidesorption oxygen capacity increased with increasing Ca content and decreased with increasing Al content in the perovskites. The Al³⁺ ions restricted, probably for kinetic reasons, the reduction of Fe⁴⁺ and the high-temperature oxygen desorption associated with it.     

Role of Ni–Co co-substitution in improving electrical and dielectric properties of La-based multiferroics nanomaterials

The synthesis of a material having ferroelectric and ferromagnetic properties in the same phase is a challenging task. In the present work, lanthanum based multiferroic materials, with composition La_1−xCo_xFe_1−yNi_yO₃ (where, x=0.0–0.5 and y=0.0–1.0), have been synthesized via sol–gel auto-combustion method. The phase of the synthesized materials was confirmed by the X-ray diffraction (XRD) analysis, while the surface morphology and particle size were determined by the scanning electron microscopy (SEM) analysis. The DC electrical resistivity and activation energy were observed to increase from 2.14×10⁷ to 3.04×10¹⁰ Ω cm and 0.64 to 0.77 eV, respectively with the increase in concentration of substituents. The drift mobility decreases from 3.27×10⁻¹³ to 2.80×10⁻¹⁶ cm² V⁻¹ S⁻¹. The dielectric constant, dielectric loss and dielectric loss factor decrease with the increase in frequency (1 MHz to 3 GHz) and Ni–Co content as well. The electrical and dielectric results are in good agreement with each other. The increase in electrical resistivity and decrease in dielectric parameters make these materials suitable for applications in microwave devices.     

Electrical properties of (La0.9Ca0.1)(Co1−xNix)O3−δ cathode materials for SOFCs

Modified perovskite ceramics (La_0.9Ca_0.1)(Co_1?xNi_x)O_3?δ (x = 0–0.3) cathodes for solid oxide fuel cells (SOFCs) were synthesized by solid state reaction. The lattice parameters, electrical conductivity, activation energy, and microstructures of these specimens were investigated systematically in this study. The results exhibited that all specimens are rhombohedron structures and their tolerance factors were greater than 0.97, indicating that the perovskite was not distorted by Ni²⁺ cation substitution for the B site of (La_0.9Ca_0.1)CoO_3?δ. The microstructures of the (La_0.9Ca_0.1)(Co_1?xNi_x)O_3?δ specimens showed good densification, and were well-sintered, with few pores. The electrical conductivity behavior conformed to the nature of a semiconductor, for all specimens. As x = 0.1, the electrical conductivity reached the maximum value of 750.3 S/cm at 800 °C, and the activation energy calculated from the Arrhenius plot of the electrical conductivity versus the reciprocal of temperature is 7.1 kJ/mol.The novelty of this study is its introduction of the concept of defect chemistry to explain the relationship between compensation mechanisms and electrical conductivity. The information gleaned regarding charge compensation mechanisms and defect formation may be valuable for a better understanding of the cathode of (La_0.9Ca_0.1)(Co_1?xNi_x)O_3?δ ceramics used for SOFCs. Moreover, the information about oxygen content versus temperature is useful for expressing the relationship between electrical conductivity and composition. Therefore, we also used thermogravimetric analysis combined with the room-temperature oxygen content which was determined by iodometric titration to investigate the oxygen content from room temperature to high temperature, in air. Based on the experimental results, the (La_0.9Ca_0.1)(Co_0.9Ni_0.1)O_3?δ specimen shows high electrical conductivity. Consequently, it is identified as a promising candidate for cathode SOFC applications.     

Rare earth-first-row transition metal perovskites as catalysts for the autothermal reforming of hydrocarbon fuels to generate hydrogen

Perovskite oxides (ABO₃) containing rare earth elements on the A-site and first-row transition metal elements on the B-site were studied as catalysts for autothermal reforming of liquid hydrocarbon fuels to produce hydrogen for fuel cell systems. Experiments were conducted in a fixed bed microreactor at temperatures of 600–800 °C and gas-hourly space velocities (GHSV) ranging from 4600 to 28,000 h⁻¹ using 2,2,4-trimethylpentane (isooctane) as a surrogate fuel. We have found that the two binary oxides, LaNiO₃ and LaCoO₃, produced high yields of H₂, but were not structurally stable. These perovskites decomposed to La₂O₃ and Ni/NiO or Co/CoO under the reducing conditions present in the reformer. Three other binary oxides, LaCrO₃, LaFeO₃, and LaMnO₃, were structurally stable but significantly less active than LaNiO₃ and LaCoO₃. The partial substitution of chromium, iron, aluminum, gallium, or manganese on the B-site of LaNiO₃ to yield LaB_xNi_1−xO₃ was shown to improve the structural stability without a significant decrease in the H₂ yield. The effects of substituting rare earth elements for La and the substitution of alkaline earth elements on the A-site (La_1−yA_yB_xNi_1−xO₃) on catalyst performance and stability were also investigated. Finally, La_0.8Sr_0.2M_0.9Ni_0.1O₃ catalysts (where M = Cr, Mn, or Fe) were tested with a “benchmark fuel” mixture containing from 0 to 50 ppmw sulfur. These tests showed that using chromium as a stabilizing element in LaNiO₃ imparts the most sulfur tolerance.     

Room temperature multiferroicity in Aurivillius compounds Bi6Fe2−xNixTi3O18 (0≤x≤1)

We investigate the structural, magnetic, ferroelectric, and dielectric properties of Bi₆Fe_2−xNi_xTi₃O₁₈ (0≤x≤1). The coexistence of ferroelectricity and ferromagnetism were observed at room temperature for the Ni-doped samples. The ferromagnetism in Bi₆Fe_2−xNi_xTi₃O₁₈ can be understood in terms of spin canting of the antiferromagnetic coupling of the Fe-based and Ni-based sublattices via Dzyaloshinsky-Moriya interaction. Moreover, the substitution of Ni for Fe was effective for the enhancement of ferroelectric properties. The x=0.6 sample exhibits a maximum remnant polarization P_r of 37.8 μC/cm² because of a lower leakage current. The rather large activation energy in the x=0 and 0.2 samples implies that the relaxation process may be not associated with the thermal motion of oxygen vacancies inside the bulk.     

Influence of Al-substitution on structures,resistivity and magnetic properties of LaxSr(2−x)Fe(1+y)Mo(1−y)O6 (x=3y,y=0.05, 0.1, 0.15, 0.2)

Two series of single-phased La_xSr_(2−x)Fe_(1+y)Mo_(1−y)O₆ and La_xSr_(2−x)Fe_(1+0.5y)Al_0.5yMo_(1−y)O₆ (x=3y, y=0.05, 0.1, 0.15 and 0.2) double perovskites were prepared by solid-state reaction. The effects of Al-substitution on the structures, resistivity and magnetic properties of La_xSr_(2−x)Fe_(1+y)Mo_(1−y)O₆ were investigated. Although Al-replacement exhibits a negligible influence of on the B-site ordering degree, it results in the suppression of magnetisation caused by non-magnetic Al³⁺ ions. Reduction of grain sizes leads to increased resistivity, thus an optimised magnetoresistance (MR) behaviour is observed. The greatest MR extent improvement can be obtained when y is 0.15 and the MR% of the Al-doped ceramics reaches −10.5% (10 K, 1 T), which is 2 times greater than that of the undoped ceramics (−4.6%, 10 K, 1 T). Interestingly, the Curie temperature (Tc) of both Al-doped and undoped samples maintained relatively constant values of approximately 420 K and 405 K, respectively, which were different results from the data obtained for similar electron-doping systems in the literature.