Doped BiFeO3 Research Articles

On the superparamagnetic behavior of BiFeO3-PbTiO3 nanoparticles

In this work, the nanostructuration effects on the physical properties of pure and La doped (0.6)BiFeO3-(0.4)PbTiO3 magnetoelectric nanoparticles were investigated. An exotic magnetic response, mimicking a superparamagneticlike behavior that raised the magnetization to levels never reported before for these materials, is clearly observed. Another important change associated with the nanostructuration is observed in the symmetry of the perovskite-structured materials, which changes from a tetragonal to a rhombohedral one. Transmission electron microscopic and X-ray diffraction results (investigated by the Debye-Scherrer method) revealed uniform nanoparticles of spherical shape, average particle sizes close to 5 nm and with a single structural domain (only one crystallite per particle). The pronounced increase (∼20 times) in the magnetization response of the nanoparticles, compared to microparticle ones, is observed in magnetization curves. A magnetic semiempirical phenomenological model, where noninteracting single-domain magnetic nanoparticles are considered, is used to fit the magnetization curves, revealing the almost absence of surface effects in a typical superparamagneticlike behavior. All these aspects pointing to a material with immense potential for practical applications in spintronics and magnetoelectronics.

Investigation of room temperature ferromagnetism in transition metal doped BiFeO3

Spintronic functionality in ferromagnetic materials is a next-generation technique, to be used in data storage, high-frequency communications, and logic devices with minimum energy consumption. Ultra-low energy consumption in high-speed logic devices can be envisioned by inducing ferromagnetic behavior into room temperature multiferroic materials. However, there is a scarcity of room temperature multiferroic materials which have a definite spin degree of freedom. To fully exploit these technological challenges, we introduce the induced ferromagnetism in bismuth ferrite (BiFeO3, BFO) by doping transition metal (Cr, Ni, Co) elements. Our investigation initiates with the experimental study on chemically synthesized BiFe(1−x)MxO3 samples where x = 0.0625 (6.25%) and M = Cr, Ni and Co. Experimental findings are verified by theoretical simulation using density functional theory (DFT + U) and gauge including projector augmented wave (GIPAW) based calculation. All the experimental studies are done at room temperature while the theoretical verification using DFT is carried to understand the underlying mechanism behind the magnetic behavior of doped BiFeO3. It is done by optimizing the structural parameters comparable to the room temperature values. Microstructural and magnetic properties are studied using x-ray diffraction (XRD), transmission electron microscopy (TEM) and Vibrating sample magnetometer (VSM). All these experimental studies confirm the structural changes and induced ferromagnetism with doping. X-ray photoelectron spectroscopy (XPS) verified the reason behind this ferromagnetic property on the basis of oxygen vacancy content. Electron paramagnetic resonance (EPR) spectroscopy shows the tuning of Δg values due to enhanced magnetization.The density of states (DOS) calculations were performed on BFO (band-gap 1.89 eV) after structural optimization using DFT + U method, confirm our experimental findings. Magnetic moment values change drastically with doping elements (M), i.e. almost negligible for BFO (antiferromagnetic) to maximum (2.85 μB/f.u.) for Ni-doped sample. We also compute the EPR g-tensor using GIPAW method to confirm the tuning of Δg values due to enhanced magnetization. These results can highlight the impact and importance of suitable transition element doping to induce the room temperature ferromagnetism in BiFeO3.

Change in structural, ferroelectric, and magnetic properties of bismuth ferrite induced by doping with gadolinium

Abstract Bismuth ferrite, BiFeO3, was doped with a wide range of concentrations (0–30%) of Gd in order to investigate structural, ferroelectric, and magnetic properties and their correlation. Powder samples of Bi1-xGdxFeO3 (x = 0.00–0.30) were prepared using the hydro-evaporation synthesis method. Sintering process at 870 °C for 6 h of as-prepared, Gd doped BiFeO3 powders resulted in ceramic samples with relative densities in the range of (74–86) %. XRD analysis revealed that Bi1-xGdxFeO3 samples (x = 0.00–0.09) crystallized in rhombohedral phase; Bi1-xGdxFeO3 samples (x = 0.10–0.20) contained both rhombohedral and orthorhombic phases, while Bi1-xGdxFeO3 (x = 0.30) sample contained only the orthorhombic phase. The structural parameters obtained after Rietveld refinement for single-phase Bi1-xGdxFeO3 ceramic samples and microstructure image analysis were correlated with the results of ferroelectric and magnetic measurements. Bi1-xGdxFeO3 (x = 0.0625, 0.075, 0.09) samples showed improved ferroelectric properties and reduced leakage current density in comparison with undoped BiFeO3. Gradual increase in Gd content in BiFeO3 led to enhanced magnetic responses and weak ferromagnetic behavior.

Hydrogen sensing characteristics of perovskite based calcium doped BiFeO3 thin films

Perovskite oxide based thin film gas sensors have long been considered as potential alternatives to commonly investigated binary metal oxides based sensors. BiFeO3, which is a prototype of p-type perovskite based semiconducting oxides, has recently drawn significant attention for its promising gas sensing characteristics. In the present work, the hydrogen sensing characteristics of calcium doped BiFeO3 has been reported by varying the film thickness, doping concentration, operating temperature, and test gas concentration. The films were deposited on glass substrates by sol-gel route using spin coating. X-ray diffraction analyses confirmed formation of phase pure films and scanning electron microscopy confirmed their uniform and dense microstructure. The Ca-doped BiFeO3 sensors exhibit higher sensitivity compared to pure BiFeO3 sensors. It is reported that the film thickness and Ca doping concentration play major role to control hydrogen sensing characteristics of the deposited films. The sensor based on 15% Ca-doped BiFeO3 sensor exhibited very high sensitivity (∼212% at 500 ppm H2), and selectivity towards hydrogen at a moderate operating temperature (∼250 °C). The enhanced gas sensing response of the doped BiFeO3 films has been attributed to the higher oxygen vacancy concentration induced by incorporation of aliovalent Ca2+.

Effect of La doping on the ferroelectric and optical properties of BiFeO3: a theoretical-experimental study

Much work has been done already on pure and doped BiFeO3 (BFO), however, renewed interest has emerged when considered as a viable photovoltaic material. It has been shown that, for this purpose, spontaneous polarization and energy gap are relevant physical quantities. In this theoretical-experimental work, we have studied the possibility of enhancing the spontaneous polarization and reducing the BFO bandgap by replacing 10% of the site A ions of the perovskite structure with La. The structural, electronic and optical properties of (Bi0.9La0.1)FeO3 were investigated using pristine BFO as a reference. A rhombohedral crystal structure was determined after La substitution and its corresponding bandgap of 2.49 eV was smaller than that of pure BFO. Theoretically, the spontaneous polarization (Ps) of the BFO increased with the incorporation of La up to a value of 103 μC cm−2, showing its effect on the ferroelectric properties. Moreover, the theoretical results and experimental measurements of the optical properties demonstrated the reduction of the optical gap.

Photocatalytic degradation of acid red-85 dye by nickel substituted bismuth ferrite nanoparticles

Visible light driven photocatalytic property of nickel doped BiFeO3 nanoparticles were explored from the degradation of Acid red-85 dye. Fuel (citric acid) assisted auto-combustion method was adopted for the synthesis of BiFe(1−x)NixO3 (x = 0.25, 0.5, 0.75 and 0.10) nanoparticles. The synthesized particles were characterized by x-ray diffraction, from which reduction in crystallite size has been observed from 57 nm to 28 nm. Lattice parameters also have been calculated for the same which shows an increase in lattice parameters confirming the presence of nickel in the BiFeO3 lattice. Morphological studies were carried out using high resolution scanning electron microscopy on undoped and 10% nickel doped BiFeO3 samples which show a decrease in particle size in the doped sample compared to the bare BiFeO3 similar results have been observed in Transmission Electron Microscopy images. Reduction in the optical band gap from 2.2 eV to 2.02 eV of BiFeO3 compared to 10% Nickel doped bismuth ferrite is observed. Vibrating sample magnetometer studies performed at room temperature indicates that addition of nickel increases both remnant and saturation magnetization to the bismuth ferrite sample thus helping in the removal of photocatalyst after treatment. Photocatalytic study on the Acid red-85 dye by visible light irradiation showed the complete degradation of dye in 40 min. The 10% Nickel doped BiFeO3 showed the best photocatalytic activity. The enhancement in the photocatalytic activity of the doped BiFeO3 is because of efficient charge separation which has been confirmed by photoluminescence studies.

Magnetism and ferroelectricity in BiFeO3 doped with Ga at Fe sites

Theoretical calculations were performed to investigate the effect of isovalent substitution of Fe with Ga on the magnetic and ferroelectric properties in BiFeO3 doped with 16.6% and 8.3% Ga. Such substitution will break the G-type anti-ferromagnetic order and enhance the magnetism of BiFeO3 by forming a stable ferrimagnetic order. The doped system maintains robust spontaneous polarization that is sufficiently large for practical applications at room temperature. The findings explain experimentally observed magnetism. The coexistence of magnetism and ferroelectricity in the doped BiFeO3 render it a candidate material for application to information storage and spintronics devices.

Large photovoltaic response in rare-earth doped BiFeO3 polycrystalline thin films near morphotropic phase boundary composition

The photovoltaic (PV) properties of polycrystalline Bi1−xLaxFeO3 (x = 0–0.3) films have been explored. The X-ray diffraction study reveals that there is a gradual phase transition with the increase in La doping. The composition x = 0.25 is found to be the morphotropic phase boundary (MPB), beyond which the system turns into the non-/antipolar orthorhombic phase. The polarization measurements reveal improved ferroelectric properties with the maximum remanent polarization observed for the x = 0.25 film. A systematic study on the direct and indirect bandgaps of the films has shown a decreasing trend with composition. Interestingly, the PV studies exhibit a maximum open-circuit voltage of 1.30 V for the x = 0.25 film which is three times larger than the value observed for pure BiFeO3 (BFO) (0.47 V). The enhanced PV response in La-doped BFO correlates with the polarization and the change in direct/indirect bandgaps associated with structural instability near the MPB composition. The approach used in this work for enhancing the PV performance in ferroelectric BFO through the combined effects of polarization, bandgaps and competing structures provides a better pathway for improving the ferroelectric PV effect.

Ferroelectric Origin and Distortion Modes in Doped BiFeO <sub>3</sub> by Crystallography Approach

There are conflicts about ferroelectric origin and relationship of two distortion modes, namely, ferroelectric (FE) mode and antiferrodistortive (AFD) mode. Here two distortion modes and enhanced ferroelectric properties (Pr=89.5 μC cm-2) are observed in BiFeO3 thin films doped with small radius Mg2+ (A-site) deposited on (111) Pt/Ti/SiO2/Si substrates by a sol-gel method, relationship of two distortion modes turns from cooperative to competitive as FE mode strengthens. A new explanation of ferroelectric origin and distortion modes in doped BiFeO3 by Defect Dipoles Driven Distortions Theory (abbreviate DDD) from crystallography is reported. Meanwhile, crystal structure regulation mechanism of doped BiFeO3 thin films with substantially enhanced ferroelectric properties is also put forward. The distances of positive and negative charge center in [FeO6] octahedrons evidently increase by Mg2+ doping compared with that in Bi0.9Sm0.1Fe0.95Mn0.05O3 (BSFMO). The change of distances is contributed to law of atomic migration from move of oxygen vacancies and attraction of defect dipoles. A positive effect on ferroelectric properties of appropriate increase in oxygen vacancies is explored, which is explained by the probability of possible position that oxygen vacancies occurred and the relationship between leakage current and overlapping oxygen positions. The potential structure and more suitable doping ions are successfully predicted by crystal structure regulation mechanism. In addition, it brings a new direction for search of other structure regulation ions so as to realize perfect ferroelectric properties for practical application, which will become an important beginning of ferroelectric materials design.

Thermal stability and electrical properties of BiFe1−xMxO3 (M = Al3+, Ga3+) ceramics

BiFeO3 (BFO) is a fascinating multiferroic material, exhibiting ferroelectric and G-type antiferromagnetic characteristics simultaneously. In this work, non-magnetic Al3+ and Ga3+ doped BFO (BFAO and BFGO) ceramics were synthesized via sol–gel and conventional sintering methods. Structural, thermal stability and electrical properties of samples were analyzed in detail. X-ray diffraction (XRD) patterns of powder and ceramic samples demonstrated efficient crystallization, consisting of rhombohedral structures with R3c space group for small amounts of added dopant. Thermal analysis exhibited that BFO decomposes into Bi25FeO39 and Bi2Fe4O9 at 950 °C. It is found that Al3+ and Ga3+ doping readily contribute to decomposition, as supported by calculations from first-principles. BiAlO3 and BiGaO3 are unstable and would spontaneously decompose, if they could be synthesized using ordinary technology. As a result, decomposition temperatures of doped powders decreased to ~ 680 °C. Dielectric behavior can be explained through the Maxwell–Wanger model and Koop’s theory. Dielectric loss decreased with increasing substitution. Leakage current density of doped ceramics became 2–3 orders of magnitude lower than that of BFO ceramic, improving performance and championing applications of modified BFO in future.

Growth of pure and BFO doped KCl crystals by Czochralski technique and fabrication of microstrip patch antenna for GHz applications

Large sized pure and 0.1 mol% BiFeO3 (BFO) doped KCl single crystals were grown by Czochralski technique. Structural parameters were confirmed by powder XRD and single crystal XRD. EDX analysis was used to determine the elemental composition of BFO and KCl crystal. Photoluminescence confirmed that BFO acts as defect quenching agent for KCl single crystal which improved its optical transparency. Doping of BFO in KCl single crystal increased its hardness as measured by a Vicker’s microhardness test. The effect of BFO on dielectric constant and dielectric loss of KCl crystal, at various temperature and frequencies was studied. The low dielectric constant viz. 4.5 and 5.8 for pure and BFO doped KCl was helpful in designing and fabricating patch antenna in GHz frequency. As a result of BFO doping, the resonant frequency of microstrip patch antenna can be tuned at 5.40 GHz from 6.01 GHz for the KCl single crystal.

Structural, dielectric, ferroelectric and magnetic properties of Gd doped BiFeO3

Gadolinium doped BiFeO3 samples were successfully synthesized by sol–gel method. The X-ray diffraction patterns show the effect of Gd doping in bismuth ferrite on structural changes as well as reduction of impurity phases. Microstructural analysis explores the morphology of pure and Gd doped BiFeO3. M-H plot for doped BFO shows that magnetization reach the value ∼0.20 emu/g. The ferroelectric polarization for applied electric field approaches ∼0.35 μC/cm2 for Bi0.85Gd0.15FeO3. The dielectric data also justified that there is the improvement of dielectric properties of BiFeO3 on doping with Gd.

Tailoring dielectric properties and multiferroic behavior of nanocrystalline BiFeO3 via Ni doping

In order to tailor the multiferroic as well as dielectric properties, we have synthesized BiFeO3 (BFO) and Ni-doped BFO nanoparticles of composition BiFe1-xNixO3 (0 ≤ x ≤ 0.07) via the sol-gel method. The influence of Ni-doping at the Fe site of BiFeO3 on the structural, dielectric, and multiferroic properties was investigated via various characterization techniques such as Raman spectroscopy, vibrating sample magnetometer, polarization-electric field (P-E) loop tracer, and LCR meter. The Raman spectra reveal the rhombohedral distorted perovskite structure of the samples in the R3c space group. The frequency dependent dielectric measurements at room temperature show that the dielectric constant for the pristine sample is an order of magnitude lower than the 1% Ni-doped BFO sample. It is also evident that the ac conductivity increases with the increase in frequency as well as Ni content. Further, the universal dielectric response (UDR) phenomenon is responsible for the dielectric behavior of all the samples in the whole frequency range except for the BiFeO3 and BiFe0.97Ni0.03O3 samples. These samples exhibit deviation from UDR phenomenon at higher frequencies. The room temperature hysteresis (M-H) loops exhibit the saturation in magnetization at higher magnetic fields. The result demonstrates a significant influence on the saturation magnetization with the Ni doping in BFO, prompting the ferromagnetic nature of the samples. The saturation magnetization of the 7% Ni-doped sample is approximately six times higher than the pristine one. The saturated P-E hysteresis loops establish the ferroelectric nature of the synthesized samples that are enhanced significantly on Ni doping. Thus, the doping of Ni in BiFeO3 improves the multiferroic properties of the matrix.

Effect of Pr3+ substitution on structural, dielectric, electrical and magnetic properties of BiFe0.80Ti0.20O3 [Bi1-xPrxFe0.80Ti0.20O3, x = 0.05, 0.10, 0.15] ceramics

Pr3+ and Ti4+ doped BiFeO3; Bi1-xPrxFe0.80Ti0.20O3 (x = 0.05, 0.10 and 0.15) ceramics were prepared by two stage solid state sintering method. The X-ray diffraction shows large structural distortion or the phase transformation from rhombhohedral to orthorhombic with the increase in Pr3+ percentage on the A site of BiFe0.80Fe0.20O3 ceramic. An anomaly peak was found close to magnetic transition temperature of BiFeO3 in dielectric constant versus temperature study for all the prepared samples. The value of activation energy is found to lie between 0.78 and 1.28 eV at an applied frequency of 1 kHz and decreases slightly with the increase in frequency from 1 kHz to 100 kHz. The value of frequency exponent ‘s’ lies between 0.80 and 0.98 and decreases with the increase in temperature range from 473 to 673 K. In the magnetic study, magnetization (2Mr) is found to increase from 0.146 to 0.250 emu/g with the increase in Pr3+ content.

Structural and magnetic properties of BiFeO3-PbTiO3 polycrystals

In this manuscript, microstructural, structural and magnetic properties of pure and La doped (0.6)BiFeO3-(0.4)PbTiO3 polycrystals were investigated. X-ray diffraction, transmission and scanning electron microscopy results reveal morphologically uniform particles of rhombohedral and rhombohedral/tetragonal symmetries for pure and La doped samples. An enhanced magnetization, characterized by slim loop hysteretic curves of null remnant magnetization and 2 emu/g saturation (15 kOe) magnetization, is observed at room temperature for samples with average particle size close to 5 nm.

Effects of Gd and Cr co-doping on multiferroic properties of Bi0.9Gd0.1Fe(1-x)CrxO3 (x = 0−0.08)

Gd and Cr co-doped Bi0.9Gd0.1Fe(1 − x)CrxO3 (x = 0 − 0.08) nanoparticles were synthesized via sol-gel method. Precursor salts of bismuth, iron, gadolinium and chromium were used as starting raw materials. Obtained nanoparticles were annealed at various temperatures between 400 − 700 °C. Optimal properties were found to display at 600 °C. X-ray diffraction (XRD) patterns revealed that the partial substitution of Bi3+ by Gd ions and Fe3+ by Cr ions in BiFeO3 (BFO) resulted in evolution of phase transition from rhombohedral to orthorhombic structure. The average crystallite size was found to vary from 16 to 68 nm depending on doping level. The field emission scanning electron micrographs demonstrated a distinct morphology of spherical and nano-sized particles. Ferroelectric measurements showed a substantial improvement in polarization in doped BFO under an applied field of ± 30 kV/cm. Magnetic properties measured at room temperature showed enhanced ferromagnetic characteristics with increasing doping level of Cr up to 6 % (at.) compared to that of un-doped BFO. An asymmetric shift both in the field and magnetization axes of M-H loops was also observed.

Magnetic Sm-BFO and Ce-BFO nanoflakes as protective coating layers for C-steel in acidic chloride environments

This investigation suggests a straightforward method for the C-steel coating by two different doped BiFeO3 (BFO) nanomaterials, specifically Ce and Sm doped BFO nanoflakes for protection against corrosion. The Ce and Sm doped BFO nanoflakes were prepared by sol–gel process and the C-steel was coated by the dip coating method. Influences of the number of layers and surfaces morphology of Ce-and Sm-BFO nanoflakes on the corrosion resistance of coated C-steel were studied. Different techniques such as FT-IR; X-ray diffraction and scanning electron microscopy were performed in order to describe the coatings structure. Moreover, the electrical and magnetic properties were also examined. Electrochemical impedance spectroscopy (EIS) and potentiodynamic measurements (PDP) were utilized to investigate the corrosion protection execution in HCl solution. Both samples are weak ferromagnetic ordering and have a semiconductor feature. It was found that the C-steel corrosion can be controlled by coating with Ce-and Sm-BFO nanoflakes. The corrosion protection results indicated that the efficiency of Sm-BFO nanoflakes as a corrosion protective material is very high compared to that of Ce-BFO nanoflakes due to the smooth surface of Sm-BFO and rough mesoporous surface of Ce-BFO as shown in SEM results.

Influence of BiFeO3 doping on structural and electrical properties of KNN–LS based lead-free piezoceramics

(0.94 − x)K0.48Na0.52NbO3–0.06LiSbO3–xBiFeO3 (KNN–LS–xBF, x = 0, 0.1, 0.3, 0.5 and 0.7%) lead- free piezoelectric ceramics were prepared by a traditional two-step solid-state reaction method. The effect of BiFeO3 (BF) content on the phase structure, microstructure, electrical properties and dielectric relaxation of KNN–LS ceramics was investigated. The XRD patterns show that all ceramics have a pure perovskite structure. BiFeO3 doping can promote the grain growth and improve electrical properties. When x = 0.3% in KNN–LS–BF, it exhibits optimum electrical performance: d33 = 285 pC/N, kp = 54.3%, er = 2185, tanδ = 1.92%, Qm = 66, Tc = 340 °C, γ = 1.4732, Ec = 17.76 kV/cm, Pr = 21.48 µC/cm2. It is indicated that the ceramics have the potentialities to be used as the high temperature low frequency filters.

Structural stability, enhanced magnetic, piezoelectric, and transport properties in (1-x)BiFeO3–(x)Ba0.70Sr0.30TiO3 nanoparticles

Multiferroic samples with composition (1-x)BiFeO3-(x)(Ba0.70Sr0.30)TiO3 (BFO-BST) were synthesized using a sol-gel route to study the effect of BST doping on structural, transport, and magnetic properties in BiFeO3 (BFO). X-ray diffraction studies with Rietveld analysis revealed that a phase transition occurred from rhombohedral (R3c) (0.0 ≤ × ≤ 0.15) to tetragonal (P4 mm) for x = 0.20 and nanocrystalline nature confirmed by transmission electron microscopy measurements. Piezoelectric properties improved as x increased from x = 0.0 (58 pC/N) to x = 0.20 (112 pC/N) increasing distortion in the crystal structure as evinced by Williamson-Hall analysis. Ferromagnetism was observed in doped BFO, different from the antiferromagnetic ordering in bulk BFO, indicating the noteworthy size effects and Fe-O-Fe bond angle variations in the magnetic ordering of BFO. An improvement in ferroelectric properties is observed with doping of BST compared to pristine BFO. Thermally activated conduction behavior occurred at low and high temperature regions as revealed by temperature dependent dc resistivity measurement. Effective improvements in dielectric response, meaning high dielectric constant with a low dielectric loss, were found in the doped samples.

High energy-storage density of lead-free BiFeO3 doped Na0.5Bi0.5TiO3-BaTiO3 thin film capacitor with good temperature stability

In this work, the lead-free and BiFeO3 doping Na0.5Bi0.5TiO3-BaTiO3 thin films [(0.94-x)Na0.5Bi0.5TiO3-0.06BaTiO3-xBiFeO3 (NBT-6BT-xBFO), x = 0.00, 0.01, 0.03, 0.05 and 0.07] were deposited on Pt/Ti/SiO2/Si substrates by a sol-gel method. The enhanced ferroelectric properties and high energy-storage density have been achieved at 5% BiFeO3 substitution. Moreover, the 5% BiFeO3 doping Na0.5Bi0.5TiO3-BaTiO3 (0.89NBT-0.06BT-0.05BFO) thin-films exhibited a high dischargeable energy density (Wrecovered) of 42.9 J/cm3 with a corresponding energy-storage efficiency (η) of 65.7% under an electric field of 1720 kV/cm, and the high temperature stabilities with regard to their energy-storage properties over temperatures ranging from room temperature to 260 °C were demonstrated, and the charging and discharging characteristics demonstrate the faster microsecond discharge and large dielectric strength of the thin film capacitor. The results indicated that the 0.89NBT-0.06BT-0.05BFO thin-films might be promising environmentally friendly lead-free materials for energy-storage capacitor application.