Effect of particle morphology on the photocatalytic activity of BiFeO3 microcrystallites

Pure BiFeO₃ (BFO) microcrystallites with different morphologies were synthesized by a controlled hydrothermal process. The crystal structure, morphology and photocatalytic property of BFO microcrystallites were investigated. The results and analysis revealed that the OH^? concentration intensely affected the morphology, dimension and phase purity of BFO. Furthermore, it was found that the smallest and coarse BFO particles exhibited the lowest photocatalytic efficiency, which suggested that the particle morphology, especially defects intensely influence the photocatalytic activity of BFO, other than the particle size.     

Effect of Mg Substitution on Room Temperature Dielectric and Magnetic Properties of Sr-Doped Bismuth Ferrite

Mg-co-substituted BiFeO₃ was synthesized. We investigated the structure and multiferroic properties of Sr- and Mg-co-substituted bismuth ferrite. The purity and structural changes induced by Mg doping are confirmed by X-ray powder diffraction and Raman spectra. It was found that a small amount of Mg doping leads to dramatic enhancement in dielectric permittivity, along with an apparent improvement in ferromagnetism .Meanwhile, the co-substitution can effectively reduce the leakage current and increase the dielectric constant. The release of latent magnetization after Sr and Mg co-doping is stronger than the sum of two single dopings, indicating a nonlinear enhancement in Sr and Mg codoping. The ferromagnetism can be ascribed to the creation of unbalanced Fe³⁺ spins and relative long-range coupling mediated by the oxygen vacancies trapped localized electrons. Compared to the pristine bismuth ferrite, Bi_0.95Sr_0.05Fe_0.9Mg_0.1O₃ exhibits more than fivefold improved magnetization with simultaneously improved electrical properties demonstrating the possibility of co-doped BiFeO₃ for practical applications.     

Characterization and photocatalytic activity of Y-doped BiFeO3 ceramics prepared by solid-state reaction method

Singe phase bismuth ferrite doped by yttrium (Bi_1?xY_xFeO₃, x = 0, 0.05, 0.1, 0.15, 0.2 and 0.25) was synthesized by solid-state reaction followed by sintering. Their structural, morphological, ferroelectric, magnetic and optical properties were examined by X-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS), vibrating sample magnetometer (VSM) and UV–visible spectrophotometry. Rhombohedrally-distorted perovskite structure of bismuth ferrite was confirmed by XRD analysis and Rietveld refinement. Microstrain and crystallite size were analyzed using Williamson-Hall model. SEM micrographs showed agglomerated particles. The doping of yttrium into the BiFeO₃ (BFO) lattice enhanced the ferroelectric and magnetic properties and the leakage current density was reduced. The energy band gap was also decreased by increasing yttrium content, leading to an enhancement of light absorption capability. The photocatalytic activity of all samples has been evaluated by the decolorization of methyl orange (MO) under visible light irradiation. The results indicated that increasing the concentration of yttrium into the BiFeO₃ (BFO) structure improved the photodegradation up to 71%.     

Sol–gel synthesis and properties of multiferroic BiFeO3

High purity multiferroic BiFeO₃ (BFO) powders were synthesized by sol–gel process using bismuth nitrate and iron nitrate as sources. The BFO powders annealed at 600 °C in nitrogen environment were mainly composed of a rhombohedral BiFeO₃ phase (R-phase) with a minor impurity phase of Bi₂O₃. A pure BFO R-phase has been obtained by leaching out the minor Bi₂O₃ phase using diluted nitric acid. A reversible phase transformation of the BFO R-phase has been detected at 836 °C by a differential thermal analysis. The powders of BFO R-phase exhibits a uniform feature with the particle size of ∼200 nm. The dielectric constant of the BFO R-phase is measured to be ∼15 in the frequency range of 10⁴∼10⁶ Hz at room temperature.     

Alumina powder assisted carbon nanotubes reinforced Mg matrix composites

Mg matrix composites reinforced by carbon nanotubes (CNTs)-Al₂O₃ mixture, which was synthesized by in situ growing CNTs over Al₂O₃ particles through chemical vapor deposition (CVD) using Ni catalyst, were fabricated by means of powder metallurgy process, followed by hot-extrusion. By controlling synthesis conditions, the as-grown CNTs over Al₂O₃ particles possessed high degree of graphitization, ideal morphology, higher purity and homogeneous dispersion. Due to the ‘vehicle’ carrying effect of micrometer-level A₂O₃, CNTs were easy to be homogeneously dispersed in Mg matrix under moderate ball milling. Meanwhile, Al₂O₃ particles as catalyst carriers, together with CNTs, play the roles of synergistic reinforcements in Mg matrix. Consequently, the Mg matrix composites reinforced by CNTs-Al₂O₃ mixture exhibited remarkable mechanical properties.     

Effect of novel (Gd,Cu) substitution on the electrical properties and magnetoelectric coupling of bismuth ferrite ceramics

The influence of Gd dopant and (Gd, Cu) dopants on the ferroelectric, dielectric and magnetoelectric properties of single phase BiFeO₃ (BFO) were investigated. Nanoparticles of undoped BiFeO₃, Bi_0.95Gd_0.05FeO₃ and Bi_1?xGd_xFe_0.98Cu_0.02O₃ (x?=?1, 2, 3, 4 and 5%) were prepared by sol–gel method. X-ray diffraction reveals that all the samples crystallize in rhombohedral phase. The simultaneous Gd and Cu doping at BFO lattice has significantly enhanced the ferroelectric properties of BFO compared to that of BFO. Substitution of Gd alone at the Bi site, gave rise to attractively enhanced remnant polarization. Though the (Gd, Cu) doped BFO samples exhibit relatively less enhancement, their values of remnant polarization are appreciable. Doping of (Gd, Cu) in the BFO lattice leads to an appreciable dielectric properties. An effective magnetoelectric coupling has been recorded for doped BFO when compared to BFO.     

Effect of Ba(Cu1/3Nb2/3)O3 content on multiferroic properties in BiFeO3 ceramics

The crystal structure, microstructural evolution, and ferroelectric and magnetic properties of BiFeO₃ (BFO) ceramic materials, widely known as room temperature multiferroic materials, were investigated with various contents of Ba(Cu_1/3Nb_2/3)O₃ (BCN). The ceramics were synthesized through the solid state reaction and pressureless sintering in air. By adding tetragonal structured BCN phase to the rhombohedral structured BFO phase, a solid solution was formed. The rhombohedral crystal structure and lattice parameter, densification behavior, and microstructure of BFO were noticeably changed upon the addition of BCB. The electrical properties of BFO were slightly enhanced by the addition of BCN. However, the magnetic properties of BFO which is the most critical issue in BFO multiferroics were drastically changed and the 20 mol% BCN added BFO ceramics showed extremely enhanced magnetization characteristics compared to the other BFO–BCN ceramics. It is expected that BFO–BCN ceramics with this composition could be one of the most promising multiferroic materials for future multiferroic applications.     

Structural,Magnetic, Optical and Electrical Properties of Ba Substituted BiFeO3

Ba²⁺ substituted bismuth ferrite (BFO) (Bi_1?x Ba _x FeO₃, x=0, 0.1, 0.2, and 0.3) was prepared by chemical route using nitrates as a precursor, and sintered at 830 °C for 1 hr. The structural, optical, magnetic, and electrical properties of the prepared samples were investigated to understand the effect of Ba²⁺ substitution in BFO. The samples were characterized by X-ray diffraction (XRD), and shows an impurity phase which reduces drastically on substitution. Magnetic measurements were carried out at room temperature up to a field of 20 kOe. Magnetic hysteresis loops revealed a significant change in magnetization on Ba²⁺ substitution in BFO with unsaturated nature. Further, the Fourier transformed Infrared (FTIR) and Ultra Violet Visible (U-V Vis) spectra of the samples at room temperature shows as a change in spectral behavior on substitution. The AC electrical conductivity analyzed through Impedance analyser at different temperatures and frequencies. The ac conductivity and its activation energies for the samples are found to be frequency and temperature dependent and also vary with Ba²⁺ substitution in BFO.     

Multiferroic properties of BiFeO3/Bi4Ti3O12 double-layered thin films fabricated by chemical solution deposition

Multiferroic BiFeO₃/Bi₄Ti₃O₁₂ (BFO/BTO) double-layered film was fabricated on a Pt(111)/Ti/SiO₂/Si(100) substrate by a chemical solution deposition method. The effect of an interfacial BTO layer on electrical and magnetic properties of BFO was investigated by comparing those of pure BFO and BTO films prepared by the same condition. The X-ray diffraction result showed that no additional phase was formed in the double-layered film, except BFO and BTO phases. The remnant polarization (2P_r) of the double-layered film capacitor was 100 μC/cm² at 250 kV/cm, which is much larger than that of the pure BFO film capacitor. The magnetization-magnetic field hysteresis loop revealed weak ferromagnetic response with remnant magnetization (2M_r) of 0.4 kA/m. The values of dielectric constant and dielectric loss of the double-layered film capacitor were 240 and 0.03 at 100 kHz, respectively. Leakage current density measured from the double-layered film capacitor was 6.1 × 10^− 7 A/cm² at 50 kV/cm, which is lower than the pure BFO and BTO film capacitors.     

Effect of Cr doping on the phase structure,surface appearance and magnetic property of BiFeO<Subscript>3</Subscript> thin films prepared via sol–gel technology

Multiferroic BiFeO₃ (BFO) and BiFe_1?xCr_xO₃ (BF_1?xC_xO, x?=?0.05, 0.1, 0.15 and 0.2) thin films were successfully synthesized on silicon (111) substrates via sol–gel technology. The effect of Cr³⁺ ion doping on the phase structure, surface morphology, valence states for Fe element and magnetic property was investigated. The introduction for simulation images of ionic space arrangement was to better comprehend the substitution site and superexchange interaction between the Fe³⁺ (Cr³⁺) and O^2? ions. The phase structure of Cr-doping thin films transition from rhombohedral to orthorhombic was confirmed by the X-ray diffraction (XRD) and Raman measurements, and the obtained results also demonstrated that the Cr³⁺ ions successfully located in Fe²⁺ and Fe³⁺ ions sites of BFO lattice system. The Field Emission Scanning Electron Microscopy (FESEM) patterns clearly exhibited that the grains sizes were remarkably decreased by Cr³⁺ ions doping, and the surfaces textures got glossier and smoother judging from the Atomic Force Microscope (AFM) images. The dense surface structure can restrict the O^2? ions escaping from the lattices system, which is beneficial for the release of magnetic property due to superexchange interaction of improvement. It was found that the saturation magnetization (Ms) was significantly linearly increased accompanying the adding of Cr-doping due to destroying of spatial modulation helical structure and enhancing of superexchange interaction. Moreover, the Hall-effect results firstly revealed that the carrier concentration and mobility rate played significant roles in magnetoelectric effect behaviors.     

Improved magnetic and piezoresponse behavior of cobalt substituted BiFeO3 thin film

Piezoresponse force microscopy (PFM) technique has been utilized to characterize the nanoscale electromechanical properties of chemical solution deposited pristine (BiFeO₃, BFO) and 5% Cobalt-substituted bismuth ferrite (BiFe_0.95Co_0.05O_3, BFCO) thin films. Distinct PFM contrast was observed on the films' surface both in out-of-plane and in-plane modes. Piezoresponse signal was larger in BFCO film than BFO and the former film also exhibited negative self polarization. The calculated self polarization factor was − 0.94. Lateral signal (in-plane) changed its sign upon 180° sample rotation which ruled out spurious electrostatic contribution and confirmed piezoelectric nature of the effect. Square patterns were written by local poling and reversible nature of the piezoresponse behavior was established. Well saturated piezoelectric hysteresis loops were acquired with improved piezoelectric coefficient value in case of Co-substituted BFO film. The effect has been described on the basis of polarization rotation with doping. Magnetization of BFO film also increases with Co-substitution. The enhancement in magnetic and ferro/piezoelectric properties should be useful for multiferroic applications.     

Hydrogen-induced degradation in multiferroic BiFeO3 ceramics

Chemical stability of multiferroic BiFeO₃ (BFO) ceramics has been studied through electrochemical hydrogen charging, in which BFO ceramic pellets were placed in 0.01 M NaOH solution with hydrogen deposited on their electrodes from the electrolysis of water. The properties of the samples were greatly degraded after the treatment: The leakage current was increased by orders of magnitude, the capacitance and the dielectric loss were dramatically increased in low frequency region. It was proposed that hydrogen entered into BFO lattice and existed at interstitial sites and the degradation was explained by hydrogen's reduction of some Fe³⁺ to Fe²⁺ in BFO. Hydrogen-induced degradation occurs relatively easily in BFO and much attention should be paid to it for BFO-based devices.     

Ferromagnetic–Antiferromagnetic Coupling by Distortion of Fe/Mn Oxygen Octahedrons in (BiFeO3)m(La0.7Sr0.3MnO3)n Superlattices

Interface enhanced magnetism attracts much attention due to its potential use in exploring novel structure devices. Nevertheless, the magnetic behavior at interfaces has not been quantitatively determined. In this study, abnormal magnetic moment reduction is observed in La_0.7Sr_0.3MnO₃ (LSMO)/BiFeO₃ (BFO) superlattices, which is induced by ferromagnetic (FM)/antiferromagnetic (AFM) coupling in the interface. With reduced repetition of the superlattice's unit cell [(LSMO)_n /(BFO)_n ]_60/n (n = 1, 2, 5, 10) on a SrTiO₃ substrate, magnetic moment reduction from 25.5 emu cc^?1 ([(LSMO)₁₀/(BFO)₁₀]₆) to 1.5 emu cc^?1 ([(LSMO)₁/(BFO)₁]₆₀) is obtained. Ab initio simulations show that due to the different magnetic domain formation energies, the magnetic moment orientation tends to be paramagnetic in the FM/AFM interface. The work focuses on the magnetic domain formation energy and provides a pathway to construct artificial heterostructures that can be an effective way to tune the magnetic moment orientation and control the magnetization of ultrathin films.     

Structural, Dielectric, Ferroelectric and Magnetic Properties of Bi0.80A0.20FeO3 (A=Pr,Y) Multiferroics

Here we studied the effect of homovalent Pr³⁺ and Y³⁺ substitution on the crystal structure, dielectric, electronic polarization and magnetic properties of the BiFeO₃ multiferroic ceramic. The samples were synthesized by the conventional solid-state reaction method. Pure phase formation of Pr doped BiFeO₃ (BFO) has been obtained, while Y³⁺ doped BFO has shown a few impurity peaks. It has shown that the crystal structure of the compounds is described within the space group R3c. Pr³⁺ modified BFO has shown an anomaly in the ε _r vs. T plot around and a Néel temperature ‘T _N’～370 ^°C. P–E hysteresis loops have shown higher value of remnant polarization for Pr³⁺ modified BFO. Magnetic properties of ceramics are determined by the ionic radius of the substituting element. Experimental results propose that the increase in the radius of A-site ion leads to effective suppression of the spiral spin structure of BiFeO₃, resulting in the appearance of net magnetization.     

Role of strain and nanoscale defects in modifying the multiferroicity in nanostructured BiFeO3 films

BiFeO₃ (BFO) multifunctional oxide exhibits fascinating multiferroic properties suitable for potential application in nanoscale electronic devices such as non-volatile random accesses memory (FeRAM), field effect transistors, capacitors, logic circuits, etc. In this communication, we report the results of studies on 200 MeV Ag⁺¹⁵ ion irradiation-induced modifications in the structural, microstructural, electrical (I–V, C–V and P–E) and magnetic properties of pulsed laser deposited nanostructured BFO films grown on conducting SrTi_0.998O_0.002TiO₃ (SNTO) substrates. Role of nanoscale structural defects and oxygen vacancies in modifying the physical properties of BFO/SNTO has been discussed in this work.     

Self-assembly growth of BiFeO3 powders prepared by microwave-hydrothermal method

With FeCl₃·6H₂O and Bi(NO₃)₃·5H₂O powder as raw materials and KOH as a mineralizer, the pure phase BiFeO₃ (BFO) powder was synthesized by microwave-hydrothermal (MH) method at 200 °C, with the reaction time as little as 30 min. The range of preparing the BFO powders had been summarized. The field emission scanning electron microscopy (FE-SEM) images revealed that the little BFO plate grew together forming rock sugar-like BFO powders, and then they grew further to form the mussel-like BFO powders. The transmission electron microscope (TEM) images also improved the self-assembly growth of BFO powders. The X-ray diffraction (XRD), the high resolution transmission electron microscopy (HRTEM) and the selected area electron diffraction (SAED) results indicated that the BFO powders grew along the [110] and [104] crystal orientation. The B-H loops of BFO indicated that the weak magnetism existed in the pure phase BFO powders.     

Two-Phase Coexistence and Multiferroic Properties of Cr-Doped BiFeO3 Thin Films

Pure BiFeO ₃ (BFO) and Cr-doped BiFe _0.97 Cr _0.03 O ₃ (BFCO) thin films were successfully prepared on F-doped SnO ₂ conductive film (FTO)/glass (SnO ₂: F) substrates by a sol–gel method. The effect of Cr doping on the structure, ferroelectric, and ferromagnetic properties of the BFO and BFCO thin films have been investigated. X-ray diffraction, Rietveld refined X-ray diffraction (XRD) patterns, and Raman spectroscopy results clearly reveal that the BFCO thin film is characterized by the coexistence of two phases (trigonal and tetragonal). Moreover, the various leakage mechanisms of both thin films have also been studied. The improved ferroelectricity with remnant polarization (Pr) of about 2Pr = 68.68 μC/cm ² under an applied electric field of 1,181.8 kV/cm and enhanced ferromagnetism with saturation magnetization (M _s) of M _s = 0.93 emu/cm ³ have been observed in the BFCO thin film. The improved electrical properties of the BFCO thin film are ascribed to the coexistence of trigonal and tetragonal phase and high valence of Cr ⁶⁺, and the Fe–O ₆ octahedron distortion is enhanced due to the overlap and hybridization of Fe _3d/Cr _3d and O _2p orbits by Cr doping.     

Temperature Dependent Magnetic, Dielectric Studies of Sm-Substituted Bulk BiFeO3

Bismuth ferrite- (BiFeO₃) ceramic is the most studied and attractive multiferroic material with low magnetization, moderate leakage current, and low polarization. Samarium substituted bulk BiFeO₃ prepared at low synthesis temperature ??600?°C by the sol?Cgel process. Room temperature X-ray diffraction (XRD) patterns confirmed the formation of perovskite structure Bi_0.9Sm_0.10FeO₃ (BSFO) phases. Present compositions possess high dielectric constant (????199) and low dielectric loss (tan????0.009) at room temperature for 100?Hz frequency. Room temperature dielectric permittivity and dielectric loss decreased with increasing frequency from 100?Hz to 10 MHz. As the temperature increased, an enormous increase in both dielectric permittivity and dielectric loss is observed at all frequency regions. Temperature dependent M?CH hysteresis loops were saturated. Spin glass-like ferromagnetic behavior is retained in M?CH hysteresis loops measured from the low temperature region and normal ferromagnetic behavior is observed in the high temperature region, both at room temperature and above ??350?K, 400?K, respectively. The origin of the ferromagnetic property in BSFO may be due to the presence of rare earth metal ions at the lattice sites of?BFO.     

Wet chemical synthesis of iron pyrite and characterization by Mössbauer spectroscopy

Iron pyrite in powder form was synthesized by using FeCl₃ and either thiourea or Na₂S as the starting materials. Attempts were made to achieve pure pyrite phase through a simple wet chemical reaction in the liquid state followed by high temperature annealing in the argon atmosphere. The phase purity and the composition of the sample were determined by X-ray diffraction (XRD) and energy dispersive analysis of X-ray (EDAX), respectively. The phase purity and its intrinsic magnetic behavior were further clarified by Fe⁵⁷ Mössbauer spectroscopic investigations.     

Spatially Resolved Large Magnetization in Ultrathin BiFeO3

Here, a quantitative magnetic depth profile across the planar interfaces in BiFeO₃/La_0.7Sr_0.3MnO₃ (BFO/LSMO) superlattices using polarized neutron reflectometry is obtained. An enhanced magnetization of 1.83 ± 0.16 μ_B/Fe in BFO layers is observed when they are interleaved between two manganite layers. The enhanced magnetic order in BFO persists up to 200 K. The depth dependence of magnetic moments in BFO/LSMO superlattices as a function of the BFO layer thickness is also explored. The results show the enhanced net magnetic moment in BFO from the LSMO/BFO interface extends 3–4 unit cells into BFO. The interior part of a thicker BFO layer has a much smaller magnetization, suggesting it still keeps the small canted AFM state. The results exclude charge transfer, intermixing, epitaxial strain, and octahedral rotations/tilts as dominating mechanisms for the large net magnetization in BFO. An explanation—one suggested by others previously and consistent with the observations—attributes the temperature dependence of the net magnetization of BFO to strong orbital hybridization between Fe and Mn across the interfaces. Such orbital reconstruction would establish an upper temperature limit for magnetic ordering of BFO.