Mn Co-substituted BiFeO3 Research Articles

Growth of heteroepitaxial La and Mn co-substituted BiFeO3 thin films on Si (100) substrate by pulsed laser deposition

Growth of heteroepitaxial La and Mn co-substituted BiFeO3 thin films on Si (100) substrate by pulsed laser deposition

Optical bandgap tuning of ferroelectric semiconducting BiFeO3-based oxide perovskites via chemical substitution for photovoltaics

Ferroelectrics exhibit bulk photovoltaic effect, of which spontaneous electric polarization separates photo-excited carriers and results in above-bandgap open-circuit voltage. Compared with organometal halide perovskites, ferroelectric oxide perovskites are much more stable in a wide range of mechanical, chemical and thermal conditions and able to fabricate using low-cost facilities. The bottlenecks for ferroelectric photovoltaic applications are their poor optical absorption of visible light and high electric resistivity owing to wide bandgap. In this work, bandgap of BiFeO3-based oxide perovskites was judiciously tuned in a range of 1.10-2.06 eV through B-site chemistry, which makes ferroelectric semiconducting oxide perovskites feasible for photovoltaic solar cell applications with wide light absorption and high efficient energy conversion. In particular, La and Mn co-substituted BiFeO3 solid solution exhibits a narrow direct bandgap around 1.20 eV, which extends photon absorptions over ∼80% sunlight spectrum covering ultraviolet-visible-infrared region and increases electrical conductivity (σdc) up to about 10-5 S/m. The electronic subshell configuration, the reduced mass of unit cell, and the tolerant factor/octahedral factor related to ionic size were attempted as descriptors to classify and map out the relationship between compositions and bandgap properties of BiFeO3-based oxide perovskites. This essay paves the road for ferroelectric semiconducting oxide perovskites to make photovoltaic solar cells with power conversion efficiency promising beyond the Shockley-Queisser limit of conventional p-n junction solar cells.

Theoretical and Experimental Evidence of Modified Structure, Magnetism and Optical Properties in Ba and Mn Co-Substituted BiFeO3

To improve the vacancy related defects associated with pure BiFeO3 (BFO) different Ba-Mn co-doped samples (Bi1-xBaxFeyMnyO3; x = 0.1; y = 0.0, 0.05, 0.1, 0.15) were prepared from sol-gel method. The influence of co-doping on structural, magnetic and optical properties of BiFeO3 has been studied in different experimental conditions. Particle size and magnetic properties were measured from SEM and VSM analysis respectively. Again band gap was calculated from UV analysis. The co-doped samples showed reduced particles size as well as better surface morphology. M-H loops suggest that the presence of Mn2+ increased ferromagnetic quantity at some levels. Still room temperature occurrence of ferromagnetism obtained from M-H loops of all samples showed weak ferromagnetic behaviour. In addition the incorporation of Ba2+ at Bi3+ site and Mn2+ at Fe3+ site decreased the band gap which was evidenced by increased optical property.

Structural, magnetic and ferroelectric properties of Sm and Mn co-substituted BiFeO3 ceramics with composition near the morphotropic phase boundary

The polycrystalline Sm and Mn co-substituted bismuth ferrites Bi1-xSmxFe0.95Mn0.05O3 (x = 0.08 and 0.16) with composition near the morphotropic phase boundary have been prepared using a modified solid-state reaction method. The Bi0.92Sm0.08Fe0.95Mn0.05O3 ceramic possesses single rhombohedral phase, whereas the Bi0.84Sm0.16Fe0.95Mn0.05O3 ceramic consists of both the R3c and Pnma phases. The addition of Sm and Mn not only enhances density and grain uniformity but also suppresses the spiral spin modulation which improves the polarization and magnetization. Excellent ferroelectric properties of microdomains are attained in the Bi0.92Sm0.08Fe0.95Mn0.05O3 ceramic. The transition of the nonpolar/antipolar phase to the polar phase and the polarization switching induced by the electric field occur in the Bi0.84Sm0.16Fe0.95Mn0.05O3 ceramic.

Phase diagram of Sm and Mn co-doped bismuth ferrite based on crystal structure and magnetic properties

Sm and Mn co-substituted BiFeO3 (BFO) ceramic has been synthesized by the tartaric acid modified sol-gel method to understand the effect of structural modification on the magnetic properties. The secondary phases (i.e., Bi2Fe4O9, Bi25FeO39 etc.) present in the parent compound BiFeO3 have been significantly reduced with increase in the substitution percentage of Sm and Mn at Bi and Fe sites of BFO respectively. The Rietveld refinement of the X-ray diffraction (XRD) patterns reveals a structural transition from rhombohedral (R3c, space group) to orthorhombic (Pbnm, space group) crystal symmetry with the concentration of Sm and Mn in the sample. The chemical state and elemental ratio of the samples have been studied by analyzing the X-ray photoelectron spectroscopy (XPS) data. Crystallite size obtained from the transmission electron microscopy analysis is in well comparable with the XRD analysis. Fourier transform infrared spectroscopy (FTIR) analysis supports the XRD results. A change in the phonon frequency and line widths in Raman spectra confirms the lattice distortion in the co-substituted samples which are in agreement with the XRD analysis. Ferromagnetic signature has been observed in Sm and Mn co-substitution. The magnetic hysteresis loop at room temperature has been analyzed by considering both ferromagnetic and antiferromagnetic contribution. The saturation magnetization for ferromagnetic phase increases up to 10% substitution of Sm and Mn in BFO and, decrease with the further increase of percentage of substituents. A correlation between crystal structure and magnetic behavior has been observed for Sm and Mn co-substituted sample. Phase diagram of crystal symmetries percentage, saturation magnetization (MS), remnant magnetization (Mr) and coercivity (HC) of Bi1-xSmxFe1-xMnxO3 for x = 0.000, 0.025, 0.050, 0.075, 0.100 and 0.150 ceramics.

Effect of structural transition on magnetic and dielectric properties of La and Mn co-substituted BiFeO3 ceramics

Bi1−xLaxFe1−xMnxO3 (x = 0.000–0.300) ceramics prepared by the tartaric acid modified sol–gel technique have been studied to analyze the effect of composition driven structural transition on the magnetic properties of bismuth ferrite (BiFeO3). It was found that the co-substitution of La & Mn at Bi & Fe sites leads to suppression of impurity phases (Bi2Fe4O9 and Bi25FeO40) which generally appear in BiFeO3. The quantitative crystallographic phase analysis has been carried out by double phase Rietveld analysis of all the XRD patterns which indicates the existence of compositional driven crystal structure transformation from rhombohederal (space group R3c) to the orthorhombic (space group Pbnm) with the increase in substitution concentration. The 5% co-substituted sample exhibit high remnant magnetization i.e. about 15 times that of BiFeO3 which is due to the suppression of cycloid spin structure and enhanced canting angle of antiferromagnetically ordered spins caused by the crystal lattice distortion. However, further increase in substitution results in the reduction of remnant magnetization and coercivity due to the appearance of complete antiferromagnetic ordering in the orthorhombic structure because of the significant contribution from the crystallographic phase of Pbnm space group (as obtained from double phase Rietveld analysis). The frequency independent higher dielectric constant and lower dielectric loss were observed for 5% co-substitution. Hence, this particular composition may be interesting for device applications.

Effect of Tb–Mn substitution on the magnetic and electrical properties of BiFeO3 ceramics

Tb–Mn co-substituted BiFeO3 of stoichiometry Bi1−xTbxFe1−xMnxO3 (x=0, 0.10, 0.15, 0.20) has been synthesized by a solid-state reaction method. Inferences from X-ray photoelectron spectroscopy indicate the presence of Fe in mixed oxidation states i.e., Fe3+ and Fe2+. But in the Tb–Mn substituted samples, Fe is predominantly in +3 oxidation state and Mn is found to be in +3 state. There is a decrease in cell volume due to Tb3+ substitution as Mn3+ ions have similar ionic radii as that of Fe3+. The grains of substituted samples have regular dimensions with agglomeration when compared to irregular shaped grains of BiFeO3. No major characteristic transitions are found in substituted samples as observed from the thermal measurements, but BiFeO3 shows a ferroelectric−paraelectric transition at ~820°C. A weak magnetic ordering is observed from the magnetization measurements at room temperature with the increase in substitution percentage. Impedance spectroscopy studies show the impact of Tb–Mn substitution on the conductivity of BiFeO3. The ferroelectric loop and leakage current studies with an applied electric field on samples also supplement the results obtained from impedance measurements.

Leakage current under high electric fields and magnetic properties in Co and Mn co-substituted BiFeO3 polycrystalline films

Co and Mn co-substituted BiFeO3 films were prepared on Pt/Ti/SiO2/Si(100) substrates, and the effect of co-substitution on the structural, electric, and magnetic properties of the prepared films was systematically investigated. Cross-sectional transmission electron microscopy observations showed that the films were homogeneous; moreover, no magnetic secondary phases were observed in the films. Nano-beam energy-dispersive X-ray spectroscopy, carried out at various points in the films, revealed that Co and Mn were distributed in the films. Further, the average concentration of Bi, Co, Mn, and Fe was 59, 2, 2, and 37at.%, respectively; these values were almost consistent with the nominal composition of the precursor solution used. X-ray diffraction profiles of the films showed that the (012)-d-spacing decreased and (104) and (110) peaks merged into a single peak with increasing co-substitution content. The leakage current in films under high electric fields drastically decreased upon co-substitution without any degradation of ferroelectricity; moreover, this effect was also observed for single Mn-substituted materials. Saturation magnetization of the films monotonically increased with the co-substitution content, and this increase was quantitatively identical to that in the case of single Co-substituted materials. These results indicate that Co and Mn play significant roles in determining the properties of co-substituted BiFeO3 films.

Effects of γ-ray irradiation on ferroelectric properties of Pr and Mn co-substituted BiFeO3 thin films

γ-ray irradiation effects on ferroelectric properties of (Bi0.85Pr0.15)(Fe0.95Mn0.05)O3 (BPFMO) thin films are investigated by pizeoresponse force microscopy and polarization–voltage hysteresis measurements. The irradiated BPFMO thin-film capacitors show reduced polarizations and imprinted hysteresis loops. The loss of nonvolatile polarization increases with the decrease of polarization values prior to the irradiation and the imprint of hysteresis loops depends on the direction of polarization. These are discussed in terms of separation of γ-ray-excited electron–hole pairs by the depolarization field and subsequent charge trapping and aggregation on charged domain walls in BPFMO thin films.

Mechanical milling assisted synthesis of Ba–Mn co-substituted BiFeO3 ceramics and their properties

Samples of composition Bi1−xBaxFe1−xMnxO3 (x = 0, 0.1, 0.2) were synthesized by initial mixing of precursors by high-energy ball milling and subsequent sintering of the milled powders. The co-substitution of Ba–Mn controls the formation of impurity phases, as evident from x-ray diffraction analysis. Evidence of Fe in mixed oxidation states of +3 and +2 and Mn in +3 state is found from x-ray photoelectron spectroscopy. Electron microscopy exhibits a decrease in grain size due to inhibition in grain growth by Ba–Mn co-substitution. The magnetization value at 20 kOe increases as the percentage of substitution increases. The x = 0.2 sample exhibits a comparable and stable resistivity curve in the experimental temperature range and has a higher value of remanent polarization (Pr) when compared with the x = 0 sample.

Polarization fatigue of Pr and Mn co-substituted BiFeO3 thin films

Polarization fatigue of (Bi0.85Pr0.15)(Fe0.95Mn0.05)O3 (BPFMO) films is studied as functions of switching fields and frequencies. Poor fatigue resistance is observed at low switching frequencies and small cycling fields. An increase of dielectric constant is observed in accompany with the suppression of switchable polarization. Both the switchable polarization and the dielectric constant can be restored to their original values simultaneously by high electric field cycling. These characteristics are discussed in terms of migration and aggregation of oxygen vacancies on domain walls to block domain switching. The results support the domain wall pinning scenario for polarization degradation in BPFMO films.

Temperature-dependent leakage current characteristics of Pr and Mn cosubstituted BiFeO3 thin films

Leakage current characteristics of (Bi0.86Pr0.14)(Fe0.95Mn0.05)O3 (BPFMO) thin films are studied at various temperatures from 293 down to 93 K. Space charge limited current and Poole–Frenkel (PF) emission are found to be the dominant mechanism in the low and the high electric fields, respectively. The trap ionization energy at zero-field in BPFMO films is deduced to be around 0.29 eV, which indicates the existence of shallow traps. A negative differential resistivity behavior is observed before the onset of PF emission at 93 K, which is discussed in terms of the competition between electron trapping and field-assisted thermal emission.

Theoretical and experimental evidence of enhanced ferromagnetism in Ba and Mn cosubstituted BiFeO3

Ba and Mn doped BiFeO3 prepared through the pyrolysis of xerogel precursors are characterized by powder x-ray diffraction, high resolution transmission electron microscopy, superconducting quantum interference device magnetometry, and polarization measurements. Structural studies by x-ray diffraction and transmission electron microscopy show a tetragonal lattice for Ba substituted BiFeO3 and a rhombohedral lattice for Mn substituted BiFeO3. A large ferromagnetic hysteresis loop is observed for Ba doped BiFeO3. Coexistence of distorted rhombohedral and tetragonal phases is observed in Ba and Mn codoped BiFeO3, where enhanced ferroelectric and ferromagnetic properties are produced by the internal strain. Density functional calculations are used to substantiate the results.

Influences of Pr and Mn co-substitution on crystallinity and electric properties of BiFeO3 thin films

Pr and Mn co-substituted BiFeO3 (BFO) thin films were fabricated on a Pt-coated (100) SrTiO3 substrates by pulsed laser deposition (PLD) method. X-ray diffraction patterns indicated that the formation of impurity phases was suppressed by Pr substitution in the BFO thin films. Furthermore, by combining with small amount of Mn substitution, Pr substitution was effective for reducing the leakage current density. The polarization vs electric field curves showed well-saturated hysteresis loops with measurement frequency of 20 kHz at room temperature. The remnant polarization at a maximum electric field of 1500 kV/cm was approximately 50–70 µC/cm2.