FeO3 Ceramics Research Articles

Structural Transition and Enhanced Ferromagnetic Properties of La, Nd, Gd, and Dy-Doped BiFeO3 Ceramics

The structural, electrical, and magnetic properties of rare-earth-doped Bi0.8 RE0.2FeO3 ceramics (rare-earth (RE) = La, Nd, Gd, and Dy) synthesized by solid-state reaction are reported and discussed. The x-ray diffraction (XRD) patterns of Bi0.8La0.2FeO3 and Bi0.8Nd0.2FeO3 were indexed to rhombohedral (R3c) and triclinic (P1) structures, respectively. Rietveld refinement of the XRD pattern of Bi0.8Dy0.2FeO3 confirmed its biphasic nature (Pnma + R3c space groups) whereas for Bi0.8Gd0.2FeO3 the orthorhombic phase with Pna21 symmetry made a major contribution, with minor contributions from the orthorhombic (Pnma) and rhombohedral (R3c) phases. Raman spectroscopy revealed changes in BiFeO3 mode positions, in addition to structural changes, on RE ion substitution. The effect of RE ion substitution on dielectric constant and loss tangent were studied at room temperature in a wide range of frequency, 50 Hz–1 MHz. Room temperature magnetization–magnetic field (M–H) measurements indicated that magnetization increased with increasing structural distortion and with partial destruction of the spin cycloid as a result of doping of BiFeO3 ceramics with rare earth ions. These compounds, with improved remnant magnetization and coercive field, are suitable for use in spin-based electronic devices.

Properties of Bi0.8Ln0.2FeO3 (Ln=La, Gd, Ho) multiferroic ceramics

Bi0.8Ln0.2FeO3 (Ln=La, Gd, Ho) multiferroic ceramics were prepared by a solid state reaction method and their properties were investigated in detail. In the Rietveld refinements of the XRD data for these ceramics, good agreement between the measured and calculated patterns was observed. The leakage current density and resistivity of ceramics were analyzed, and it was found that Bi0.8Ho0.2FeO3 (BHFO) ceramics possessed much better electric insulation than Bi0.8La0.2FeO3 (BLFO) and Bi0.8Gd0.2FeO3 (BGFO) ceramics. Considerable differences in magnetic properties among BLFO, BGFO, and BHFO ceramics were observed and might be correlated with the suppression of the spatially modulated spiral spin structure. A notable exchange bias effect was observed in these ceramics at room temperature, which may be correlated with exchange interaction at the interfaces of weakly ferromagnetic and antiferromagnetic components. Moreover, the magnetoelectric (ME) coupling effect was confirmed in Bi0.8Ln0.2FeO3 (Ln=La, Gd, Ho) ceramics by the magnetic field induced polarization and dielectric investigations.

Effect of sintering temperature on structure and multiferroic properties of Bi0.825Sm0.175FeO3 ceramics

The Bi0.825Sm0.175FeO3 (BSFO) ceramics were prepared by solid state reaction route and the structural, vibrational, and multiferroic properties of these ceramics was studied. The properties of BiFeO3 (BFO) ceramics at higher sintering temperature i. e. 850 °C were compared to that of ceramic at normal sintering temperature (i. e. 830 °C). The Rietveld-refined structure revealed the rhombohedral R3c symmetry in BFO and a mixed phase symmetry (R3c 38.26% + Pnma 61.74%) in BSFO for ceramics sintered at 830 °C. The formation separate phase of SmFeO3 (Pbnm; 12.16%) along with BSFO (Pn21a; 87.84%) phase was observed for ceramics sintered at 850 °C. The small change in sintering temperature led to changes in Raman spectra accompanying abrupt changes in ferroelectric properties. The BSFO shows strong ferromagnetic characteristic due to the spin canting in bulk BFO. The net macroscopic magnetization in bulk BFO due to Sm doping exist due to suppression of spin modulated spiral structure. The BSFO ceramics sintered at 830 °C shows ferroelectric polarization (2Pr) 7.3723 μC/cm2 at an applied field of 15 kV/cm but the BSFO ceramics sintered at 850 °C shows no ferroelectric polarization at room temperature. The presence of magnetization and reasonable ferroelectric polarization at room temperature in BSFO ceramics sintered at 830 °C make this material suitable for multiferroic application.

Small polaron conduction in lead modified lanthanum ferrite ceramics

In the present work we have illustrated the physics of the electrical characteristics of nanocrystalline La1−xPbxFeO3, (0⩽x⩽0.2) powder prepared using auto-combustion synthesis. The effect of lead doping on the dielectric, impedance and ac conductivity characteristics of lanthanum ferrite has systematically been investigated. The synthesized powders were phase pure and crystallized into centro-symmetric Pnma space group. As compared to pure LaFeO3 ceramics (dielectric constant∼14,000), the dielectric constant is grossly increased (∼30,000) in Pb doped LaFeO3. The temperature dependence of dielectric constant of 10.0 at.% Pb doped LaFeO3 exhibits dielectric maxima similar to that observed in ferroelectric ceramics with non-centrosymmetric point group. For La0.8Pb0.2FeO3 ceramics, the frequency dependence of the dielectric constant and loss tangent at various temperatures (300–450K) exhibit typical colossal dielectric constant (CDC) like behavior. From the impedance spectroscopy we have estimated the grain and grain boundary resistance and capacitance of Pb doped LaFeO3 that follow a small polaron hopping conduction model. Long range movement of the charge carriers govern the CDC behavior.

Structural and Ferroic Properties of La, Nd, and Dy Doped BiFeO3 Ceramics

Polycrystalline samples of Bi0.8RE0.2FeO3 (RE = La, Nd, and Dy) have been synthesized by solid-state reaction route. X-ray diffraction (XRD) patterns of Bi0.8La0.2FeO3 and Bi0.8Nd0.2FeO3 were indexed in rhombohedral (R3c) and triclinic (P1) structure, respectively. Rietveld refined XRD pattern of Bi0.8Dy0.2FeO3 confirms the biphasic (Pnma + R3c space groups) nature. Raman spectroscopy reveals the change in BiFeO3 mode positions and supplements structural change with RE ion substitution. Ferroelectric and ferromagnetic loops have been observed in the Bi0.8RE0.2FeO3 ceramics at room temperature, indicating that ferroelectric and ferromagnetic ordering coexist in the ceramics at room temperature. The magnetic measurements at room temperature indicate that rare-earth substitution induces ferromagnetism and discerns large and nonzero remnant magnetization as compared to pristine BiFeO3.

Effect of temperature on structural expansion for Bi0.8−xPrxBa0.2FeO3 (x ≤ 0.1) ceramics

At temperature range of 25–800°C, in situ X-ray diffraction studies on Bi0.8−xPrxBa0.2FeO3 (x≤0.1) ceramics are performed. The unit cell symmetry remains unchanged, while temperature dependence of lattice parameters and unit-cell volumes vary. There are no phase decomposition and structural phase transition detected in the system, which indicate that Pr, Ba co-doping may stabilize the structure of BiFeO3 ceramic. The unit cell thermal expansion and thermal expansion coefficients are positive. However, for sample with x=0.1, i.e., Bi0.7Pr0.1Ba0.2FeO3, the unit cell structural stability decreases, and the thermal expansion coefficient increases. The thermal expansion coefficients abnormal around the Néel temperature are observed, which may suggest a possible antiferromagnetic–paramagnetic transition. It is probably the explanation for their low thermal expansion coefficient during the heating process.

The microstructure and ferroelectric property of Nd-doped multiferroic ceramics Bi0.85Nd0.15FeO3

The microstructures of Bi0.85Nd0.15FeO3 ceramics were investigated by using transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM), selected area electron diffraction (SAED), convergent-beam electron diffraction (CBED) and X-ray energy dispersive spectrometry (EDS). The superstructure phase related to 1/4(hh0)p and 1/4(00l)p diffraction spots were observed in the samples. It is found that the superstructure phase can co-exist with R3c phase in a single grain. The bright and dark field images accompanied with SAED patterns provided evidences of the superstructure phase dispersed into the matrix with R3c symmetry and evolution of the domain. The ferroelectric domain wall was observed according to the displacement of Fe3+ ions respect to Bi3+ sub-lattice in the ferroelectric R3c phase based on HRTEM observations. The space group Pnam of the superstructure was identified by combining SAED and CBED techniques. EDS measurement revealed that the concentration of Nd in the Pnam phase is higher than that in R3c phase. This might mean that the transition from R3c to Pnam structure is due to the inhomogeneous distribution of the Nd concentration and the weakened stereochemical activity of Bi3+ lone electron pair, arisen from the increase in Nd content.

Phase coexistence in Bi1−x Pr x FeO3 ceramics

Bi1−x Pr x FeO3 ceramics across the rhombohedral–orthorhombic phase boundary have been studied by X-ray diffraction, transmission electron microscopy, and differential scanning calorimetry. The structural phase transitions in Bi1−x Pr x FeO3 driven by doping concentration and temperature are significantly different from those in BiFeO3 compounds doped with other rare-earth elements. The features of the structural transformations have been discussed based on the specific character of the chemical bonds associated with praseodymium ions. The detailed study of the crystal structure evolution clarified the ranges of both single-phase and phase coexistence regions at different temperatures and dopant concentrations. For x = 0.125, compound extraordinary three-phase coexistence state has been observed in a narrow temperature range at about 400 °C. The results explicate driving forces of the structural transitions and elucidate the origin of the remarkable physical properties of BiFeO3-based compounds near the morphotropic phase boundary.

Dielectric, multiferroic properties and resistance modulation of Ta-doped Bi0.97Ba0.03FeO3 ceramics

Single phase Bi 0.97 Ba 0.03 Fe 1-x Ta x O 3 ceramics with x = 0, 0.01, 0.03, 0.05 were synthesized by modified rapid sintering process method. The formation of rhombohedral perovskite-like structure was confirmed by X-ray diffraction investigation for all the samples. Dielectric and leakage current measurements indicated that the content of the oxygen vacancy in the samples decreased as a function of the substitution of Ta 5+ ions. A distinct threshold switching behavior was observed in the leakage current density. The impedance measurements suggested that the grain effect made a major contribution to the resistance. The changes in dielectric, multiferroic properties and resistance modulation of the Ta 5+ and Ba 2+ co-doped BiFeO 3 ceramics could have a huge potential for material application.

Structural, Electrical and Magnetic Properties of Bi0.75Dy0.25FeO3 Ceramics

Structural, Electrical and Magnetic Properties of Bi0.75Dy0.25FeO3 Ceramics

Mg-doped Bi0.8Ca0.2FeO3 with enhanced ferromagnetic properties

We report the structural and enhanced ferromagnetic properties of Mg doped Bi0.8Ca0.2FeO3 ceramics. Oxygen vacancies are introduced into Bi0.8Ca0.2Fe1−xMgxO3 ceramics doping confirmed by XRD and Raman spectra, which induce a significantly enhanced macroscopic ferromagnetism from antiferromagnetism to ferromagnetism. The enhanced ferromagnetism can be ascribed to the creation of unbalanced Fe3+ spins induced by the collapse of the space modulated cycloidal spin structure and relative long-range coupling mediated by the oxygen vacancies trapped localized electrons. The improved ferromagnetic properties obtained by co-doping demonstrate the possibility of bulk BFO in practical multiferroic applications.

Improvement of electrical conductivity and leakage current in co-precipitation derived Nd-doping BiFeO3 ceramics

BiFeO3 and Bi0.925Nd0.075FeO3 ceramics were prepared by co-precipitation method. The crystal structure and electrical properties of the samples were characterized by X-ray diffraction (XRD), impedance spectra and leakage current measurement. XRD result implied that the impurity phases are weakened by suitably doping Nd. The impedance spectra of BiFeO3 sample indicate that low grain boundary resistance and non Debye-type relaxation below the Neel temperature. Complex impedance spectra suggested that the doped samples are closer to Debye-type, and the grain boundary resistance increases which lead to low leakage current density.

Effect of Mn doping on multiferroic properties in Bi0.8Ba0.2FeO3 ceramics

Multiferroic ceramics Bi0.8Ba0.2Fe1−xMnxO3 with x=0.0, 0.05, 0.1, and 0.15 were prepared by using conventional solid state reaction method. The structural, magnetic, ferroelectric and dielectric properties were investigated. The phase transition from rhombohedral to tetragonal structure with increasing Mn substitution concentration was confirmed by using X-ray diffraction. Magnetic measurements indicated that the remanent magnetization of Bi0.8Ba0.2Fe0.95Mn0.05O3 (x=0.05) is around 2.64 times higher than that of Bi0.8Ba0.2FeO3 (x=0.0), which is probably due to the coexistence of rhombohedral and tetragonal phase that destructs the cycloidal spin structure and enhances the ferromagnetic properties significantly. The ferroelectric measurements revealed that the leakage current is significantly suppressed with an increase in the Mn content. In addition, Mn doping was found to be helpful to reduce dispersion in dielectric constant and loss, especially at lower frequencies. Furthermore, an anomaly in the dielectric constant was observed in the vicinity of the magnetic transition temperature, which could be attributed to the intrinsic magnetoelectric coupling effect. It is also worth noting that the intensity of the dielectric anomaly peak for the sample of x=0.05 was highest (~183.4) among all the samples, which indicated that the magnetoelectric coupling effect could be greatly improved by enhanced ferromagnetic properties.

Effects of Co doping on electronic structure and electric/magnetic properties of La0.1Bi0.9FeO3 ceramics

In this work, we report the influence of Co-doping on the electronic band structure, dielectric and magnetic properties of La0.1Bi0.9Fe1−xCoxO3 ceramics. X-ray photoelectron spectroscopy investigation shows that Co dopant can shift the valence band spectrum and core-level lines of constituent elements towards higher bind energy regions simultaneously increase the concentration of oxygen vacancies in ceramics. The effects of dopant are discussed with focus given to the Co-doping induced enhancement of electrical conductivity and resistive switching phenomena.

Room temperature structure and multiferroic properties in Bi0.7La0.3FeO3 ceramics

Single phase Bi0.7La0.3FeO3 ceramic samples were successfully synthesized by sol–gel combustion and co-precipitation methods, performing a final sintering at 820–870°C from 10 up to 180min. Rietveld refinements of the XRD data detected small satellite peaks that were successfully indexed by an incommensurated modulated structure model. Lanthanum doping improves magnetic response, reduces the leakage current and dielectric losses. The piezoelectric coefficient was reported for the first time in the Bi0.7La0.3FeO3 composition.

Effects of Structural Collapse and Magnetic Moment on Magnetization in ${\rm Bi}_{0.8-{\rm x}}{\rm Pr}_{\rm x}{\rm Ba}_{0.2}{\rm FeO}_{3}$$({\rm x}\leq 0.1)$ Multiferroics

The structural, magnetic, and ferroelectric properties of polycrystalline Bi <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.8-x</sub> Pr <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</sub> Ba <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.2</sub> FeO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> (0.0 ≤ x ≤ 0.1) ceramics are studied systematically. The symmetry of the unit cell with Ba and Pr substitution in BiFeO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> in the substitution range remains the cubic with the space group of <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">Pm</i> 3 <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">m</i> . The additive of Pr in (BiBa)FeO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> reduces the unit cell volume together with varying the lattice parameters without changing the unit cell symmetry. The distortion of FeO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">6</sub> octahedron resulting from the additive of Pr may induce the collapse of the spatial spin structure, which may enhance the spontaneous magnetization significantly. The remnant magnetization ( <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">Mr</i> ) increases monotonically with increasing the content of Pr, which probably originates from the ferromagnetic arrangement of Pr-ions and Fe-ions in the unit cell. The conductivity increases in a wide temperature regime result in an obvious leakage observed with increasing the Pr concentration.

Enhanced dielectric and magnetic properties in Ru-substituted Bi0.9La0.1FeO3 ceramics

Multiferroic ceramics Bi0.9La0.1Fe1−xRuxO3 (x = 0%, 1% and 3%) were prepared using a solid-state reaction process. The structural transformation from rhombohedral to pseudocubic was confirmed in the Ru-doped samples via x-ray diffraction investigation, and the improved dielectric properties are ascribed to the morphotropic phase boundary and decreased oxygen defects existing in the Ru-substituted samples. The enhancement of magnetization is interpreted by the increased Fe3+ ions and the formation of a local ‘ferrimagnetic’ structure in the Ru-doped samples.

Structure, ferroelectric and piezoelectric properties of multiferroic Bi0.875Sm0.125FeO3 ceramics

With a rhombohedra-like structure, the Bi0.875Sm0.125FeO3 ceramics show much-improved ferroelectric and piezoelectric properties: a saturated ferroelectric polarization of 40μC/cm2 and a piezoelectric d33 of 45pC/N at 20°C. After the polarized sample was annealed at 600°C, its d33 decreases to ∼20pC/N. Besides, Bi0.875Sm0.125FeO3 shows dielectric or impedance resonances at 20–550°C, suggesting that the ferroelectric component with high Curie temperature still partially remained at 550°C. However, the impedance and the resistance decrease so fast with temperature increasing that both of them are below 1000ohm above 420°C. Even so, this piezoelectric resonance method can explore leaky ferroelectrics at high temperature.

Improved leakage current and ferromagnetic properties in magnetic field annealed BiFeO3-based ceramics

Single-phase Bi0.85La0.15FeO3 ceramics were synthesized under various magnetic fields (Ha=0T, 3T, 5T). Substantially reduced leakage current and hence modified ferroelectric (FE) properties were obtained with magnetic field annealing (MA). The largest magnetization and lowest leakage current with large FE polarization (Pr∼33μC/cm2) were found in the sample annealed with Ha=3T. Great changes were also observed in the Raman spectra. All the observed features originate mainly from the different FE domain wall structures induced by MA. These results demonstrate that MA is an effective way to tune the multiferroic and magnetoelectric properties in BiFeO3-based materials.

Structure and piezoelectric properties of BiFeO3 and Bi0.92Dy0.08FeO3 multiferroics at high temperature

Multiferroic BiFeO3 and Bi0.92Dy0.08FeO3 ceramics were prepared to study their crystal structures and piezoelectric properties. BiFeO3 exhibits rhombohedral phase below 810°C. Although Bi0.92Dy0.08FeO3 ceramic also shows rhombohedral phase at room temperature, it allows the coexistence of rhombohedral phase and orthorhombic phase at 460–650°C. Both samples have maximum polarizations of >21μC/cm2 and piezoelectric d33 values of ∼37pC/N at room temperature. Their polarized slices show the dielectric anomalies and impedance anomalies because of vibrating resonances below 500°C, and the thickness vibration electromechanical coupling factor is ∼0.6 and ∼0.4 for BiFeO3 and Bi0.92Dy0.08FeO3, respectively. The vibrating resonances confirm piezoelectric responses. Furthermore, samples' impedance and resistance decrease fast with temperature increasing, which screens piezoelectric response above 550°C.