BiFeO3 Samples Research Articles

Enhanced multiferroic properties and magneto-dielectric effect analysis of La/Co modified BiFeO3

Multiferroic BiFeO3 (BFO), La/Co modified BiFeO3 (BLFO/BLCFO) ceramics prepared by solid state reaction route were characterised and their magneto-dielectric properties were investigated. The phase purity of La/Co modified BFO ceramics was confirmed from powder X-ray diffraction studies and distortion observed in the lattice due to compression was found to be associated with polarization switching on La/Co co-substitution into BFO. An enhancement and stability in saturation magnetization with low coercive field (Hc ≈ 400 Oe) has been achieved at room temperature in La/Co modified BFO samples. Dielectric and impedance measurements reveal the enhanced values of dielectric constant, real and imaginary part of impedance and modulus on account of modification by La/Co in BFO. The modulus studies confirm the presence of ionic polarization due to the co-substitution of La2+/Co3+ ions and dominant grain effects in BLFO and BLCFO samples. The magneto-dielectric effect (% MDE) is found to be maximum value of 1.8 in magnitude at 8 kOe for BLFO-10%Co, and increases with increase in Co content in to BLFO. The substitution of Co into BLFO generates polarization at La and Fe layer, which provides the enhanced value of %MDE and stability in purity of BFO ceramic.

Raman spectroscopy probed spin-two phonon coupling and improved magnetic and optical properties in Dy and Zr substituted BiFeO3 nanoparticles

In this study BiFeO3 (BFO), Bi0.9Dy0.1FeO3 (BDFO) and Bi0.9Dy0.1Fe0.97Zr0.03O3 (BDFZO) nanoparticles were prepared by sol-gel route. X-ray diffraction study showed rhombohedral structure for BFO while orthorhombic structure for BDFO and BDFZO samples. X-ray photoelectron spectroscopy indicated the dominancy of Fe3+ oxidation state in BDFO sample which decreases in BDFZO sample. All samples showed ferromagnetic behaviour with saturation magnetization of 0.5, 5.5 and 5.2 emu/g for BFO, BDFO and BDFZO samples, respectively. A slight decrease in magnetization for BDFZO sample as compared to BDFO sample may be attributed to the substitution of non-magnetic Zr4+ ion which does not take part directly in magnetization. Two-phonon Raman mode located at ∼1260 cm−1 showed anomalous intensity in pure BFO which subsequently suppressed for BDFO and BDFZO samples indicating spin-two phonon coupling in these samples. UV–Visible diffuse spectroscopy results showed that the band-gap of BFO, BDFO and BDFZO samples were 2.26, 2.22 and 2.24 eV, respectively, suitable for photocatalytic applications.

Highly resistive fast-sintered BiFeO3 ceramics

ABSTRACTThis contribution presents an alternative synthesis route to produce highly resistive BiFeO3 multiferroic samples. The use of a fast sintering process followed by quenching to room temperature resulted in ceramic bulks with a resistance higher than 1012 Ohm.m at room temperature. In addition, anomalies found in the dielectric and resistivity characterization indicated the presence of magnetoelectric coupling in the synthesized BiFeO3 sample.

Investigations on the structure, defects, electrical and magnetic properties of Ni-substituted BiFeO3 ceramics

Polycrystalline BiFe1−xNixO3 (x = 0.00–0.30) ceramics were synthesized by solid state reaction method followed by rapid liquid phase sintering. The effect of Ni substitution on the structure, defects, electrical and magnetic properties of BiFeO3 ceramics was investigated. X-ray diffraction measurements reveal that Ni-substituted samples exhibit distorted rhombohedral perovskite structure as that of unsubstituted BiFeO3, and the impurity phases such as Bi2Fe4O9 decrease due to Ni substitution. The changes in the phonon frequencies in Raman spectra reveal the lattice distortion in Ni-substituted samples which are in agreement with the XRD analysis. Positron annihilation lifetime measurements reveal that cation vacancy-type defects exist in all samples, the vacancy concentration increases with increasing Ni content from 0.00 to 0.30. The enhancement in leakage current and dielectric properties is observed in Ni-substituted samples due to the suppression of impurity phases and Fe2+. Magnetic measurements indicate that the Ni-substituted BiFeO3 samples exhibit weak ferromagnetism. The magnetic properties are found to increase with increasing Ni concentration due to the internal structural distortion, cation vacancy, the ferromagnetic exchange between the neighboring Fe3+ and Ni2+ ions and the change in Fe–O–Fe bond angle.

Influence of Eu and Sr co-substitution on multiferroic properties of BiFeO3

Pure BiFeO3 (BFO), and Eu-Sr co-substituted BFO samples were prepared by a sol–gel method. The effects of Eu and Sr codoped on the structural, morphological, magnetic and ferroelectric properties were systematically investigated. The X-ray diffraction and Fourier transform infrared spectroscopy reveal that substitution of Eu and Sr at the Bi site results in structural change and single phase formation. The maximum remnant magnetization of 0.287emu/g and coercive field of 10.305kOe are observed in the Bi0.85Eu0.05Sr0.10FeO3 sample. The suppression of spin cycloid caused from the structural distortion can play an important role in the improvement of magnetic properties. The Eu and Sr co-doped samples also exhibit good ferroelectric properties, which may be attributed to suppressing the formation of oxygen vacancies by Eu substitution.

Influence of lightly Sm-substitution on crystal structure, magnetic and dielectric properties of BiFeO3 ceramics

Multiferroic Bi1−xSmxFeO3 (x = 0.00–0.05) ceramics were prepared by a conventional solid-state reaction method, and effects of Sm3+ ions on crystal structure, magnetic and dielectric properties were investigated. TGA-DSC studied the synthesis process of pure BiFeO3 sample and confirmed the synthesis temperature was about 825 °C. For all samples, they were rhombohedral distorted perovskite structure of R-3c space group. Lattice constants and particle sizes decreased based on the data of XRD and Rietveld refinement. Morphologies of the samples were determined with Scanning Electron Microscopy proving the grain to be shrinkage. With increasing of Sm3+ ions, saturation magnetization of Bi0.97Sm0.03FeO3 material at 5000 Oe was about 1.5 times larger than that of pure BiFeO3, while the coercivity increased from 87.3 Oe to 121.3 Oe. In a wide range frequency from 1 MHz to 1.6 GHz, dielectric constant kept a stable value. With Sm3+ increasing, dielectric constant increased from 27 to 46, while dielectric loss kept a low value.

Effect of Annealing on Room Temperature Multiferroics of BiFe<sub>1-x</sub>Co<sub>x</sub>O<sub>3</sub>

We studied the effect of annealing and Co ion doping on the structure, leakage current, ferroelectric polarization and magnetism of BiFeO3 samples. X-ray diffraction patterns demonstrate that an appropriate Co doping concentration is favor of suppressing the secondary phase but annealing treatment is apt to the growth of both the main and the secondary phases. The current density as a function of an electric field indicates that Co doping increases the leakage current density as samples before annealing but suppresses it after annealing. Annealing treatment improves the leakage for Co-doped sample and reduces it for the undoped sample. Ferroelectric hysteresis loops reflect that Co ions doping is liable to increase the ferroelectric polarization, while the sample is annealed it will do opposite effect. However the annealing treatment do improve the ferroelectricity for pure BiFeO3 sample. The magnetic hysteresis at room temperature shows the obvious enhancement of ferromagnetic properties with the sample after annealing.

A systematic study on multiferroics Bi1−xCexFe1−yMnyO3: Structural, magnetic and electrical properties

Bi1−xCexFe1−yMnyO3 samples (0≤x≤0.10 and 0≤y≤0.05) were synthesized by the sol–gel technique to investigate the effect of Ce and Mn doping on the structural, magnetic and electrical properties of multiferroic BiFeO3 (BFO). It is observed that Ce–Mn doping induces structural changes in BFO and also decreases the secondary phases noticeably. Frequency dependent dielectric properties of pure and doped BFO samples were measured from 300K to 675K in the frequency range of 1KHz–1MHz. Enhanced ferromagnetism and improved ferroelectric properties were observed which may be correlated with the structural transformation and grain morphology. As Ce concentration increased in BFO, the leakage current density decreased gradually. We have observed here exciting properties in doped bismuth ferrites which provide the possibility to create new functional material.

Improved photoelectrocatalytic properties of Ti-doped BiFeO3 films for water oxidation

Because of the potential application in photoelectrochemical cells for water splitting, the synthesis of BiFeO3 (BFO) is receiving increasing attention. A simple, low-cost drop-casting route was used to synthesize Ti ions doped BiFeO3 thin films and the effects of the Ti ions doping on their crystal structure, morphology, and photoelectrocatalytic activity had been studied. X-ray diffraction results of Ti-BFO films showed that the doping of Ti ions lead to the distortion of crystal structure. Photoelectrochemical measurements indicated that the photoelectrocatalytic activities of BFO film doped with 5 % Ti ions showed the best performance. The UV–Vis absorption spectra of Ti-BFO films showed that the absorption wavelength of BFO doped with 5 % Ti had obvious shift and the band gap decreased. From the results of Mott–Schottky (M–S) plots, we calculated the positions of valence band (VB) and conduction band (CB) for Ti-BFO film. The outcomes showed that the band gap structure of BFO samples doped with 5 % Ti had more positive CB and more negative VB. The calculated photogenic charge carrier concentrations of BFO and Ti (0.05)-BFO were 5.07 × 1019 and 6.32 × 1019 cm−3, respectively. The reduced band gap and increased photogenic charge carrier concentration are the major factors contributing to the enhancement of photoelectrocatalytic performances of Ti-BFO films.

Magnetic properties in BiFeO3 doped with Cu and Zn first-principles investigation

Based on first-principles spin-polarized density functional theory calculations, the electronic structures, and magnetic properties of Cu and Zn-doped BiFeO3 are investigated. The calculated formation energies show that Cu prefers to occupy Fe site, while the Zn prefer to occupy Bi site. All the doped BiFeO3 systems turn out to be favorable for G-type antiferromagnetic arrangement. The substitution of Cu and Zn for Fe produces a magnetic moment of 3.0 and 4.0 μB per dopant, respectively. The net magnetic moments are from the broken symmetry of the AFM spin ordering network. For the substitution of Cu and Zn for Bi, the net magnetic moment originates from Cu/Zn itself and hole introduced by Cu/Zn. Two-Cu/Zn-doped cases show various magnetic behaves. If O vacancy is far away from dopants, the O vacancies don't affect the net magnetic moment of the substitution of Cu and Zn for Fe, but have notable effect for Bi site doping. The O vacancies result in metallicity in all doped cases. Our study demonstrates that the nonmagnetic Cu and Zn doping will lead to the diversity and complexity of magnetic properties depending on doping sites, distance between dopants, intrinsic defect, and so on, which could be responsible for the observed various magnetic behaviors in Cu/Zn-doped BiFeO3 samples.

Effect of co-doping on the optical, magnetic and photocatalytic properties of the Gd modified BiFeO3

Gd doped, and (Gd, Sm), (Gd, La) co-doped BiFeO3 particles are synthesized by solvothermal method. XRD pattern analysis confirms rhombohedral structure of all the samples. Absorbance spectra reveal bands corresponding to charge transfer of Bi–O, and Fe–O and Fe d–d transitions. The band gap of the co-doped BiFeO3 samples is found to be less than Gd-doped sample. Photoluminescence spectra of all the samples show transition peaks in the visible region upon excitation at 325 and 365 nm. The magnetization increases for the co-doped samples and Sm co-doped sample exhibits an enhanced coercivity. Photocatalytic tests are carried out at room temperature for the degradation of methylene blue dye in the presence of catalyst and H2O2 at pH value 2.24. Sm co-doped sample shows better photocatalytic activity compared to La co-doped sample. The main species for the degradation of dye is found by introducing the scavengers in the reaction. The possible reaction mechanism for the degradation of methylene blue is explained.

A comparative study on structural, vibrational, dielectric and magnetic properties of microcrystalline BiFeO3, nanocrystalline BiFeO3 and core–shell structured BiFeO3@SiO2 nanoparticles

The manifestation of dimensionalities and SiO2 shell induced modifications in structural, vibrational, dielectric and magnetic properties of microcrystalline, nanocrystalline BiFeO3 and core–shell structured BiFeO3@SiO2 nanoparticles are reported. Transmission electron microscopy image confirmed the core–shell structure of BiFeO3@SiO2 nanoparticles with BiFeO3 core ∼50–90 nm and SiO2 shell ∼16 nm. X-ray diffraction and Raman spectroscopy results show the presence of distorted rhombohedral structure with R3c space group in all samples. Bond valance sum (BVS) calculations shows the SiO2 shell includes very small compressive stress of Bi–O and increases tensile stress of the Fe–O bonds in comparison with BiFeO3 nanoparticles. SiO2 shell on BiFeO3 nanoparticles reduces the dielectric constant and loss in comparison with microcrystalline and nanocrystalline BiFeO3 samples. The magnetic measurement indicated the existence of room-temperature weak ferromagnetism in BiFeO3 and core–shell BiFeO3@SiO2 nanoparticles, whereas bulk BiFeO3 showed antiferromagnetic nature (almost paramagnetic in nature), which is also endorsed by second order Raman modes. The influence of the grain boundary specific area (SGB) on ferromagnetism in microcrystalline and nanocrystalline BiFeO3 samples has been deciphered.

Sintering, microstructure and electrical properties of 0.4(Bi0.5K0.5)TiO3-0.6BiFeO3 lead-free piezoelectric ceramics

The effect of sintering temperature on the densification, microstructure and structure of 0.4(Bi0.5K0.5)TiO3-0.6BiFeO3 lead-free piezoelectric ceramics is assessed. The 0.4(Bi0.5K0.5)TiO3-0.6BiFeO3 powders were prepared by using the mixed-oxide method and were sintered at temperatures of 1000, 1050 and 1100 °C for 1 to 5 hrs. Unlike earlier work, the sintered samples showed high densities even when sintered at 1000 °C. X-ray diffraction revealed that the sintered samples shared the same rhombohedral structure as BiFeO3. With increasing sintering temperature, the rhombohedral distortion of the unit cell decreased. In addition to the relaxor-like broad peak around 400 °C, a low-temperature dielectric peak was found at temperatures below 190 °C by employing a low-frequency sweep down to 10 mHz. The DC conductivity of the 0.4(Bi0.5K0.5)TiO3-0.6BiFeO3 sample exhibited three temperature regions with activation energy values of 0.56 eV (T > 500°C), 0.72 eV (400°C < T <200°C) and 0.81 eV (T < 190°C). The characteristic transitions in the conductivity could be related to the Neel temperature (370°C) and the conductivity anomaly observed at ca. 190°C in BiFeO3.

Specific features of the evolution of magnetic properties of bismuth ferrite modified with rare-earth element cations

The dependences of magnetic properties of multiferroics obtained upon isovalent substitution of bismuth cations in the BiFeO3 sample by rare-earth metal cations have been investigated experimentally. The crystal-chemical characteristics and indices of global instability of the samples have been calculated in the approximation of the valence bond method using the SPuDS program. Correlations of the structure and magnetic properties depending on the chemical composition of multiferroics have been established based on the analysis of experimental and theoretical data.

Substitution-driven Structural and Magnetic Transformation of Mn- and Co-doped BiFeO3

Mn- and Co-doped and pure BiFeO3 (BFO) nanoparticles were synthesized through sol-gel method. The result of the Rietveld refinement based on X-ray diffraction (XRD) data revealed that BFO and 10 % Co-doped BFO sample crystallize in rhombohedral symmetry (R3c) structure. The transition of crystal structure can be found when Fe atoms were replaced by Mn atoms partially (from R3c to P4/mbm). Besides, Raman spectra further confirm the structure transformation which is consistent with the results of Rietveld refinements. Analysis of vibrating sample magnetometer (SQUID–VSM) magnetic hysteresis data shows the enhancement of magnetism in Mn- and Co-doped samples which were explained by the changing of bond length and angle which were obtained from the Rietveld refinement.

Structural modification and enhanced magnetic properties with two phonon modes in Ca–Co codoped BiFeO3 nanoparticles

Bi1−xCaxFe1−xCoxO3 nanoparticles with x=0.0, 0.05, 0.10 and 0.15 were successfully synthesized by cost effective tartaric acid based sol–gel route. The alkali earth metal Ca2+ ions and transition metal Co3+ ions codoping at A and B-sites of BiFeO3 results in structural distortion and phase transformation. Rietveld refinement of XRD patterns suggested the coexistence of rhombohedral and orthorhombic phases in codoped BiFeO3 samples. Both XRD and Raman scattering studies showed the compressive lattice distortion in the samples induced by codoping of Ca2+ and Co3+ ions. Two-phonon Raman spectra exhibited the improvement of magnetization in these samples. X-ray photoelectron spectroscopy (XPS) showed the dominancy of Fe3+ and Co3+ oxidation states along with the shifting of the binding energy of Bi 4f orbital which confirms the substitution Ca2+ at Bi-site. The magnetic study showed the enhancement in room temperature ferromagnetic behavior with co-substitution consistent with Rama analysis. The gradual change in line shape of electron spin resonance spectra indicated the local distortion induced by codoping.

Synthesis of nano-structured Bi1−xBaxFeO3 ceramics with enhanced magnetic and electrical properties

Bi1−xBaxFeO3(x = 0, 0.2) compounds were synthesized by conventional solid-state reaction method. Structural, morphological, magnetic and ferroelectric properties of the products were investigated systematically by employing X-ray diffraction, field emission scanning electron microscope, vibrating sample magnetometer as well as electrical evaluation techniques, respectively. The XRD results demonstrated distorted rhombohedral BiFeO3 crystal structure with the space group of R3c. However, 20wt% Ba doped sample underwent a structural phase transition from rhombohedral to distorted pseudo-cubic structure. FESEM images of the BiFeO3 sample calcined at 850 °C showed agglomerated nano-particles with a mean particle size of 60 nm, while Bi0.8Ba0.2FeO3 sample showed uniform cubic particles with a mean particle size of 220 nm. For Bi0.8Ba0.2FeO3 sample calcined at 850 °C, an anomaly in permittivity was observed in the vicinity of 370 °C which is around the Neel temperature of bismuth ferrite and is in agreement with the recent reports.

Effect of A and B-site substitution with Pb, La and Ti on phase stabilization and multiferroic properties of BiFeO3

The work presents a comparative study of the effects of divalent (Pb), trivalent (La) and tetravalent (Ti) substituents on the multiferroic properties of BiFeO3 (BFO). Both A and B-sites were substituted to obtain the compositions i.e. (Bi1−x−yLaxPby)(Fe1−zTiz)O3 (x, y=0, 0.1, 0.2 and z=0, 0.05, 0.1, 0.15). Each of the substituent was particularly chosen i.e. Pb was chosen to keep the lone pair character which is the similar case as Bi ion. Additionally isovalent La was chosen to achieve single phase by reducing Bi volatization. Both these ions, on substitution, stabilized the crystal structure and suppressed the formation of extra phases which are unavoidable in pure BFO. All the Ti substituted and Bi0.8La0.2FeO3 compositions exhibited rhombohedral perovskite (R3c) phase, while Bi0.8Pb0.2FeO3 and Bi0.8La0.1Pb0.1FeO3 exhibited cubic phase. Mössbauer measurements revealed that impurity phase in case of compositions with divalent and trivalent substituents, disappeared completely when Ti substituted Fe. For all the compositions Fe ions were found in +3 state. High temperature dielectric properties showed that all the compositions were ferroelectric with paraelectric transition lying well above the room temperature. Weak ferromagnetism was found in Ti substituted compositions where coercivity was found to increase as the Ti concentration increases. All the BFO samples substituted with Pb, exhibited a dielectric anomaly in the temperature range, 100°C⩽T⩽250°C. A systematic reduction in the intensity of the dielectric anomaly peak was observed as a function of Ti concentration which indicates that the anomaly is related to the conductivity and is element specific. However, Mössbauer data revealed absence of Fe2+ state, which ensured that it was not related with the presence of Fe2+ ions. Saturation polarization was found to increase as Ti concentration increased from 0% to 10%.

Structural, Magnetic and Dielectric Properties of Bi0.9Re0.1FeO3(Re = La, Sm, Gd and Y)

With a view to understand the influence of rare earth ion doping on the structural, magnetic and dielec- tric properties of bismuth ferrite, a series of samples were prepared by the solidstate reaction method. After character- izing the samples with XRD and SEM studies, the magnetic and dielectric measurements were carried out. It is interest- ing to note that the two impurity phases, viz., orthorhombic Bi2Fe4O9 and cubic Bi25FeO40 observed in undoped bis- muth ferrite are found to vanish with rare earth doping. Further, the remnant magnetization (Mr) values are found to increase with increasing ionic radii of dopant rare earth ion. Interestingly, among all the samples, La doped BiFeO3 sample is exhibiting negative magnetization at low tem- peratures. The dielectric constant is found to exhibit two transitions above room temperature. Efforts have been made to explain the observed structural, magnetic and dielectric behaviours of the materials.

Positron annihilation spectroscopic studies of multiferroic Bi1-xPrxFeO3 nanocrystalline compounds

Positron lifetime (PL) and coincidence Doppler broadening spectroscopic (CDBS) experiments were carried out on BiFeO3 samples doped with Pr(in place of Bi) in concentrations (x) = 0, 0.05, ..., 0.35. In the initial stages of doping, the existing Bi3+ vacancies are occupied by Pr3+ ions and positron trapping is reduced. From x = 0.15 to 0.35, more number of positrons diffused to the surfaces of the nanocrystallites. The CDBS ratio curves showed enhancement of the peak at pL = 10.2×10-3 m0c due to positron annihilation with the 2p electrons of oxygen. It suggested increased trapping of positrons in newly created cationic vacancy-type defects due to the mismatch of ionic radii of Pr3+ substituting Bi3+ ions. The lattice parameters decreased (3.9661-3.9248Å) while the nanocrystallitesizes reduced and increased (28.6-34.2nm) during the doping.