Structural,vibrational, optical,magnetic and dielectric properties of Bi1−xBaxFeO3 nanoparticles

Bi_1−xBa_xFeO₃ (x=0.05, 0.10 and 0.15) nanoparticles were synthesized by the sol–gel method. X-ray diffraction and Raman spectroscopy results showed the presence of distorted rhombohedral structure of Bi_1−xBa_xFeO₃ nanoparticles. Rietveld refinement and Williamson–Hall plot of the x-ray diffraction patterns showed the increase in lattice parameters, unit cell volume and the particle size. Infrared spectroscopy and Raman analysis revealed the shifting of phonon modes towards the higher wavenumber side with increasing Ba concentration. These samples exhibited the optical band gap in the visible region (2.47–2.02 eV) indicating their ability to absorb visible light. Magnetic measurement showed room temperature ferromagnetic behavior, which may be attributed to the antiferromagnetic core and the ferromagnetic surface of the nanoparticles, together with the structural distortion caused by Ba substitution. The magnetoelectric coupling was evidenced by the observation of the dielectric anomaly in the dielectric constant and the dielectric loss near antiferromagnetic Neel temperature in all the samples.     

Improved dielectric and ferroelectric properties in Ti-doped BiFeO3-PbTiO3 thin films prepared by pulsed laser deposition

Ti-modified thin films of multiferroic 0.72Bi(Fe_1 − xTi_x)O₃-0.28PbTiO₃ (BFPT, x = 0 and 0.02) solid solution were prepared by pulsed laser deposition. The BFPT (x = 0 and 0.02) films possess a tetragonal structure with highly preferential (001) orientation. The effects of the ionic substitution on the properties of BFPT (x = 0 and 0.02) films have been investigated. The leakage current of the BFPT (x = 0.02) thin film is significantly reduced, and the dielectric and ferroelectric properties greatly improved by the aliovalent ionic substitution of Ti⁴⁺ for Fe³⁺. The BFPT (x = 0.02) thin film exhibits a reasonably high remnant polarization P_r with 2P_r up to 90 μC/cm² at 312 kV/cm and a switchable polarization up to 92 μC/cm² at 417 kV/cm.     

Effect of Gd substitution on ferroelectric and magnetic properties of Bi4Ti3O12

Polycrystalline samples Bi_4 − xGd_xTi₃O₁₂ (x = 1, 1.5, 2) were investigated by X-ray diffraction, piezoresponse force microscopy and SQUID-magnetometry techniques. Increasing the gadolinium content was shown to suppress the spontaneous polarization in Bi_4 − xGd_xTi₃O₁₂, resulting in a polar-to-nonpolar phase transition near x = 1.5. In contrast to previous expectations, all these samples were found to be paramagnets. It was thus proven that introducing magnetically-active Gd ions into the lattice of the ferroelectric Aurivillius-type compound should not be considered as an effective way to achieve multiferroic behavior.     

Investigation of temperature-dependent polarization, dielectric, and magnetization behavior of multiferroic layered nanostructure

The PbZr_0.53Ti_0.47O₃(PZT)/CoFe₂O₄(CFO) layered nanostructures show lowering of dielectric constant and polarization, and an enhanced magnetization with a decrease in temperature from 400 K to 100 K. The temperature dependence of the real part of the dielectric constant illustrates a step-like behavior, whereas the imaginary part gives a relaxation peak near the step maxima temperature. A slow decrease in the polarization was observed from 300 K to 200 K, with an eventual collapse of polarization at ~ 100 K, and a complete polarization recovery with heating, these phenomena is reproducible over cycles of experiment. Remanent magnetization of the layered nanostructure was found to be three times higher at 100 K than that at room temperature. There is a slow enhancement in remanent (internal) magnetization with lowering of temperature, resulting in slow polarization switch and finally the collapse. The temperature-dependent dielectric, polarization and magnetization were different from the parent layer, indicating a kind of dynamic magneto-electric coupling in the layered nanostructures.     

Enhancement of magnetic and dielectric properties of KNbO3–CoFe2O4 multiferroic composites via thermal treatment

In this work, multiferroic composites were produced from CoFe₂O₄ and KNbO₃ mixtures via control of the heat treatment temperature. For this, CoFe₂O₄ nanoparticles were produced by sol-gel method, while KNbO₃ was synthesized by microwave-assisted hydrothermal synthesis. The powders were homogenized and subjected to heat treatment at 300, 400 and 500 °C for 5 h. The structural, electrical and magnetic properties were characterized. The results of X-ray diffraction indicated that there was no formation of secondary phases with heat treatment. Raman vibrational modes confirmed the presence of KNbO₃ and CoFe₂O₄ in the prepared composites. SEM analysis showed that the composite microstructure consists of smaller ferrite particles arranged on the surface of largest cubic KNbO₃ particles. The improvement of coercivity (H_C = 382.1Oe) and dielectric constant (?’~7860) was observed for the composite thermally treated at 300 °C. The obtained results show the potential application of KN:CFO composites for multifunctional devices.     

Enhanced multiferroism in Gd-doped BiMn2O5 ceramics

Gd doped Bi_1-xGd_xMn₂O₅ (x = 0.00–0.12) ceramics are synthesized via solid state reaction technique and they have been investigated for their structural, morphological, magnetic and electric properties. X-ray diffraction experiments and their corresponding Rietveld refinement confirm that the major phase characteristics of all the compositions are orthorhombic. The morphological studies reveal that the average grain size gradually decreases from $～500\mathrm{nm}$ to $～200\mathrm{nm}$ due to Gd substitution. The temperature dependent zero field cooled (ZFC) and field cooled (FC) magnetization curves demonstrate an antiferromagnetic Néel transition at 42 K. Due to the substitution of 4% Gd, the phase transition remains unaltered, however, both ZFC and FC curves coincide with each other at an applied magnetic field of 500 Oe. The remanent magnetizations and coercive fields are also found to enhance at room temperature due to the substitution of Gd. The polarization vs. electric field hysteresis loops exhibits the ferroelectric behavior of Bi_1-xGd_xMn₂O₅ (x = 0.00–0.12) ceramics at room temperature. Gd substitution also results in enhanced stability of the dielectric constant at wide range of high frequencies with significant suppression of low frequency dispersion particularly for 12% Gd doping. Moreover, a correlation among leakage current, remanent polarization, dielectric constant and microstructure has also been observed while investigating the Gd doped BiMn₂O₅ ceramics.     

Effect of calcination temperature on structural,magnetic and optical properties of multiferroic YFeO3 nanopowders synthesized by a low temperature solid-state reaction

Nanosize multiferroic YFeO₃ powders have been synthesized via the low temperature solid-state reaction. Differential scanning calorimetry (DSC), X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and Raman spectroscopy all indicated that the phase-pure orthorhombic YFeO₃ powders were obtained at 800 °C with a size below 150 nm. X-ray photoelectron spectroscopy (XPS) showed the Fe³⁺ ions to be predominant. Magnetic hysteresis loops exhibited some ferromagnetic behaviour of the YFeO₃ nanopowders at ambient temperature. The maximum and remnant magnetizations of the powders were about 2.49 and 0.88 emu/g, respectively. Moreover, optical measurements demonstrated that the optical band gap of the nanopowders was around 2.4 eV, proving that they can strongly absorb visible light. So an easy and efficient way to synthesize YFeO₃ nanopowders with promising application in the magnetic and optical fields has been successfully developed.     

Impedance spectroscopy studies on PbFe0.5Nb0.5O3 –BiFeO3 multiferroic solid solution

(1-x) PbFe_0.5Nb_0.5O₃ (PFN) – (x) BiFeO₃ (BFO) (PFN – BFO) multiferroic solid solution (x = 0.1, 0.2, 0.3 and 0.4) were synthesized by single step solid state reaction method. Single phase was confirmed in all the samples through room temperature (RT) X-ray Diffraction (XRD) with monoclinic structure (Cm space group). Transmission Electron Microscopy (TEM) studies confirm the high crystallinity of the materials with the average particle size of 100 nm. The temperature (313–528 K) and frequency (100 Hz – 5 MHz) dependent impedance, modulus and DC conductivity of PFN – BFO solid solutions were investigated. An impedance spectroscopy result shows a significant contribution from the grains (bulk) to the conductivity and exhibits non-Debye type of relaxation. The bulk (grain) resistance reduces as the temperature increases corresponds to negative temperature coefficient of resistance (NTCR) behaviour. Electric modulus exhibits thermally dependent relaxation phenomena in the material. The Bergmann modified KWW function fitted to the imaginary modulus with non-Debye type of relaxation. DC conductivity of PFN – BFO solid solutions was found to follow the Arrhenius behaviour.     

Structural,vibrational, electrical and magnetic properties of Bi1−xPrxFeO3

Crystalline solid solution of Bi_1−xPr_xFeO₃ (x=0.05, 0.1, and 0.15) ceramics has been successfully synthesized by a low temperature assisted co-precipitation method. Rietveld-refinement of the X-ray diffraction data reveals rhombohedral structure for Bi_1−xPr_xFeO₃ (x=0.05, 0.10) and triclinic for Bi_1−xPr_xFeO₃ (x=0.15). The crystallite sizes of the Bi_1−xPr_xFeO₃ (x=0.05, 0.1 and 0.15) are found to be approximately 33, 27 and 22 nm respectively calculated using Debye–Scherrer equation. The SEM images of Bi_1−xPr_xFeO₃ (x=0.05, 0.10 and 0.15) ceramics show grains with almost spherical morphology. 4A1 and 7E Raman modes have been observed in the range 100–650 cm⁻¹ and two phonon modes centered around 1150–1450 cm⁻¹ have also been observed corresponding to 2A₄ (LO), 2E₈ (TO) and 2E₉ (TO) modes of Bi_1−xPr_xFeO₃ (x=0.05, 0.1 and 0.15). The changes in Raman modes such as prominent frequency shift, line broadening and intensity have been noticed with the increase of Pr concentration in BiFeO₃ (BFO) suggesting a structural transformation as revealed by the Rietveld refinement. An anomaly in the temperature dependent dielectric studies has been noticed in all the samples at the vicinity of Neel temperature (T_N) indicating a magnetic ordering and an increase in magnetization with increase of Pr concentration is noticed from the room temperature magnetic studies. Further, the leakage current density is found to be reduced with increasing Pr concentration.     

Origin of enhanced magnetization in (La,Co) codoped BiFeO3 at the morphotropic phase boundary

In this work, we studied the effect of Co doping on the crystal structure and magnetic properties of Bi_0·84La_0·16FeO₃ polycrystalline ceramics. The analysis of X-ray diffraction patterns (XRD) demonstrates the coexistence of the R3c rhombohedral and Imma orthorhombic structures, along with small Bi₂₅FeO₄₀ and CoFe₂O₄ parasitic impurities in the samples. The study of phonon vibration with different excitation wavelengths identifies an individual contribution of two structures on the Raman spectra. Scanning electron micrographs identify small and large grains in the Imma and R3c phases, respectively. The magnetic impurity strongly impacted the magnetic properties of the compounds, while La and Co substitution for BiFeO₃ at the polymorphs induces the phase boundary ferromagnetism. The enhancement of magnetization mainly originates from magnetic impurity and phase boundary ferromagnetism. The variation in magnetization confirms a formation of phase boundary spins, and distinguishes the individual contributions of the coexisting phases and CoFe₂O₄ on the magnetic properties of the compounds.