Synthesis and thermodynamic stability of multiferroic BiFeO3

Ceramic BiFeO₃ samples were prepared by the sol gel combustion method using urea as fuel. The obtain powders were thermal treated at different temperatures (300-840  °C) and times (1-64  h) to investigate the best synthesis conditions of the material. The resulting materials were analysed by TGA, FTIR, SEM/EDS and XRD. Rietveld analysis was applied to the diffraction data. The temperature and time of the heat treatment are critical for a high BiFeO₃ phase content. Thermal treatment of 1  h at 600  °C yielded 99% molar of the BiFeO₃ phase with a mean particle size of 120  nm. Upper or lower calcinations temperatures yielded higher content of the secondary phases Bi₂Fe₄O₉ and Bi₂₅FeO₃₉. Further heat treatment in air or in argon, up to 64  h, induces a decomposition of the BiFeO₃ phase according to the reaction 49 BiFeO₃ BiFeO₃ → 12 Bi₂Fe₄O₉ + Bi₂₅FeO₃₉ pointing out that BiFeO₃ is not thermodynamically stable at 600  °C. The BiFeO₃ decomposition follows Avrami-Erofeev law with a slope of 1 indicating a one-dimensional kinetics.     

Investigation of structural,optical, dielectric and magnetic studies of Mn substituted BiFeO3 multiferroics

A series of Mn doped BiFeO₃ with composition BiMn_xFe_1−xO₃ (x = 0.0, 0.025, 0.05, 0.075, 0.1) was synthesized via a citrate precursor method. Structural, morphological, optical, electrical and magnetic properties were investigated by using various measurement techniques. XRD patterns confirmed that the materials possess distorted rhombohedral structure with space group R3c. Average crystallite size was found to be in the range 18–36 nm. A decrease in the value of lattice parameters has been observed due to contraction of unit cell volume with Mn doping. Higher tensile strain for the prepared nanoparticles was observed in Hall-Williamson Plot. Field Emission Scanning Microscopy (FESEM) showed the spherical, uniform, dense nanoparticles in the range 80–200 nm. Reduction in grain size was observed which may be due to suppression of grain growth with Mn doping. FTIR studies reported two strong peaks at 552 cm⁻¹ and 449 cm^-1 which confirmed the pervoskite structure. Dielectric properties were studied by measuring the dielectric constant and loss in the frequency range 1 kHz to 1 MHz. Magnetic hysteresis loop showed the retentivity (M_r) increasing from 0.0514 emu/g of BFO to 0.0931 emu/g of 10% Mn doping. Coercivity was found to increase upto 0.0582 T for 5% Mn doping and then reduced to 0.0344 T for 7.5% Mn doping. Saturation magnetization was observed to increase from 0.6791 emu/g for BFO to 0.8025 emu/g for 7.5% and then reduced to 0.6725 emu/g for 10% Mn doping in BFO. Improvement in dielectric and magnetic properties makes this material as a promising candidate for multifunctional device applications.     

Structural properties of multiferroic 0.5BiFeO3–0.5Pb(Fe0.5Nb0.5)O3 solid solution

Magnetoelectric multiferroics are very promising materials because of their practical applications and fundamental interests. The most widely studied magnetoelectric oxides are ABO₃ perovskites. In the paper structural properties of BiFeO₃ and Pb(Fe_0.5Nb_0.5)O₃ solid solution are described. The material crystallizes in rhombohedral R3c crystal structure which parameters are presented. Mössbauer spectroscopy was used to study local changes in an iron environment due to Fe/Nb substitution and hyperfine interaction parameters of different local surroundings of iron atoms are presented. The random distribution of B-site sublattice cations was confirmed. Ab initio calculations of the studied solid solution were conducted and theoretical crystal structure parameters were compared with the experimental data. The theoretical magnetic and electric properties are discussed. The local iron magnetic moments were estimated and their dependence on the local surrounding changes is shown. The calculated electrons densities and Bader's topological analysis were used to describe chemical bonding properties.     

Role of Ni–Co co-substitution in improving electrical and dielectric properties of La-based multiferroics nanomaterials

The synthesis of a material having ferroelectric and ferromagnetic properties in the same phase is a challenging task. In the present work, lanthanum based multiferroic materials, with composition La_1−xCo_xFe_1−yNi_yO₃ (where, x=0.0–0.5 and y=0.0–1.0), have been synthesized via sol–gel auto-combustion method. The phase of the synthesized materials was confirmed by the X-ray diffraction (XRD) analysis, while the surface morphology and particle size were determined by the scanning electron microscopy (SEM) analysis. The DC electrical resistivity and activation energy were observed to increase from 2.14×10⁷ to 3.04×10¹⁰ Ω cm and 0.64 to 0.77 eV, respectively with the increase in concentration of substituents. The drift mobility decreases from 3.27×10⁻¹³ to 2.80×10⁻¹⁶ cm² V⁻¹ S⁻¹. The dielectric constant, dielectric loss and dielectric loss factor decrease with the increase in frequency (1 MHz to 3 GHz) and Ni–Co content as well. The electrical and dielectric results are in good agreement with each other. The increase in electrical resistivity and decrease in dielectric parameters make these materials suitable for applications in microwave devices.     

Raman and dielectric spectroscopic analysis of magnetic phase transition in Y(Fe0.5Cr0.5)O3 multiferroic ceramics

Here, we report the Raman and dielectric spectroscopic studies as a function of temperature of orthorhombically distorted Y(Fe_0.5Cr_0.5)O₃ (YFC) ceramics, measured from 80 to 300 K. The dc-magnetization measurements under field cooled (FC)-zero field cooled (ZFC) protocol indicate a small onset of magnetic ordering at T_N∼270 K. The field dependent magnetization plot recorded at 50 K, 150 K and 200 K show a clear opening in hysteresis loops. The linear dependence of magnetization plot at high field without any saturation of magnetization indicates the coexistence of weak ferromagnetic (WFM) component within the canting antiferromagnetic (CAFM) matrix. Temperature evolution of Raman line-shape parameter of B_2g(4) phonon mode clearly exhibits an anomalous behavior of phonon shift near T_N∼270 K, indicating the spin-phonon coupling in the ceramics. From the temperature dependent dielectric permittivity (ε(T)) study, two dielectric relaxation peaks are detected below 200 K and above 250 K. The appearance of former relaxation peak is responsible for polaronic conduction mechanism, while the later one is associated with magnetic phase transition which might be relevant to the presence of magnetoelectric coupling in YFC ceramics. The observed P-E hysteresis loops at room temperature indicate weak ferroelectric nature of the ceramics.     

Exploring multiferroicity in BiFeO3 - NaNbO3 thermistor electroceramics

The BiFeO₃ –NaNbO₃ electroceramics, synthesized by the ceramic method, are studied aiming to obtain materials with a well-defined thermistor response coexisting with a relevant magnetic response. XRD data and Raman analysis reveal a structural transition as a function of composition. Compositional features explored from ICP, XPS and EDS measurements, suggest compositional heterogeneity leading to a cluster-type scenario implying NNO-rich and BFO-rich regions in the samples. Impedance spectroscopy data reveal the development of a PTCR thermistor response for x ≥ 0.5 near room temperature. The x = 0.9 ceramic shows resistivity changes of about six orders of magnitude in the first thermal cycle and maximum permittivity values of ∼ 10⁵, much higher than those previously reported for BFO-doped ceramics. Magnetization data are interpreted in terms of the stabilization of superparamagnetic clusters. The response displayed by the x = 0.9 ceramic makes it a promising multifunctional material for device applications.     

Magnetic and electrical properties of single-phase multiferroic (1-x)Pb(Zr0.52Ti0.48)O3–xPb(Fe0.5Nb0.5)O3 thin films prepared by sol-gel route

Single-phase multiferroic (1-x)Pb(Zr_0.52Ti_0.48)O₃-xPb(Fe_0.5Nb_0.5)O₃ (0≤x≤0.5) thin films were synthesized by sol-gel route and characterized to understand their structural, electrical, and magnetic properties. The films were thermally treated by conventional furnace (CFA) and rapid thermal annealing (RTA). A pyrochlore-free perovskite phase is stabilized only by RTA in samples with high Fe³⁺/Nb⁵⁺ content. The films displayed excellent dielectric and ferroelectric properties in the whole concentration range, with saturated hysteresis loops and remanent polarization values of ～15μC/cm². Films with x>0.3 showed ferromagnetic behavior at room temperature. Consequently, the multiferroic behavior in the films occurs in a different concentration range than that observed in bulk ceramics. The origin of the weak ferromagnetism is discussed.     

Multiferroic composite for combined detection of static and alternating magnetic fields

Magneto-electric (ME) effects in bi-layer multiferroics consisting of single crystal hexagonal SrAl_xFe_12 − xO₁₉ ferrites (hexaferrites, where x = 0, 0.8, 1.2) and PbZrTiO₃ (PZT) have been studied. The main objective of the study was to determine the effect of the Al doping concentration on the ME coupling. We experimentally demonstrate that the Al doping improves dramatically the magnetically induced ME voltage. Moreover, our data indicate that the ME response is a linear function of both ac and dc applied magnetic fields. This is in contrast to previously reported multiferroic laminates that show a non-linear magneto-electric induced voltage as a function of the applied dc magnetic field. The multiferroic structures reported in this work have therefore enhanced functionality because they can linearly operate as a dual ac/dc magnetic field sensors.     

Dielectric tunability and magnetoelectric coupling in LuFe2O4 epitaxial thin film deposited by pulsed-laser deposition

C-axis orientated LuFe₂O₄ thin films on (001) sapphire substrates are epitaxially deposited by pulsed-laser deposition. Temperature-dependent resistance characterization reveals the ferrimagnetic transition at 237 K and charge-ordering transition at 340 K in the film. Importantly, the dielectric constant of the film can be significantly changed by both electric and magnetic fields. The dielectric tunability reaches 35% when an electric field of 5 V is applied, while this value reduces to 20% and 15%, respectively, when a magnetic field of 0.83 T is applied perpendicular and parallel to the film normal direction. This suggests a magnetically controlled dielectric tunability and strong magnetoelectric coupling, and is therefore promising for tunable device applications in film form.