Reduced leakage current of multiferroic BiFeO3 ceramics with microwave synthesis

Preparation of multiferroic BiFeO₃(BFO) is reported using microwave heating. The prepared sample is characterized using x-ray diffraction, scanning electron microscopy, differential scanning calorimetry and leakage current measurements. It is observed that the BFO can be prepared with microwave heating at a fast heating rate, consisting of more homogeneous microstructure and better electrical properties. Phase purity of the sample is confirmed from x-ray diffraction measurements. Uniform grain size distribution is observed for the sample prepared with microwave heating. More than an order of magnitude reduction in the leakage current is observed for the sample prepared with microwave heating as compared to conventional radiant heating.     

Effect of Dy-substitution on structural,electrical and magnetic properties of multiferroic BiFeO3 ceramics

The polycrystalline samples of dysprosium (Dy)-modified bismuth ferrite (i.e., Bi_1−xDy_xFeO₃; x=0–0.2 with the interval of 0.05) (BDFO) were synthesized using a high-temperature solid-state reaction method. Preliminary X-ray structural analysis showed that the reported crystal structure of BiFeO₃ (rhombohedral) is invariant even with Dy-substitution at the Bi-site upto x=0.2. The scanning electron micrograph of the compounds showed (i) the uniform distribution of grains on the sample surface with high density and (ii) reduction of grain size on increasing Dy content in BiFeO₃ (BFO). Studies of impedance, electrical modulus and electric conductivity of the materials in wide frequency (10–1000 kHz) and temperature (30–500 °C) ranges using a complex impedance spectroscopy technique have provided new and interesting information on the contribution of grains, grain boundary and interface in these parameters. Detailed studies of impedance spectroscopy clearly exhibit the dielectric relaxation of non-Debye type. The ac conductivity of the Dy-substituted BFO obeyed Jonscher's universal power law. An increase in Dy-content in BDFO results in the increase of spontaneous magnetization of BFO due to the collapse of spin cycloid structure.     

Room-temperature multiferroic behaviour in Co/Fe co-substituted layer-structured Aurivillius phase ceramics

Room-temperature single-phase multiferroic materials have attracted the considerable attention of may scientists due to their technological applications in the next-generation electronic devices. Here, we report the structural evolution and its relationship with the ferroelectric and magnetic properties of Aurivillius Bi_3.25Sm_0.75Ti_{(3-x) (}Fe,Co)_xO₁₂ (BSmT:FCx); for (0≤x ≤ 0.4) ceramics to elucidate the room-temperature multiferroics induced by Fe/Co co-substitution. The XRD analysis and structural refinements reveal that incorporating of Co, and Fe ions into Ti sites of BSmT host lattices gives rise to a single orthorhombic phase with no evidence of secondary phases. The X-ray photoelectron spectroscopy (XPS) spectra indicate the existence of oxygen vacancies in the investigated samples. Pure BiT samples exhibit diamagnetic behavior, while BSmT:FCx; for (0≤x ≤ 0.4) exhibits ferromagnetic behavior at room temperature. As a result, the sample with x = 0.4 displayed the highest remnant and saturation magnetization values. It is believed that the occurrence of magnetism causes by the magnetic double exchange interaction between the non-equivalent magnetic ions and oxygen. The reversible polarization induced by an electric field indicates the ferroelectric character of the ceramics at room temperature. The co-substituting samples show unsaturated P-E hysteresis loops, which could be caused by the domain pinning induced by the accumulation of oxygen vacancies near the domain boundaries. We expect these compounds are promising for developing electronic devices for future applications.     

Correlation between structure,dielectric and multiferroic properties of lead free Ni modified BaTiO3 solid solution

Present study highlights the influence of Ni doping on the structural, optical, dielectric, ferroelectric and magnetic properties of nanocrystalline BaTi_1-xNi_xO₃ (0 ≤ x ≤ 0.06) ceramics synthesized by sol-gel auto combustion process. Phase identification and crystal structure are examined through Rietveld refinement analysis that ensures mono-phase nature with tetragonal crystal structure in P4mm space group. The observed variation in the structural parameters viz. lattice constants, unit cell volume, bond lengths (Ti-O) and bond angles (Ti-O-Ti) are the direct evidence of distortion produce in the unit cell under the effect of Ni doping. The FTIR studies confirm perovskite structure and presence of stretching/bending vibrations of the various bands present in the samples. The optical properties divulge a minor alteration in optical bandgap under the influence of Ni content. Dielectric studies reveal higher value of the dielectric constant for pristine sample in the low frequency region, but its value decreases on Ni doping. The dielectric response, analyzed through UDR model, exhibits deviation from linear behavior at higher frequencies. Ferroelectric measurements demonstrate that the pristine sample has higher values of remnant polarization (P_r) and maximum polarization (P_m) which decrease linearly with the increase in Ni doping. Magnetic hysteresis loops at room temperature establish a weak ferromagnetic nature of the samples that arises due to the carrier-mediated exchange interactions with smaller values of magnetic parameters. These investigations ensure that the ferromagnetism can be induced in BaTiO₃ by appropriate doping of Ni ions, which may find their potential use in the field of multiferroics.     

Characterization of the superconductor-multiferroic type materials based on YBa2Cu3O7-δ–YMnO3 composites

The effect of YBa₂Cu₃O_7-δ (YBCO) inoculated with a small amount of magnetoelectric YMnO₃ (YMO) is presented. The particulate composites YBCO‒x%YMO (x = 0.10, 0.25, 0.50, 0.75, 1.00 and 5.00 wt%) were investigated for microstructural and physicochemical properties. The YBCO precursor exhibits two characteristic peaks of crystallite sizes around 50 μm and 300 μm after the second calcining process. The vibrational spectra shows a small deterioration of the superconducting state of YBCO−YMO ceramics across the YMO content, which is due to the vanishing of the orthorhombic phase seen as a O(4) band moves towards the higher wave numbers on the Raman scale. From the magnetic measurements one can find that a small addition of YMO (around 1.00 wt%) does not significantly change the superconducting properties of YBCO crystallites, and most of the composites exhibit critical temperature T_c close to 92 K. The direct magneto-resistance measurements at 77 K show critical current densities (J_c) of 205 A cm⁻² for YBCO−0.25%YMO and 35 A cm⁻² for YBCO−5.00%YMO, respectively.     

Microstructural and multiferroic properties in layered perovskite-related Sm6Ti4Fe2O20

Layered perovskite-related Sm₆Ti₄Fe₂O₂₀ compound was successfully synthesized through the intercalation of bilayer SmFeO₃ into the Sm₂Ti₂O₇ with pyrochlore structure by means of floating-zone melting technique. The microstructural properties were characterized using aberration-corrected scanning transmission electron microscopy and X-ray diffraction. Electron energy-loss spectroscopy investigation reveals that the Fe³⁺ ions prefer to occupy the inner sites within the perovskite-like layers. This compound exhibits clearly the spin glass-like behavior as demonstrated by the magnetic properties measurement. Such complex magnetic behavior could be attributed to the partial chemical order of Ti/Fe over the B sites and the interactions between magnetic ions including Sm³⁺ and Fe³⁺. In addition, the multiferroic behavior with the coexistence of the ferroelectricity and ferromagnetism was well established by magnetic and piezoresponse measurements.     

Microstructure and magnetic properties of BaTiO3-(Ni,Zn)Fe2O4 multiferroics

The microstructures of composite xBaTiO₃-(1−x)(Ni_0.5Zn_0.5)Fe₂O₄ (BT-NZF) multiferroics with various mixing ratios (x = 0.50, 0.60 and 0.70) are investigated by means of electron backscatter diffraction (EBSD) and magnetic force microscopy (MFM). The EBSD measurements reveal a change in the texture of the ferrite and the BaTiO₃ grains upon increasing the ferrite content in the sample. The sample with x = 0.70 exhibits the best ferrite texture, where only some directions are present. Furthermore, the resulting grain sizes vary from several µm (x = 0.50) to about 100 nm in the sample with x = 0.70. The MFM images reveal the presence of magnetic domains being extended over several adjacent grains, which according to the EBSD data may comprise different crystallographic orientations. In this way, we can explain the differences in the magnetic contrast obtained.     

BiFeO3 thin films prepared using metalorganic chemical vapor deposition

BiFeO₃ thin films were grown on (001) SrTiO₃ and (001) ZrO₂(Y₂O₃) substrates by single source metalorganic chemical vapor deposition in the temperature range T = 500 ÷ 800 °C using Fe(thd)₃ and Bi(C₆H₅)₃ as volatile precursors. X-ray diffraction analysis shows cube-on-cube epitaxial growth of BiFeO₃ on (001) SrTiO₃. The strongly reduced bismuth transfer into the film due to the high thermal stability of Bi(C₆H₅)₃ was counterbalanced by the increase of the total pressure as well as of the residence time of the precursor flow in the reactor; the Bi/Fe ratio in the film thus becomes close to that in the precursor mixture. Optical second harmonic generation measurements have evidenced the ferroelectric ordering in BiFeO₃ films and the apparent decrease of the Curie temperature of the strained films as compared to BiFeO₃ single crystal.     

Structural transformation and multiferroic properties of Sm and Ti co-doped BiFeO3 ceramics with Fe vacancies

Sm and Ti co-doped BiFeO₃ (BFO) ceramics with Fe vacancies—Bi_0.86Sm_0.14FeO₃, Bi_0.86Sm_0.14Fe_0.99Ti_0.01O₃, and Bi_0.86Sm_0.14Fe_0.9Ti_0.05O₃—were prepared by a solid-state method using a rapid liquid process. X-ray diffraction indicated that all samples exhibited a rhombohedral structure with a minor secondary phase. The structural transformation from a rhombohedral (space group: R3c) to orthorhombic structure (space group: Pnma) was observed in the sample of Bi_0.86Sm_0.14Fe_0.9Ti_0.05O₃, which was also confirmed by Raman scattering spectra. Microstructural investigations with scanning electron microscopy showed a reduction in grain size with doping of BFO. The dielectric loss of Bi_0.86Sm_0.14Fe_0.9Ti_0.05O₃ reaches 0.05 (at 100 Hz) owing to the introduction of Ti and Fe vacancies. Ferroelectromagnetic measurements revealed the existence of ferroelectricity with a remanent polarization of 0.24 µC/cm² in Bi_0.86Sm_0.14FeO₃, paraelectricity in Bi_0.86Sm_0.14Fe_0.9Ti_0.05O₃, and weak ferromagnetism with a remanent magnetization of 0.2 emu/g in Bi_0.86Sm_0.14Fe_0.99Ti_0.01O₃. The two composition-driven phases exist simultaneously and the different coercive field might be related to the jumps in the ferromagnetic hysteresis loops. Both the ferroelectric and magnetic properties were shown to correlate with the composition-driven structural evolution.     

Liquid-phase sintered bismuth ferrite multiferroics and their giant dielectric constant

Rapid liquid-phase sintering method has been modified and used to synthesize Bi_1-xLa_xFeO_3-δ compositions (0 ≤ x ≤ 0.5). The temperature and sintering procedures to obtain stable single-phase samples have been defined. This provides a significant reduction in the synthesis time, which is especially important for mass production. Furthermore, giant dielectric constants have been observed in Bi_1-xLa_xFeO_3-δ compositions, the values of which are one order of magnitude larger than those of similar Bi-based multiferroics obtained by conventional methods. It potentially makes them one of the most promising materials for modern technological applications.