Multiferroic Properties of (Bi0.9Gd0.1FeO)1−x (BaTiO3) x Ceramics

Conventional solid-state reaction method has been employed for the synthesis of polycrystalline (Bi_0.9Gd_0.1FeO)_1?x (BaTiO₃) _x for x=0.1, 0.2 and 0.3, ceramics samples. The effect of BaTiO₃ content on the multiferroic properties of Gd-doped BiFeO₃ ceramics has been presented. Pure perovskite phase with high density has been obtained by optimizing the synthesis approach, calcination and sintering strategies. Structural analysis carried out using X-ray diffraction confirms the formation of desired morphotropic phase. The dielectric properties have been investigated at different concentration of BaTiO₃ as function of temperature, revealing that by increasing the BaTiO₃ content dielectric constant increases while dielectric losses decrease. Magnetic study shows that initially saturation magnetization increases with increase in BaTiO₃ content up to x=0.1; however, afterwards it decreases for higher concentration of BaTiO₃. According to ferroelectric measurements, PE loops (with low coercive field) are observed at room temperature. The remnant polarization (P _r ) has been found to be 0.169, 0.619 and 0.760 μC/cm², respectively, for samples with x=0.1, 0.2 and 0.3. Magnetoelectric coupling in as-synthesized samples has been indirectly deduced by an anomaly observed at magnetic transition temperature.     

Structural and Magnetic Properties of Co(Cr1−y Al y )2O4 (y=0.0–0.2) Compounds

We present the structural and magnetic properties of Co(Cr_1?y Al _y )₂O₄ compounds prepared by the sol-gel technique for y=0.00,0.025,0.05,0.075,0.10,0.15 and 0.20. It has been observed that non-magnetic Al⁺³ substitution for Cr⁺³ enhances the magnetization at low temperatures compared to that of the parent compound. With increase in Al concentration spiral magnetic transition observed at around 24 K becomes less prominent, without affecting the ferrimagnetic transition temperature at around 97 K. The saturation magnetization values after subtracting the paramagnetic contribution vary from 0.036 μ_B/f.u to 0.376 μ_B/f.u. The enhanced ferromagnetic interaction and resultant magnetization could be explained in terms of decrease in Cr⁺³–O–Cr⁺³ bond angle with increase in doping concentration.     

Effect of Hot Pressing Conditions on the Microstructure and Optical Properties of MgO–Y2O3 Composite Ceramics

Inorganic Materials - We have studied the effect of hot pressing conditions on the microstructure and optical characteristics of Y2O3–MgO composite ceramics. The hot pressing conditions have...     

NTC characteristic of SnSb0.05O2–BaTi0.8Fe0.2O3−δ composite materials

SnSb _x O₂ (x = 0.003, 0.01, 0.02, 0.05 and 0.07) ceramics and SnSb_0.05O₂–BaTi_0.8Fe_0.2O_3-δ composite ceramics were prepared by using wet-chemical synthesis methods. The phases and related electrical properties of the ceramics were investigated. The results show that all the prepared ceramics have the effect of negative temperature coefficient (NTC) of resistivity over a wide temperature range. The room-temperature resistivities (ρ ₂₅) and material constants (B _25/85) of the SnSb _x O₂ NTC ceramics increase with the Sb concentration increases. The B _25/85 of the ceramics can be enhanced obviously when a certain content of BaTi_0.8Fe_0.2O_3-δ was added in SnSb _x O₂. The analysis of impedance spectra reveals that both grain and grain boundary contribute to the NTC effect of the ceramics. The conduction mechanisms combining the electron-hopping model and band conduction are proposed for the NTC effect in the SnSb_0.05O₂–BaTi_0.8Fe_0.2O_3-δ composite ceramics.     

Synthesis and Para-conductivity Analyses of TlBa2(Ca 3 − y Mg y )Cu4 O 1 2 − δ Superconductors

TlBa ₂(Ca _3?yMg _y)Cu ₄ O _12?δ (y=0,0.5,1.0, 1.5,2) superconductors are synthesized at normal pressure, and the influence of doped Mg atoms on the superconductivity parameters at the microscopic level is investigated by carrying out para-conductivity analyses of conductivity data. The samples have shown tetragonal structure, and the unit cell volume decreases with increased Mg doping. The onset temperature of superconductivity [ T _c(onset)] and zero resistivity critical temperature [ T _c(R=0)] decreases with Mg. Maximum magnitude of diamagnetism is observed in the samples with Mg of y=2. The apical oxygen mode of the type Tl-O _A-Cu(2) and CuO ₂ planar oxygen modes are softened as observed in fourier transform infrared spectrometer (FTIR) absorption measurements. The fluctuation-induced conductivity (FIC) analyses of conductivity data have shown the enhancement of inter-plane coupling and coherence length along the c-axis. These analyses have shown an increase in the coherence length along the c-axis and the inter-layer coupling J. The enhancement of inter-plane may possibly be arising due to a small decrease in the value of the order parameter of the carriers from ∣ψ∣ ²=1 in the CuO ₂ planes in Mg-doped samples. The values of B _c0(T), B _c1(T), J _c(0) are suppressed with the increased incorporation of Mg, which most likely arises due to the weak coupling of the grains induced by Mg doping.     

Temperature and Frequency Dependent Dielectric Properties of Cu0.5Tl0.5Ba2Ca3(Cu4−y Cd y )O12−δ Bulk Superconductor

The temperature and frequency dependent dielectric properties of polycrystalline Cd-doped Cu_0.5Tl_0.5Ba₂Ca₃(Cu_4?y Cd _y )O_12?δ (y=0,0.25,0.5,0.75) bulk superconductor samples are investigated. The zero resistivity critical temperature {T _c(R=0)} has decreased and normal state resistivity has increased with the increase of Cd-doping in Cu_0.5Tl_0.5Ba₂Ca₃(Cu_4?y Cd _y )O_12?δ samples. The dielectric properties such as dielectric constants (ε′,ε″), dielectric loss tangent (tanδ) and ac-conductivity (σ _ac ) are investigated by measuring the capacitance (C) and conductance (G) in the frequency range of 10 KHz to 10 MHz at different temperature from 80 K to 300 K. The negative capacitance (NC) is observed in all Cu_0.5Tl_0.5Ba₂Ca₃(Cu_4?y Cd _y )O_12?δ samples. The large values of NC observed at lower frequencies and temperatures may be due to reduced thermal vibrations and enhanced polarizability of the material. The effect of Cd-doping on bulk properties, dc-resistivity (ρ) and ac-electrical conductivity (σ _ac ) of these superconductor samples are investigated. The polarization in Cu_0.5Tl_0.5Ba₂Ca₃(Cu_4?y Cd _y )O_12?δ samples is most likely arising from the displacement of charges in CuO₂/CdO₂ planes relative to the static charges at Ba²⁺, Tl³⁺, and Cu²⁺ sites in Cu_0.5Tl_0.5Ba₂O_4?δ charge reservoir layers by external applied field.     

Oxygen release–absorption properties and structural stability of Ce0.8Fe0.2O2−x

Oxygen release–absorption properties and structural stability of Ce–Fe mixed oxides (Ce_0.8Fe_0.2O_2?x) with different calcination temperatures (600–1000 °C) were investigated and correlated to their oxygen storage capacity. Iron ions could be incorporated into the CeO₂ lattice to form a solid solution after calcination at low temperatures, but such solid solution was unstable under high-temperature thermal treatments. High-temperature (≥800 °C) calcination resulted in the appearance of exposed Fe₂O₃ phases on the surface of the solid solution, and this structural evolution finally affected the reduction behavior. The Fe³⁺ reduction from the Ce–Fe oxide solid solution was easier than the bulk Fe₂O₃ particles, while the small Fe₂O₃ particles in close contact with CeO₂ could enhance the reducibility of cerium oxides. The strong interaction between the exposed small Fe₂O₃ particles and the solid solution made the Ce–Fe mixed oxides possess good reduction stability and high oxygen storage capacity (OSC) even after repeated redox treatments. Such interactions were absent toward the physically mixed sample. An unusual enhancement on the reducibility of Ce–Fe mixed oxides was observed after a successive redox treatment. Large oxygen evolution appeared at around 600 °C for the recycled samples, and the OSC rose to 1.31 mmol-O₂/g after six redox cycles. The XRD, Raman, and TEM analyses revealed that the material structure of the mixed oxides was stabilized to have an inter-region between the Fe₂O₃ particles and the solid solution after the redox treatment. It was concluded that such microstructural evolutions of composite particle from solid solution under redox conditions brought beneficial property to the OSC of the Ce–Fe mixed oxides.     

Enhancement of Magnetic and Dielectric Properties in Chemically Modified Multiferroic (0.90)BiFe1−x Cr x O3–(0.10)BaTiO3 Nanocomposite

Multiferroic BiFeO₃ and Cr-doped (0.90)BiFe_1?x Cr _x O₃–(0.10)BaTiO₃ (x=0.00, 0.03, and 0.05) nanocomposites were prepared by sol–gel method. Optimum calcination and sintering strategies for obtaining pure perovskite phase have been determined. X-ray diffraction and transmission electron microscope (TEM) were used for the structural and particle size analysis, whereas LCR and SQUID magnetometer was used for dielectric and magnetic measurements. TEM measurements show that the average particle sizes of all the samples were ～19 nm. The dielectric constant was found to be increased twofold in low frequency region with the Cr-doping for x=0.05 in (0.90)BiFe_1?x Cr _x O₃–(0.10)BaTiO₃. The hysteresis curve (M–H) exhibits ferromagnetic nature for all the samples (x=0.0, 0.03, and 0.05) and the spontaneous magnetization at room temperature was found to be 0.63 emu/gm in pure BiFeO₃, which increased to 0.99 emu/gm for x=0.05. Zero-field-cooled (ZFC) and field-cooled (FC) magnetization curves show large discrepancy suggesting spin glass behavior.     

Doping Effect of Y2Ba4CuMoO y Particles on Microstructure and Superconducting Properties in Polycrystalline YBa2Cu3O7−δ

Synthesis of polycrystalline YBa₂Cu₃O_7?δ (YBCO) + x mol.% Y₂Ba₄CuMoO _y (Y-24Mo1) (x=0.0, 0.1, 0.2, 1.0, 2.0 and 20.0) superconductors have been done in air via a standard solid state reaction technique. The effects of Y-24Mo1 particles on the microstructure and the superconducting transition temperature were systematically investigated through XRD and SEM techniques. When Y-24Mo1 was added to the YBCO, the orthorhombic structure maintained even at the highest concentration. Tiny peaks of Y-24Mo1 having (022) orientation and some other impurity were detected in the XRD spectrum when x=20.0. SEM results reveal the presence of some entities, and these entities changed with increase of Y-24Mo1 content. The Y-24Mo1 addition modifies the normal state resistivity of samples from metallic to semiconducting, and is accompanied by a two-step transition. Moreover, low Y-24Mo1 concentration leads to a relatively sharp superconducting transition and a broad transition take place for Y-24Mo1 addition greater than 0.2 mol.%. These results can be induced by the inhomogeneities, by the change of the effective coupling strength, and by the variation of the coupling between grains.     

Synthesis and magnetic properties of polycrystalline films of Co x Fe y Cr3 − x − y O4 and Cr2O3/CoFe2O4 multiferroics

Magnetic properties of first obtained polycrystalline films of FeCr₂O₄, CoCr₂O₄, and CoFe_0.5Cr_1.5O₄ multiferroics and films of a Cr₂O₃/CoFe₂O₄ composite multiferroic have been studied. In particular, magnetization curves and temperature dependences of the magnetic moment of the samples were measured in the temperature range 4.2–300 K in fields of up to 10 kOe. It was shown that the Curie point of a multiferroic depends on its cation composition. It was found that an exchange bias of the hysteresis loop exists in films of the Cr₂O₃/CoFe₂O₄ composite multiferroic at temperatures below the Néel point of Cr₂O₃ (330 K).     

Ion Beam Analysis and Physical Properties Measurements of (Tl0.8Hg0.2−x Sb x )Ba2Ca2Cu3O9−δ

The superconducting phase (Tl_0.8 Hg_0.2−x Sb _x ) Ba₂Ca₂Cu₃O_9−δ , with 0.0 ≤x≤0.2, was characterized by means of X-ray powder diffraction (XRD), scanning electron microscope (SEM), proton induced X-ray emission (PIXE), Rutherford backscattering spectrometry (RBS), electrical resistivity (ρ) and transport critical current density (J _ct). The tetragonal structure of (Tl, Hg)-1223 remains unchanged with antimony-substitution, whereas the lattice parameters were changed. The actual elemental-content of Tl, Hg, Sb, Ba, Ca and Cu, determined from PIXE, were very close to those of the nominal elemental-content. The oxygen-stoichiometry of the prepared samples was determined by non-Rutherford backscattering cross section at 3 MeV H⁺. A little increase in the oxygen stoichiometry was observed as Sb-content increased. The superconducting transition temperature T _c, determined from electrical resistivity, increased from 124.8 K to 131.5 K as x increased from 0.0 to 0.1 and then it decreased with the further increase in x. The same behavior is obtained for J _ct vs. x. This indicates that the optimal Sb-substitution in (Tl, Hg)-1223 is at about x=0.1.     

Oxygen vacancy concentration variation, diffusivity and thermo-chemical stability of La0.2Ba0.8Co0.2Fe0.8O3−δ membranes

Perovskite type La_0.2Ba_0.8Co_0.2Fe_0.8O_3– (LBCF) powders were synthesized by modified citrate pyrolysis and LBCF membranes were prepared by isostatic pressing. The oxygen vacancy concentration variation of the LBCF powders becomes substantial above 973 K due to order-disorder transition. The unsteady-steady oxygen permeation flux of the LBCF membrane decreases with time and it takes about 40 min to get the steady state. In the temperature range of 1073–1223 K, the oxygen vacancy diffusivity of the LBCF membrane increases almost a magnitude and the increasing of oxygen permeation flux is mainly attributed to the increasing of oxygen vacancy diffusivity. An in-situ high-temperature X-ray diffraction was used to study the stability of the LBCF membrane, indicating that it has good thermochemical stability at elevated temperatures and low oxygen partial pressures.     

Structural and Magnetic Study of Zn-Substituted NiGa y Fe 2−y O 4 Ferrite

Single-phase spinel ferrite Ni _1?x Zn _x Ga _y Fe _2?y O ₄ with (0.0≤x≤0.5) and y=0.5 samples were synthesized using solid-state reaction technique. These ferrites were investigated using X-ray diffraction (XRD), vibrating sample magnetometer (VSM) and Mössbauer effect (ME) spectroscopy. XRD patterns confirmed the formation of single-phase cubic spinel ferrites for all samples. Lattice parameter was found to decrease with the introduction of Ga ³⁺ in NiFe ₂ O ₄, and then to increase with the increasing of Zn ²⁺ content x. ME measurements illustrated a strong dependence on the zinc concentration. The cation distribution calculated from the ME spectra at room temperature indicated that the Ga ³⁺ ion substituted iron in both octahedral B and tetrahedral A- sites. Zn ²⁺ ions firstly introduced in A- site, and for higher x, they distribute in both B- and A- sites. VSM measurements indicated that the change in the value of saturation magnetization can be explained using the cation distribution obtained from ME measurements. The coercivity values can be interpreted on the basis of magneto-crystalline anisotropy.     

Pressure and Grain-Size Dependences of the Chiral-Glass Transition in Superconductive Ceramics of YBa2Cu3O7−y

Ceramic high-T _c superconductors may be viewed as a weakly coupled random Josephson network containing -junctions, and a chiral-glass transition is predicted to be occurred. Grain-size and pressure dependences of the chiral-glass transition have been investigated on YBa₂Cu₃O_7–y ceramics. With decreasing temperature, a superconducting order occurs at first inside each grain at T _c1 and furthermore among the grains at T _c2 (<T _c1). The increase in T _c2 with increase of synthesis temperature is considered as the change of strength of inter-grain Josephson coupling accompanying the growth of grain size. The T _c2 also increases with increase of pressure, however, the sharpness of the transition at T _c2 does not change. In spite of difference in T _c2's, the increasing rates of T _c2 by pressure are nearly the same.     

Multiferroic properties of multilayered BaTiO3–CoFe2O4 composites via tape casting method

Multiferroic magnetoelectric (ME) BaTiO₃–CoFe₂O₄ (BTO–CFO) ceramic composites with different thicknesses were fabricated via tape casting technique. The interfacial morphology of the composite demonstrates the presence of plate-like grains with a thickness of ~400 nm. This could be associated with the residual stresses originated from lattice mismatch and different thermal expansion coefficients between BTO and CFO layers. The dielectric constant, piezoelectric constant, and ferroelectric properties of the multilayered composite are degraded in the presence of CFO layers in comparison with those of BTO bulk. Furthermore, the dielectric constant and polarization of the composite decrease with increasing frequency. The leakage current density and magnetic remanence ratio of the composite reach up to the order of 10^?6 A/cm² and 40 %, respectively. The direct and converse ME coefficients were measured to be 8.1 μV/cm Oe and 1.1 × 10^?3 G/V, respectively. Based on the converse ME effect, an electrically controlled ME inductor was designed using the composite as its core. The inductance and tunability of the inductor increase with increase of applied dc electric field.     

Magnetodielectric effect in Ni0∙5Zn0∙5Fe2O4–BaTiO3 nanocomposites

Composites comprising of nanoparticles of Ni_0?5Zn_0?5Fe₂O₄ (NZF) and BaTiO₃ (BT), respectively were synthesized by a chemical method. The particles had diameters in the range of 15–31 nm. NZF was prepared by a coprecipitation technique. This was soaked in a sol containing BT. Compositions synthesized were xNZF-(1 – x) BT, where x = 0?7, 0?5 and 0?3, respectively. The composites showed ferromagnetic hysteresis loops due to NZF phase. The analysis of coercivity variation as a function of temperature gave blocking temperatures in the range of 306–384 K depending on the diameter of the ferrite nanoparticles. This implied that superparamagnetic interactions are above these temperatures. The nanocomposites also exhibited ferroelectric behaviour arising due to the presence of BT. The remanent polarization of the samples was small. This was adduced to the nanosize of BT. The specimens showed magneto-dielectric (MD) effect in the magnetic field range 0–0?7 Tesla. The MD parameter measured at the maximum magnetic field was around 2%. This was one order of magnitude higher than that reported so far in similar composite systems. This was explained on the basis of a two-phase inhomogeneous medium model with an interface between them, the phases possessing drastically different electrical conductivities.