Magneto-electric coupling in multiferroic La0.8Bi0.2Fe0.7Mn0.3O3 ceramic

We present here the structural, electrical, magnetic and magneto-electric properties of La_0.8Bi_0.2Fe_0.7Mn_0.3O₃ multiferroic sample synthesized by conventional solid state reaction route. X-ray diffraction (XRD) data show single phase character of the sample. Temperature (150-475 K) variation of dielectric constant exhibits step like escalation which consists of two transitions corresponding to the respective peaks in temperature vs. loss tangent curve. Slight shift in peak positions to higher temperatures with the increase of frequency indicates the presence of relaxor behavior. Magnetic field dependent magnetization (M-H) curve at 300 K demonstrates weak ferromagnetic conduct. Coupling between the electric and magnetic orders has been explored by means of magneto-capacitance measurement. Presence of magneto-electric (ME) coupling ascertains the candidature of this material for device application.     

Combustion synthesis of Sr0.5Ba0.5Nb2O6 and effect of fuel on its microstructure and dielectric properties

Nano-sized Sr_0.5Ba_0.5Nb₂O₆ (SBN50) powder has been synthesized, at very short reaction time, for the first time by a novel combustion method. Ba(NO₃)₂ and Sr(NO₃)₂ were used as source of Sr and Ba, respectively, while Nb-oxalate was used as the source of niobium. Urea, hexamethyltetramine (HMT) and glycine were used as fuel. The crystallite sizes in the powder ranged between 14-125 nm. X-ray diffraction analysis showed complete SBN50 phase formation at 700 °C, when urea/HMT was used as fuel, and at 800 °C when glycine was used as fuel. Ferroelectric-paraelectric phase transition temperature (T_c) close to 40 °C was observed when urea and HMT were used and the T_c was −49 °C when glycine was used. When urea was used as fuel highest dielectric constant was observed for the pellets sintered at 1250 °C for 4 h. Low dielectric loss was observed when HMT was used as fuel. Larger grain sizes in the sintered pellets were observed when glycine was used as fuel.     

Polaronic relaxation in La0.8Bi0.2Fe0.7Mn0.3O3

In this work, the dielectric properties of La_0.8Bi_0.2Fe_0.7Mn_0.3O₃ ceramics have been investigated in a temperature range of 76–320 K and a frequency range of 300 Hz–10 MHz. Two thermally activated dielectric relaxations were observed with the activation energy around 0.283 ± 0.007 eV for the low-temperature relaxation and 0.268 ± 0.007 eV for the high-temperature relaxation. Annealing in N₂ and O₂ can destroy and create the high-temperature relaxation, respectively. But the treatments have no significant influence on the low-temperature relaxation. The low-temperature relaxation was found to be bulk effect related to the dipolar effect due to the hopping polarons, and the high-temperature relaxation was associated with the Maxwell–Wagner relaxation due to surface-layer effect caused by hopping polarons blocked and trapped at the surfaces of the sample.     

Enhanced ferroelectric properties of multilayer SrBi2Ta2O9/SrBi2Nb2O9 thin films for NVRAM applications

Ferroelectric SrBi₂Ta₂O₉/SrBi₂Nb₂O₉ (SBT/SBN) multilayer thin films with various stacking periodicity were deposited on Pt/TiO₂/SiO₂/Si substrate by pulsed laser deposition technique. The X-ray diffraction patterns indicated that the perovskite phase was fully formed with polycrystalline structure in all the films. The Raman spectra showed the frequency of the O–Ta–O stretching mode for multilayer and single layer SrBi₂(Ta_0.5Nb_0.5)₂O₉ (SBNT) samples was 827–829 cm⁻¹, which was in between the stretching mode frequency in SBT (813 cm⁻¹) and SBN (834 cm⁻¹) thin films. The dielectric constant was increased from 300 (SBT) to 373 at 100 kHz in the double layer SBT/SBN sample with thickness of each layer being 200 nm. The remanent polarization (2P_r) for this film was obtained 41.7 μC/cm², which is much higher, compared to pure SBT film (19.2 μC/cm²). The coercive field of this double layer film (67 kV/cm) was found to be lower than SBN film (98 kV/cm).     

Effect of sintering temperature on magneto-transport properties of La0.67Ba0.33MnO3/Ba0.7Sr0.3TiO3 composites

La_0.67Ba_0.33MnO₃-20 wt.%-Ba_0.7Sr_0.3TiO₃ composites were sintered at different temperatures in order to explore the possibility of improving the magneto-transport properties of the composites. Detail studies on the magnetic and electrical transport properties for the sintered composite samples have been performed. Results show that the sintered composites have identical ferromagnetic to paramagnetic transition temperature and filamentary feature of metallic phase. When sintering temperature higher than 1300 °C, the composites show Efros-Shklovskii-like variable-range hopping in the temperature range lower than Curie temperature. For samples sintered lower than 1100 °C, a dome-like resistance peak appears at a temperature well below the Curie temperature. Magnetoresistance behavior indicates the existence of spin polarized tunneling in the low temperature range. Considering the contributions from Efros-Shklovskii-like variable-range hopping and spin polarized tunneling, the resistance peak can be well fitted.     

Enhancement of electrical resistivity of Sr0.5Ba0.5Fe12O19 nanomaterials by doping with lanthanum and nickel

From the viewpoint of electronic and telecommunication devices, the electrical resistivity is required to be high to curb the eddy current losses for efficient performance at high frequencies. In the present work, Sr_0.5Ba_0.5Fe₁₂O₁₉ hexaferrite has been doped with a binary mixture of lanthanum and nickel using chemical co-precipitation method of synthesis. The crystallite size of the synthesized samples is estimated in the range of 36–58 nm and their structural analyses have confirmed a single magnetoplumbite phase. The magnitude of the dc-electrical resistivity is enhanced, by almost 100 times, but Curie temperature (T_C) is reduced by doping with La–Ni, which has been explained on the basis of the exchange interactions. In addition, the doped samples exhibit very low dielectric constant (έ = 11–13) and low dielectric loss tangent (tan δ = 0.07–0.10) determined at a frequency of 1 MHz. These characteristics may be suitable for their potential application in electromagnetic attenuation materials and microwave devices.     

Structural and dielectric properties of ferroelectric Sr1−xBaxBi2(Nb0.5Ta0.5)2O9 and Sr0.5Ba0.5Bi2(Nb1−yTay)2O9 ceramics

Ferroelectric Sr_1−xBa_xBi₂(Nb_0.5Ta_0.5)₂O₉ and Sr_0.5Ba_0.5Bi₂(Nb_1−yTa_y)₂O₉ were synthesized by solid state reaction route. X-ray diffraction studies confirm the formation of single phase layered perovskite solid solutions over a wide range of compositions (x=y=0.0, 0.25, 0.50, 0.75 and 1). The lattice parameters and the Curie temperature (T_c) have been found to have linear dependence on x and y. Transmission electron microscopy (TEM) suggest the lowering of orthorhombic distortion with increasing Ba²⁺ substitution. Variations in microstructural features as a function of x and y were monitored by scanning electron microscopy (SEM). The dielectric constant at room temperature increases with increase in both x and y. Interestingly Ba²⁺ substitution on Sr²⁺ site induces diffused phase transition and diffuseness increases with increasing Ba²⁺ concentration.     

Effect of internal strain on magnetic properties of La0.7Sr0.3MnO3 films

Oriented La_0.7Sr_0.3MnO₃ (LSMO) films were grown on (100) LaAlO₃ (LAO) and SrTiO₃ (STO) substrates by the method of radio frequency magnetron sputtering. X-ray diffraction patterns of LSMO films deposited on LAO and STO substrates showed a slight shift of diffraction peaks as compared to the corresponding peaks of the bulk LSMO. The shift of diffraction peaks was believed to result from the in-plane lattice mismatch between the film and substrate. The distinguished strain effects of substrates were found to be effective on the magnetic properties and the surface morphology of LSMO films on both substrates. The results indicate that STO substrate may be the promising candidate for room-temperature applications of LSMO film.     

Dielectric properties of the spin-1/2 dimer compounds Ba3Cr2O8 and Sr3Cr2O8

We have investigated the dielectric and magnetic properties of the spin-1/2 compounds Ba₃Cr₂O₈ and Sr₃Cr₂O₈. For Ba₃Cr₂O₈, the real part dielectric constant ε′(T) exhibit a frequency-independent peak near the Jahn-Teller transition temperature T_JT = 70 K. However, no anomaly in the ε′(T) curve is observed near T_JT = 275 K for the isostructural compound Sr₃Cr₂O₈. The difference in the ε′(T) behaviors may be attributed to the different onset temperatures of the phonon mode splitting in the samples. Dielectric relaxation analysis revealed that extrinsic contribution due to Maxwell–Wagner effect dominated at high temperatures for both compounds. The magnetization data reveal the occurrence of antiferromagnetic interactions between the Cr⁵⁺ ions and the χ(T) curves can be described by the Bleaney–Bowers equation for both compounds.     

La2(Zr0.7Ce0.3)2O7—A new oxide ceramic material with high sintering-resistance

Oxide ceramics with high sintering-resistance above 1473 K have very important applications in thermal barrier coatings (TBCs), catalytic combustion and high-temperature structural materials. Lanthanum zirconate (La₂Zr₂O₇, LZ) is an attractive TBC material which has higher sintering-resistance than yttria stabilized zirconia (YSZ), and this property could be further improved by the proper addition of ceria. The composite La₂(Zr_0.7Ce_0.3)₂O₇ (LZ7C3) has the highest sintering-resistance, and it is observed that fine particles of the second phase in the composite concentrate at the grain boundary to prevent the grain growth of the main phase. The formation of hollow fibers is helpful to the further improvement of sintering-resistance of LZ7C3, and this technique may be widely applied to protect the catalyst support against thermal sintering.     

Effect of doping with Nd ions on the structural and ferroelectric properties of Ca0.28Ba0.72Nb2O6 single crystal

The crystal structure of Ca_0.28Ba_0.72Nb₂O₆ (CBN-28) crystal with Nd-doping has been determined from X-ray single crystal diffraction data, in the tetragonal system with space group P4bm and the following parameters: a = b = 12.458 Å, c = 3.954 Å, V = 613.688 Å³, and Z = 5. X-ray diffraction results on a Nd-doped CBN-28 single crystal also have demonstrated that Nd³⁺ and Ca²⁺ occupy the same site in the crystal structure. Dielectric and ferroelectric measurements have been performed. Transition from ferroelectric to paraelectric at around 223 °C has been observed. The Nd-doped crystal has a lower Curie temperature (T_m) than that of undoped CBN-28 crystal. The spontaneous polarization (P_s) and coercive electric field (E_c) also decrease compared with their values in the undoped CBN-28 crystal.     

Dielectric properties and high-temperature dielectric relaxation of Ba3Ti4Nb4O21 ceramic

A₃B₈O₂₁ hexagonal perovskite-like Ba₃Ti₄Nb₄O₂₁ ceramics were synthesized by a conventional solid-state reaction technique, which exhibit high dielectric constant at room temperature. A high-temperature dielectric relaxation was observed in frequency range from 40 Hz to 1 MHz above the Curie temperature. The real and imaginary parts of the impedance (Z′ and Z″) as functions of frequency indicate the presence of two relaxation processes at high temperatures, which are attributed to grain and grain boundary responses. The conductivities of both grain and grain boundary obey the Arrhenius law with activation energies of 1.04 eV and 1.27 eV, respectively. The dielectric relaxation processes are related to the oxygen vacancies inside the ceramic, indicating that the Q × f value of Ba₃Nb₄Ti₄O₂₁ might be improved via eliminating oxygen vacancies.     

Microstructure and nano-scratch behaviors of La0.7Sr0.3MnO3 films

The La_0.7Sr_0.3MnO₃ (LSMO) films were prepared at various substrate temperatures on si(100) by DC magnetron sputtering method. The microstructure and nano-scratch behaviors of the films were investigated. The results indicate that the films are single phase with perovskite distorted cubic structure and the texture orientation changes obviously with the increase of substrate temperature. A smooth and dense nanocrystalline LSMO film is obtained at high substrate temperature. The (110) preferred orientation growth is beneficial to the improvement of nano-scratch resistance of the films. The friction coefficient between the films and the diamond tip depends on the critical load (L_c). Elastic deformation is the dominant deformation mechanism and the friction coefficient is about 0.08-0.14 for all the films when the loading normal load is less than L_c. When the loading load is larger than L_c, the delamination or detachment of the films occur and the friction coefficient increases abruptly near the L_c. The films deposited at 480 °C and 680 °C possess higher L_c which is about 77 mN due to lower hardness. The suitable decrease in hardness can enhance cohesion strength and scratch resistance of the films.     

Effects of step-edge conditions on the magnetoresistance of La0.7Sr0.3MnO3 tunneling junctions

La_0.7Sr_0.3MnO₃ (LSMO) tunneling magnetoresistance (TMR) junctions have been successfully fabricated on step-edge (001) SrTiO₃ substrates with various step-edge conditions. We report the fabrication techniques of LSMO step-edge TMR junctions in which the standard photolithography and ion-beam etching techniques are used to control the step angle and step height. We study the influence of step-edge conditions on the resistance and MR behavior of LSMO TMR junctions. It is found that the magnitude of junction resistance increases with an increase of step angle or with a decrease of d/H ratio, where d is the film thickness and H the step height. Based on our results, we conclude that a suitable d/H ratio of ∼0.5-0.9 with a high step angle of φ≥70° can be regarded as the optimum conditions for the fabrication of step-edge TMR junctions.     

Color emission and dielectric properties of Eu-doped Gd2O3 gate oxide thin films

High-k Gd₂O₃ used for thin film transistor (TFT) gate insulators has been synthesized via a simple solution process. Phase analysis and capacitive performance reveal that a high dielectric constant of ~ 20 and a low leakage current level of < 10⁻⁸ A/cm² at 1 MV/cm with a good transparency under the visible wavelength region are readily produced by the sol-gel method. Eu³⁺ doping leads to an increased dielectric constant induced by the additional electric dipole transition, which is evidently visualized by the photoluminescence behavior and/or by the defect-controlled thin film microstructures. Thus, the solution-processed (Gd,Eu)₂O₃ film is a viable gate insulator to be considered for the proposed “color emissive” switching devices as well as for the low power-driven TFT devices.     

Effects of reduction treatment on the photorefractive properties of Pb0.5Ba0.5Nb2O6

Lead barium niobate is a new photorefractive material of high interest for a variety of applications including holographic storage. Pb_0.5Ba_0.5Nb₂O₆ crystals have been grown by the Bridgman method, and the effects of heat treatments on their photorefractive properties were investigated using Ar ion laser at λ=514.5 nm. The color and absorption spectrum of the crystals varied depending on the oxygen partial pressure during heat treatment. The oxygen diffusivity was estimated to be in the order of 10⁻⁶ and 10⁻⁵ cm²/h at 425 and 550 °C, respectively. Reduction treatment at an oxygen pressure of 215 mTorr increased the effective density of photorefractive charges about three times from 8.0×10¹⁵ to 2.2×10¹⁶ cm⁻³ and made the charge transport more electron-dominant. As a result, the maximum gain coefficient improved from 5.5 to 13.8 cm⁻¹. A diffraction efficiency as high as 70% was achieved in a reduced crystal.     

Synthesis and electrical properties of sodium bismuth niobate Na0.5Bi2.5Nb2O9

The crystal structure of Na_0.5Bi_2.5Nb₂O₉ was refined from its powder X-ray diffraction (XRD) data. Sodium bismuth niobate at 23 °C is orthorhombic, A2₁am, with a=5.4998(3) Å, b=5.4602(2) Å, c=24.952(1) Å, and Z=4. The piezoelectric properties were investigated using the dense bulk ceramics. The electromechanical coupling coefficients (k_ij) and electrical quality factors (Q_m) are k₃₁=3.2%, k_t=10.0%, and Q_m=3800. Single crystals were also grown from stoichiometric melts using a slow cooling technique. The dielectric and ferroelectric properties of the crystal indicates that a spontaneous polarization clearly occurs in the crystallographic a-b plane.     

Dielectric properties of Y2O3 donor-doped Ba0.8Sr0.2TiO3 ceramics

Ba_0.8Sr_0.2TiO₃ ceramics doped with Y₂O₃ from 0 to 0.10 mol% exhibit normal ferroelectric phase transition, while the ceramics doped with Y₂O₃ from 0.20 to 0.30 mol% show a giant dielectric constant behavior with loss less than 0.15 at 1 kHz from −40 °C to 140 °C, which is suggested due to semiconductive grain and the Maxwell–Wagner effect by structure disordering in grain boundary. The analyses of unipolar charge for the semiconductive grain indicate three kinds of dielectric processes: thermally stimulated process of unipolar hopping, dispersion process of dielectric constant with frequency, and phase transition process accompanied with disappearance of giant dielectric constant in cubic phase. The XPS results confirm that some of the barium ions are in low energy state to form e-Ba²⁺ and to provide hopping sites for electrons. The ceramics doped with Y₂O₃ from 0.50 to 0.75 mol% recover the normal ferroelectricity. The possible mechanics are relevant to binding effect of cation vacancies on electrons.     

Low temperature preparation of nanocrystalline Sr0.5Ba0.5Nb2O6 powders using an aqueous organic gel route

Nano-sized Sr_0.5Ba_0.5Nb₂O₆ (SBN50) ceramic powders have been synthesized by an aqueous organic gel route. Homogeneous Sr-Ba-Nb precursor gels are prepared with Ba-EDTA, Sr-EDTA, and Nb-citrate complex as source of Sr, Ba, and Nb, respectively. Citric acid and ethylenediaminetetraacetic acid (EDTA) were used as the chelating agents. The structural variation of the SBN powder with annealing temperature was studied by TG-DTA, FT-IR and XRD. The precursor gel on calcination at 800 °C for 2 h produces a pure tungsten bronze SBN phase and the corresponding average particle size is 30-50 nm. The influences of the pH and the molar ratio of citric acid:Nb cation on the formation of homogeneous Sr-Ba-Nb precursor gels were also studied. The results show that a homogeneous Sr-Ba-Nb precursor gel with no precipitate is formed at pH 8 and the optimum molar ratio of citric acid and the metal cations is 3:1.     

Czochralski single crystal growth of Sr.61Ba.39Nb2O6 for surface acoustic wave applications

Ferroelectric Sr_.61Ba_.39Nb₂O₆ (SBN) single crystals approximately 2 cm. in diameter and 5–6 cm. long have successfully been grown by the Czochralski technique. All the crystals were grown in the [001] direction in argon or oxygen, and it was found that the degree of difficulty of growth increased as the diameter of the crystals increased. Temperature dependence measurements on the poled Z-cut SBN crystals showed that this composition has temperature compensated orientations which make this material useful for surface acoustic wave devices.