Structural and electrical impedance study of Pb(Sr1/3Nb2/3)O3

Lead-based ceramic Pb(Sr_1/3Nb_2/3)O₃ (PSN) is prepared by the columbite precursor method and structurally characterized using XRD. The X-ray diffraction shows a perovskite structure with cubic pyrochlore phase. Detailed studies of ε′ and ε″ of the compounds show that the compounds exhibit dielectric anomaly. Impedance spectroscopy is used to characterize the electrical behaviour. Results indicate that the relaxation mechanism of the material is temperature dependent and has dominant bulk contribution in different temperature ranges. Modulus spectroscopic data were used to gain an insight into the electrical properties of the samples and with a view to observe the relaxations in them. Frequency dependence dielectric permittivity shows typical Debye-type dielectric dispersion. Temperature-dependent DC resistivity shows that resistance decreases with the increase in temperature and follows Arrhenius behaviour in different temperature regions.     

Magnetic ordering and instability investigation of multiferroic Pb(Fe2/3W1/3)O3 ceramics by microwave dielectric spectroscopy

The microwave dielectric and magnetic properties of Pb(Fe_2/3W_1/3)O₃ multiferroic ceramics were investigated. A dielectric dispersion occurring in the frequency range 100 MHz-3 GHz and in a broad temperature range showed itself to be a powerful tool to detect magnetostrictive effects. The experimental results revealed the following remarkable features: the temperature dependence of f_R (characteristic frequency) and the dielectric strength Δε (characteristic of the dispersion) enabled us to identify not only the para-ferroelectric (T_C≈180 K) but also the para-antiferromagnetic (T_N≈340 K) phase transitions, while magnetic measurements revealed the para-antiferromagnetic ordering and a weak superexchange interaction (T_N2∼15 K). Additionally, both characterizations confirmed the existence of structural or magnetic instabilities around 250 K.     

铁电磁体Pb(Fe1/2Nb1/2)O3的磁电性能研究

对铁电磁体Pb(Fe_1/2Nb_1/2)O₃单晶样品中的 介电和磁性能进行了研究. 认为在其反铁磁相变点观察到的介电常数和损耗的异常来自于自发极化序和自旋序的相互作用 引起的磁电耦合. 磁矩与温度的关系曲线在Nel点以下的低温段呈上升趋势，测得的磁滞 回线证明有弱铁磁性出现. 对铁电磁体磁电相互作用的Monte Carlo模拟得到与实验类似的 结果. 关键词： 铁电磁体 1/2Nb_1/2)O₃')" href="#">Pb(Fe_1/2Nb_1/2)O₃ 磁电耦合 Monte Carlo模拟     

AC conductivity and dielectric relaxation in Ba(Sm1/2Nb1/2)O3 ceramic

The complex perovskite oxide a barium samarium niobate (BSN) synthesized by solid-state reaction technique has single phase with cubic structure. The scanning electron micrograph of the sample shows the average grain size of BSN∼1.22 μm. The field dependence of dielectric response and loss tangent were measured in the temperature range from 323 to 463 K and in the frequency range from 50 Hz to 1 MHz. The complex plane impedance plots show the grain boundary contribution for higher value of dielectric constant in the low frequency region. An analysis of the dielectric constant (ε′) and loss tangent (tan δ) with frequency was performed assuming a distribution of relaxation times as confirmed by the scaling behaviour of electric modulus spectra. The low frequency dielectric dispersion corresponds to DC conductivity. The logarithmic angular frequency dependence of the loss peak is found to obey the Arrhenius law with an activation energy of 0.71 eV. The frequency dependence of electrical data is also analyzed in the framework of conductivity and electric modulus formalisms. Both these formalisms show qualitative similarities in relaxation times. The scaling behaviour of imaginary part of electric modulus M″ and dielectric loss spectra suggest that the relaxation describes the same mechanism at various temperatures in BSN. All the observations indicate the polydispersive relaxation in BSN.     

Ferroelectric relaxor behaviour in Pb(Fe0.5Ta0.5)O3

The relaxor ferroelectric lead iron tantalate, Pb(Fe_0.5Ta_0.5)O₃ (PFT) is synthesized by Coulombite precursor method. The X-ray diffraction pattern of the sample at room temperature shows a cubic phase. The field dependence of dielectric response is measured in a frequency range 0.1 kHz — 1 MHz and in a temperature range from 173–373 K. The temperature dependence of permittivity (ɛ′) shows broad maxima at various frequencies. The frequency dependence of the permittivity maximum temperature (T _m ) has been modelled using Vogel-Fulcher relation.      

Structural, magnetic and electric properties of HoMnO3 films on SrTiO3(001)

HoMnO₃ films were grown on pure and Nb-doped SrTiO₃ (001) substrates by pulsed laser deposition. The films grew epitaxially with the c-axis along the substrate normal. Varying the deposition temperature between 650 and 850 °C did not significantly affect the structural and magnetic properties of the films, whereas growth in oxygen partial pressures below 0.01 mbar lead to a degradation of the structural properties. Some of the films had a ferromagnetic-like magnetic phase transition at about 45 K, probably related to Mn₃O₄ precipitates; this magnetic response was isotropic. The Ho sublattice was found to be paramagnetic down to 5 K, but showing a pronounced anisotropy with the c-axis being the hard axis. The films showed a distinct dielectric anomaly at 16 K that depended on voltage and slightly on frequency in the range between 1 kHz and 1 MHz. The magnetoelectric effect was large with an in-plane field of 8 T suppressing the dielectric anomaly completely.     

Acoustic properties of multiferroic PbFe1/2Ta1/2O3

The longitudinal acoustic wave velocity and attenuation in PbFe_1/2Ta_1/2O₃ ceramics have been measured by pulse-echo technique in the temperature range from 4.2 to 530 K. The anomalies observed in the sound velocity and attenuation behavior versus temperature were correlated with Burns temperature, temperature range of the coexistence of relaxor ferroelectric and antiferromagnetic states, and a suggested second antiferromagnetic phase transition at low temperatures.     

A comparative study of dielectric and Raman spectroscopy of Pb(Y b1/2Ta1/2)O3 and Pb(Y b1/2Nb1/2)O3

A comparative study of the properties of two highly ordered lead based complex perovskites Pb(Y b_1/2Ta_1/2)O₃ and Pb(Y b_1/2Nb_1/2)O₃ has been carried out through x-ray diffraction, dielectric and Raman scattering measurements. These two compounds differ significantly in their structure, dielectric response and phonon vibration although the ionic radii and valencies are same for Ta and Nb. The room temperature x-ray diffraction pattern and Raman spectra show that the symmetry of lead ytterbium tantalate is lower than that of lead ytterbium niobate. The Raman spectra of Pb(Y b_1/2Ta_1/2)O₃ also indicates the presence of local distortion in the lattice which may be one of the factors responsible for the existence of a secondary transition.     

Dielectric and impedance measurements on (1−x)Ba(Fe1/2Ta1/2)O3-xBa(Zn1/3Ta2/3)O3 ceramics

In this work, ((1−x)Ba(Fe_1/2Ta_1/2)O₃-xBa(Zn_1/3Ta_2/3)O₃), ((1−x)BFT-xBZT) ceramics with x = 0.00–0.12 were synthesized by the solid–state reaction method. X-ray diffraction data revealed that both the powders and ceramics were of a pure-phase cubic perovskite structure. All ceramics showed large dielectric constants. For the x = 0.12 sample, a very high dielectric constant (>20,600) was observed. A lowering in the dielectric loss compared to pure BFT ceramics was observed with the BZT addition. The impedance measurements indicated that BZT has a strong effect on the bulk grain and grain boundary resistance of BFT ceramics. These results are in agreement with the measured dielectric properties. Based on dielectric and impedance results, (1−x)BFT-xBZT ceramics could be of great interest for high performance dielectric materials applications due their giant dielectric constant behavior.     

Tuning the multiferroic properties of Pb(Fe1/2Nb1/2)O3 by cationic substitution

A series of substituted lead iron niobate compounds with the general formula Pb²⁺_(1−x)A^Z_x(Fe_{{(1−(2−Z)x)/2}}Nb_{{(1+(2−Z)x)/2}})O₃ (0<x<0.6 and A=La³⁺, K⁺ or Sr²⁺) were prepared by a modified solid-state synthesis. The relative concentrations of Fe³⁺ and Nb⁵⁺ were adjusted to compensate the charge imbalance due to the aliovalent substitution. The dielectric constant and magnetic susceptibilities were studied as a function of temperature. The temperature of the dielectric maximum, T_M, of the substituted compounds decreased linearly with increasing concentration of the substituent ions. The magnetic measurements showed an antiferromagnetic transition at temperatures T_N1 due to the superexchange interactions mediated by Fe–O–Fe and an additional antiferromagnetic-type transition at T_N2. T_N1 linearly increased with the increasing concentration of Fe³⁺ ion at the B-site of ABO₃-type substituted compounds. T_M is shown to be directly dependent on the concentration of the ferroactive Nb⁵⁺ ions at the B-site and Pb²⁺ ions at the A-site.     

The structural and multiferroic properties of (Bi1−xLax)(Fe0.95Co0.05)O3 ceramics

La and Co co-doped BiFeO₃ ((Bi_1−xLa_x)(Fe_0.95Co_0.05)O₃ (x=0, 0.10, 0.20, 0.30)) ceramics were prepared by tartaric acid modified sol–gel method. The X-ray diffraction patterns indicate a transition from rhombohedral structure to tetragonal structure at x=0.20, which has been confirmed by the Raman measurements. The band gap increases with increasing x to 0.20, and then decreases with further increasing x to 0.30. The structural transition has significant effects on the multiferroic properties. The remnant magnetization and saturate ferromagnetic magnetization decrease abruptly with increasing x to 0.10, and then gradually increase with further increasing x up to 0.30. The coercivity is significantly reduced with increasing La doping concentration. The ferroelectricity has been improved by La doping, and the polarization increases with increasing x to 0.10, then decreases with further increasing x up to 0.30. The simultaneous coexistence of soft ferromagnetism and ferroelectricity at room temperature in tetragonal Bi_0.70La_0.30Fe_0.95Co_0.05O₃ indicates the potential multiferroic applications.     

Structural, ferroelectric and impedance spectroscopy properties of Y modified Pb(Fe0.5Nb0.5)O3 ceramics

Some ceramic samples of Pb_1−xY_x(Fe_0.5Nb_0.5)_1−x/4O₃ (PYFN) (0.00≤x≤0.08) were synthesized by a mixed oxide route. X-ray diffraction patterns of all the above samples confirm the formation of single phase material crystallizing in monoclinic structure. Dielectric properties (ε_r and tan δ) were analyzed in a wide temperature (30-350 °C) and frequency range (100 Hz-1 MHz). Ferroelectric properties of these compounds were confirmed from polarization (P-E hysteresis loop) measurements at room temperature. All the room temperature hysteresis loops of PYFN ceramics were well simulated using the ferroelectric capacitor model. Yttrium substitution resulted in notable enlargement of room temperature remnant polarization (2P_r). The 2P_r of PYFN (x=0.02) reaches to a large value (23 μC/cm²), which is nearly 5 times greater than that of PFN ceramic (4.6 μC/cm²). All the compounds exhibits negative temperature coefficient of resistivity (NTCR) type behavior as that of semiconductors. Dc conductivity (estimated via bulk resistivity) variation with temperature of all the samples follows Arrhenius type of electrical conductivity.     

Orientation-dependent dielectric properties of Ba(Sn0.15Ti0.85)O3 thin films prepared by sol-gel method

The orientation-dependent dielectric properties of barium stannate titanate (Ba(Sn_0.15Ti_0.85)O₃, BTS) thin films grown on (1 0 0) LaAlO₃ single-crystal substrates through sol-gel process were investigated. The nonlinear dielectric properties of the BTS films were measured using an inter-digital capacitor (IDC). The results show that the in-plane dielectric properties of BTS films exhibited a strong sensitivity to orientation. The upward shift of Curie temperature (T_c) of the highly (1 0 0)-oriented BTS thin films is believed to be attributing to a tensile stress along the in-plane direction inside the film. A high tunability of 47.03% was obtained for the highly (1 0 0)-oriented BTS films, which is about three times larger than that of the BTS films with random orientation, measured at a frequency of 1 MHz and an applied electric field of 80 kV/cm. This work clearly reveals the highly promising potential of BTS films for application in tunable microwave devices.     

铁电磁体Pb(Fe1/2Nb1/2)O3相变特征

借助与示差扫描量热法、磁化率测量、电子自旋共振、铁电与介电性质测量及电子衍射系统地研究了Pb(Fe_1/2Nb_1/2)O₃(PFN)的电、磁性质和相变特征．结果表明发生在380K附近的顺电－铁电转变和发生在145K附近的顺磁 反铁磁转变分别为一级相变和二级相变或弱一级相变．在室温下,PFN的剩余极化与矫顽场分别为11.5μC/cm²和3.04kV/cm.介电测量表明PFN的顺电-铁电相变为弥散型相变．其弥散指数为1.62.电子衍射表明Fe³⁺与Nb⁵⁺离子在B位置上是无序分布的,正是这种与无序分布相关联的成分涨落导致铁电相变的弥散性. 关键词：     

Elastic behavior of some manganite-based orthorhombic RMnO3 (R=Sm,Eu, Gd,Dy) multiferroics

A series of multiferroic materials with the compositional formula, RMnO₃ (where R=Sm, Eu, Gd, Dy) were prepared by the well-known citrate sol–gel technique. After characterizing the samples structurally, a systematic investigation of elastic behavior has been undertaken and it has been found that the elastic moduli are increasing continuously with decreasing ionic radii of the dopant ion. Variation of longitudinal velocity with temperature for all the samples, over a temperature range 80–300 K has also been studied and the observed experimental behavior is explained using four theoretical models. It has been concluded that Lakkad's model explains the variation of Young's modulus with temperature in a better way than the other ones.     

Microwave dielectric properties of high quality factor La(Mg0.5−xCaxSn0.5)O3 ceramics

The microwave dielectric properties of La(Mg_0.5−xCa_xSn_0.5)O₃ ceramics were examined with a view to their exploitation for wireless communications. The La(Mg_0.5−xCa_xSn_0.5)O₃ ceramics were prepared by the conventional solid-state method with various sintering temperatures. The La(Mg_0.5−xCa_xSn_0.5)O₃ ceramics contained Ca₂SnO_4, CaSnO₃, and La₂O₃. The amount of Ca₂SnO₄ increased with increasing sintering temperature. However, the relative amount of CaSnO₃ decreased with increasing sintering temperature. An apparent density of 6.52 g/cm³, a dielectric constant (ε_r) of 20.2, a quality factor (Q×f) of 80,500 GHz, and a temperature coefficient of resonant frequency (τ_f) of −79 ppm/°C were obtained for La(Mg_0.4Ca_0.1Sn_0.5)O₃ ceramics that were sintered at 1500 °C for 4 h.     

Synthesis and characterization of Sr(ZnZr)x Fe12−2xO19-PANI composites

Strontium zinc zirconium hexaferrites/polyaniline (Sr(ZnZr)_xFe_12−2xO₁₉-PANI, x=0, 0.5, 1.0) composites were synthesized by oxidative chemical polymerization of aniline in the presence of ammonium peroxydisulfate (APS). The structure and morphology of the product was characterized by FTIR, TGA and SEM. The particle size of the core material was found to be about 250-500 nm. After coating with polyaniline, the particle size of Sr(ZnZr)_0.5Fe₁₁O₁₉-PANI composites grew upto 0.5-1.0 μm. XRD of the ferrites indicated that the structure of the core materials is hexagonal, with lattice constants around 5.886-5.885 Å. It was found that the saturation magnetization (M_S) and coercivity (H_C) for Sr(ZnZr)_xFe_12−2xO₁₉-PANI composites decreased after polyaniline coating. The composite under applied magnetic field, exhibited ferromagnetic hysteretic loops with high saturation magnetization (M_S=18.9-3.8 emu/g) and coercivity (H_C=3850.0-583.91 Oe).     

Magnetic properties of off-stoichiometric R2Co3Zn14 (R=Y,Gd) single crystals

Single crystal X-ray diffraction data indicate that the R₂Co₃Zn₁₄ (R=Gd, Y) phase crystallizes non-stoichiometrically with a mixed occupancy of Co/Zn atoms on the 12-coordinated transition metal site and one of the three zinc sites. The crystals are rhombohedral with R-3m space group. Magnetization measurements provide no evidence of localized 3d electron moment in Y₂Co_2.3Zn_14.7 which is non-magnetic down to 1.8 K. Thermodynamic and transport measurements on two Gd₂Co_3+xZn_14−x crystals reveal that the extra cobalt influences temperature below which the samples enter into an antiferromagnetic state: T_N=31.5(3) K for Gd₂Co₃Zn₁₄ and 28(1) K for Gd₂Co_4.2Zn_12.8. A lower magnetic ordering temperature of T_mag=6.0(2) K is common in both Gd samples.     

Structural and magnetic properties of Mn3−xCdxTeO6 (x=0, 1, 1.5 and 2)

Mn₃TeO₆ exhibits a corundum-related A₃TeO₆ structure and a complex magnetic structure involving two magnetic orbits for the Mn atoms [Ivanov et al., 2011 [3]]. Mn_3−xCd_xTeO₆ (x=0, 1, 1.5, and 2) ceramics were synthesized by solid state reaction and investigated using X-ray powder diffraction, electron microscopy, and calorimetric and magnetic measurements. Cd²⁺ replaces Mn²⁺ cations without greatly affecting the structure of the compound. The Mn and Cd cations were found to be randomly distributed over the A-site. Magnetization measurements indicated that the samples order antiferromagnetically at low temperature with a transition temperature that decreases with increasing Cd doping. The nuclear and magnetic structure of one specially prepared ¹¹⁴Cd containing sample: Mn_1.5¹¹⁴Cd_1.5TeO₆, was studied using neutron powder diffraction over the temperature range 2-295 K. Mn_1.5¹¹⁴Cd_1.5TeO₆ was found to order in an incommensurate helical magnetic structure, very similar to that of Mn₃TeO₆ [Ivanov et al., 2011 [3]]. However, with a lower transition temperature and the extension of the ordered structure confined to order 240(10) Å.     

Epitaxial growth and structural characterization of Pb(Fe1/2Nb1/2)O3 thin films

We have grown lead iron niobate thin films with composition Pb(Fe_1/2Nb_1/2)O₃ (PFN) on (0 0 1) SrTiO₃ substrates by pulsed laser deposition. The influence of the deposition conditions on the phase purity was studied. Due to similar thermodynamic stability spaces, a pyrochlore phase often coexists with the PFN perovskite phase. By optimizing the kinetic parameters, we succeeded in identifying a deposition window which resulted in epitaxial perovskite-phase PFN thin films with no identifiable trace of impurity phases appearing in the X-ray diffractograms. PFN films having thicknesses between 20 and 200 nm were smooth and epitaxially oriented with the substrate and as demonstrated by RHEED streaks which were aligned with the substrate axes. X-ray diffraction showed that the films were completely c-axis oriented and of excellent crystalline quality with low mosaicity (X-ray rocking curve FWHM?0.09°). The surface roughness of thin films was also investigated by atomic force microscopy. The root-mean-square roughness varies between 0.9 nm for 50-nm-thick films to 16 nm for 100-nm-thick films. We also observe a correlation between grain size, surface roughness and film thickness.