Dielectric, magnetoelectric, structure, and dissipative properties and the Mössbauer effect in PbFe1/2Nb1/2O3 ceramics in wide frequency and temperature ranges

High-quality fine-grained ceramic samples of classical multiferroics PbFe_1/2Nb_1/2O₃ (PFN) were synthesized. Their dielectric, magnetoelectric, and magnetic characteristics, including the Mössbauer effect, were measured over wide ranges of temperatures (10–1000 K) and field frequencies (from 25 Hz to 1 MHz). The temperature dependence of the dielectric loss exhibits a maximum between 150 and 170 K, likely due to magnetic ordering. The dependence of ? on the magnetic field displays an anomalous increase near the Curie temperature (370 K) that rises to 40% upon heating.     

Phase separation with charge self-organization in the Tb0.95Bi0.05MnO3, Gd0.75Ce0.25Mn2O5, and Eu0.8Ce0.2Mn2O5 manganite multiferroics

New doped manganite multiferroics Tb_0.95Bi_0.05MnO₃, Gd_0.75Ce_0.25Mn₂O₅, and Eu_0.8Ce_0.2Mn₂O₅, which are semiconductors, have been grown and studied. The starting dielectric multiferroics TbMnO₃ and RMn₂O₅ (R = Gd and Eu) have close magnetic and ferroelectric ordering temperatures of 30–40 K. The crystals studied are multiferroics in which states with giant permittivity and ferromagnetism coexist at room temperature. An analysis of the dielectric properties suggests that, at temperatures T ≥ 180 K, these crystals undergo a phase separation involving dynamic periodic alternation of quasi-2D layers of mixed-valence manganese ions, a process accounting for the onset of charge-induced ferroelectricity. At low temperatures (T < 100 K), a small phase volume in the crystals is occupied by as-grown quasi-2D layers containing dopants and carriers. Most of the crystal volume is occupied by the carrier-free dielectric phase. Thermally activated hopping conduction involving carrier self-organization in the crystal matrix with ferroelectric frustrations drives a phase transition to the state of charge-induced ferroelectricity at T ∼ 180 K. Original Russian Text ? V.A. Sanina, E.I. Golovenchits, V.G. Zalesskiĭ, 2008, published in Fizika Tverdogo Tela, 2008, Vol. 50, No. 5, pp. 874–882.     

Na0.25K0.25Bi0.5TiO3无铅压电陶瓷的介电特性研究

用固相反应法制备了Na_0.25K_0.25Bi_0.5TiO₃ (NKBT50)陶瓷，研究了该陶瓷在室温至400℃温度范围内的介电性能.发现该陶瓷的介电温谱与烧结气氛、极化状态有关.在空气中烧结的未极化样品在70℃附近存在介电和损耗峰，而极化后及在氧气氛中烧结的样品并不存在该介电、损耗峰.分析认为70℃的介电和损耗峰与氧空位形成的缺陷偶极子的极化弛豫有关.热激电流显示，陶瓷的去极化温度为225℃，与此相对应的介电、损耗峰也 关键词： 介电性能 氧空位 极化弛豫 钛酸铋钠钾     

Antiferromagnetic resonance and dielectric properties of rare-earth ferroborates in the submillimeter frequency range

The magnetoresonance and dielectric properties of a number of crystals of a new family of multiferroics, namely, rare-earth ferroborates RFe₃(BO₃)₄ (R = Y, Eu, Pr, Tb, Tb_0.25Er_0.75), are studied in the sub-millimeter frequency range (ν = 3–20 cm⁻¹). Ferroborates with R = Y, Tb, and Eu exhibit permittivity jumps at temperatures of 375, 198, and 58 K, respectively, which are caused by the R32 → P3₁21 phase transition. Antiferromagnetic resonance (AFMR) modes in the subsystem of Fe³⁺ ions are detected in the range of anti-ferromagnetic ordering (T < T _N = 30–40 K) in all ferroborates that have either an easy-plane (Y, Eu) or easy-axis (Pr, Tb, Tb_0.25Er_0.75) magnetic structure. The AFMR frequencies are found to depend strongly on the magnetic anisotropy of a rare-earth ion and its exchange interaction with the Fe subsystem, which determine the type of magnetic structure and the sign and magnitude of an effective anisotropy constant. The basic parameters of the magnetic interactions in these ferroborates are found, and the magnetoelectric contribution to AFMR is analyzed.     

Comparison of optical properties between Wurtzite and zinc-blende Ga0.75Al0.25N

In order to compare optical properties of Wurtzite and zinc-blende Ga_0.75Al_0.25N and lay a foundation for preparation of Ga_0.75Al_0.25N photocathodes, absorption coefficient, complex refractive index, dielectric function, reflectivity and loss function of Ga_0.75Al_0.25N in two forms are calculated using first principle based on density functional theory. Results show that zinc-blende Ga_0.75Al_0.25N owns smaller band gap than Wurtzite Ga_0.75Al_0.25N, and its conduction band is more broad. The optical properties differences of two structures mainly occur at the range of 8.6–26.0 eV. The peaks of imaginary dielectric function and absorption curves are at higher energy point for zinc-blende Ga_0.75Al_0.25N. The highest absorption of Wurtzite Ga_0.75Al_0.25N is 351386.171/cm at 11.05 eV, which is smaller than zinc-blende of 437809.895/cm at 13.84 eV. The average reflectivity of Wurtzite Ga_0.75Al_0.25N is lower than that of zinc-blende Ga_0.75Al_0.25N. The results are conductive for designing component structures of Ga_0.75Al_0.25N photocathodes.     

Giant dielectric response and polaronic hopping in charge-ordered Nd1.75Sr0.25NiO4 ceramics

The structure and dielectric properties of charge-ordered Nd_1.75Sr_0.25NiO₄ ceramics are presented. The giant dielectric constant about 30 000 is observed in the present ceramics even the frequency is up to 5 MHz. There are three dielectric relaxations at the curve of temperature dependence of dielectric constant in the considered temperature range. Based on the comparison of activation energies of dielectric relaxation and electrical conductivity, the giant dielectric response should be attributed to the bulk factor, that is, thermally activated small polaronic hopping in the present ceramics, while the dielectric relaxation around room temperature should cause from the effect of grain boundaries.     

Dielectric response of Ba0.75Sr0.25TiO3 epitaxial films to electric field and temperature

The structure and dielectric parameters of the intermediate ferroelectric layer in the (001)SrRuO₃ ∥ (100)Ba_0.75Sr_0.25TiO₃ ∥ (001)SrRO₃ heterostructure grown by laser ablation on (001)La_0.294Sr_0.706Al_0.647Ta_0.353O₃ were studied. Tensile mechanical stresses accounted for the polar axis in the ferroelectric, being oriented predominantly parallel to the substrate plane. The remanent polarization in the Ba_0.75Sr_0.25TiO₃ layer increased approximately linearly with decreasing temperature in the interval 320–200 K. The real part of the dielectric permittivity of the intermediate ferroelectric layer reached a maximum ?′/?₀=4400 at T _M≈285 K (f=100 kHz). The narrow peak in the temperature dependence of the dielectric loss tangent for the Ba_0.75Sr_0.25TiO₃ ferroelectric layer, observed for T<T _M, shifted toward lower temperatures with decreasing frequency and increasing bias voltage applied to the electrodes.     

Magnetoelectric effects in multiferroic Y-type hexaferrites Ba(0.3)Sr(1.7)CoxMg(2-x)Fe(12)O(22)

Y-type hexaferrites with tunable conical magnetic structures are promising single-phase multiferroics that exhibit large magnetoelectric effects. We have investigated the influence of Co substitution on the magnetoelectric properties in the Y-type hexaferrites Ba(0.3)Sr(1.7)CoxMg(2-x)Fe(12)O(22)(x = 0.0, 0.4, 1.0, 1.6). The spin-induced electric polarization can be reversed by applying a low magnetic field for all the samples. The magnetoelectric phase diagrams of BaBa(0.3)Sr(1.7)CoxMg(2-x)Fe(12)O(22) are obtained based on the measurements of magnetic field dependence of dielectric constant at selected temperatures. It is found that the substitution of Co ions can preserve the ferroelectric phase up to a higher temperature, and thus is beneficial for achieving single-phase multiferroics at room temperature.     

Increase in the electric polarization in a multiferroic induced by the flexoelectric effect

It has been shown that the electric polarization in multiferroic crystals exhibiting the magnetoelectric effect can be increased in the presence of the flexoelectric effect. The polarization in manganite-based perovskite-like multiferroics can be increased if the long axes of the rhombohedra formed upon the rhombohedral distortion of MnO₆ octahedra can undergo fan spatial rotations, leading to the flexoelectric effect.     

Optical constants of layered structured Ga0.75In0.25Se crystals from the ellipsometric measurements

We have carried out the spectroscopic ellipsometry measurements on Ga_0.75In_0.25Se single crystals in the 1.2–6.0 eV spectral range at room temperature. The optical constants, real and imaginary parts of the dielectric function, refractive index and extinction coefficient, were found as a result of analysis of ellipsometric data. The critical point analysis of the second derivative spectra of the dielectric function revealed four interband transition structures with critical point energy values of 3.19, 3.53, 4.10 and 4.98 eV. The results of the analysis were compared with those of the ellipsometric studies performed on GaSe which is the main constituent of the Ga_0.75In_0.25Se crystal. The obtained critical point energies are in good agreement with the energies of the GaSe crystal reported in the literature.     

Effect of gamma irradiation on the dielectric response of a Sr0.75Ba0.25Nb2O6 relaxor single crystal

The effect of gamma irradiation from a ⁶⁰Co source at a dose D = 4.75 × 10⁴ R on the dielectric properties of a Sr_0.75Ba_0.25Nb₂O₆ (SBN-75) single crystal with a smeared phase transition is investigated in the low-and infralow-frequency ranges. The behavior of the dielectric response is compared with the behavior of the depolarization currents induced in unirradiated and irradiated samples of the Sr_0.75Ba_0.25Nb₂O₆ compound. The observed changes in the electrical properties of the Sr_0.75Ba_0.25Nb₂O₆ single crystal in the temperature range of the smeared phase transition are associated with the stabilization of the ferroelectric phase of the material in an internal bias field generated by radiation-induced defects.     

磁电耦合对于多铁性材料CdCr2S4介电性质的影响

多铁性材料CdCr2S4在磁相变温度附近的介电异常现象,为一定温度区间内共存的弛豫铁电性和铁磁性提供了磁电耦合的依据.基于Heisenberg模型,我们计算出了最近邻磁自旋对关联,最近邻磁自旋对关联通过磁电耦合修正阻碍极化翻转的激活能大小,使得被冻结在某一特定方向上的极化又能够在其它的可能取向上进行热翻转,改变了极化弛豫时间的分布,从而导致了介电性质在磁相变温度附近发生介电异常.同时,考虑温度、测量频率和磁场对介电性质的影响,得出的结果和实验数据很好吻合.     

Antiferromagnet with an antisymmetry center in an external static magnetic field as a left-handed medium

Using the example of a tetragonal easy-axis antiferromagnet with a 4 _z ⁻2 _x ⁺ I ⁻ structure, the conditions have been determined under which both the negative refraction effect and the negative phase velocity effect are simultaneously possible in an external field orthogonal to the easy axis. The results are extended to multiferroics with the uniform magnetoelectric effect and to centrosymmetric magnets.     

First-principles study of structural,electronic, linear and nonlinear optical properties of Ga<Subscript>2</Subscript>PSb ternary chalcopyrite

Epitaxial magnetic oxide films can be reversibly strained on piezoelectric monocrystalline substrates of Pb(Mg_1/3Nb_2/3)_0.72Ti_0.28O₃(PMN-PT). The magnetic film/piezoelectric substrate sample that undergoes well-defined biaxial, nearly linear strains exceeding 0.1% provides a model system for the inspection of strain-dependent magnetic properties which are exploited in many two-phase magnetoelectric multiferroics. We review the properties of the applied pseudocubic PMN-PT(001) substrates, and summarize results for the strain-dependent magnetization of ferromagnetic manganites (La, A)MnO₃ (A = Sr or Ca).     

Spatially modulated antiferromagnetic structures in an easy-plane multiferroic

Possible types of spatially modulated periodic antiferromagnetic structures in a uniaxial rhombohedral multiferroic with BiFeO₃ crystal symmetry have been studied depending on the ratio of the uniaxial anisotropy and magnetoelectric interaction parameters. It has been shown that, along with symmetric cycloid antiferromagnetic structures with zero transverse component of the antiferromagnetism vector, there are changes in the antiferromagnetism vector direction with both right and left nonzero components of the antiferromagnetic moment, which are branched from the high-symmetry spatially modulated distribution. These solutions degenerate into a homogeneous state at a critical value of the normalized easy-plane anisotropy parameter. The existence of the found spatially inhomogeneous antiferromagnetic states with an incommensurate period can lead to additional features in magnetoelectric properties in multiferroics of the type under consideration near magnetic phase transitions in electric and magnetic fields.     

Dielectric investigations in Sr<Subscript>0.75</Subscript>Ba<Subscript>0.25</Subscript>Nb<Subscript>2</Subscript>O<Subscript>6</Subscript> relaxor ferroelectric thin films

The dielectric properties of Sr_0.75Ba_0.25Nb₂O₆ relaxor ferroelectric thin films were carefully analyzed. In contrast to bulk samples which present three distinct dielectric relaxation phenomena Sr_0.75Ba_0.25Nb₂O₆ thin films present only two of them. The suppression of the third anomaly can be mainly attributed to the narrow grain size distribution of nanograins and weak tensile strains imposed to the film from the substrate. The whole set of results point to the interpretation of a dielectric response characteristic of mesoscopic structure, which is composed of clusters and nanodomains.     

Phase formation and dielectric study of Bi doped BaTi0.75Zr0.25O3 ceramic

Perovskite types Ba_1−xBi_2x/3Ti_0.75Zr_0.25O₃ (with x = 0.0, 0.025, 0.05) ceramics have been prepared through solid state reaction route. The room temperature XRD study suggests that all the compositions have single phase cubic symmetry with space group Pm-3m. Temperature dependent dielectric studies of the ceramics have been investigated in the frequency range of 50 Hz–1 MHz. The densities of the samples are determined using Archimedes’ principle and found to be ∼98% of X-ray density. The dielectric study revealed diffuse phase transition of second order. A broad dielectric anomaly coupled with the shift of dielectric maxima toward a higher temperature with increasing frequency indicates the relaxor-type behavior in the ceramics. The diffusivity increases with increase in Bi contents in the studied composition range. The transition temperature decreases with increase in Bi contents due to the decrease in grain size.     

Magnetic and magnetoelectric properties of the Tb<Subscript>0.75</Subscript>Ho<Subscript>0.25</Subscript>Fe<Subscript>3</Subscript>(BO<Subscript>3</Subscript>)<Subscript>4</Subscript> ferroborate

Single crystals of the Tb_0.75Ho_0.25Fe₃(BO₃)₄ ferroborate have been grown by the group method from a solution–melt based on bismuth trimolybdate. The magnetic and magnetoelectric properties of the ferroborate single crystals have been investigated in the temperature range from 4.2 to 300 K and in magnetic fields up to 9 T. Magnetically, this material is an antiferromagnet with the Néel temperature T _N = 38.8 K and easy-axis anisotropy. The magnitude of the magnetoelectric polarization has been found to be more than 1.5–2.0 times greater than the sum of the polarizations induced by the magnetic field for the ferroborates TbFe₃(BO₃)₄ and HoFe₃(BO₃)₄ taken in the corresponding shares.     

Strain effects on multiferroic BiFeO3 films

The field of multiferroics has experienced a rapid progress resulting in the discovery of many new physical phenomena. BiFeO₃ (BFO) compound, which is one of the few room-temperature single-phase multiferroics, has contributed subsequently to this progress. As a result, significant review articles have been devoted specifically to this famous system. This chapter is dedicated to the strain effects on the structure stability and property changes of BFO thin films. It is a short and non-exhaustive topical overview that may be seen as an invitation for interested readers to go beyond. There is a very active and prolific research in this field and we apologize to the authors whose relevant work is not cited here. After a short introduction, we will thus review the effect of strain on BFO films by describing the consequences on the structure and the phase transitions as well as on polar, magnetic and magnetoelectric properties.     

Mechanical properties and electronic structure of TiC,Ti0.75W0.25C,Ti0.75W0.25C0.75N0.25, TiC0.75N0.25 and TiN

The first-principles calculations are performed to investigate the mechanical properties and electronic structure of TiC, Ti_0.75W_0.25C, Ti_0.75W_0.25C_0.75N_0.25, TiC_0.75N_0.25 and TiN. Density functional theory and ultrasoft pseudopotentials are used in this study. From the formation energy, it is found that nitrogen can increase the stability of TiC. The calculated elastic constants and elastic moduli of TiC compare favorably with other theoretical and experimental values. Tungsten and nitrogen are observed to significantly increase the bulk, shear and Young's modulus of TiC. Through the analysis of B/G and Cauchy pressure, tungsten can significantly improve the ductility of TiC. The electronic structure of TiC, TiN, Ti_0.75W_0.25C, Ti_0.75W_0.25C_0.75N_0.25, and TiC_0.75N_0.25 are used to describe nonmetal–metal and metal–metal bonds. Based on the Mulliken overlap population analysis, the hardness values of TiC, Ti_0.75W_0.25C, Ti_0.75W_0.25C_0.75N_0.25, TiC_0.75N_0.25 and TiN are estimated.