Electric and magnetic behaviour in double doped La2/3＋4x/3Sr1/3-4x/3Mn1-xMgxO3

The double-doped La2/3＋4x/3Sr1/3-4x/3Mn1-xMgxO3 samples with fixed Mn^3＋/Mn^4＋ ratio equal to 2/1 are investigated by means of magnetism and transport measurements. Phase separation is observed at temperature higher than T^onset c for x = 0.10 and 0.15. For x = 0.10, rather strong phase separation induces drastic magnetic random potential and results in the localization of carriers. Thus, the varlable-range hopping process dominates. For other samples, there is no or only weak phase separation above T^onset c. Thus, thermal activation mechanism is responsible for the high temperature transport behaviour. For x = 0.20 and 0.25, unexpected AFM behaviour is observed at low temperature. All these results are well understood by considering the special role of the ＂double-doping＂.     

Phase transitions in La1−x CaxMnO3−x/2 manganites

The crystal structure parameters and magnetic and electrical properties of La_1?x Ca_xMnO_3?x/2 reduced manganites with 0≤x≤0.5 are established. These investigations contribute to the understanding of magnetic interactions in manganites without Mn⁴⁺ ions. It is found that these manganites show a long-range antiferromagnetic order up to x=0.09 and transform into spin glasses at 0.09<x≤0.35. The compositions in the range 0.35<x≤0.5 show a strong increase in the spontaneous magnetization and critical point associated with the appearance of spontaneous magnetization and can therefore be viewed as inhomogenious ferromagnets. The magnetic and crystal structure peculiarities of La_0.5Ca_0.5MnO_2.75 are established by the neutron diffraction method. The strongly reduced samples show a large magnetoresistance below the point where the spontaneous magnetization develops. The magnetic phase diagram of La_1?x Ca_xMnO_3?x/2 is established by magnetization measurements. The magnetic behavior is interpreted assuming that the Mn³⁺-O-Mn³⁺ magnetic interaction is anisotropic (positive-negative) in the orbitally ordered phase and isotropic (positive) in the orbitally disordered phase. Introduction of the oxygen vacancies changes the magnetic interaction sign from positive to negative, thereby leading to a spin glass state in strongly reduced compounds. The results obtained reveal unusual features of strongly reduced manganites such as a large ferromagnetic component, a high magnetic ordering temperature, and a large magnetoresistance despite the absence of Mn³⁺-Mn⁴⁺ pairs. In order to explain these results, the oxygen vacancies are supposed to be ordered.     

Structural,dielectric and Raman scattering studies of (1− x )PbMg1/3Nb2/3O3– x PbTiO3 single crystals (0 ≤ x ≤ 0.38)

The X-ray powder diffraction, dielectric and Raman studies of the (1???x) PbMg_1/3Nb_2/3O₃–xPbTiO₃ (0?≤?x?≤?0.38) single crystals were performed in a wide temperature range. The results of these complementary studies revealed that with the increase in PT content, the relaxor behaviour transforms into the ferroelectric one and a sequence of different structural phase transitions occur. On the base of the structural and dielectric results the phase diagram was determined. In addition, these results were used as a base for the group theory analysis and interpretation of the observed Raman spectra. The Raman spectra were deconvoluted using the independent damped oscillator model. From this model the temperature dependencies of the wavenumbers, reduced intensities and line widths were determined.     

Kinetics of the phase transition in crystals PbIn1/2Nb1/2O3–PbMg1/3Nb2/3O3–x PbTiO3

The time dependences of the optical transmission, velocity of sound, and elastic constants at room temperature in a number of the [001]-oriented PbIn_1/2Nb_1/2O₃–PbMg_1/3Nb_2/3O_3–x PbTiO₃ single crystals with the compositions lying both far from the morphotropic phase boundary and in its vicinity have been investigated. The analysis of the data obtained has been carried out. It has been shown that, in all the studied crystals, phase transitions induced by an electric field occur in two stages: the first stage is an incubation period associated with a small increase in the polarization of a part of the sample being in the glass phase, and the second phase (after the incubation period τ) is a rapid increase in the polarization and the formation of a long-range order. It has been found that the time τ depends on how close is the temperature of measurements to the Vogel–Fulcher temperature T _f . The closer is the temperature of measurements to the Vogel–Fulcher temperature T _f , the weaker is the electric field required to be applied to the sample in order to induce a ferroelectric phase. It has been demonstrated that the phase state of the studied crystals is unstable, which manifests itself in a strong dependence of the incubation period τ on the time between the annealing of the sample and the beginning of the measurements. The stability of the phase state depends on the PbTiO₃ concentration: an increase in the concentration leads to a significant increase in the stability.     

Al,F-doped new perovskite lithium fast ion conductor Li3x La2/3−x□1/3−2x Ti1−y Al y O3−y F y (x=0.11)

Al, F-doped new perovskite lithium ion conductors (x=0.11) have been prepared by solid state reaction. It is found that a pure perovskite-structured phase with space group of P4mm(99) exits in the composition range of 0<y≤0.10. The sample with y=0.02 possesses the highest ionic conductivity of 1.06×10⁻³ S/cm at room temperature, and its decomposing voltage is 2.3 V. The factors affecting the conductivity of this system are discussed.     

La2/3Ca1/3Mn1-xCuxO3(x=4%)中的热弛豫现象

采用溶胶-凝胶法并经不同温度烧结而制备了名义组分为La2/3Ca1/3MnO3和La2/3Ca1/3Mn1-xCuxO3(x=4%)的系列样品,对它们进行了阻温关系测量.从试验结果观察到:在La2/3Ca1/3MnO3样品中,降温和升温测量的阻温曲线几乎完全重合,没有观察到明显的热弛豫现象.而在不同温度烧结的La2/3Ca1/3Mn1-xCuxO3(x=4%)样品中,阻温关系有明显的热弛豫现象.同时试验还观察到该热弛豫现象随样品烧结温度(颗粒尺寸)的增加逐渐消失.     

Ultrasonic evidence for short-range charge-ordering state in the low-doping regime of La1– x Sr x FeO3 (1/3 ≤ x ≤ 0.45)

The longitudinal ultrasonic velocity (V) and attenuation at a frequency of 10?MHz, as well as magnetization and resistivity, have been measured in the single-phase polycrystalline La_{1– x} Sr _x FeO₃ (x?=?1/3, 0.4, 0.45). Upon cooling down from high temperature, a slight softening in V and dramatic stiffening are observed in all samples, and this stiffening is coincided with a big attenuation peak. The relative increase of V is proportional to the Sr concentration, which implies that this anomaly is strongly correlated with Fe⁴⁺. However, no obvious changes in magnetic and electric properties are found at this temperature range. The analysis suggests that this feature may be caused by the Jahn–Teller effect of Fe⁴⁺ and correspond to the formation of short-range charge-ordering state.     

A longitudinal (1−x)Pb(Mg1/3Nb2/3)O3-xPbTiO3 single-crystal piezoelectric transformer

In present work, a simple longitudinal piezoelectric transformer with the size of 16-mm length, 2-mm width, and 2-mm thickness was designed and fabricated with relaxor ferroelectric single-crystal (1−x)Pb(Mg_1/3Nb_2/3)O₃-xPbTiO₃. Both the input and output parts utilized the ultrahigh longitudinal extensional electromechanical coupling coefficients. Compared to the Rosen-type transformer, this transformer has the advantages of smaller-size, simpler structure, no spurious vibration, and no acoustic mismatch between the input and output parts. The electrical measurement results indicated that under the resonance frequency of 114 kHz, a step-up voltage of about 30 can be obtained under a 10-MΩ load resistance, which agreed well with the calculated results. Besides, the frequency and load dependences of this transformer were also systematically studied and discussed. The results indicated that this transformer has potential applications in the small-size high-voltage amplifier and magnetoelectric/piezoelectric-transformer composite transducers.     

Temperature impedance spectroscopy of (1 − x)Na1/2Bi1/2TiO3-xLaMg1/2Ti1/2O3 solid solutions

The dielectric properties of (1 ? x)Na_1/2Bi_1/2TiO₃-xLaMg_1/2Ti_1/2O₃ solid solutions (0 ≤ x ≤ 0.4) were studied over the temperature range 650–1030 K by measuring the impedance spectra over the frequency range 25–10⁶ Hz. The Curie temperature T _C was determined as a function of the composition of the solid solutions. It is shown that T _C decreases linearly with increasing x. The temperature dependences of the dc component (σ_dc) and the polarization component (σ′_ac) of the real part of the conductivity are found. The activation energy for the conductivity σ_dc is shown to increase abruptly as the temperature increases above approximately 770 K for all solid solution compositions. Over the temperature range corresponding approximately to the region of existence of the tetragonal phase of Na_1/2Bi_1/2TiO₃, the polarization component σ′_ac of the solid solutions, as well as that of the pure compound, is anomalously high at low frequencies, which can be due to structural inhomogeneities.     

Crystallisation of magnetron sputtered amorphous Si1− x C x films (x = 1/3) studied by grazing incidence X-ray diffractometry

The crystallisation of amorphous Si_{1? x} C _x films (x = 1/3) produced via magnetron sputtering on silicon substrates was investigated. Grazing incidence X-ray diffractometry was used to analyse the crystalline precipitates obtained after annealing at temperatures between 1200°C and 1350°C. After annealing times of 15 h at 1200°C and 15 h at 1350°C, crystallisation of SiC is complete. The average crystallite size, d, was determined using the Scherrer equation. The rate constants for the initial growth of the crystallites were determined by straight line fits in the d ? t diagrams (t being the annealing time), which obey the Arrhenius law. The activation enthalpy of 4.0 ± 0.7 eV is, within error limits, the same as that found for the growth of silicon carbide crystallites in magnetron sputtered Si_{1? x} C _x films (x = 1/2).     

(La1-xYx)2/3Ca1/3MnO3(x=0.195)系统中的磁电阻行为

本文实验上探讨了 (La1-xYx)2/3Ca1/3MnO3(x=0.195)窄带系统不同磁场下的磁电阻行为.实验表明,最大磁电阻效应随磁场增加而实质性提高,例如,7T磁场下最大磁电阻效应高达1.8×105%.同时,我们在远远低于半导体-金属转变温度下也观察到明显的磁电阻效应.文中就磁电阻的起因进行了讨论,特别是基于最近提出的自旋-极化子模型,对实验数据进行了定量的比较性分析,结果表明,该模型虽能预言实验的主要特征,但理论预言随磁场增加而明显偏离实验.     

Dielectric properties and specific heat of the (1 ? x)PbNi1/3Nb2/3O3-xPbTiO3 ferroelectric ceramics

The temperature dependences of the permittivity and specific heat of a mixed (1 ? x)PbNi_1/3Nb_2/3O₃-xPbTiO₃ system (where x = 0.3, 0.4, 0.5) have been studied over a wide temperature range 290?C700 K. It has been shown that the composition with x = 0.3 undergoes a diffuse phase transition at T _m ?? 315 K. A specific heat anomaly that is characteristic of the phase transition has been revealed at T ?? 315 K in all the compositions under study.     

Electro-optic effect in SrTiO3 and Sr1–x Ca x TiO3 (x = 0.014)

The results of the investigation of the quadratic electro-optic effect in Sr_1–x Ca _x TiO₃ with x = 0.014 (SCT) and in nominally pure SrTiO₃ (STO) at room temperature in applied direct-current (dc) and alternating-current (ac) electric fields have been presented. It has been shown that the quadratic (in polarization) electro-optic coefficients of STO and SCT crystals coincide within the accuracy of the determination (±5%). It has been found that, in nominally pure STO measured in a dc electric field, there is a relaxation of the electro-optic effect with a relaxation time τ ≈ 30 s due to the formation of a space charge in the sample. No similar effect in SCT has been observed. A possible mechanism for the formation of a space charge in STO and SCT has been discussed.     

High-Pressure Synthesis of SeCu 1− x Zn x O 3 Perovskites

The SeCu 1 m x Zn x O 3 solid solution, with a distorted perovskite-type structure, has been synthesized under high pressures and temperatures. X-ray diffraction analysis indicates that the zinc ions occupy the copper sites, a solid solution being formed. It seems that high-pressure stabilises a small cation such as Se 4+ in the A site of the perovskite structure ABO 3 although the material is better described as formed by selenite anions \rm{SeO}_{3}^{-} and Cu 2+ /Zn 2+ cations.     

Heat capacity of multiferroics Bi1–x Pr x FeO3

Physics of the Solid State - The heat capacity of multiferroics Bi1–x Pr x FeO3 (0 ≤ x ≤ 0.20) has been studied in the temperature range 130–800 K. An insignificant...     

In替代(La2/3Ca1/3)(Mn(3-2x)/3In2x/3)O3体系巨磁电阻效应的研究

采用固态反应法制备了In替代的(La_2/3Ca_1/3)(Mn_(3-2x)/3In_2x/3)O₃(x=0.00，0.10，0.15)体系.通过测量其零场和1.6T磁场下样品的电阻-温度关系以及一定温度下磁电阻率与磁场的关系.发现随In³⁺替代量的增加其磁电阻峰和电阻峰均向低温方向移动，同时巨磁电阻效应减弱，磁电阻峰也展宽.这是由于In³⁺替代量的变化，引起 关键词：     

Phase transitions and magnetic-transport phenomena in the system La2/3Ba1/3(Mn1−x Cox)O3

A study is performed of the crystalline structure, magnetization, and magnetotransport properties of the system La_2/3Ba_1/3(Mn_1−x Co_x)O₃ with perovskite structure. It is shown that cubic solid solutions exist over the entire range of cobalt concentrations 0⩽x⩽1. Compositions with x⩽0.2 are ferromagnets with maximum resistance near T _C . Compositions with 0.2<x<0.4 manifest properties of inhomogeneous ferromagnets. Measurements of magnetic properties indicate the absence of long-range magnetic order in compositions with 0.5⩽x⩽0.9, which are probably spin glasses. The spontaneous magnetization of cobaltate (2μ _B per formula unit) corresponds to ferromagnetic ordering of the moments of the Co³⁺ and Co⁴⁺ ions found in the intermediate spin state. It is conjectured that the magnetoresistance consists of an extrinsic and an intrinsic contribution. The first arises as a result of intergrain transport of spin-polarized charge carriers, and the second, as a result of magnetic ordering near T _C . The magnetoresistance is essentially independent of the spontaneous magnetization and decreases abruptly as the cobalt concentration is increased with a corresponding transition from long-range to short-range magnetic order. Zh. éksp. Teor. Fiz. 116, 604–610 (August 1999)