Charge ordering modulations in a Bi_0.4Ca_0.6MnO₃ film with a thickness of 110 nm

The low temperature sample stage in a transmission electron microscope is used to investigate the charge ordering behaviours in a Bi 0.4 Ca 0.6 MnO 3 film with a thickness of 110 nm at 103 K.Six different types of superlattice structures are observed using the selected-area electron diffraction(SAED) technique,while three of them match well with the modulation stripes in high-resolution transmission electron microscopy(HRTEM) images.It is found that the modulation periodicity and direction are completely different in the region close to the Bi 0.4 Ca 0.6 MnO 3 /SrTiO 3 interface from those in the region a little further from the Bi 0.4 Ca 0.6 MnO 3 /SrTiO 3 interface,and the possible reasons for this are discussed.Based on the experimental results,structural models are proposed for these localized modulated structures.     

Improved multiferroic properties of La-doped 0.6BiFeO3—0.4SrTiO3 solid solution ceramics

The 0.6（Bi1-xLax）FeO 3-0.4SrTiO 3（x = 0,0.1） multiferroic ceramics are prepared by a modified Pechini method to study the effect of substitution of SrTiO3 and La in BiFeO3.The X-ray diffraction patterns confirm the single phase characteristics of all the compositions each with a rhombohedral structure.The magnetic properties of the ceramics are significantly improved by a solid solution with SrTiO3 and substitution of La.The values of the dielectric constant ε r and loss tangent tan δ of all the samples decrease with increasing frequency and become constant at room temperature.The La-doped 0.6BiFeO3-0.4SrTiO3 ceramics exhibit improved dielectric and ferroelectric properties,with higher dielectric constant enhanced remnant polarization（Pr） and lower leakage current at room temperature.Compared with a anti-ferromagnetic BiFeO3 compound,the 0.6（Bi0.9La0.1）FeO3-0.4SrTiO3 sample shows the optimal ferromagnetism with remnant magnetization M r ～ 0.135 emμ/g and ferroelectricity with Pr ～ 5.94 μC/cm 2 at room temperature.     

Influence of the Multiplet Levels of Sm3+ and Eu3+ on the Heat Capacity of Multiferroic BiFeO3

Physics of the Solid State - The temperature dependence of the heat capacity of multiferroics BiFeO3, Bi0.90Sm0.10FeO3, and Bi0.90Eu0.10FeO3 has been investigated. It is found that the substitution...     

Homogeneous interface-type resistance switching in Au/La_0.67Ca_0.33MnO₃/SrTiO₃/F:SnO₂ heterojunction memories

La0.67Ca0.33MnO3 thin films are fabricated on fluorine-doped tin oxide conducting glass substrates by a pulsed laser deposition technique with SrTiO3 used as a buffer layer. The current-voltage characteristics of the heterojunctions exhibit an asymmetric and resistance switching behaviour. A homogeneous interface-type conduction mechanism is also reported using impedance spectroscopy. The spatial homogeneity of the charge carrier distribution leads to field- induced potential-barrier change at the Au-La0.67Ca0.33MnO3 interface and a concomitant resistance switching effect. The ratio of the high resistance state to the low resistance state is found to be as high as 1.3 × 10 4 % by simulating the AC electric field. This colossal resistance switching effect will greatly improve the signal-to-noise ratio in nonvolatile memory applications.     

Dielectric, magnetic and magnetoelectric properties of La and Nb codoped bismuth ferrite

Single-phase Bi(0.80)La(0.20)FeO(3) (BLFO) and Bi(0.80)La(0.20)Fe(1-x)Nb(x)O(3) (BLFNO) samples were prepared in order to study the dielectric, magnetic and magnetoelectric properties of La and Nb codoped BiFeO3. Rietveld refinement of x-ray diffraction patterns of La and Nb codoped samples has been performed using the R3c space group. Magnetic hysteresis loops revealed that codoping can effectively increase the spontaneous magnetization due to change in the bond angle of Fe-O-Fe as a result of distortion created by the Nb5+ doping. Magnetic field-induced relative change of the dielectric constant for BLFO and BLFNO samples is a signature of magnetoelectric coupling.     

The phase transition,hygroscopicity, and thermal expansion properties of Yb_2-xAl_xMo₃O₁₂

Materials with the formula Yb 2-xAlxMo3O12(x =0.1, 0.2, 0.3, 0.4, 0.5, 0.7, 0.9, 1.0, 1.1, 1.3, 1.5, and 1.8) were synthesized and their structures, phase transitions, and hygroscopicity investigated using X-ray powder diffrac- tion, Raman spectroscopy, and thermal analysis. It is shown that Yb2-xAlxMo3O12 solid solutions crystallize in a single monoclinic phase for 1.7 ≤ x ≤ 2.0 and in a single orthorhombic phase for 0.0 ≤ x ≤ 0.4, and exhibit the characteristics of both monoclinic and orthorhombic structures outside these compositional ranges. The monoclinic to orthorhombic phase transition temperature of Al2Mo3O12 can be reduced by partial substitution of Al 3+ by Yb3+, and the Yb2-x AlxMo3O12 (0.0 < x ≤ 2.0) materials are hydrated at room temperature and contain two kinds of water species. One of these interacts strongly with and hinders the motions of the polyhedra, while the other does not. The partial substitution of Al3+ for Yb3+ in Yb2Mo3O12 decreases its hygroscopicity, and the linear thermal expansion co- efficients after complete removal of water species are measured to be 9.1×10 6 /K, 5.5×10 6 /K, 5.74×10 6 /K, and 9.5 × 10 6 /K for Yb1.8 Al0.2 (MoO4)3 , Yb1.6Al0.4 (MoO4 )3, Yb0.4 Al1.6 (MoO4)3 , and Yb 0.2Al1.8 (MoO4)3 , respectively.     

Interface-related switching behaviors of amorphous Pr_0.67Sr_0.33MnO₃-based memory cells

The resistive switching properties in amorphous Pr0.67Sr0.33MnO3 films deposited by pulsed laser deposition are investigated.Reproducible and bipolar counter-8-shape and 8-shape switching behaviours of Au/Pr0.67Sr0.33MnO3 /F:SnO2 junctions are obtained at room temperature.Dramatically,the coexistence of two switching polarities could be reversibly adjusted by an applied voltage range.The results allocated those two switching types to areas of different defect densities beneath the same electrode.The migration of oxygen vacancies and the trapping effect of electrons under an applied electric field play an important role.An interface-effect-related resistance switching is proposed in an amorphous Pr0.67Sr0.33MnO3-based memory cell.     

Effect of the la concentration on the dielectric and optical properties of the transparent ferroelectric ceramics 75PbMg<Subscript>1/3</Subscript>Nb<Subscript>2/3</Subscript>O<Subscript>3</Subscript>-25PbTiO<Subscript>3</Subscript>

The dielectric, optical, and electro-optical properties of transparent ferroelectric ceramics 75PbMg_1/3Nb_2/3O₃-25PbTiO₃ (75PMN-25PT) with different La concentrations (2, 3, and 4 at %) have been studied. It has been shown that all the samples under study undergo a diffuse phase transition, and the degree of diffuseness increases with an increase in the La concentration. The temperature dependences of the optical transmission in different regimes of applying an electric field and the quadratic electro-optical effect have been investigated. It has been found that, at room temperature, the maximum electro-optical effect is observed in the ceramics with a La concentration of 3 at %. This is explained by the fact that the measurement temperature is close to the Vogel-Fulcher temperature at which the minimum electric field is required to induce ferroelectric states from the relaxor phase.     

Nonlinear dielectric response of epitaxial Ba<Subscript>0.6</Subscript>Sr<Subscript>0.4</Subscript> TiO<Subscript> 3</Subscript> thin films

Based on Landau-Devonshire (LD)-type phenomenological thermodynamic theory, the electric field dependence of the dielectric properties of tetragonal single-domain barium strontium titanate(Ba_1-xSr_xTiO₃) films on cubic substrates is theoretically investigated by taking into account the high order terms of the polarization. At room temperature, the nonlinear dielectric responses of epitaxial Ba_0.6Sr_0.4TiO₃ films are provided by adjusting the film thickness and growth temperature. The strong nonlinearity of relative dielectric constant and pyroelectric coefficient are attained around critical film thickness on MgO (69 nm) and LaAlO₃ (132 nm) substrates or critical growth temperature on MgO (337 °C) substrate with respect to epitaxy-induced lattice misfit and thermal stresses during deposition. This can be explained that small compressive stresses are effective to support high nonlinearity of dielectric constant and pyroelectric coefficient for Ba_0.6Sr_0.4TiO₃ films irrespective of whether they are on compressive substrate or tensile substrate. It is also predicted that a large tunability may be achieved by altering processing conditions, such as the film thickness and growth temperature for different substrates. Our theoretical results are in good agreement with the experimental data reported in literature.     

Evolution of the 251 cm^-1 infrared phonon mode with temperature in Ba_0.6K_0.4Fe₂As₂

The far-infrared optical reflectivity of an optimally doped Ba1-xKxFe2As2(x =0.4) single crystal is measured from room temperature down to 4 K. We study the temperature dependence of the in-plane infrared-active phonon at 251 cm-1 . This phonon exhibits a symmetric line shape in the optical conductivity, suggesting that the coupling between the phonon and the electronic background is weak. Upon cooling down, the frequency of this phonon continuously increases, following the conventional temperature dependence expected in the absence of a structural or magnetic transition. The intensity of this phonon is temperature independent within the measurement accuracy. These observations indicate that the structural and magnetic phase transition might be completely suppressed by chemical doping in the optimally doped Ba0.6K0.4Fe2As2 compound.     

高温原位拉曼光谱研究BiFeO3陶瓷反应烧结相变过程

多铁性材料BiFeO3(BFO)由于具有潜在的磁电耦合效应而备受关注,但纯相陶瓷的制备始终是一个难点,部分原因在于对其反应烧结相变规律的认识尚不充分。高温原位拉曼光谱技术(HT-Raman)是表征复杂的固体相变及反应的有力手段。首次利用HT-Raman,研究了不同配比(1∶1, 1.03∶1和1.05∶1)的Bi2O3-Fe2O3在不同升降温速率(10和100℃·min-1)下的反应烧结相变过程,以及降温时反应产物的收缩效应。结果表明:Bi2O3-Fe2O3反应烧结生成BiFeO3的过程中,会产生中间过渡相Bi2Fe4O9和Bi25FeO39∶Bi2O3-Fe2O3配比为1.03∶1、升降温速率较快时,产物中杂相含量最少,可见Bi过量及较快的升降温速率能有效抑制杂相的生成。降温过程中,发现BFO的A1-1峰位随着温度降低发生蓝移,且二者呈良好的线性关系,这说明降温过程中BFO仅因温度变化产生晶格收缩,并没有结构相变。此外,还利用二维X射线衍射(2D-XRD)及背散射电子衍射(EBSD),表征了烧结产物的相组成及形貌。XRD结果也显示Bi过量时杂相含量较少,与拉曼结果一致。结合2D-XRD和EBSD的结果可知, Bi过量时烧结产物晶粒尺寸较大且均一,可见快速升降温有利于晶粒的成核与生长。研究结果可帮助进一步认清反应烧结规律,并指导纯相BiFeO3基陶瓷的制备。     

Structural, spectroscopic, magnetic and electrical characterization of Ca-doped polycrystalline bismuth ferrite, Bi(1-x)Ca(x)FeO(3-x/2) (x ≤ 0.1)

The crystal structure and physical properties of multiferroic polycrystalline Ca(2+)-doped BiFeO(3) samples have been investigated. The present experimental investigation suggests that Bi(1-x)Ca(x)FeO(3-x/2) (x ≤ 0.1) can be considered as a solid solution between BiFeO(3) and CaFeO(2.5). The oxidation state of Fe in these materials is + 3 and charge balance occurs through the creation of oxygen vacancies. For each composition, two structural phase transitions are revealed as anomalies in the variable-temperature in situ x-ray diffraction data which is consistent with the well-established high-temperature structural transformation in pure BiFeO(3). All compositions studied show antiferromagnetic behaviour along with a ferromagnetic component that increases with Ca(2+) doping. The resistivities of the Bi(1-x)Ca(x)FeO(3-x/2) samples at room temperature are of the order of 10(9) Ω cm and decrease with increasing Ca(2+) content. Arrhenius plots of the resistivity show two distinct linear regions with activation energies in the range of 0.4-0.7 and 0.03-0.16 eV. A correlation has been established between the critical temperatures associated with the structural phase transitions and the multiferroic properties. A composition of x = 0.085 is predicted to show maximum magneto-electric coupling.     

The doping effects of BiFe_1-xCo_xO₃（x=0.0-0.8） in layered perovskite Bi₄Ti₃O₁₂ ceramics

Bi5Fe1-xCoxTi3O15(x = 0.0, 0.2, 0.4, 0.5, 0.6, and 0.8) multiferroic ceramics are synthesized in two steps using the solid state reaction technique. X-ray diffraction patterns show that the samples have four-layer Aurivillius phases. At room temperature (RT), the samples each present a remarkable coexistence of ferromagnetism (FM) and ferroelectricity (FE). The remnant polarization (2P r ) reaches its greatest value of 14 μC/cm 2 at x = 0.6. Remnant magnetization (2M r ) first increases and then decreases, and the greatest 2M r is 7.8 menu/g when x = 0.5. The magnetic properties for x = 0.4 are similar to those for x = 0.6, indicating that the magnetic properties originate mainly from the coupling between Fe 3+ and Co 3+ ions, rather than from their own magnetic moments.     

Dielectric and polar order behaviors of BaTiO<Subscript>3</Subscript>-Bi(Mg<Subscript>1/2</Subscript>Ti<Subscript>1/2</Subscript>)O<Subscript>3</Subscript> ceramics

New perovskite solid solution ceramics of (1−x)BaTiO₃-xBi(Mg_1/2Ti_1/2)O₃ ((1−x)BT-xBMT, x≤0.09) were synthesized by the solid-state reaction technique. X-ray diffraction studies have revealed a stable single perovskite structure for all samples. Dielectric measurements were carried out at different frequencies and temperatures. The polarization evolutions with temperatures were measured to investigate the ferroelectric properties. All the compositions show features of ferroelectrics with diffuse phase transition, though the temperature T _m of their dielectric constant maximum ε _m is frequency dependent. The dielectric constant peak ε(T) of (1−x)BT-xBMT ceramics become broad with increasing BMT content. During the temperature range of ε(T) peak summit, (1−x)BT-xBMT ceramics present quasi-linear dielectric phenomenon under high electric field with very high dielectric constant.     

Spectroscopic characterization of Yb:Sc₂O₃ transparent ceramics

Yb:Sc2O3 transparent ceramics are fabricated by a conventional ceramic process and sintering in H2 atmosphere. The room-temperature spectroscopic properties are investigated, and the Raman spectrum shows an obvious vibration characteristic band centred at 415 cm 1 . There are three broad absorption bands around 891, 937, and 971 nm, respectively. The strongest emission peak is centred at 1.04 μm with a broad bandwidth (11 nm) and an emission cross-section of 1.8×10 20 cm 2 . The gain coefficient implies a possible laser ability in a range from 990 nm to 1425 nm. The energy-level structure shows that Yb:Sc 2 O 3 ceramics have large Stark splitting at the ground state level due to their strong crystal field. All the results show that Yb:Sc2O3 transparent ceramics are a promising material for short pulse lasers.     

Bi4-xLax Ti3O12-SrBi4-y Lay Ti4O15共生结构铁电材料拉曼光谱研究

在常温下 ,对La掺杂共生结构铁电陶瓷Bi4 -xLaxTi3O1 2 _SrBi4 -yLayTi4 O1 5[BLT_SBLT(x y) ,x y =0 0 0 ,0 2 5 ,0 5 0 ,0 75 ,1 0 0 ,1 2 5 ,1 5 0 ]进行了拉曼光谱研究 .结果表明 ,在掺杂量低于 0 5 0时 ,La只取代类钙钛矿层中的Bi,当掺杂量高于 0 5 0后 ,部分La开始进入 (Bi2 O2 ) 2 层 .La取代 (Bi2 O2 ) 2 层中的部分Bi以后 ,(Bi2 O2 ) 2 层结构发生变化 ,原有的绝缘层和空间电荷库的作用减弱 ,导致材料剩余极化下降 .La掺杂量增至 1 5 0时 ,样品出现弛豫铁电性 ,这与 30cm- 1 以下模的软化相对应 ,说明La掺杂可引起材料的结构相变     

PLD and RF-PLD synthesis of Ba<Subscript>0.6</Subscript>Sr<Subscript>0.4</Subscript>TiO<Subscript>3</Subscript> ferroelectric thin films for electrically controlled devices

Ba_0.6Sr_0.4TiO₃ (BST) bulk ceramic synthesized by solid state reaction was used as target for thin films grown by pulsed laser deposition (PLD) and radiofrequency beam assisted PLD (RF-PLD). The X-ray diffraction patterns indicate that the films exhibit a polycrystalline cubic structure with a distorted unit cell. Scanning Electron Microscopy investigations showed a columnar microstructure with size of spherical grains up to 150 nm. The capacitance–voltage (C–V) characteristics of the BST films were performed by applying a DC voltage up to 5 V. A value of 280 for dielectric constant and 12.5% electrical tunability of the BST capacitor have been measured at room temperature.     

An investigation into ultra-thin pseudobinary oxide (TiO<Subscript>2</Subscript>)<Subscript>x</Subscript>(Al<Subscript>2</Subscript>O<Subscript>3</Subscript>)<Subscript>1-x</Subscript> films as high-<Emphasis Type="Italic">k</Emphasis> gate dielectrics

As potential gate dielectric materials, pseudobinary oxide (TiO₂)_x(Al₂O₃)_1-x (0.1≤x≤0.6) films (TAO) were deposited on Si (100) substrates by pulsed-laser deposition method and studied systematically via various measurements. By a special deposition process, including two separate steps, the TAO films were deposited in the form of two layers. The first layer was deposited at room temperature and the second layer was completed at the substrate temperature of 400 °C. Detailed data show that the properties of the TAO films are closely related to the ratio between TiO₂ and Al₂O₃. The existence of the first layer deposited at room temperature can effectively restrain the formation of the interfacial layer. And according to the results of X-ray photoelectron spectroscopy and high-resolution transmission electron microscopy performed on the films, no other information belonging to the silicon oxide could be observed. For the (TiO₂)_0.4(Al₂O₃)_0.6 film, the best result has been achieved among all samples and its dielectric constant is evaluated to be about 38. It is valuable for the amorphous TAO film as one of the promising dielectric materials for high-k gate dielectric applications. PACS 77.55.+f; 73.40.Qv; 81.15.Fg     

Fabrication and dielectric properties of modified calcium (Pb<Subscript>0.75</Subscript>Ba<Subscript>0.25</Subscript>)(Zr<Subscript>0.7</Subscript>Ti<Subscript>0.3</Subscript>)O<Subscript>3</Subscript> ceramics

The aim of the present work is to report investigations concerning the influence of homovalent modificators on relaxor properties of PBZT 25/70/30 ceramics. The selection of the proper homovalent additive was very important. Literature reports as well as data taken from the periodic table indicated, that calcium ions substitute themselves for lead ions with high likelihood of occurrence. The investigations showed that the substitution significantly changed the microstructure of ceramics – with grains of calcium modified ceramics decreasing and density increasing. The XRD measurements indicate that the pure PBZT ceramics as well as calcium dopant were characterised by tetragonal structure with space group I4 /mmm. Addition of calcium leads to a slight decrease in the lattice constant and crystal structure. The calcium modification also changes the dielectric properties. The temperature characteristic of the dielectric constant achieved a broadened maximum at temperature T_m, which decreases with increasing Ca content. The properties typical for ferroelectric relaxors weaken with increasing calcium dopant.     

Temperature-dependent electrical, elastic and magnetic properties of sol-gel synthesized Bi(0.9)Ln(0.1)FeO3 (Ln = Nd, Sm)

This report details correlated electrical, mechanical and magnetic behaviour in BiFeO(3) ceramics doped with 10% Ln (Ln = Sm, Nd) ions on the Bi, or perovskite A, site and synthesized by a sol-gel method. The ceramics exhibit bulk piezoelectric and ferroelectric properties and clear ferroelectric domain patterns through piezoresponse force microscopy. Resonant ultrasound spectroscopy, dielectric spectroscopy and magnetometry studies show correlated magnetoelectromechanical behaviour and the existence of weak ferromagnetism for both compositions. An anomaly with simultaneous mechanical and magnetic signatures is discovered in both materials near room temperature, while previously reported transitions and anomalies are found to exhibit electro- and/or magnetomechanical coupling. Magnetism is significantly enhanced in the Sm doped sample, which is a promising multiferroic material.