Effects of thickness on the infrared optical properties of Ba<Subscript>0.9</Subscript>Sr<Subscript>0.1</Subscript>TiO<Subscript>3</Subscript> ferroelectric thin films

Using infrared spectroscopic ellipsometry (IRSE), the optical properties of the Ba_0.9Sr_0.1TiO₃ (BST) ferroelectric thin films with different film thicknesses on Pt/Ti/SiO₂/Si substrates prepared by a modified sol-gel method have been investigated in the 2.5–12.6 m wavelength range. By fitting the measured ellipsometric parameter ( and ) data with a three-phase model (Air/BST/Pt) and the classical dispersion relation for the BST thin films, the optical constants and thicknesses of the thin films have been obtained. The average thickness of the single layer decreases with increasing film thickness. The refractive index of the BST films decreases with increasing thickness in the wavelength range 2.5–11 m, and increases with increasing thickness in the wavelength range 11–12.6 m. However, the extinction coefficient of the BST films monotonously decreases with increasing thickness. It is closely associated with the crystallinity of the thin films, the crystalline size effect and the influence of the interface layer. The absorption coefficient of the BST films with different thicknesses decreases with increasing thickness. PACS 77.55.+f; 78.20.Ci; 78.30.Am; 81.70.Fy; 81.40.Tv     

Influence of ambient oxygen pressure on the preferred orientation, microstructures, and dielectric properties of (Ba1-xSrx)TiO3 thin films with compositionally graded structures

Compositionally graded (Ba_1-xSr_x)TiO₃ (BST) thin films, with x decreasing from 0.25 to 0.0, were deposited on Pt(111)/Ti/SiO₂/Si(100) substrates by pulsed-laser ablation at 600 °C and under ambient oxygen pressures ranging from 50 to 400 mTorr. The influence of the ambient gas pressure on the preferred orientation, microstructures, and dielectric properties of compositionally graded BST films was investigated by X-ray diffraction, scanning electron microscopy, and dielectric frequency spectra, respectively. As the ambient oxygen pressure was increased, the preferred orientation evolved in the order: (100)+(110)(110)+(111) random orientation, and the surface roughness of the graded BST films also increased. The graded BST films deposited at high ambient oxygen pressures (300400 mTorr) exhibited a grainy structure with polycrystalline grains throughout the film thickness, whereas the graded films deposited at low ambient oxygen pressures (50200 mTorr) possessed a columnar structure. The evolution of the microstructure was ascribed to the different physical and chemical properties of the species that were incident onto the substrates at the various oxygen pressures. The dielectric properties of the graded BST films were dependent upon the ambient oxygen pressures. The graded BST films deposited at 200 mTorr exhibited the highest dielectric constant. PACS 77.55.+f; 77.22.Ch; 81.15.Fg     

Magnetic and electrical investigations of Fe<Subscript>85-x</Subscript>Co<Subscript>x</Subscript>B<Subscript>15</Subscript> metallic glasses

The magnetic and electrical properties of metallic glasses with the general formula Fe_85-xCo_xB₁₅ were investigated over a large temperature range to study their concentration-dependent physical parameters. All of the samples investigated (x=17,21,30, and 40) were soft ferromagnets with coercive fields H_c1 Oe and high Curie temperatures slightly above 1200 K. The temperature-dependent magnetization behaved irregularly, and exhibited hysteresis during heating and subsequent cooling through the Curie temperature. The variation of the magnetization with temperature demonstrates that one or more phase transformations (crystallization) occurred in the course of the heating. The electrical resistivities exhibited positive temperature coefficients and minima at temperatures below 50 K. We did not observe a nonmonotonic variation of the magnetic and electrical properties with a monotonic change of the Fe_85-xCo_xB₁₅ composition that would correlate with the earlier proposed formation of strong nanoclusters in the vicinity of particular stoichiometrically close Fe:Co ratios. The good soft magnetic characteristics make the Fe_85-xCo_xB₁₅ metal glasses promising candidates for engineering materials in inductive applications. PACS 71.23.Cq; 75.75.+a     

Iron-induced magnetic,transport and magnetoresistance behavior in Nd<Subscript>0.67</Subscript>Sr<Subscript>0.33</Subscript>MnO<Subscript>3</Subscript> epitaxial films

The effects of Fe doping on Mn site in the colossal magnetoresistive film, Nd_0.67Sr_0.33MnO₃ have been studied by preparing the series Nd_0.67Sr_0.33Mn_1-xFe_xO₃ (x=0,0.05 and 0.1). Upon doping, no structural changes have been found. However, the Curie temperature, the associated metal-to-insulator transition temperature and the magnetization decrease drastically with Fe doping. The resistivity in the paramagnetic regime for all the samples follows Emin–Holsteins theory of small polaron. The polaron activation energy, W_p and resistivity coefficient, A increase with Fe doping. This effect may be ascribed to the fact that upon Fe doping, the long-range ferromagnetic order is destroyed and, therefore, W_p is enhanced in the system. As compared to the La-based system, Fe doping has a stronger tendency to destabilize the long-range ferromagnetic order in the Nd-based system. Large MR (as high as 90%) observed in the epitaxial NSMFO film may be attributed to the good lattice-matching between the grown film and substrate. PACS 75.47.Gk; 75.47.Lx; 75.70.-i     

Structural and thermoelectric properties in (Sb<Subscript>1-x</Subscript>Bi<Subscript>x</Subscript>)<Subscript>2</Subscript>Te<Subscript>3</Subscript> thin films

We have investigated the structural and thermoelectric properties of (Sb_1-xBi_x)₂Te₃ thin films on CdTe(111)B. Analysis of X-ray diffraction patterns (–2 scans and rocking curves) of the films shows that they are of high quality and that they are well aligned with their (00.1) axis normal to the substrates. Measurements of the temperature-dependent thermoelectric power, resistivity, and Hall coefficient of the films were performed with respect to the binary composition, x. For the samples in the range 0.2<x<0.3, the room-temperature thermopower values were in the range 159–184 V/K, the room-temperature carrier concentrations were 3.93–5.13×10¹⁹ cm^-3, and the room-temperature mobilities were 24.6–64.0 cm²V^-1s^-1. PACS 72.20.Pa; 72.80.Jc; 73.6l.Le     

Subsolidus phase relation and crystal structure in the Pr<Subscript>1+x-y</Subscript>Ba<Subscript>2-x-z</Subscript>Ca<Subscript>y+z</Subscript>Cu<Subscript>3</Subscript>O<Subscript>7±δ</Subscript> system

The Pr_1+xBa_2-xCu₃O_7± solid solution was investigated by means of X-ray powder diffraction combined with Rietveld analysis. A Pr123 single phase could be synthesized under Pr-rich conditions by sintering at 950 °C in air. The solubility range of Pr_1+xBa_2-xCu₃O_7± solid solution is 0.08x0.80. The structure of Pr_1+xBa_2-xCu₃O_7± for 0.08x<0.30 is orthorhombic. The structure transforms into tetragonal for 0.30x0.80. To form the Pr123 single phase, the Ba sites in the Pr123 structure must have partial Pr ions, and the least amount is x=0.08. Ba ions cannot occupy the sites of Pr ions. In the Pr123 structure, Ca ions can replace Pr ions; the highest value is x=0.4 in the PrBa_2-xCa_xCu₃O_7± system under our experimental conditions. However, Ca ions cannot replace Ba ions. The ionic radius plays a more important role than the chemical properties in the substitution between Pr, Ba and Ca ions in the Pr123 structure. PACS 61.50.Ah; 64.70.Kb; 61.72.Ff     

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.     

Large magnetic entropy change in Nd<Subscript>2/3</Subscript>Sr<Subscript>1/3</Subscript>MnO<Subscript>3</Subscript>

In order to search for new materials for the application of magnetic refrigeration, the polycrystalline perovskite compound Nd_2/3Sr_1/3MnO₃ was prepared by a solid-state method. The dependence of the magnetization on the applied field and temperature was measured near the Curie temperature. In terms of Maxwells equation, the temperature dependence of the absolute value of the isothermal magnetic entropy change |S_M| at various applied fields from 1 T to 5 T was determined. The results showed that a large magnetic entropy change was observed in this compound. The maximum magnetic entropy change |S_M^max|can reach 3.25 J/kgK with an applied field of 1 T at the Curie temperature of 257.5 K, which equals that of Gd. At 5 T applied field, it is 7.57 J/kgK. Such good magnetocaloric properties make this compound a promising candidate for the application of magnetic refrigeration in the room-temperature range. PACS 74.25.Ha; 75.30.-m; 75.30.Sg; 75.50.-y; 75.60.-d     

Structure,transport and magnetic properties of the double perovskite (Ca<Subscript>2-2x</Subscript>Sr<Subscript>2x</Subscript>)FeMoO<Subscript>6</Subscript>

In this paper we report the results of a detailed investigation of the double perovskite (Ca_2-2xSr_2x)FeMoO₆ system. Chemical size effects on structural, electrical, and magnetic properties caused by the substitution of isovalent, larger Sr ions into the smaller Ca sites, resulting in (Ca_2-2xSr_2x)FeMoO₆, have been examined. The compounds crystallize in the monoclinic space group P2₁/n for 0.0x<0.2, the orthorhombic space group Pbnm for 0.2x<0.4, and the tetragonal space group I4/m and I4/mmm for x0.4. Examination of the resistivity of all compounds reveals a metallic behavior which is well described by a Tⁿ dependence except for x=1.0. These n values change from 1 to 2 as T decreases lower than T_c. This is indicative of a variation in the transport mechanism at T_c. The ferrimagnetic transition temperature T_c increases with increasing x from 318 (x=0.0) to 393 K (x=1.0). For all samples, the saturation magnetization at 82 K obeys M_s3.5_B/(formula unit), compared to a theoretical spin-only moment of 4_B/(formula unit) for a perfectly ordered compound. PACS 74.25.Fy; 74.25.Ha     

Manganese doping effects on interband electronic transitions,lattice vibrations,and dielectric functions of perovskite-type Ba<Subscript>0.4</Subscript>Sr<Subscript>0.6</Subscript>TiO<Subscript>3</Subscript> ferroelectric ceramics

The Ba_0.4Sr_0.6−x Mn _x TiO₃ (BSMT) ceramics with different Mn composition (from 1% to 10%) have been prepared via the conventional solid-state reaction sintering. The X-ray diffraction analysis shows that the ceramics are polycrystalline with the single perovskite phase. The lattice vibrations and optical properties have been investigated using Raman scattering, spectroscopic ellipsometry (SE), and infrared reflectance spectra. It was found that the optical bandgap for the BSMT ceramics is varied between 3.40 and 3.65 eV. The three first-order Raman-active phonon modes can be observed, and the frequency of the A ₁(LO₃)/E(LO) mode shows a blue shift of 8 cm⁻¹ with the Mn composition, which can be attributed to the distortion of the TiO₆ octahedron. With increasing Mn composition, the frequency of the infrared-active TO₄ mode decreases from 532 to 520 cm⁻¹, owing to the local variation of the lattice constant induced by the Mn incorporation. Moreover, the optical functions of the ceramics from the far-infrared to ultraviolet region are obtained based on the SE and reflectance spectra, which is useful for the potential applications in ferroelectric-based optoelectronic devices.     

High-temperature complex impedance and modulus spectroscopic studies of doped Na<Subscript>0.5</Subscript>Bi<Subscript>0.5</Subscript>TiO<Subscript>3</Subscript>-BaTiO<Subscript>3</Subscript> ferroelectric ceramics

Lead-free Na_0.5Bi_0.5TiO₃ (NBT) and (1 ? x)Na_0.5Bi_0.5TiO₃ + xBaTiO₃ with x = 0.1 and 0.2 (where x = 0.1 and 0.2 are named as NBT1 and NBT2, respectively), (1 ? y)Na_0.5Bi_0.5TiO₃ + yBa_0.925Nd_0.05TiO₃ with y = 0.1 and 0.2 (where y = 0.1 and 0.2 are named as NBT3 and NBT4, respectively)-based relaxor ferroelectric ceramics were prepared using the sol-gel method. The crystal structure was investigated by X-ray diffraction (XRD) at room temperature (RT). The XRD patterns confirmed the presence of the rhombohedral phase in all the samples. The electrical properties of the present NBT-based samples were investigated by complex impedance and the modulus spectroscopy technique in the temperature range of RT–600 °C. The AC conductivity was found to increase with the substitution of Ba²⁺ ions to the NBT sample whereas it significantly decreased with the addition of Nd³⁺ ions. The more anion vacancies in Ba-added samples and the lower anion vacancies in Nd-added samples were found to be responsible for higher and lower conductivities, respectively.     

Resonance-enhanced two-photon sum-frequency generation in NiO and KNiF<Subscript>3</Subscript>

Resonance-enhanced sum-frequency generation is introduced as a novel tool for investigation of magnetically ordered compounds. A tunable laser at frequency ₁ is used to excite an intermediate electronic transition resonantly while a second laser at frequency ₂ is used to scan the nonlinear spectrum at ₁+₂. The technique is particularly useful for investigation of centrosymmetric compounds since resonance enhancement at ₁ leads to large nonlinear signals even in the case of weakly allowed nonlinear processes. The technique is demonstrated on antiferromagnetic NiO and KNiF₃ and also shown to be useful for investigation of samples with large thickness or absorption. PACS 42.65.Ky; 78.20.Ls; 42.62.FiThis revised version was published in October 2004 with a correction to the name of Takuya Satoh.     

Effect of Mg doping on the structural and dielectric properties of strontium titanate ceramics

The influence of Mg incorporation into A- and B-sites of the perovskite lattice of SrTiO₃ ceramics on the structural, microstructural and low-frequency dielectric properties is studied in this work. Compounds in the systems Sr_1-xMg_xTiO₃ and SrTi_1-yMg_yO_3- were synthesised by a conventional solid-state method. The solid solubility of Mg is restricted to x<1% for A-site occupancy (Sr_1-xMg_xTiO₃) and to y<15% for B-site occupancy (SrTi_1-yMg_yO_3-). The lattice parameter is found to increase with Mg content for the SrTi_1-yMg_yO_3- system, while it is almost invariant in the Sr_1-xMg_xTiO₃ one. The dependence on the lattice-site occupancy is also verified for the grain growth of ceramics. For SrTi_1-yMg_yO_3- the average grain size markedly decreases with increasing Mg content. For Sr_1-xMg_xTiO₃ the inverse dependence is observed. Contrary to expectations, Mg does not induce ferroelectricity or relaxor-like behaviour in strontium titanate, located in either A- or B-sites of the SrTiO₃ lattice. Moreover, fitting the dielectric behaviour to Barretts law demonstrates that B-site doping drives the system away from the ferroelectric instability. In Mg-doped strontium titanate ceramics the dielectric permittivity and dielectric losses decrease. The results are discussed based on the correlation between cation-site occupancy, charge and chemical stoichiometry in both systems. PACS 61.10.Nz; 68.37.Hk; 68.37.Lp; 77.22.Ch; 77.22.Gm; 77.84.Dy     

Interaction of running gratings of the space charge and conductivity in photorefractive Bi<Subscript>12</Subscript>Si(Ti)O<Subscript>20</Subscript> crystals

The simultaneous excitation and nonlinear interaction of the space-charge and photoconductivity gratings are studied experimentally in photorefractive Bi₁₂SiO₂₀ and Bi₁₂TiO₂₀. The measurements are performed using the diffraction technique, which implies the illumination of the crystal by an oscillating interference pattern (_r=532 nm) along with the application of dc and ac electric fields and detection of the diffracted probe beam (_p=650 nm). Such illumination excites the running photoconductivity grating, which interacts with the ac component of the applied field giving rise to the space-charge wave. Being the eigenmode of the space-charge oscillations, this wave reveals itself as a low-frequency resonant maximum at the frequency-transfer function of the detected signal. The drift mobilities of electrons are estimated using the developed technique: =(1.1–1.4)×10^-2 cm²/Vs (Bi₁₂SiO₂₀, T=296–298 K) and =2.8×10^-3 cm²/Vs (Bi₁₂TiO₂₀, T=293 K). PACS 42.65.Sf; 42.70.Nq     

<Emphasis Type="Bold">(Ba,Sr)TiO</Emphasis><Subscript>3</Subscript> thin films grown by pulsed laser deposition with low dielectric loss at microwave frequencies

We report a method for producing BST films with consistently high figures of merit for tunable microwave applications. (Ba_1-x,Sr_x)TiO₃ (x=0.4, target doped with 1% W) thin films have been deposited using pulsed laser deposition onto (100)MgO substrates. Films were deposited at low partial pressures of oxygen (50 mTorr) at a substrate temperature of 730 °C. An analysis of the X-ray diffraction data indicates that the film has a nearly cubic structure, with the overall lattice parameter enlarged relative to the bulk material due to the presence of oxygen vacancies. A post-deposition anneal of the film in flowing oxygen (1000 °C for 6 h) resulted in a decrease in the lattice parameter while remaining nearly cubic. An analysis of the microwave dielectric properties (1–20 GHz) showed that the annealed film exhibited about 10% tunability for an applied bias field of 67 kV/cm with a dielectric Q(1/tan)>600. Investigation of the films by time-resolved confocal scanning optical microscopy (CSOM) has revealed that there is an out-of-plane polarization at zero applied field (E_DC=0). The results show that the paraelectric response is relatively insensitive to applied field, while the ferroelectric response is correlated with the growth of in-plane nanodomains. We find these results to be consistent with a large number of studies that show that strain-relief is of paramount importance if ferroelectric films are to be developed as microwave circuit components. PACS 81.15.Fg; 85.50.-n     

Crystal structure and dielectric properties of a new complex perovskite oxide Ba<Subscript>2</Subscript>LaSbO<Subscript>6</Subscript>

The crystal structure and dielectric properties of a new perovskite material Ba₂LaSbO₆ are reported. Ba₂LaSbO₆ was synthesized for the first time by conventional solid state reaction process. X-ray diffraction (XRD) measurements and Rietveld analysis revealed that, contrary to the other Ba₂RESbO₆ (RE=Rare-Earth other than La) complex perovskites, which are cubic, Ba₂LaSbO₆ crystallizes in a monoclinic system having space group P2₁/n. Dielectric properties of Ba₂LaSbO₆ at frequencies up to 40 GHz and in temperatures ranging from 50 to 350 K were determined by analyzing the resonant modes in cavities totally filled with the measured material. The dielectric constant ()=15.8±0.2% and at 77 K loss factor (tg)9×10^-4. Preliminary studies reveal that at the processing temperatures, Ba₂LaSbO₆ do not react with YBa₂Cu₃O_7- (YBCO) and Bi₂Sr₂CaCu₂O_x [Bi (2212)] superconductors. Having appropriate dielectric properties along with chemical stability with superconductors Ba₂LaSbO₆ can be used as a precursor as substrate for deposition of various passive HTS devices, or as an insulator in the active SIS structures. PACS 61.66.Fn; 61.10.Nz; 77.22.-d; 74.72.-h     

Structural transformations in MoO<Subscript>x</Subscript> thin films grown by pulsed laser deposition

In this work, laser-induced crystallization in MoO_x thin films (1.8x2.1) is reported. This transformation involves a MoO_x oxidation and subsequently a crystallization process from amorphous MoO₃ to crystalline MoO₃. For comparison purposes crystallization is induced thermally, in an oven, as well. The crystallization kinetics is monitored by Raman spectroscopy; a threshold in the energy density necessary to induce the phase transformation is determined in the case of photo-crystallization. This threshold depends on the type of substrate on which the film is deposited. For the thin films deposited on glass substrates, the structural transformation is from amorphous MoO_x to the thermodynamically stable MoO₃ crystalline phase. For the thin films deposited on Si(100) the structural transformation is from amorphous MoO_x to a mixture of MoO₃ and the thermodynamically unstable MoO₃ crystalline phases. The structural transformations are also characterized by scanning electron microscopy and light-transmission experiments. PACS 81.15.Fg; 61.80.Ba; 78.30.-j     

Genesis,structural, and transport properties of La<Subscript>2</Subscript>Mo<Subscript>2-x</Subscript>W<Subscript>x</Subscript>O<Subscript>9</Subscript> prepared via mechanochemical activation

Fast oxide-ion conductors La₂Mo_2-xW_xO₉ (x = 0–1) have been prepared using mechanochemical activation (MA) of starting oxides in a high-power planetary ball mill. Studies of La₂Mo_2-xW_xO₉ genesis and structural properties using thermal analysis, XRD, SEM, IR, and Raman spectroscopy have revealed that MA results in the formation of an amorphous precursor, while the cubic β-phase is formed after calcination at 700–900 °C. Due to a high dispersion of powders, high-density pellets of W-LAMOX ceramics have been obtained already after sintering at 950 °C. Their electrical conductivity measured by the impedance spectroscopy depends on the W concentration being sufficiently high (up to 5.6?10^?3 S/cm at 630 °C) at temperatures below 650 °C.     

Structural and optical characterization of polycrystalline CdSe<Subscript>x</Subscript>Te<Subscript>1-x</Subscript> thin films

Solid solutions of CdSe_xTe_1-x (0.7x1) were synthesized by vacuum fusion of stoichiometric amounts of CdSe and CdTe constituents in a silica tube. X-ray and electron microscope diffractometry techniques revealed that the CdSe_xTe_1-x thin films were polycrystalline with a hexagonal structure. The variation of lattice constants with composition was found to obey Vegards law. The compositional dependence of the optical constants, the refractive index n and the absorption index k, of the films was determined in the spectral range of 400–2000 nm. The dispersion of the refractive index of the films could be described using the Wemple–DiDomenico single oscillator model. Changes of the dispersion parameters were also studied as a function of the mole fraction x. A plot representing ²=f(h) showed that the CdSe_xTe_1-x thin films of different compositions have two direct transitions corresponding to the energy gaps E_g and E_g+. The variation in either E_g or E_g+ with x indicates that this system belongs to the amalgamation type. The variation follows a quadratic dependence and the bowing parameters were found to be 0.4 and 0.5 eV, respectively. PACS 78.20.-e; 81.15.-z     

Ba1-xSrxTiO3及Ba0.6-xPbxSr0.4TiO3陶瓷的制备和介温特性研究

用固相反应法制备了Ba_1-xSr_xTiO₃(BST)及Ba_0.6-xPb_xSr_0.4TiO₃(BPST)陶瓷,通过XRD,FESEM和拉曼谱分析了Pb掺杂对Ba_0.6Sr_0.4TiO₃样品的晶格、相变及表面形貌的影响.测试了BST及B 关键词： BST BPST 弥散相 介温特性