Electroresistance and field effects in epitaxial thin films of Pr0.7Sr0.3MnO3

Highly epitaxial thin films of Pr_0.7Sr_0.3MnO₃ were grown on (100) SrTiO₃ single crystal substrates by laser ablation. Similar to other manganite compounds, these Pr_0.7Sr_0.3MnO₃ films exhibited remarkable magnetoresistance. Application of electric currents could induce a remarkable reduction in resistivity, demonstrating a strong electroresistance effect. The ratio of the resistance variation, ER=[R(0)−R(I)]/R(I), is about 33% at metal-insulator transition temperature. Using a planar field effect configuration, significant field modulation of the metal-insulator transition was achieved. The observed field effects were discussed based on the strong interactions between carrier spins and localized spins in Mn ions, as well as the percolative mechanism of phase separation.     

Dielectric and piezoelectric properties of (1−x)Ba0.7Sr0.3TiO3−xBa0.7Ca0.3TiO3 perovskites

Dielectric and piezoelectric properties of (1?x)Ba_0.7Sr_0.3TiO₃?xBa_0.7Ca_0.3TiO₃ (BST?xBCT) (x=0.2–0.9) perovskite ceramics have been investigated. BCT has fully incorporated into BST lattice, forming a complete perovskite solid solution, whose lattice constant χ decreases almost linearly with increase in x from 0.2 to 0.4, while showing an anomalous expansion at 0.4<x≤0.6. This, together with the deviation of tetragonal–orthorhombic phase transition temperature (T_O–T) from the linear relation T_O–T (K)=?103.7x+239.3 at x=0.5, suggests that a small amount of Ca²⁺ has substituted for Ti⁴⁺. Curie temperature T_C increases linearly with increase in x from 0.2 to 0.9, which is mainly contributed to the increase of the Ba/Sr ratio. The calculated degree of relaxation (γ) is in the range of 1.41–1.53, indicating that the BST–xBCT ceramics are ferroelectric materials with diffuse phase transition. Strain and piezoelectric constant (d₃₃) decrease with increasing x, whereas planar electromechanical coefficient (k_p) reaches a maximum (17.0%) at x=0.6.     

The effect of Mg doping on the dielectric and tunable properties of Pb0.3Sr0.7TiO3 thin films prepared by sol–gel method

Mg doped Pb_0.3Sr_0.7TiO₃ (PST) thin films were fabricated by the sol–gel method on a Pt/Ti/SiO₂/Si substrate. The microstructure, surface morphology, dielectric and tunable properties of PST thin films were investigated as a function of Mg concentration. It is found that proper Mg doping dramatically improves the dielectric loss (0.0088 @ 1 MHz), furthermore, the crystallinity, dielectric constant, and tunability of films simultaneously decrease with the increase of Mg content. The 2 mol% Mg doped PST thin film shows the highest figure of merit (FOM) value of 36.8 for its the smallest dielectric loss and upper tunability. The dependence of Rayleigh coefficient on the doping concentration was examined, which indicated that the reduction of dielectric constant and tunability of films should be related to the $\mathrm{Mg}''_{\mathrm{Ti}}$ – $\mathrm{V}_{\mathrm{O}}^{\bullet\bullet}$ defect dipoles pinning the domain wall motion of residual polar clusters in PST.     

Improvement of dielectric （Ba,Sr）TiO3 thin tunability and loss tangent of films with K doping

Ba0.6Sr0.4 TiO3 thin films doped with K were deposited on Pt/Ti/SiO2 /Si substrates by the chemical solution deposition method. The structure, surface morphology and the dielectric and tunable properties of Ba0.6Sr0.4 TiO 3 thin films have been studied in detail. The K content in Ba0.6Sr0.4TiO3 thin films has a strong influence on the material’s properties including surface morphology and the dielectric and tunable properties. It was found that the Curie temperature of K-doped Ba0.6Sr0.4 TiO3 films shifts to a higher value compared with that of undoped Ba0.6Sr0.4TiO3 thin films, which leads to a dielectric enhancement of K-doped Ba0.6Sr0.4 TiO3 films at room temperature. At the optimized content of 0.02 mol, the dielectric loss tangent is reduced significantly from 0.057 to 0.020. Meanwhile, the tunability is enhanced obviously from 26% to 48% at the measured frequency of 1 MHz and the maximum value of the figure of merit is 23.8. This suggests that such films have potential applications for tunable devices.     

Effect of the film thickness on the impedance behavior of sol–gel Ba0.6Sr0.4TiO3 thin films

Perovskite Ba_0.6Sr_0.4TiO₃ sol–gel thin films with different thicknesses are fabricated as MFM configuration to study the effect of the film thickness on the dielectric relaxation phenomenon and the ionic transport mechanism. The frequency dependent impedance, electric modulus, permittivity and AC conductivity have been investigated in this context. Z? plane for all the tested samples shows two regions, corresponding to the bulk mechanism and the distribution of the grain boundaries–electrodes process. Electric modulus versus frequency plots reveal non-Debye relaxation peaks. The observed decrease in both the impedance and permittivity with the increase in film thickness is attributed to the grain size effect. The frequency dependent conductivity plots show three regions of conduction processes, i.e. low-frequency region due to DC conduction, mid-frequency region due to translational hopping motion and high-frequency region due to localized hopping and/or reorientational motion.     

Microstructure and dielectric properties of Li2CO3 doped 0.7(Ba,Sr)TiO3–0.3MgO ceramics

Microstructure and dielectric properties of Li₂CO₃ doped 0.7(Ba,Sr)TiO₃–0.3MgO ceramics for the low temperature sintering and microwave applications will be presented. In these days, low temperature sintering process has been widely spread out for the integrated electronic modules for the communication systems such as front-end modules, antenna modules, and switching modules. We have added Li₂CO₃ and MgO to (Ba,Sr)TiO₃ material to reduce the sintering temperature and improve dielectric properties such as loss tangent, and frequency dispersion.In this paper, we have discussed the crystalline properties, dielectric properties, and the microstructures of Li₂CO₃ doped 0.7(Ba,Sr)TiO₃–0.3MgO ceramics. No pyro phase was observed in the X-ray diffraction method. Very weak frequency dispersion (<0.7%) of dielectric permittivity was observed from the 1 kHz to 1 MHz range. We found that the grain size of BST is around 2 μm, while the grain size of Li₂CO₃ dope 0.7BST–0.3MgO is around 4 μm from the SEM analysis.     

Ultrafast Photoelectric Effects in Heterojunctions of La0.7Sr0.3MnO3 and Si

Ultrafast photoelectric effects have been observed in p-n heterojunctions of La0.7Sr0.3MnO3(LSMO)/Si and LSMO/SrTiO3_/Si for the first time. The rise time was about 1 ns and the full width at half maximum was about 2ns for the photovoltaic pulse when the heterojunction was irradiated by a laser of -25 ps pulse duration and 1064 nm wavelength. The photovoltaic sensitivity was as large as 435 mV//mJ for a 1064 nm laser pulse. No such pulse was observed with irradiation from a pulsed 10.6 μm CO2 laser.     

Oxygen dynamics in epitaxial YBa2Cu3O7-δ thin films

We have investigated epitaxial YBCO films by Perturbed γγ-Angular Correlation Spectroscopy (PAC) using ¹¹¹In probe atoms. The probes substitute on the yttrium lattice site and hyperfine parameters have been determined unambiguously. Excellent agreement is achieved with full potential electric field gradient (efg) calculations. Good reproducibility has been achieved by our preparation method allowing us to study the influence of oxygen motion on the structural orthorhombic to tetragonal phase transition. This revised version was published online in August 2006 with corrections to the Cover Date.     

Ultrafast optical probes of dynamic behavior in La0.7Sr0.3MnO3/YBa2Cu3O7 − δ/La0.7Sr0.3MnO3 heterostructure

Ultrafast pump-optical probe spectroscopy was used to analyze carriers dynamics behavior in La_0.7Sr_0.3MnO₃/YBa₂Cu₃O_{7 ? δ}/La_0.7Sr_0.3MnO₃ heterostructure. Our results show the pump signal ΔR/R for higher laser power (such as 21 and 41 mW), below T _c , first goes positive, then crosses zero and goes negative, before relaxing back to equilibrium over a time scale of a few ten picoseconds. We extract the characteristic relaxation time of the different process by fitting the data at these powers with a three-exponential decay. For higher laser power, the long characteristic relaxation time are obtained, which implies the competition between FM order and SC order in the La_0.7Sr_0.3MnO₃/YBa₂Cu₃O_{7 ? δ}/La_0.7Sr_0.3MnO₃ heterostructure.     

择优取向MgO缓冲层上制备的硅基Ba0.7Sr0.3TiO3薄膜的结构和性能研究

分别采用sol-gel法和磁控溅射法在Si(001)单晶衬底上制备出(111)和(001)取向的MgO缓冲层薄膜,随后在其上生长Ba0.7Sr0.3TiO3(BST30)铁电薄膜.通过X射线衍射,扫描电子显微镜,原子力显微镜等方法研究了薄膜的微结构.实验结果发现,在较厚的MgO(001)缓冲层上可长出(101)取向的BST30薄膜,而在较薄的MgO(111) 缓冲层上则表现出(101)和(111)取向相互竞争的现象,随着MgO(111)缓冲层厚度的增加,BST30薄膜的(101)取向被抑制,而(001)取向逐渐增强.利用反射率测定仪、阻抗分析仪研究了BST30薄膜的光学和电学性能. 通过改进的单纯形法拟合反射率曲线,得到了BST30及其MgO缓冲层薄膜的光学常数, 由BST30薄膜的电压-电流特性(I-V曲线),发现MgO缓冲层对BST30薄膜的漏电流有明显的阻隔作用,可以有效消除BST30膜层的p-n结效应.     

Investigations of sol–gel-derived highly (100)-oriented Ba0.5Sr0.5TiO3 : MgO composite thin films for phase-shifter applications

Sol–gel deposition of highly oriented Ba_0.5Sr_0.5TiO₃:MgO composite thin films has shown desirable dielectric constant reduction and higher figure of merit for phase-shifter applications. In this multilayer configuration, MgO distributed homogeneously through the Ba_0.5Sr_0.5TiO₃ (BST50) matrix, and it helped in tailoring the dielectric constant and reducing the loss tangent significantly. In the present study, the high-frequency dielectric behavior of the films has been evaluated by fabricating an eight-element coupled microstrip phase shifter and measuring the degree of phase shift and insertion loss as a function of applied voltage at room temperature. An increase in phase-shifter figure of merit (degree of phase shift per dB insertion loss) from 28°/dB for pure BST50 to 71°/dB for a BST50:MgO film (at 14 GHz and 333 kV/cm) has been observed. PACS 77.55.+f; 81.20.Fw; 73.40.-c; 85.50.-n     

Studies of some physical properties of PrFe0.7Ni0.3O3 thin films after 200 MeV Ag15+ irradiation

PrFe_0.7Ni_0.3O₃ thin films (thickness ~ 200 nm) were prepared by pulsed laser ablation technique on LaAlO₃ substrate. These films were irradiated with 200?MeV Ag¹⁵⁺ ions at various fluencies, ranging from 1 × 10¹¹ to 1 × 10¹² ions/cm². These irradiated thin films were characterized by using X-ray diffraction, dc conductivity, dc magnetization and atomic force microscopy. These films exhibit orthorhombic structure and retain it even after irradiations. The crystallite size (110–137?nm), micro strain (1.48 × 10^?2–1.75 × 10^?2 line^?2?m^?4) and dislocation density (79.7 × 10¹⁴–53.2 × 10¹⁴ line/m²) vary with ion fluencies. An enhancement in resistivity at certain fluence and then a decrease in its value (0.22175–0.21813?Ω?cm) are seen. A drastic change in observed magnetism after ion irradiation is seen. With ion irradiation, an increase in surface roughness, due to the formation of hillocks and other factors, is observed. Destruction of magnetic domains after irradiation can also be visualized with magnetic force microscopy and is in close agreement with magnetization data. The impact on various physical properties in these thin films after irradiation indicates a distortion in the lattice structure and consequently on single-particle band width caused by stress-induced defects.     

Structure and microstructure of epitaxial Sr n Fe n O3 n −1 films

Thin films of SrFeO_{3??? x} (0?≤?x?≤?0.5) (SFO) grown on a (LaAlO₃)_0.3 (SrAl_0.5Ta_0.5O₃)_0.7 (LSAT) substrate by pulsed laser deposition have been structurally investigated by electron diffraction and high resolution transmission electron microscopy for different post-deposition oxygen treatments. During the deposition and post-growth oxidation, the oxygen-reduced SFO films accept extra oxygen along the tetrahedral layers to minimize the elastic strain energy. The oxidation process stops at a concentration SFO_2.875 and/or SFO_2.75 because a zero misfit with the LSAT substrate is reached. A possible growth mechanism and phase transition mechanism are suggested. The non-oxidized films exhibit twin boundaries having a local perovskite-type structure with a nominal composition close to SFO₃.     

溶胶-凝胶法制备(Ba0.7Sr0.3)TiO3/LaNiO3异质薄膜及其结构和介电性质研究

采用改进的溶胶-凝胶方法在单晶Si(100)衬底上制备了介电性能优异的(Ba0.7Sr0.3)TiO3/LaNiO3 异质薄膜.实验发现,在750 ℃下、O2气氛中晶化的LaNiO3薄膜的电阻率最小.C-V与I-V特性测量表明(Ba0.7Sr0.3)TiO3薄膜具有优异的介电性能,在频率为50 kHz、零偏压下的相对介电常数εr>300,偏压为6 V时漏电流密度JL<1.2×10-6 A/cm2.     

Role of Ti 3d carriers in mediating the ferromagnetism of Co∶TiO2 anatase thin films

We study the surface and bulk electronic structure of the room-temperature ferromagnet Co∶TiO(2) anatase films using soft- and hard-x-ray photoemission spectroscopy with probe sensitivities of ～1 and ～10 nm, respectively. We obtain direct evidence of metallic Ti(3+) states in the bulk, which get suppressed to give a surface semiconductor, thus indicating the difference in electronic structure between surface and bulk. X-ray absorption and resonant photoemission spectroscopy reveal Ti(3+) electrons at the Fermi level (E(F)) and high-spin Co(2+) electrons occurring away from E(F). The results show the importance of the charge neutrality condition: Co(2+)+V(O)(2-)+2Ti(4+)?Co(2+)+2Ti(3+) (V(O) is oxygen vacancy), which gives rise to the elusive Ti 3d carriers mediating ferromagnetism via the Co 3d-O 2p-Ti 3d exchange interaction pathway of the occupied orbitals.     

Composition-dependent ferroelectric properties of Ba1− x Sr x TiO3 ceramics

Pure and homogeneous nanopowders of Ba_{1? x} Sr _x TiO₃ with compositions x?=?0, 0.10, 0.20, 0.25, 0.30 and 0.35 were prepared by a polymerized complex method based on the Pechini-type reaction route, wherein mixed solutions of citric acid, ethylene glycol, titanium isopropoxide, barium carbonate and strontium carbonate were polymerized to form transparent resins used as precursors for the final oxide powders. X-ray diffraction data indicated the formation of tetragonal BaTiO₃ and cubic (Ba,?Sr)TiO₃ solid solutions, free from secondary phases. Ceramics with relative densities of 85–93% were obtained after sintering for 3?h at 1300°C and 1350°C, respectively. The Sr content influences the microstructure of the ceramic samples and their ferroelectric characteristics. The P(E) loops are strongly composition-dependent, with reducing the remnant polarization and coercive fields with increasing the Sr addition. The First Order Reversal Curve (FORC) analysis is used for describing the local switching properties and the ac-tunability characteristics. The maximum of the FORC distribution is located at low fields, meaning that small fields are necessary to switch the large majority of the domains of these systems. The tunability determined in the FORC experiment depends not only on the actual field, but also on the reversal field. For two compositions, higher FORC tunability at room temperature was found for the sample closer to its ferro-para phase transition. This result was interpreted in relationship with the domain walls mobility, which is higher for the ferroelectric sample close to its ferro-para phase transition.     

Thermal Conductivity Anomalies Related to the Double-Bump of Resistivity in Nd0.7Sr0.3Mn1-xCrxO3

The temperature dependence of the thermal conductivity κ(T) and electrical resistivity ρ(T) has been measured for perovskite Nd0.7Sr0.3Mn1-xCrxO3 (0.01≤x≤0.15) in the temperature range of 10-300 K. The double-bump feature in p(T) is observed for x = 0.08 and 0.10 samples. Below the insulating-metallic transition temperature TIM, κ(T) shows a sharp increase and such an increase is depressed gradually by the increasing Cr content.While κ(T) displays an anomalous decrease around the temperature where the second bump of ρ(T) emerges.We suggest that the enhancement of κ(T) below TIM is related to both phonons and magnons, the ferromagnetic interaction of Mn^3 -O-Cr^3 should be super-exchange interaction rather than double-exchange interaction, and the Cr doping just weakens the DE interaction in this system.     

Synthesis and characterization of nanostructured (Pb1− x Sr x )TiO3 thin films by a modified chemical route

The coating solutions of nanostructured (Pb_{1– x} Sr _x )TiO₃ (PST) thin films have been prepared by the sol–gel combined metallo-organic decomposition method. The coating solutions were deposited on Pt/Ti/SiO₂/Si substrates using a spin-coating technique with spinning speed of 4300 rpm and annealed at 650°C. The effect of Sr content in reducing the grain size and tetragonal distortion of PST films has been studied. The optimum conditions for crystalline phase formation in the films have been analyzed by thermogravimetric, differential thermal analysis and Fourier transform infrared spectroscopy. The phase and microstructure of the films were studied by X-ray diffraction (XRD) and atomic force microscopy (AFM). The XRD pattern shows that the PST thin films are crystallized into tetragonal structures without any impurity phase and the distortion ratio reduces with increasing Sr concentration. The AFM results indicate an increase in grain size with increasing annealing temperature of the film and reduction in grain size with increasing Sr concentration.     

Growth of Ba2−x Sr x NaNb5O15 thin film on La0.05Sr0.95TiO3 substrate by pulsed laser deposition

Ba_2−x Sr _x NaNb₅O₁₅ thin films were prepared on La_0.05Sr_0.95TiO₃ substrates by pulsed laser deposition. The structural and ferroelectric properties of the thin films depended on substrate temperature (T _sub) and Sr concentration. When T _sub was fixed at 700 °C, the Ba_2−x Sr _x NaNb₅O₁₅ (x = 0, 0.6, 1.0, and 1.4) thin films exhibited a high c-axis orientation. The thin films consisted of well-developed grains and exhibited a smooth surface. The c-axis-oriented Ba_0.6Sr_1.4NaNb₅O₁₅ thin film with the lowest Curie temperature also exhibited a high c-axis orientation and a P-E hysteresis loop with a high ferroelectricity at T _sub 650 °C. Thus, its remanent polarization (P _r) and coercive field (E _c) were 2P _r 24.9 μC/cm² and 2E _c 107 kV/cm, respectively. These values indicate that Ba_2−x Sr _x NaNb₅O₁₅ has ferroelectricity in the thin film form.