Reduced dielectric loss and leakage current in CaCu3Ti4O12/SiO2/CaCu3Ti4O12 multilayered films

The CaCu₃Ti₄O₁₂/SiO₂/CaCu₃Ti₄O₁₂ (CCTO/SiO₂/CCTO) multilayered films were prepared on Pt/Ti/SiO₂/Si substrates by pulsed laser deposition method. It has been demonstrated that the dielectric loss and the leakage current density were significantly reduced with the increase of the SiO₂ layer thickness, accompanied with a decrease of the dielectric constant. The CCTO film with a 20 nm SiO₂ layer showed a dielectric loss of 0.065 at 100 kHz and the leakage current density of 6×10⁻⁷ A/cm² at 100 kV/cm, which were much lower than those of the single layer CCTO films. The improvement of the electric properties is ascribed to two reasons: one is the improved crystallinity; the other is the reduced free carriers in the multilayered films.     

Experimental and numerical investigation of a ceramic dielectric resonator (DRA): CaCu3Ti4O12 (CCTO)

In this study, the CaCu₃Ti₄O₁₂ (CCTO) ceramic phase was synthesized by microwave heating in a much shorter time compared to the conventional heating methods. The results indicate that microwave processing is a promising method for preparing CCTO ceramics. CCTO was prepared using a domestic microwave oven operated at 2.45 GHz with 800 W. After a few minutes of microwave irradiation the formation of CCTO was confirmed by X-ray powder diffraction. The CCTO ceramic was studied in the medium-frequency (MF) range (100 Hz-1 MHz) and in the microwave range of frequencies. The experimental and theoretical characteristics of the dielectric resonator antenna are investigated.     

Growth of highly-oriented CaCu3Ti4O12 thin films on SrTiO3 (1 0 0) substrates by a chemical solution route

Highly-oriented CaCu₃Ti₄O₁₂ (CCTO) thin films deposited directly on SrTiO₃ (1 0 0) substrates have been developed successfully using a chemical solution coating method. X-ray diffraction (XRD), scanning electron microscopy (SEM), and atomic force microscopy (AFM) were employed to characterize the structure and the morphology. It was observed that the CCTO thin films had the 1 μm × 1 μm domain-like microstructure that consists of compact grains of about 0.1 μm in size. The cross sectional SEM image shows that the CCTO grains grow regularly close to the clear interface between the CCTO film and the SrTiO₃ substrate. The result was discussed in terms of lattice mismatch between CCTO and SrTiO₃.     

Nanoparticles of the giant dielectric material, CaCu3Ti4O12 from a precursor route

A method of preparing the nanoparticles of CaCu₃Ti₄O₁₂ (CCTO) with the crystallite size varying from 30 to 200 nm is optimized at a temperature as low as 680 °C from the exothermic thermal decomposition of an oxalate precursor, CaCu₃(TiO)₄(C₂O₄)₈·9H₂O. The phase singularity of the complex oxalate precursor is confirmed by the wet chemical analyses, X-ray diffraction, FT-IR and TGA/DTA analyses. The UV-vis reflectance and ESR spectra of CCTO powders indicate that the Cu(II) coordination changes from distorted octahedra to nearly flattened tetrahedra (squashed) to square-planar geometry with increasing annealing temperature. The HRTEM images have revealed that the evolution of the microstructure in nanoscale is related to the change in Cu(II) coordination around the surface regions for the chemically prepared powder specimens. The nearly flattened tetrahedral geometry prevails for CuO₄ in the near surface regions of the particles, whereas square-planar CuO₄ groups are dominant in the interior regions of the nanoparticles. The powders derived from the oxalate precursor have excellent sinterability, resulting in high-density ceramics which exhibited giant dielectric constants upto 40,000 (1 kHz) at 25 °C, accompanied by low dielectric loss <0.07.     

磁性隧道结Ni80Fe20/Al2O3/Co的研究

用等离子体氧化形成中间绝缘层的方法可重复制备出具有隧道磁电阻（TMR)效应的Ni₈₀Fe₂₀/Al₂O₃/Co磁性隧道结.光透射谱等实验结果表明等离子体氧化能可控制地制备较致密的Al₂O₃绝缘层.样品的TMR比值在室温下最高可达6.0％,反转场可低于800A/m,相应的平台宽度约为2400A/m.结电阻R_j的变化范围从百欧到几百千欧,并且TMR比值随零磁场结偏压增大单调减小. 关键词：     

Influence of Lu2O3 on electrical and microstructural properties of CaCu3Ti4O12 ceramics

In this work, the influence of Lu₂O₃ doped on the dielectric and electrical properties of CaCu₃Ti₄O₁₂ was reported. Lu₂O₃-doped CCTO was prepared by a conventional solid state technique using CuO, TiO₂, and CaCO₃ as starting materials. The samples were studied by X-ray diffraction (XRD) and scanning electron microscopy (SEM); dielectric measurements were measured in the 10² Hz–10⁷ Hz frequency range at room temperature; and the nonlinear behavior of all samples was measured. The doping of Lu₂O₃ resulted in an increase in the dielectric constant of CCTO, but decreased the stability of the frequency dependence. Increasing concentrations of Lu₂O₃ resulted in decreasing nonlinear coefficients.     

Influence of sintering conditions and doping on the dielectric relaxation originating from the surface layer effects in CaCu3Ti4O12 ceramics

Detailed investigations into the dielectric dispersion phenomenon in the giant dielectric constant material CaCu₃Ti₄O₁₂ (CCTO) around room temperature revealed the existence of two successive dielectric relaxations. In the temperature domain, a new dielectric relaxation was clearly observed around 250 K, in addition to the well-investigated dielectric relaxation close to 100 K. The effect of sintering and doping (La³⁺) on the strength of these dielectric relaxations were studied in detail. The sintering temperature as well as its duration was found to have tremendous influence on the dielectric relaxation that was encountered around 250 K. This Maxwell-Wagner (M-W) type of relaxation was found to be originating from the surface layer containing the Cu-rich phase, which was ascribed to the difference in the oxygen content between the surface and the interior of the sample. Interestingly, this particular additional relaxation was not observed in La_2/3Cu₃Ti₄O₁₂, a low dielectric constant member of the CCTO family, in which the segregation of Cu-rich phase on the surface was absent. Indeed the correlation between the new relaxation and the presence of Cu-rich phase in CCTO ceramics was further corroborated by the absence of the same after removing the top and bottom layers.     

Schottky-type grain boundaries in CCTO ceramics

In this work we studied electrical barriers existing at CaCu₃Ti₄O₁₂ (CCTO) ceramics using dc electrical measurements. CCTO pellets were produced by solid state reaction method and X-ray diffractograms showed which single phase polycrystalline samples were obtained. The samples were electrically characterized by dc and ac measurements as a function of temperature, and semiconductor theory was applied to analyze the barrier at grain boundaries. The ac results showed the sample’s permittivity is almost constant (10⁴) as function of temperature at low frequencies and it changes from 100 to 10⁴ as the temperature increases at high frequencies. Using dc measurements as a function of temperature, the behavior of barriers was studied in detail. Comparison between Schottky and Poole-Frenkel models was performed, and results prove that CCTO barriers are more influenced by temperature than by electric field (Schottky barriers). Besides, the behavior of barrier width as function of temperature was also studied and experimental results confirm the theoretical assumptions.     

添加Nb对CaCu3Ti4O12陶瓷介电性能的影响

采用固相反应法制备了CaCu₃Ti_4-xNb_xO₁₂(x=0，0.01，0.04，0.08，0.2)陶瓷，样品在x取值范围内形成了连续固溶体.在40Hz—110MHz频率范围对样品进行了介电频谱分析，实验结果表明，与纯CaCu₃Ti₄O₁₂不同，含Nb试样除了在频率大于10kHz范围内出现的德拜弛豫 关键词： 巨介电常数 德拜弛豫 阻挡层电容 等效电路     

Enhanced nonlinear current-voltage behavior in Au nanoparticle dispersed CaCu3Ti4O12 composite films

An enhanced nonlinear current-voltage behavior has been observed in Au nanoparticle dispersed CaCu₃Ti₄O₁₂ composite films. The double Schottky barrier model is used to explain the enhanced nonlinearity in I-V curves. According to the energy-band model and fitting result, the nonlinearity in Au: CCTO film is mainly governed by thermionic emission in the reverse-biased Schottky barrier. This result not only supports the mechanism of double Schottky barrier in CCTO, but also indicates that the nonlinearity of current-voltage behavior could be improved in nanometal composite films, which has great significance for the resistance switching devices.     

Dielectric property of CaCu3Ti4O12 thin film grown on Nb-doped SrTiO3(100) single crystal

Thin film of CaCu₃Ti₄O₁₂ (CCTO) has been deposited on Nb-doped SrTiO₃(100) single crystal using pulsed laser deposition. The dielectric constant and AC conductivity of CCTO film in the metal–insulator–metal capacitor configuration over a wide temperature (80 to 500 K) and frequency (100 Hz to 1 MHz) range have been measured. The small dielectric dispersion with frequency observed in the lower temperature region (<300 K) indicates the presence of small defects in the deposited CCTO thin film. The frequency-dependent AC conductivity at lower temperature indicates the hopping conduction. The dielectric dispersion data has been analyzed in the light of both conductivity relaxation and Debye type relaxation with a distribution of relaxation times. Origin of dielectric dispersion is attributed to the distribution of barrier heights such that some charge carriers are confined between long-range potential wells associated with defects and give rise to dipolar polarization, while those carriers which do not encounter long-range potential well give rise to DC conductivity.     

Dielectric dispersion in PbO-Sb2O3-As2O3:V2O5 glasses

Dielectric properties, viz. dielectric constant ε′, loss tan δ and a.c conductivity σ_ac (over a wide range of frequency and temperature) and dielectric breakdown strength of PbO-Sb₂O₃-As₂O₃ glasses doped with V₂O₅ (ranging from 0 to 0.5 mol%) are studied. Analysis of these results, based on optical absorption and ESR spectra, indicates that the insulating strength of the glasses is comparatively high when the concentration of V₂O₅ is about 0.3 mol% in the glass matrix.     

Strain induced phase separation on La0.5Ca0.5MnO3 thin films

The effect of epitaxial strain on La_0.5Ca_0.5MnO₃ films of various thicknesses grown on SrTiO₃, SrLaAlO₄, and SrLaGaO₄ substrates is studied by Raman spectroscopy, magnetic, and resistivity measurements. The transport and magnetic properties as well as Raman spectra of the films are affected by epitaxial strains. The energy of the A_g(2) mode and the tilting angle of the MnO₆ octahedra is affected by the strain imposed by the substrate. In the spectra of the films deposited on the (1 0 0) SrTiO₃ substrate strong Jahn-Teller (JT) modes appear, which couple with charge-ordering. In all other films these modes are suppressed and no additional Raman lines are present at low temperatures contrary to the bulk compound. The low frequency continuum scattering decreases at low temperatures indicating a coupling with both the charge and orbital transitions. Comparison of the Raman spectra with the magneto-transport properties suggests an interpretation in terms of a strain induced phase separation between ferromagnetic metallic and antiferromagnetic insulating states.     

Electron spin resonance study of bulk and nanosized La0.875Sr0.125MnO3

The structural, transport and electron spin resonance properties of bulk and nanosized La_0.875Sr_0.125MnO₃ prepared by a sol-gel method have been investigated. The bulk sample has an orthorhombic structure and a ferromagnetic insulating ground state. The ESR spectra indicate the coexistence of the ferromagnetic insulating and ferromagnetic metallic phases below T_C. In addition to a sharp peak in the vicinity of T_C, another sharp peak close to is clearly observed in the intensity of the spectra, which may be correlated with the structural transition and orbital ordering at this temperature. For the nanosized sample, a drastically different behavior is found. With a rhombohedral structure down to 70 K, the nanosized sample shows a ferromagnetic metallic ground state. The ESR studies reveal the coexistence of the paramagnetic and ferromagnetic resonance signals. The resonance intensity shows a broad peak around 200 K, which may be due to the wide ferromagnetic transition in the nanoparticle.     

The suppression of structural phase transformation in LaVO3 and La1−xSrxVO3 thin films fabricated by pulsed laser deposition

Highly oriented (100) thin films of LaVO₃ and La_1−xSr_xVO₃ have been fabricated by pulsed laser deposition in a reducing atmosphere. The films show a transition from insulating to metallic behaviour in the composition region of x, 0.175<x<0.200. In the single crystals of the antiferromagnetic insulating phase, a first-order structural phase transition is observed few degrees below the magnetic transition, which manifests itself as a kink in the temperature dependence of resistivity. In the highly oriented thin films of LaVO₃ and La_1−xSr_xVO₃ fabricated on lattice matched substrates in this study, the structural phase transformation in the insulating phase has been suppressed. The electrical conduction is found to take place via hopping through localized states at low temperatures. The metallic compositions show a non-linear (T^1.5) behaviour in the temperature dependence of resistivity. V (2p) core level spectra of these films show a gradual change in the relative intensities of V³⁺ and V⁴⁺ ions as the value of x increases.     

Scanning tunneling spectroscopy of a superlattice superconductor: Bi2Sr2CaCu2O8

It is clear that many of the cuprate superconductors are truly superlattices, composed of sheets whose individual superconducting critical temperatures may approach bulk values. Such a cuprate is Bi₂Sr₂CaCu₂O₈, often referred to as BSCCO-2212. Scanning tunneling spectroscopy (vacuum tunneling) applied toa-bBiO cleavage planes ofT_c≈90 K BSCCO single crystals under liquid helium simultaneously provides topography and local dI/dVspectra (superconducting density of states: DOS). The spectra, which are similar to those obtained from angle-resolved photoemission spectroscopy, confirm a large gap parameter Δ(x,y) on the uppermost layer. The dI/dVspectra do not unequivocally select order parameter symmetry, but are probably consistent with d-wave or anisotropic s-wave states. Spatial variations of Δ on a 100 Å scale are attributed to variation in BiO metallicity, originating in oxygen stoichiometry variations. A model is presented to explain the different dI/dVspectra which are seen, and associated with different local oxygen concentrations. This model, based on the superconducting proximity effect, assumes that in some regions the BiO uppermost layer is insulating and in other regions it is metallic, in the latter case induced superconductivity by proximity to the CuO₂planes. Our STM measurements appear to sample only the uppermost half cell of the crystal, and contain no obvious superlattice features. Recent measurements have confirmed Josephson radiation from voltage biasedc-axis pillars of BSCCO. From the point of view of the present work, the superconducting systems which weakly couple along thec-direction to create Josephson junctions are probably half-cell slabs of height 15.4 Å, each containing two CuO₂and two BiO layers, which act as single composite electrodes for the Josephson junctions.     

填充式方钴矿化合物CeOs4Sb12近藤相互作用的非弹性中子散射研究

CeOs₄Sb₁₂晶体中由于导电电子与Ce³⁺ 4f¹电子之间存在c-f杂化作用导致费米面附近存在能量间隙.这种c-f近藤相互作用和能量间隙是理解CeOs₄Sb₁₂物理性质，如近藤绝缘体行为、Ce³⁺磁矩在低温下猝灭以及重费米性等电、磁性质的关键.当用LAM-D中子谱仪对粉末CeOs₄Sb₁₂进行测量时，可以得到不同温度下CeOs₄Sb₁₂的非弹性中子散射谱.结果表明CeOs₄Sb₁₂中存在近藤相互作用，其作用强度为3.1 meV，证实了CeOs₄Sb₁₂为近藤绝缘体.中子测量得出CeOs₄Sb₁₂德拜温度为317 K. 关键词： 非弹性中子散射 填充式方钴矿 近藤绝缘体     

Influence of sintering atmosphere and thermobaric treatment (TBT) on dielectric behaviors of CaCu3Ti4O12 ceramics

Copper titanate (CaCu₃Ti₄O₁₂, or CCTO) ceramics sintered in oxygen and vacuum at 1100°C for 12 h have been treated by thermobaric treatment (TBT) at 9 GPa and 1000°C for 10 min and then quenched in liquid nitrogen (LN₂). Pure cubic body-centered perovskite-related structure was confirmed from XRD results. Besides, after TBT and quenching, additional minor peaks of TiO₂ and Cu were indexed, as well as a little cell volume expansion. A little reduction of the grain size as well as fuzzy grain boundaries can be observed in FE-SEM after TBT. The dielectric constant ε' of CCTO ceramics sintered in oxygen (～4600 @ 100 Hz) were a half of those treated by TBT and quenching (～9600 @ 100 Hz), whereas the value decreased almost by an order of magnitude (from 10⁵ to 10⁴) after TBT and quenching for the samples sintered in vacuum. The complex impedance spectra at high temperature showed three semicircles so a three RC model was used to explain the different relaxation regions consisting of grain, grain boundary and domain boundary. Besides, two relaxation peaks appeared in the frequency dependence of the imaginary part of the electric modulus formalism $M^{?}$ and the complex impedance $Z^{?}$. Moreover, the contributions of grain boundaries can be figured out by the relaxation activation energy $E_{re}$ (338–629 meV) and the conduction one $E_{dc}$ (396–823 meV) fitted by $M^{?}$ and $Z^{?}$ plots separately. As a result, the disorder and heterogeneity typical of grain boundaries should be responsible for the giant dielectric relaxation characteristics of CCTO ceramics, which can be affected by the sintering atmosphere as well as the extreme treating conditions.     

Study of the band edge in CdIn2S4 by photovoltaic effect

Photovoltaic spectra were measured at 300 and 100 K on AuCdIn₂S₄ Schottky barriers in the spectral range near the band edge of the compound. Analysis of the spectra gives the values of the direct and indirect gaps at both temperatures together with the associated phonon energies. The results are compared with the predictions of the most recent band calculations on CdIn₂S₄.     

Revival of metastable behavior in phase separated La0.5Ca0.5MnO3+δ

The La_xCa_1−xMnO_3+δ compositions close to charge ordering (x∼0.5) show a gradual relaxation from a metallic/ferromagnetic state to an insulating/antiferromagnetic state with thermal cycling. Here, we report on the magnetic relaxation in the metastable state and also the revival of the metastable state (in a relaxed sample) due to high temperature thermal treatment. We also show the changes in the magnetization and the thermoelectric power as the revived metastable state is cycled. We find that the changes in the thermoelectric power extend well into the region above the charge ordering temperatures. This suggests that the micro-structural changes accompanying the thermal cycling leave their imprint in the paramagnetic insulating state as well.