Magnetoelectric effect in cobalt ferrite-barium titanate composites and their electrical properties

CoFe₂O₄-BaTiO₃ composites were prepared using conventional ceramic double sintering process with various compositions. Presence of two phases in the composites was confirmed using X-ray diffraction. The dc resistivity and thermoemf as a function of temperature in the temperature range 300 K to 600 K were measured. Variation of dielectric constant (ɛ′) with frequency in the range 100 Hz to 1 MHz and also with temperature at a fixed frequency of 1 kHz was studied. The ac conductivity was derived from dielectric constant (ɛ′) and loss tangent (tan δ). The nature of conduction is discussed on the basis of small polaron hopping model. The static value of magnetoelectric conversion factor has been studied as a function of magnetic field.     

Electric-field dependence of dielectric properties of sol-gel derived Ba(Zr0.2Ti0.8)O3 ceramics

The effect of a dc bias field on the diffuse phase transition and nonlinear dielectric properties of sol-gel derived Ba(Zr_0.2Ti_0.8)O₃ (BZT) ceramics are investigated. Diffuse phase transitions were observed in BZT ceramics and the Curie–Weiss exponent (CWE) was γ∼2.0. The dielectric constant versus temperature characteristics and the γ in the modified Curie–Weiss law, ε ⁻¹=ε _m ⁻¹[1+(T−T _m ) ^γ /C₁](1≤γ≤2), as a function of the dc bias field was obtained for BZT ceramics. The results indicated that γ is a function of dc bias field, and the γ value decreased from 2.04 to 1.73 with dc bias field increasing from 0 to 20 kV/cm. The dielectric constant decreases with increasing dc bias field, indicating a field-induced phase transition. The dc bias field has a strong effect on the position of the dielectric peak and affects the magnitude of the dielectric properties over a rather wide temperature range. The peak temperature of the dielectric loss does not coincide with the dielectric peak and an obvious minimum value for the dielectric loss at the temperature of the dielectric peaks is observed. At room temperature, 300 K, the high tunability (K=80%), the low loss tangent (≈0.01) and the large FOM (74), clearly imply that these ceramics are promising materials for tunable capacitor-device applications.     

Transport and dielectric properties of thin polycrystalline CoO films

Transport and dielectric properties of polycrystalline CoO films were studied as functions of the applied field, frequency and temperature. TheI–V plots showed that the Poole-Frenkel field emission mechanism is responsible for conduction at fields>10⁵ V/cm. The ac conductivity σ(ω), the imaginary part of the dielectric constantε ₂, and tan δ plots as functions of frequency revealed three dispersion regions. The σ(ω) andε ₂ frequency dependence indicates a non-adiabatic hopping of charge carriers at low frequencies and adiabatic hopping at high frequencies. The activation energy of a dielectric oscillator is 0.15 eV. Work supported by the Office of Naval Research.     

A. C. Electroluminescence of CdS : Sm and CdS : Cu,Sm EL phosphors

Electroluminescent CdS : Sm and CdS : Cu, Sm phosphors have been prepared and their EL characteristics are investigated. The brightness waves of these phosphors were measured at a sinusoidal alternating voltages with a frequency of 500 Hz, 1 kHz, 2 kHz and 5 kHz. One primary and one secondary peak have been observed in each half period of the applied sinusoidal field. Time averaged EL brightness has been found to follow the Alfrey-Taylor relationB= =B ₀ exp (–b/V ^1/2) over a wide range of frequencies. Variation of the constants of this relation with frequency of the applied field have been studied. Variation of current across the EL cell with the applied voltage has also been investigated. The EL emission spectra show peaks at 600 and 650 nm in CdS : Sm phosphor whereas in the case of CdS : Cu, Sm phosphor the emission peak occurs at 600 nm. The observed results have been interpreted in terms of acceleration collision theory of the electroluminescence.One of the authors (RKT) expresses his gratitude to the University Grants Commission, New Delhi, for a fellowship and the Principal, S. D. J. Post Graduate College, Chandesar Azamgarh, for study leave.     

Modeling of transient electroluminescence overshoot in bilayer organic light-emitting diodes using rate equations

When a voltage pulse is applied under forward biased condition to a spin-coated bilayer organic light-emitting diode (OLED), then initially the electroluminescence (EL) intensity appearing after a delay time, increases with time and later on it attains a saturation value. At the end of the voltage pulse, the EL intensity decreases with time, attains a minimum intensity and then it again increases with time, attains a peak value and later on it decreases with time. For the OLEDs, in which the lifetime of trapped carriers is less than the decay time of the EL occurring prior to the onset of overshoot, the EL overshoot begins just after the end of voltage pulse. The overshoot in spin-coated bilayer OLEDs is caused by the presence of an interfacial layer of finite thickness between hole and electron transporting layers in which both transport molecules coexist, whereby the interfacial energy barrier impedes both hole and electron passage. When a voltage pulse is applied to a bilayer OLED, positive and negative space charges are established at the opposite faces of the interfacial layer. Subsequently, the charge recombination occurs with the incoming flux of injected carriers of opposite polarity. When the voltage is turned off, the interfacial charges recombine under the action of their mutual electric field. Thus, after switching off the external voltage the electrons stored in the interface next to the anode cell compartment experience an electric field directed from cathode to anode, and therefore, the electrons move towards the cathode, that is, towards the positive space charge, whereby electron–hole recombination gives rise to luminescence. The EL prior to onset of overshoot is caused by the movement of electrons in the electron transporting states, however, the EL in the overshoot region is caused by the movement of detrapped electrons. On the basis of the rate equations for the detrapping and recombination of charge carriers accumulated at the interface expressions are derived for the transient EL intensity I, time t_m and intensity I_m corresponding to the peak of EL overshoot, total EL intensity I_t and decay of the intensity of EL overshoot. In fact, the decay prior to the onset of EL overshoot is the decay of number of electrons moving in the electron transporting states. The ratio I_m/I_s decreases with increasing value of the applied pulse voltage because I_m increases linearly with the amplitude of applied voltage pulse and I_s increases nonlinearly and rapidly with the increasing amplitude of applied voltage pulse. The lifetime τ_t of electrons at the interface decreases with increasing temperature whereby the dependence of τ_t on temperature follows Arrhenius plot. This fact indicates that the detrapping involves thermally-assisted tunneling of electrons. Using the EL overshoot in bilayer OLEDs, the lifetime of the charge carriers at the interface, recombination time of charge carriers, decay time of the EL prior to onset of overshoot, and the time delay between the voltage pulse and onset time of the EL overshoot can be determined. The intense EL overshoot of nanosecond or shorter time duration may be useful in digital communication, and moreover, the EL overshoot gives important information about the processes involving injection, transport and recombination of charge carriers. The criteria for appearance of EL overshoot in bilayer OLEDs are explored. A good agreement is found between the theoretical and experimental results.     

Linear stability analysis of electrohydrodynamic instabilities at fluid interfaces in the “small feature” limit

The endeavour to effectively harness interfacial electrohydrodynamic instabilities, to create small patterns, involves reducing the wavelength of the instability. This can be accomplished by decreasing the separation between the electrodes which may not always be possible. One may therefore have to reduce the surface tension or increase the applied voltage at a fixed electrode spacing. This can result in the wavelength of the pattern becoming of the same order as the electrode separation. Pease and Russel (J. Chem. Phys. 118, 3790 (2003)) were the first to argue that the commonly used Thin-Film Approximation (TFA) that involves an asymptotic expansion in the small parameter δ = (ε ₀ φ ₀²/(γh ₀))^1/2 (where ε ₀ is the permittivity of vacuum, φ ₀ is the root mean square value of the applied potential, γ is the surface tension and h ₀ is the thickness of the thin film) need not always be valid and γ may not be small in experiments. Higher-order corrections to the TFA might therefore be necessary. We extend the Direct Current (DC) field analysis of Pease and Russel to an Alternating Current (AC) field. AC field has been suggested as an effective way of controlling the wavelength of electrohydrodynamic instabilities at fluid-fluid interfaces. Infact, the perfect and leaky dielectric limits can be realised in the same fluid at very high and very low electric field frequencies, respectively. Recently, Roberts and Kumar (J. Fluid Mech. 631, 255 (2009)) carried out an analysis using TFA to investigate AC-field-induced instabilities at air-polymer interfaces. We propose a Generalized Model (GM), without the lubrication approximation, and carry out detailed comparison with the TFA. We consider the top fluid to be air, a perfect dielectric, and the bottom fluid to be a perfect or a leaky dielectric. The analysis is carried out for both DC and AC fields, and the deviation from TFA is expressed in terms of the parameter B = γh ₀/(ε ₀ φ ₀²) = δ ^{− 2}. We discuss variation of the wavelength of the fastest growing mode with frequency of the applied field for any arbitrary value of B, unlike the analysis of Roberts and Kumar which is restricted to B ≫ 1(δ ≪ 1) . We also revisit the analysis of Pease and Russel for instabilities under DC field and present the results in terms of the single parameter, B.     

Effect of grain boundary on resistive magnetodielectric property of polycrystalline <Emphasis Type="Italic">γ</Emphasis>-Fe<Subscript>2</Subscript>O<Subscript>3</Subscript>

We have investigated the role of the grain boundary on the resistive magnetodielectric property of polycrystalline γ-Fe₂O₃ through impedance spectroscopy measurements. Depending on the sample preparation temperature, the dielectric constant of γ-Fe₂O₃ is significantly different especially at low frequencies (<10⁴ Hz) and high temperatures (>200 K). The value of the magnetodielectric effect at a specific frequency and the resonance frequency for the maximized magnetodielectric effect are different, although polycrystalline γ-Fe₂O₃ samples show a quite similar magnetoresistance. Through the experimentally obtained resistance ratio between the grain and the grain boundary, we can reproduce the magnetodielectric curves based on the Maxwell–Wagner model and the measured magnetoresistance.     

Electric and dielectric properties of solution-gas interface grown amorphous AgCl films

Electric and dielectric properties of solution-gas interface grown AgCl thin film capacitors (Al/AgCl/Al) of various thicknesses have been studied in the frequency range 10¹–10⁶ Hz at various temperatures (303–393 K). I–V characteristics show ohmic, space-charge-limited, and thermionic emission conduction mechanisms to operate at low, intermediate and high voltages respectively. Capacitance decreases with increasing film thickness and applied frequency while it increases with increase of temperature. Loss factor (tanδ), which shows a pronounced minimum with frequency, increases with the rise of temperature and (tanδ)_min shifts to a higher frequency. The large values of capacitance and dielectric constant (ɛ) in the low frequency region indicate the possibility of an interfacial polarization mechanism to operate in this region while electronic and ionic polarizations dominate in the high frequency region.     

Scanning probe microscopy investigation of ferroelectric properties of barium strontium niobate crystals

The ferroelectric properties of relaxor ferroelectric materials Sr _x Ba_{1 − x} Nb₂O₆ of different compositions have been investigated using scanning probe microscopy and macroscopic polarization methods. An analysis of the results obtained from microscopic and macroscopic measurements of the hysteresis loops has demonstrated that a frequency dependence of the coercive field E _c is observed at frequencies f in the range from 0.03 to 250 Hz, so that the coercive field E _c regularly decreases with decreasing frequency. The observation of the kinetics of domains under a direct-current voltage applied to the probe of the atomic force microscope has revealed a lateral motion of domain walls in response to electric fields E ≪ E _c . This result qualitatively explains a slow polarization relaxation occurring for a giant time in Sr _x Ba_{1 − x} Nb₂O₆.     

Ferroelectric relaxor behaviour in Pb(Fe0.5Ta0.5)O3

The relaxor ferroelectric lead iron tantalate, Pb(Fe_0.5Ta_0.5)O₃ (PFT) is synthesized by Coulombite precursor method. The X-ray diffraction pattern of the sample at room temperature shows a cubic phase. The field dependence of dielectric response is measured in a frequency range 0.1 kHz — 1 MHz and in a temperature range from 173–373 K. The temperature dependence of permittivity (ɛ′) shows broad maxima at various frequencies. The frequency dependence of the permittivity maximum temperature (T _m ) has been modelled using Vogel-Fulcher relation.      

Influence of hydrogen on the volt-ampere and volt-farad characteristics of mis silicon tunnel diodes

The results of a joint analysis of volt-ampere and volt-farad characteristics of aPd−SiO _x -n-Si structure with a thin (3.7 nm) oxide tunnel layer are presented. It is shown that the forward-bias region of the volt-ampere characteristic can be used to identify the dependence of the surface potential ϕ_δ inSi on the voltage with consideration of the dielectric layer inhomogeneity as a function of the dielectric thickness. The probability of tunneling through theSiO _x layer is estimated for local regions with the least thickness equal to 1.3 nm. When the voltage increases from 0.1 to 0.7 V, the probability of tunneling decreases from 0.78 to 0.40, whereas the potential barrier height increases from 0.04 to 0.08 eV with consideration of the mirror image forces (given that the relativeSiO _x permittivity is equal to 3.9). In going from the room atmosphere to a gas mixture comprising 50 vol.% of hydrogen, the positive-charge density in the dielectric increases by 8·10⁻⁷ C/cm². Moreover, the voltage on flat bands and j _s decrease for local regions of a specimen with thin dielectric layers much more weakly than for most of the field electrode. V. D. Kuznetsov Siberian Physical-Technical Institute at Tomsk State University. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 9, pp. 3–11, September, 1999.     

Thin porous anodic alumina films: Interface trap density determination

The electrical properties of thin porous alumina films in the form of MOS structures were studied with dielectric spectroscopy at room temperature. The thickness of the samples was found to be approximately 95 nm with a cross section area of 1.6×10⁻³ cm².C-V andG-V measurements were performed by applying a loop sweep voltage 2.0 V to −5.0 V. Correction to the measurements were performed by considering a series resistanceR _S and leakage currentI _DC. TheC-V results show hysteresis effects due to the presence of positive charges in porous alumina. These charges may be are attributed to the residual electrolyte during the anodization process in the sample preparation. Three distinct regions in theC-V results are observed, namely the inversion, the depletion and the accumulation regions. In the voltage region, where depletion of carriers is observed,C-f andG-f measurements were recorded in the frequency range 1 Hz to 10⁶ Hz. The conductance method was applied for the calculation of the density D_it of carriers, trapped in the interface between insulator and semiconductor, and of the response time τ_it as a function of the applied bias voltage were performed. The values obtained for D_it and τ_it are of the order of 3×10¹² eV⁻¹ cm⁻² and 10⁻³ s, respectively, and they are voltage dependent. Paper presented at the Patras Conference on Solid State Ionics — Transport Properties, Patras, Greece, Sept. 14 – 18, 2004.     

等静压下0.75Pb（Mg1/3Nb2/3）O3-0.25PbTiO3陶瓷的介电性能研究

研究了等静压对0.75Pb（Mg_1/3Nb_2/3）O₃-0.25PbTiO₃（PMN-25PT）陶瓷介电温谱的影响,PMN-25PT剩余极化随等静压变化和等静压压致相变.结果表明,随着压力增加,PMN-25PT的介电峰值温度T_m降低,/+{dT_m}/-{dP}≈-4℃/kbar,极化弛豫增强;剩余极化随压力增加连续减小;介电常数对压力的依赖关系与对温度场的依赖相似,压力诱导PMN-25PT发生弛豫铁电—顺电相变,相变为宽化的渐变过程,频率色散和极化弛豫更加强烈和普遍. 关键词： 铌镁酸铅-钛酸铅 等静压 介电弛豫 压致相变     

Effect of nickel doping on structural, optical and electrical properties of TiO2 nanoparticles by sol-gel method

Nickel-doped anatase TiO₂ nanoparticles have been prepared by sol-gel method. The X-ray powder diffraction study reveals that all the prepared samples have pure anatase phase tetragonal system. The average crystallite size of the prepared sample is 14 nm, when found through transmission electron microscope. A strong frequency dependence of both dielectric constant (?′) and dielectric loss (tan δ) were observed for various dopant levels at room temperature in the frequency range of 42 Hz to 5 MHz. At low frequency, the piling up of mobile charge carriers at the grain boundary produces interfacial polarization giving rise to high dielectric constant. The asymmetric shape of frequency dependence of the dielectric loss for the primary relaxation process is observed for each concentration. From the ac conductivity studies, the reduction in conductivity may arise due to the decreasing particle with the increase in Ni-dopant level.     

Magnetoresistance of LaCoO<Subscript>3</Subscript> and an insulator-metal transition induced in it by a high magnetic field

The transformation of the band structure of LaCoO₃ in the applied magnetic field has been theoretically studied. If the field is below its critical value B _C ≈ 65 T, the dielectric band gap decreases with the field, thus giving rise to negative magnetoresistance that is highest at T ≃ 300–500 K. The critical field is related to the crossover between the low- and high-spin terms of Co³⁺ ions. The spin crossover results in an insulator-metal transition induced by an increase in the magnetic field.     

Influence of the stress on magnetoelectric effect in magnetostrictive-PZT bilayers

In this letter, we investigate the influence of the stress on magnetoelectric (ME) effect in a magnetostrictive-PZT bilayer. ME voltage coefficient α*_E = δE / δH, where δE is the induced electric field for an applied alternating current (ac) magnetic field δH, is obtained by solving the stress-related piezoelectric constitutive equation and the conventional magnetostrictive equation with appropriate boundary condition. Based on the free-energy density function of the PZT film in stress state, we get the stress-related piezoelectric charge coefficient ^p d*₃₁ and dielectric permittivity ^pε*₃₃. After taking the cobalt ferrite (CFO) as magnetostrictive phase, it is found that α*_E increases with decreasing 2-d compressive stress for CFO-PZT, which not only is qualitatively consistent with previous experimental measurements, but also provides a possible route to improve the ME effect.      

Pressure dependence of the Raman frequency shifts related to the thermodynamic quantities in phase II of s-triazine

The temperature-induced frequency shifts (1/ω)(∂ω/∂T) _p are calculated from the pressure induced frequency shifts (1/ω)(∂ω/∂P) _T using the observed frequencies at various pressures for the Raman modes of II and III in phase II (P > P _C ) of s-triazine. The values of the mode Grüneisen parameters γ _T (isobaric) and γ _T (isothermal) of those Raman modes studied here, are determined through the Pippard relations in the spectroscopically modified forms for this crystal. The temperature-induced and the pressure-induced frequency shifts are then used to predict the pressure dependence of the isothermal compressibility, thermal expansion and the specific heat in phase II (P > P _C ) of s-triazine.     

High-dielectric losses in stabilized γ-iron/graphite nanocomposites

γ-iron and γ-iron/graphite nanoparticles were synthesized by a non-equilibrium cooling arc-discharge method. Experimental extinction spectra exhibit unexpected dielectric resonance of γ-iron nanoparticles at the microwave frequency, presenting the first metallic nanostructure of the high-dielectric loss. We further construct a multi-dielectric loss structure, with the enhanced dielectric loss factor of 0.7–0.9, by encapsulating the γ-iron into onion-like graphite nanoshells.     

Space-charge relaxation and electrical conduction in?K0.5Na0.5NbO3 at high temperatures

Sodium potassium niobate K_0.5Na_0.5NbO₃(KNN) ceramic was synthesized by a solid-state technique. The X-ray diffraction of the sample at room temperature showed a monoclinic phase. The real part (ε′) and imaginary part (ε″) of dielectric permittivity of the sample were measured in a frequency range from 40 Hz to 1 MHz and in a temperature range from 350 to 850 K. The ε′ deviated from Curie–Weiss law above 702 K, due to additional dielectric contributions resulting from universal dielectric response and thermally activated space charges at high temperatures. This anomaly arose from a Debye dielectric dispersion that slowed down following an Arrhenius law. We have established a link between the dielectric relaxation and the conductivity.     

Photo and electroluminescence of ZnO : Er and ZnO : Ag,Er electroluminors

A number of ZnO : Er and ZnO : Ag, Er electroluminors have been prepared and their photo (PL) and electroluminescent (EL) properties investigated. While the addition of Ag slightly shifts the PL spectra towards longer wavelength side, the EL spectra not only shift but consist of some new transitions. In ZnO : Er electroluminors, additional transitions also exist at higher frequencies of excitations. Brightness waves for this system consist of two secondary peaks during each half cycle of exciting field. Temperature dependence shows two broad peaks. While voltage dependence of ZnO : Er satisfies the relationB=B ₀ exp(−b/V ^1/2), the relationB=B ₀ V exp(−b/V ^1/2) is found to be suitable for ZnO : Ag, Er electroluminor. Possible mechanisms for these phenomena have been proposed. A preliminary account of this work was presented at the International Symposium on Solid State Physics held at the Indian Association for the Cultivation of Science, Calcutta in January 1977.