Ac Resistivity Research Articles

The Effect of Heat Treatment on Structural and Multiferroic Properties of Phase-Pure BiFeO3

Phase-pure multiferroic BiFeO3 (BFO) was prepared by the coprecipitation technique using diverse precursors bismuth oxide at temperature as low as 400°C. The dependence of structural, microstructural, thermal, electrical (AC and DC), and magnetic properties on sintering temperature was systematically investigated. Uniaxially pressed samples (O8 mm) were sintered in air at 500°C to 800°C for 4 h. X-ray diffraction analysis was used to determine the amorphous and perovskite nature of as-synthesized and calcined/sintered samples, respectively. The crystallite size of sintered powders increased from 47 nm to 67 nm. Scanning electron microscopy showed grain growth during sintering, which improved intergranular connectivity and decreased porosity in the samples. The ferroelectric to paraelectric Curie transition temperature (TC) of pure BFO powder was detected by differential scanning calorimetry analysis and found to be 820°C ± 1°C. The samples exhibited high AC resistivity and dielectric constant, and low loss tangent values. The samples exhibited weak ferromagnetic behavior with an unsaturated magnetization versus magnetic field hysteresis loop at room temperature. Ferroelectric behavior and variation in remnant polarization and coercivity were observed from polarization versus electric field loops. Enhanced capacitance in the magnetic field revealed the magnetoelectric effect in the samples.

Superconductivity in Mg/MgO interface

Metastable superconductivity at ≈50K in the interfaces formed by metallic and oxidized magnesium (MgO) has been observed by ac magnetic susceptibility and electrical resistance measurements. The superconducting interfaces have been produced by the surface oxidation of metallic magnesium under special conditions.

Electrical, Dielectrical and Magnetic Properties of Zr-Mn Doped Nano-Ferrites

We measured the dc electrical resistivity as a function of temperature and dielectric parameters in the frequency range 100 Hz to 3 MHz of nanosized Zr-Mn spinel ferrites with nominal composition CoFe2-2xZrxMnxO4 (0.1 x 0.4). The dc electrical resistivity decreased with the rise in temperature for all the samples, showing a semiconductor like behavior. From the dc electrical resistivity the activation energy and drift mobility are determined. Both the drift mobility and activation energy increase with a rise in x. The dielectric constant, dielectric (losses) and ac electrical resistivity as a function of frequency are also reported. The low field ac magnetic susceptibility measurement showed that the ferrimagentic transition temperature is in the range of 4395 K to 6585 K.

Effect of Ca-substitution on the Magnetic and Dielectric Properties of Mn-Zn Ferrites

Spinel Mn-Zn ferrites with composition MnxZn0.4Ca0.6-2xFe2+xO4, where x = 0.10, 0.15, 0.20, 0.25 and 0.30 have been prepared by conventional ceramic technique sintered at 1300 °C for 4 hours. The influence of Ca-substitution on various properties of Mn-Zn ferrites have been studied in this work. Investigations were carried out by the measurements of Curie temperature, permeability, loss tangent, Q-factor, dielectric constant and AC resistivity of the samples. Curie temperature (Tc), the real part of initial permeability (µ´), loss tangent (tan ?), and AC resistivity have been found to be decreased while the Q-factor increases with the increase in Ca-content. The frequency characteristics of the dielectric constant and AC resistivity have been found to be decreased as the frequency increases. Maxwell-Wagner interfacial type of dielectric polarization was observed with the addition of Ca-content over the entire range of frequency considered. Keywords: Spinel ferrites; Sintering temperature; Initial permeability; Dielectric constant.© 2012 JSR Publications. ISSN: 2070-0237 (Print); 2070-0245 (Online). All rights reserved.doi: http://dx.doi.org/10.3329/jsr.v4i2.9752 J. Sci. Res. 4 (2), 297-306 (2012)

Effect of Ni doping on magnetism and superconductivity in Eu0.5K0.5Fe2As2

The effect of Ni-doping on the magnetism and superconductivity in Eu0.5K0.5Fe2As2 has been studied through a systematic investigation of magnetic and superconducting properties of Eu0.5K0.5(Fe1-xNix)2As2 (x = 0, 0.03, 0.05, 0.08 and 0.12) compounds by means of dc and ac magnetic susceptibilities, electrical resistivity and specific heat measurements. Eu0.5K0.5Fe2As2 is known to exhibit superconductivity with superconducting transition temperature Tc as high as 33 K. The Ni-doping leads to a rapid decrease in Tc; Tc is reduced to 23 K with 3% Ni-doping, and 8% Ni-doping suppresses the superconductivity to below 1.8 K. In 3% Ni-doped sample Eu0.5K0.5(Fe0.97Ni0.03)2As2 superconductivity coexists with short range ordering of Eu2+ magnetic moments at Tm ~ 6 K. The suppression of superconductivity with Ni-doping is accompanied with the emergence of a long range antiferromagnetic ordering with TN = 8.5 K and 7 K for Eu0.5K0.5(Fe0.92Ni0.08)2As2 and Eu0.5K0.5(Fe0.88Ni0.12)2As2, respectively. The temperature and field dependent magnetic measurements for x = 0.08 and 0.12 samples reflect the possibility of a helical magnetic ordering of Eu2 moments. We suspect that the helimagnetism of Eu spins could be responsible for the destruction of superconductivity as has been observed in Co-doped EuFe2As2. The most striking feature seen in the resistivity data for x = 0.08 is the reappearance of the anomaly presumably due to spin density wave transition at around 60 K. This could be attributed to the compensation of holes (K-doping at Eu-site) by the electrons (Ni-doping at Fe site). The anomaly associated with spin density wave further shifts to 200 K for x = 0.12 for which the electron doping has almost compensated the holes in the system.

AC electrical properties of FeIn2S4

The frequency and temperature dependences of the ac capacitance and resistivity of FeIn2S4 semiconductors are studied. Resonances are observed at certain temperatures in the frequency range (2.5–5.0) × 105 Hz. The permittivity of the crystals and the activation energy of charge carriers are determined. It is found that electrical conduction in the given temperature interval is governed by an activation mechanism. The activation energy is frequency-dependent, because the relaxation time of barrier layers decreases with rising frequency.

Magnetic and transport properties of Pr2Pt3Si5

We have investigated the magnetic and transport properties of a polycrystalline Pr2Pt3Si5 sample through the dc and ac magnetic susceptibilities, electrical resistivity, and specific heat measurements. The Rietveld refinement of the powder X-ray diffraction data reveals that Pr2Pt3Si5 crystallizes in the U2Co3Si5-type orthorhombic structure (space group Ibam). Both the dc and ac magnetic susceptibility data measured at low fields exhibit sharp anomaly near 15K. In contrast, the specific heat data exhibit only a broad anomaly implying no long range magnetic order down to 2K. The broad Schottky-type anomaly in low temperature specific heat data is interpreted in terms of crystal electric field (CEF) effect, and a CEF-split singlet ground state is inferred. The absence of the long range order is attributed to the presence of nonmagnetic singlet ground state of the Pr3+ ion. The electrical resistivity data exhibit metallic behavior and are well described by the Bloch–Grüniesen–Mott relation.

Dielectric loss, conductivity relaxation process and magnetic properties of Mg substituted Ni–Cu ferrites

The dielectric properties, dc and ac electrical resistivities of Mg substituted Ni–Cu ferrites with general formula Ni0.5Cu0.5−xMgxFe2O4 (0.0≤x≤0.5) have been investigated as a function of frequency, temperature and composition. ac resistivity of all the samples decreases with increase in the frequency exhibiting normal ferrimagnetic behavior. The frequency dependence of dielectric loss tangent showed a maximum in between 10Hz and 1kHz in all the ferrites. The conductivity relaxation of the charge carriers was examined using the electrical modulus formulism, and the results indicate the presence of the non-Debye type of relaxation in the prepared ferrites. Similar values of activation energies for dc conduction and for conductivity relaxation reveal that the mechanism of electrical conduction and dielectric polarization is the same in these ferrites. A single ‘master curve’ for normalized plots of all the modulus isotherms observed for a given composition indicates that the distribution of relaxation time is temperature independent. The saturation magnetization and coercivity as calculated from the hysteresis loop measurement show striking dependence on composition.

Magnetization and magneto-resistance in Y(Ba1−xSrx)2Cu3O7−δ (x = 0.00–0.50) superconductors

Here we present the magnetic properties and upper critical field (BC2) of polycrystalline Y(Ba1−xSrx)2Cu3O7−δ superconductors, which are determined through detailed ac/dc susceptibility and resistivity under magnetic field study. All the samples are synthesized through the solid state reaction route. Reduction in the Meissner fraction (the ratio of field cooled to zero field cooled magnetization) is observed with increasing Sr content, suggesting the occurrence of flux pinning in the doped samples. The ac susceptibility and resistivity measurements reveal improved grain coupling in Sr substituted samples. Consequently the inter-grain critical current density (Jc), upward curvature near Tc in the temperature dependence of the upper critical field (BC2(T)), and BC2 are enhanced. Both Jc and BC2 increase in lower Sr substitution (up to x = 0.10) samples followed by decrease for higher doping due to degradation in the effective pinning and grain coupling.

Electronic Transport in Tin(IV) Oxide Nanocrystalline Films: Two-Medium Transport with Three-Dimensional Variable-Range Hopping Mechanism for the Ultrasmall Nanocrystallite Size Regime

Homogeneous, nanocrystalline films of tin(IV) oxide with controllable crystalline grains in the ultrasmall size range of 4–12 nm have been prepared by using a simple method of spin-coating followed by annealing in oxygen at different postannealing temperatures (Tanneal). These nanocrystalline films all exhibit a high optical transparency of 90–100% in the visible region with a band gap of 3.71 ± 0.05–3.87 ± 0.05 eV compared to 3.6 eV for bulk SnO2, indicating a high carrier density for all the TO films. The films obtained with Tanneal ≥ 350 °C, marking the onset of crystallization, are found to be conductive. The ac resistivity is measured as a function of temperature between 50 and 280 K for all the conductive films, and two distinct behaviors are observed between 50 and 90 K (LT) and 120–280 K (HT). The presence of two different media, i.e., the crystalline grains and the charge-depletion layer, can explain the observed resistivity behavior. The excellent fit of a parallel resistor model to the resistivity data for samples obtained with Tanneal = 350–700 °C further validates the presence of the two media, revealing energy barrier heights of 48.0 ± 0.4–60.5 ± 0.4 meV for transport across the grain boundaries. The resistivity behavior in each medium is best described by the three-dimensional variable-range hopping (3D-VRH) model, given its excellent fit to the experimental data. On the basis of the resistivity results as analyzed within this model, we conclude that increasing Tanneal leads to a reduction in the carrier density as defect density decreases. The 3D-VRH fits to the resistivity in the LT region further reveal that above the onset of exponential growth at Tanneal = 500 °C, a remarkable improvement in the charge transport occurs likely due to the observed enhanced crystallinity. Postannealing at different temperatures, therefore, has a direct effect on the extent of crystallization in the amorphous matrix and the size of the resulting nanocrystallites, both of which affect the defect density and transport channels, and can therefore be used to provide fine control on the resistivity of the nanocrystalline SnO2 film.

Studies on structural, electrical and optical properties of dibromo bis(triphenyl phosphine oxide)cadmium(II) single crystal

Single crystals of semi-organic optical material dibromo bis(triphenyl phosphine oxide)cadmium(II)[CdBr2(TPPO)2] were grown by low temperature solution growth method. The composition of grown crystals was confirmed on the basis of energy dispersive analysis of X-ray (EDAX). The optical band gap of the material was calculated from the UV–Visible–NIR spectral analysis performed. Dielectric constant and dielectric loss were measured as a function of frequency and temperature. Study confirmed the contribution of space charge polarization. Also, ac resistivity and conductivity were calculated. The growth features were observed by etching study.

Effect of cation disorder on structural, magnetic and dielectric properties of La2MnCoO6 double perovskite

The origin of dielectric anomalies and magnetodielectric response of La2MnCoO6 has been investigated by means of ultra-high resolution synchrotron x-ray powder diffraction, neutron powder diffraction, resistivity, magnetization and dielectric measurements. The study has been performed on two different bulk samples presenting different degrees of Mn/Co order: 95 and 74%. Beside the well-known influence on magnetic properties, our results show that the main effect of disorder lies on the electrical resistivity. Bond distances clearly show Mn4+/Co2+ valence states in the well-ordered sample, while for the disordered one this picture still holds. AC resistivity data show dielectric anomalies and a small magnetodielectric effect, but impedance complex plane analyses prove that these phenomena appear at the frequency–temperature region where extrinsic effects dominate the dielectric response.

On the temperature dependent dielectric properties, conductivity and resistivity of MIS structures at 1MHz

In this study, the temperature dependence of the dielectric properties, conductivity and resistivity of metal-insulator-semiconductor (MIS) structures have been investigated using capacitance (C) and conductance (G/@w) measurements in wide temperature range of 120-400K at 1MHz. Calculation of the dielectric constant (@e'), dielectric loss (@e''), loss tangent (tan @d), ac conductivity (@sac), ac resistivity (@rac) and the real and imaginary parts of electric modulus (M' and M'') were given in the studied temperature range. The values of the @e', @e'' and @sac increase exponentially with the increasing temperature between 280K and 400K. On the other hand, these values remain almost constant between 120K and 240K. In addition, the experimental dielectric data have been analyzed by considering electric modulus formalism. The [email protected]ac vs. 1000/T plot shows two linear regions with different slopes which correspond to low (120-240K) and high (280-400K) temperature ranges. The values of activation energy for two different conduction mechanisms were found as 4meV and 201meV for low and high temperature ranges, respectively.

Exploration of the superconducting properties of Y 3Ba 5Cu 8O 18 with and without Ca doping by magnetic measurements

The superconducting Y 3Ba 5Cu 8O 18 (Y-358) and Y 3Ba 5Ca 2Cu 8O 18 (YCa-358) compounds have been synthesized by using the sol–gel method. Hence, the influence of doping of Ca into the compound Y-358 has been studied by comparing the resistivities, DC magnetizations both M(H) and M(T),flux pinning properties, AC susceptibilities and critical current densities of the undoped and doped compounds, at low temperatures. The AC susceptibility and resistivity measurements showed that the superconducting transition temperature, T c, is suppressed (about 6 K) with the addition of Ca into the main compound. The hysteresis loops of YCa-358 show peak like projecting parts at temperatures below 45 K and around zero applied field that may be due to the local modulation of the composition in YCa-358. Such a behavior has been observed for the first time. The critical current densities, J c, determined from the hysteresis measurements decrease with the addition of Ca into Y-358. At 15 K, the maximum values of J c, for the compounds Y-358 and YCa-358 are found to be 8 × 10 4 A/cm 2 and 4.5 × 10 4 A/cm 2, respectively. The flux pinning force, F p, calculated from the field dependence of the J c values shows that the irreversibility line shifts to lower magnetic fields with the doping of Ca into Y-358. Furthermore, the measurements of the inphase and out off components of the ACsusceptibilities clearly demonstrate that the superconducting volume fraction of Y-358 decreases with the addition of Ca.

Signatures of spin-glass behaviour in PrIr2B2 and heavy fermion behaviour in PrIr2B2C

The magnetic and transport properties of PrIr2B2 and PrIr2B2C have been investigated by dc and ac magnetic susceptibility, specific heat, electrical resistivity and magnetoresistance measurements. PrIr2B2 forms in CaRh2B2-type orthorhombic crystal structure (space group Fddd). At low fields the dc magnetic susceptibility of PrIr2B2 exhibits a sharp anomaly near 46 K which is followed by an abrupt increase below 10 K with a peak at 6 K, and split-up in ZFC and FC data below 46 K. In contrast, the specific heat exhibits only a broad Schottky type hump near 9 K which indicates that there is no long range magnetic order in this compound. The thermo-remanent magnetization is found to decay very slowly with a mean relaxation time τ = 3917 s. An ac magnetic susceptibility measurement also observes two sharp anomalies; the peak positions strongly depend on the frequency and shift towards high temperature with an increase in frequency, obeying the Vogel–Fulcher law as expected for a canonical spin-glass system. The two spin-glass transitions occur at freezing temperatures Tf1 = 36 K and Tf2 = 3.5 K with shifts in the freezing temperatures per decade of frequency δTf1 = 0.044 and δTf2 = 0.09. An analysis of the frequency dependence of the transition temperature with critical slowing down, τmax/τ0 = [(Tf−TSG)/TSG]−zν, gives τ0 = 10−7 s and exponent zν = 8, and the Vogel–Fulcher law gives an activation energy of 84 K for Tf1 and 27.5 K for Tf2. While zν = 8 is typical for spin-glass system, the characteristic relaxation time τ0 = 10−7 s is very large and comparable to that of superspin-glass systems. An addition of C in PrIr2B2 leads to PrIr2B2C which forms in LuNi2B2C-type tetragonal structure (space group I4/mmm) and remains paramagnetic down to 2 K. The specific heat data show a broad Schottky type anomaly, which could be fairly reproduced with CEF analysis which suggests that the ground state is a CEF-split singlet and the first excited state singlet is situated 15 K above the ground state. The Sommerfeld coefficient γ∼300 mJ mol−1 K−2 of PrIr2B2C is very high and reflects a heavy fermion behaviour in this compound. We believe that the heavy fermion state in PrIr2B2C has its origin in low lying crystal field excitations as has been observed in PrRh2B2C.

Transmission line model of carbon nanotubes: through the Boltzmann transport equation

A transmission line (TL) model of a carbon nanotube (CNT) is analyzed through the Boltzmann transport equation (BTE). With the help of a numerical solution of the BTE, we study the kinetic inductance (LK), quantum capacitance (CQ) and resistivity (RS) of a CNT under a high frequency electric field. Values of LK and CQ obtained from BTE accord with the theoretical values, and the TL model is verified by transport theory for the first time. Moreover, our results show that the AC resistivity of CNTs deviates from DC, increasing along with shorter electric field wave length. This shows that changes in RS in the high frequency condition must be considered in the TL model.

Thermal and electrical properties of carbon nanotubes based polysulfone nanocomposites

Carbon nanotubes (CNTs)-reinforced polysulfone (PSU) nanocomposites were prepared through solution mixing of PSU and different weight percent of multi-walled carbon nanotubes (MWCNTs). Thermal properties of nanocomposites were characterized using thermo-gravimetric analysis (TGA) and differential scanning calorimetry (DSC). TGA studies revealed an increase in thermal stability of the PSU/MWCNTs nanocomposites, which is due to the hindrance of the nanodispered carbon nanotubes to the thermal transfer in nanocomposites and also due to higher thermal stability of CNTs. Morphological properties of nanocomposites were characterized by high resolution transmission electron microscopy (HRTEM) and field emission scanning electron microscope (FESEM). The influence of CNTs loading on electrical properties of PSU/MWCNTs nanocomposites was studied by the measurement of AC and DC resistivity. Dielectric study of nanocomposites was carried out at different frequencies (10 Hz–1 MHz) by using LCR meter. An increase in dielectric constant and dielectric loss was observed with increase in CNTs content, which is due to the interfacial polarization between conducting CNTs and PSU.

Effect of Si/Ge ratio on resistivity and thermopower in Gd5SixGe4−x magnetocaloric compounds

The effect of Si/Ge ratio on resistivity and thermopower behavior has been investigated in the magnetocaloric ferromagnetic Gd5SixGe4−x compounds with x=1.7–2.3. Microstructural studies reveal the presence of Gd5(Si,Ge)4-matrix phase (5:4-type) along with traces of secondary phases (5:5 or 5:3-type). The x=1.7 and 2.0 samples display the presence of a first order structural transition from orthorhombic to monoclinic phase followed by a magnetic transition of the monoclinic phase. The alloys with x=2.2 and 2.3 display only magnetic transitions of the orthorhombic phase. A low temperature feature apparent in the AC susceptibility and resistivity data below 100K reflects an antiferromagnetic transition of secondary phase(s) present in these compounds. The resistivity behavior study correlates with microstructural studies. A large change in thermopower of −8μV/K was obtained at the magneto-structural transition for the x=2 compound.

The effect of substitution of Mn by Fe and Cr on the martensitic transition in theNi50Mn34In16 alloy

The potential shape memory alloyNi50Mn34In16 is studied with partial substitution of Mn with Fe and Cr to investigate the effect of suchsubstitution on the martensitic transition in the Ni–Mn–In alloy system. Theresults of ac susceptibility, magnetization and electrical resistivity measurementsshow that while the substitution with Cr increases the martensitic transitiontemperature, the substitution with Fe decreases it. Possible reasons for this shift inmartensitic transition are discussed. Evidence of kinetic arrest of the austenite tomartensite phase transition in the Fe substituted alloys is also presented. Unlikethe kinetic arrest of the austenite to martensite phase transition in the parentNi50Mn34In16 alloy which takes place in the presence of high external magnetic field, the kinetic arrest ofthe austenite to martensite phase transition in the Fe doped alloy occurs even in zeromagnetic field. The Cr substituted alloys, on the other hand, show no signature of kineticarrest of this phase transition.

Effect of substitution with potassium and chromium oxides on the magnetic and electrical properties of Li-ferrite

Li-ferrite samples substituted with potassium and chromium oxides were prepared by using ceramic method. The chemical formula K y Li 0.5− y Cr x Fe 2.5− x O 4 ( x = 1.15, 1.2, … and 1.35; y = 0.0 and 0.05) was used. The Curie and compensation temperatures, T c and T o are decreased respectively with the increasing of the Cr content for all samples with potassium and without potassium. For Li–Cr ferrite, the values of T o dose not change with potassium substitution ions relative to that sample without potassium at each Cr content. However, the Curie temperature, T c and ac resistivity were increased (improved) with the potassium substitution relative to the samples without potassium at each Cr content. Furthermore, the magnetization was improved with the increasing of Cr content for, Li-ferrite at a temperature greater than the compensation temperature.