Superparamagnetic behavior of La0.67Sr0.33MnO3 nanoparticles prepared via sol-gel method

Magnetic nanoparticles of La_0.67Sr_0.33MnO₃ (LSMO) manganite were prepared by sol-gel method. Phase formation and crystal structure of the synthesized powder were examined by the X-ray diffraction (XRD) using the Rietveld analysis. The mean particle size was determined by the transmission electron microscopy (TEM). Infrared transmission spectroscopy revealed that stretching and bending modes are influenced by calcinations temperature. The temperature dependence of the ac magnetic susceptibility was measured at different frequencies and ac magnetic fields in the selected ranges of 40-1000 Hz and 80-800 A/m, respectively. The temperature dependence of ac susceptibility shows a characteristic maxima corresponding to the blocking temperature near room temperature. The frequency dependence of the blocking temperature is well described by the Vogel-Fulcher law. By fitting the experimental data with this law, the relaxation time τ₀=1.7×10⁻¹² s, characteristic temperature T₀=262±3 K, anisotropy energy E_a/k=684±15 K and effective magnetic anisotropy constant k_eff=2.25×10⁴ erg/cm³ have been obtained. dc Magnetization measurement versus magnetic field shows that some of LSMO nanoparticles are blocked at 293 K. The role of magnetic interparticle interactions on the magnetic behavior is also investigated.     

Time series of ac conductivities of terbium nitrate crystals at low temperatures

Time series of ac conductivities are measured at 2 kHz along three crystal directions in terbium nitrate crystal below 50 K. The anisotropy is found in the structure of the fluctuation observed in the time series of the conductivities. The bursts with non-periodic oscillations are found in only the measurements of the time series of the ac conductivities for one direction perpendicular to the c-axis (c2-axis). The anisotropy is seen in probability distribution functions and power spectral ones derived from the time series of the conductivities. The characteristic behaviors are found in the time series of the conductivity for the one crystal direction perpendicular to the c-axis. Asymmetric non-Gaussian line shape of the probability distribution function is seen for the c2-axis. Chaotic behavior is also found in the function of the correlation exponent to the embedding dimension derived from the time series of the conductivities.     

Dielectric properties and conductivity in CuO and MoO3 doped borophosphate glasses

A novel set of glasses of the type (B₂O₃)_0.10-(P₂O₅)_0.40-(CuO)_0.50−x-(MoO₃)_x, 0.05≤x≥0.50, have been investigated for dielectric properties in the frequency range 100 Hz-100 kHz and temperature range 300-575 K. From the total conductivity derived from the dielectric spectrum the frequency exponent, s, and dc and ac components of the conductivity were determined. The temperature dependence of dc and ac conductivities at different frequencies was analyzed using Mott's small polaron hopping model, and the high temperature activation energies have been estimated and discussed. The observed initial decrease in conductivity (ac and dc) and increase in activation energy with the addition of MoO₃ have been understood to be due to the hindrance offered by the Mo⁺ ions to the electronic motions. The observed peak-like behavior in conductivity (dip-like behavior in activation energy) in the composition range 0.20-0.50 mol fractions of MoO₃ may be due to mixed transition effect occurring in the present glasses. The temperature dependence of frequency exponent, s, has been analyzed using different theoretical models. It is for the first time that the mixed transition metal ion (TMI) doped borophosphate glasses have been investigated for dielectric properties and conductivity over wide temperature and frequency ranges and the data have been subjected to a thorough analysis.     

Effect of iron doping on the characterization and transport properties of calcium phosphate glassy semiconductors

X-ray diffraction (XRD), differential scanning calorimeter (DSC), density (d) and dc conductivity (σ) of the glasses in Fe₂O₃-CaO-P₂O₅ system were reported. The dc conductivity in the temperature range 303-453 K was measured. The overall features of these XRD curves confirm the amorphous nature of the present samples. The density of glasses increases from 2.750 to 2.892 g/cm³ with increasing Fe₂O₃ content as a result of a strengthening of cross-linking within glass network. The glass temperature values (T_g) of the present glasses were larger than those of tellurite glasses. This indicates a higher thermal stability of the glass in the present system. The glasses had conductivities ranging from 10⁻⁹ to 10⁻⁵ Sm⁻¹ at temperatures from 303 to 453 K. Electrical conduction of the glasses was confirmed to be due to non-adiabatic small polaron hopping and the conduction was primarily determined by hopping carrier mobility.     

Temperature dependence of thermal property for lead nitrate crystal

Thermal property was measured in a lead nitrate crystal, Pb(NO₃)₂, at temperatures from 90 to 340 K by use of ac calorimetry technique. The heat capacity derived from the measurements showed temperature dependence with thermal hysteresis, in the temperature region from 240 to 300 K. The anomaly of the heat capacity was found in the vicinity of 275.22 K. The broad temperature variation in the heat capacity was observed in the region from 235 to 260 K.     

Effect of Pb on the electrical properties of a-Ge20Se80 glasses

The present paper reports the effect of Pb impurity (low ∼2 at% and high ∼10 at%) on the ac conductivity (σ_ac) of a-Ge₂₀Se₈₀ glass. Frequency-dependent ac conductance and capacitance of the samples over a frequency range ∼100 Hz to 50 kHz have been taken in the temperature range ∼268 to 358 K. At frequency 2 kHz and temperature 298 K, the value of σ_ac increases at low as well as at higher concentration of Pb. σ_ac is proportional to ω^s for undoped and doped samples. The value of frequency exponent (s) decreases as the temperature increases. The static permittivity (ε_s) increases at both Pb concentrations. These results have been explained on the basis of some structural changes at low and higher concentration of Pb impurity.     

Electrical and chemical characteristics of nano-meter gold encapsulated in mesoporous and microporous channels and cages of FSM-16 and Y zeolites

The dc and ac conductivities as well as the dielectric constant () were measured for different zeolites encapsulated gold (AuCl₃) samples at different temperatures (300-500 K) and various frequencies (5 kHz-1 MHz). The conductivity was found to change in the order Au/FSM-27>Au/NaY>Au/FSM-47. Sorbed water contained inside zeolites assists greatly the proton mobility (zeolite protons) and the ion mobility (Na⁺ and Au⁺) and hence enhance the electric conduction in the temperature range 300-373 K. Raising the temperature over 373 K induces dehydration effect that assists the metallic gold formation and thus a dramatic loss in conductivity was revealed. The conduction mechanism was expected to be partially ionic and partially electronic. The IR study showed that the exposure of Au zeolites to CO gas produced a characteristic band of Au⁺-CO at 2180 cm⁻¹ that tends to decrease with temperatures and even vanishes at 376 K in favor of Au⁰-CO at 2128 cm⁻¹. Similarly, a phase transition at 338 K, that occurs in the range 300-376 K, was confirmed by DTA to further emphasize the temperature regions of either Au⁺ cations (338 K) or Au⁰ (376 K) formation.     

Refractive index, oscillator parameters and temperature-tuned energy band gap of Tl4In3GaS8-layered single crystals

Transmission and reflection measurements in the wavelength region 450-1100 nm were carried out on Tl₄In₃GaS₈-layered single crystals. The analysis of the room temperature absorption data revealed the presence of both optical indirect and direct transitions with band gap energies of 2.32 and 2.52 eV, respectively. The rate of change of the indirect band gap with temperature dE_gi/dT=-6.0×10⁻⁴ eV/K was determined from transmission measurements in the temperature range of 10-300 K. The absolute zero value of the band gap energy was obtained as E_gi(0)=2.44 eV. The dispersion of the refractive index is discussed in terms of the Wemple-DiDomenico single-effective-oscillator model. The refractive index dispersion parameters: oscillator energy, dispersion energy, oscillator strength and zero-frequency refractive index were found to be 4.87 eV, 26.77 eV, 8.48×10¹³ m⁻² and 2.55, respectively.     

Heat capacity of paramagnetic nickelocene: Comparison with diamagnetic ferrocene

Nickelocene [bis(η⁵-cyclopentadienyl)nickel: Ni(C₅H₅)₂, electron spin S=1, the ground state configuration ³A_2g] is paramagnetic and belongs to a typical molecule-based magnet. Heat capacities of nickelocene have been measured at temperatures in the 3−320 K range by adiabatic calorimetry. By comparing with those of diamagnetic ferrocene crystal, a small heat capacity peak centered at around 15 K and a sluggish hump centered at around 135 K were successfully separated. The low-temperature peak at 15 K caused by the spin is well reproduced by the Schottky anomaly due to the uniaxial zero-field splitting of the spin S=1 with the uniaxial zero-field splitting parameter D/k=45 K (k: the Boltzmann constant). The magnetic entropy 9.7 J K⁻¹ mol⁻¹ is substantially the same as the contribution from the spin-manifold R ln 3=9.13 J K⁻¹ mol⁻¹ (R: the gas constant). The sluggish hump centered at around 135 K arises from rotational disordering of the cyclopentadienyl rings of nickelocene molecule. The enthalpy and entropy gains due to this anomaly are 890 J mol⁻¹ and 6.9 J K⁻¹ mol⁻¹, respectively. As the hump spreads over a wide temperature region, separation of the hump from the observed heat capacity curve involves a little bit ambiguity. Therefore, these values should be regarded as being reasonable but tentative. The present entropy gain is comparable with 5.5 J K⁻¹ mol⁻¹ for the sharp phase transition at 163.9 K of ferrocene crystal. This fact implies that although the disordering of the rings likewise takes place in both nickelocene and ferrocene, it proceeds gradually in nickelocene and by way of a cooperative phase transition in ferrocene. A reason for this originates in loose molecular packing in nickelocene crystal. Molar heat capacity and the standard molar entropy of nickelocene are larger than those of ferrocene beyond the mass effect over the whole temperature region investigated. This fact provides with definite evidences for the loose molecular packing in nickelocene crystal.     

Electrical conductivity of 4-tricyanovinyl-N,N-diethylaniline

Physical characterizations of 4-tricyanovinyl-N,N-diethylaniline, TCVA, have been reported. The differential scanning calorimetry measurements of TCVA showed that this compound is stable up to 423 K. The temperature dependence of electrical conductivity, in the temperature range from 298 to 403 K, was studied on pellet samples of TCVA with evaporated ohmic Au electrodes. The electrical conductivity was found to be 7.01×10⁻⁹ Ω⁻¹ cm⁻¹ at room temperature. The temperature dependence of the electrical conductivity is typical for semiconducting compounds. The current density-voltage (J-V) characteristics of TCVA pellet samples have been investigated at different temperatures. In low-voltage region, the conduction current obeys Ohm's law while the charge transport phenomenon appears to be space-charge-limited current in the higher voltage regions.     

Electron hopping mechanism in hematite (α-Fe2O3)

The frequency dependence of the real (?′) and imaginary (?″) parts of the dielectric constant of polycrystalline hematite (α-Fe₂O₃) has been investigated in the frequency range 0-100 kHz and the temperature range 190-350 K, in order to reveal experimentally the electron hopping mechanism that takes place during the Morin transition of spin-flip process. The dielectric behaviour is described well by the Debye-type relaxation (α-dispersion) in the temperature regions T<233 K and T>338 K. In the intermediate temperature range 233 K<T<338 K a charge carrier mechanism takes place (electron jump from the O²⁻ ion into one of the magnetic ions Fe³⁺) which gives rise to the low frequency conductivity and to the Ω-dispersion. The temperature dependence of relaxation time (τ) in the −ln τ vs 10³/T plot shows two linear regions. In the first, T<238 K, τ increases with increasing T implying a negative activation energy −0.01 eV, and in the second region T>318 K τ decreases as the temperature increases implying a positive activation energy 0.12 eV. The total reorganization energy (0.12-0.01) 0.11 eV is in agreement with the adiabatic activation energy 0.11 eV given by an ab initio model in the literature. The temperature dependence of the phase shift in the frequencies 1, 5, 10 kHz applied shows clearly an average Morin temperature T_Mo=284±1 K that is higher than the value of 263 K corresponding to a single crystal due to the size and shape of material grains.     

Growth and characterization of pure and potassium iodide-doped zinc tris-thiourea sulphate (ZTS) single crystals

Single crystals of pure and potassium iodide (KI)-doped zinc tris-thiourea sulphate (ZTS) were grown from aqueous solutions by the slow evaporation method. The grown crystals were transparent. The lattice parameters of the grown crystals were determined by the single-crystal X-ray diffraction technique. The grown crystals were also characterized by recording the powder X-ray diffraction pattern and by identifying the diffracting planes. The FT-IR spectrum was recorded in the range 400-4500 cm⁻¹. Second harmonic generation (SHG) was confirmed by the Kurtz powder method. The thermo gravimetric analysis (TGA) and differential thermal analysis (DTA) studies reveal that the materials have good thermal stability. Atomic absorption studies confirm the presence of dopant in ZTS crystals. The electrical measurements were made in the frequency range 10²-10⁶ Hz and in the temperature range 40-130 °C along a-, b- and c-directions of the grown crystals. The present study shows that the electrical parameters viz. dc conductivity, dielectric constant, dielectric loss factor and ac conductivity increase with increase in temperature. Activation energy values were also determined for the ac conduction process in grown crystals. The dc conductivity, dielectric constant, dielectric loss factor and ac conductivity of KI-doped ZTS crystal were found to be more than those of pure ZTS crystals.     

Electronic structure and magnetic properties of the compound CeCuIn

Magnetization, magnetic susceptibility, electrical resistivity, thermoelectric power and X-ray photoemission measurements were performed on a polycrystalline sample of CeCuIn. This compound crystallizes in a hexagonal structure of the ZrNiAl type. The magnetic data indicate that CeCuIn remains paramagnetic down to 1.9 K with a paramagnetic Curie temperature of −13 K and an effective magnetic moment equal to 2.5 μ_B. The electrical resistivity has metallic character, yet in the entire temperature range studied here, it is a strongly nonlinear function of temperature. The temperature dependence of the thermoelectric power is dominated by a small positive maximum near 76 K and a deep negative minimum at about 16 K. Above 150 K the thermopower exhibits a Mott's type behavior. The positive sign of the Seebeck coefficient in this temperature region indicates that the holes are dominant charge and heat carriers. The structure of Ce 3d_5/2 and Ce 3d_3/2 XPS spectra has been interpreted in terms of the Gunnarsson-Schönhammer theory. Three final-state contributions f⁰, f¹ and f² are clearly observed, which exhibit a spin-orbit splitting Δ_SO≈18.7 eV. The appearance of the 3d⁹f⁰ component is a clear evidence of the intermediate valence behavior of Ce. From the intensity ratio I(f⁰)/[I(f⁰)+I(f¹)+I(f²)] the 4f-occupation number is estimated to be 0.95. In turn, the ratio I(f²)/[I(f¹)+I(f²)]=0.08 yields a measure of the hybridization energy that is equal to 45 meV.     

High sensitivity absorption spectroscopy of methane at 80 K in the 1.58 μm transparency window: Temperature dependence and importance of the CH3D contribution

The high resolution absorption spectrum of methane in the 1.58 μm transparency window has been recorded at room temperature and at 79 K by CW-Cavity Ring Down Spectroscopy using a cryogenic cell and a series of Distributed Feed Back (DFB) diode lasers. The achieved sensitivity (α_min ∼ 3 × 10⁻¹⁰ cm⁻¹) has allowed for a detailed characterization of the 6289-6526 cm⁻¹ region which corresponds to the lowest opacity of the transparency window. A list of 6868 and 4555 transitions with intensities as weak as 1 × 10⁻²⁹ cm/molecule was constructed from the recordings at 297 and 79 K, respectively. By comparison with a spectrum of CH₃D recorded separately by Fourier Transform Spectroscopy, 1282 and 640 transitions of monodeuterated methane, CH₃D, in natural abundance in our sample were identified at 297 and 79 K, respectively.The rotational temperature determined from the intensity distribution of the 3ν₂ band of CH₃D (79.3 K) was found in good agreement with the temperature value previously obtained from the Doppler line broadening. The reduction of the rotational congestion by cooling down to 79 K reveals a spectral region near 6300 cm⁻¹ where CH₃D transitions are dominant.The low energy values of the transitions observed both at 79 K and at room temperature were derived from the variation of their line intensities. These transitions with lower energy determination represent 93.9% and 68.4% of the total absorbance in the region, at 79 K and room temperature, respectively. The quality of the obtained empirical low energy values is demonstrated for CH₄ by the marked propensity of the empirical low J values to be close to integers. The line lists at 79 K and room temperature provided as Supplementary Material allow accounting for the temperature dependence of methane absorption between these two temperatures. The investigated region covering the 5ν₄ band of the ¹²CH₄ isotopologue will be valuable for the theoretical treatment of this band which is the lowest energy band of the icosad.     

DC and AC conductivities of (As2S3)100−x(AsSe0.5Te0.5I)x chalcogenide glasses

The DC and AC conductivities of samples from the system (As₂S₃)_100−x(AsSe_0.5Te_0.5I)_x, where x=0, 5, 10, 15, 20, 25, 30, 35, 50, 70 and 90 mol%, were measured as a function of temperature. Besides, the AC conductivities of the samples with x=10 and 30 were measured as a function of frequency from room temperature to the glass transition temperature. The DC conductivity dependence on temperature is of the Arrhenius type, whereas the value of the pre-exponential factor suggests the electrical conduction by localized states in the band tails and by localized states near the Fermi level. The small values of the conduction activation energy (10⁻²-10⁻¹ eV) obtained at higher frequencies suggest that the conduction in these materials is due to hopping of charge carriers between close defect states near the Fermi level.     

Thermomagnetic transitions and coercivity mechanism in bulk composite Nd60Fe30Al10 alloys

The thermomagnetic behaviour (within the temperature range 553-300 K) for the bulk composite Nd₆₀Fe₃₀Al₁₀ alloy is described in terms of a transition from paramagnetic to superferromagnetic state at T=553 K, followed by a ferromagnetic ordering for T<473 K. For the superferromagnetic regime, the alloy thermomagnetic response was associated to a homogeneous distribution of magnetic clusters with mean magnetic moment and size of 1072 μ_B and 2.5 nm, respectively. For T<473 K, a pinning model of domain walls described properly the alloy coercivity dependence with temperature, from which the domain wall width and the magnetic anisotropy constant were estimated as being of ≈8 nm and ≈10⁵ J/m³, typical values of hard magnetic phases. Results are supported by microstructural and magnetic domain observations.     

Ferromagnetism in amorphous Ge1−xMnx grown by low temperature vapor deposition

Magnetic properties of amorphous Ge_1−xMn_x thin films were investigated. The thin films were grown at 373 K on (100) Si wafers by using a thermal evaporator. Growth rate was ∼35 nm/min and average film thickness was around 500 nm. The electrical resistivities of Ge_1−xMn_x thin films are 5.0×10⁻⁴∼100 Ω cm at room temperature and decrease with increasing Mn concentration. Low temperature magnetization characteristics and magnetic hysteresis loops measured at various temperatures show that the amorphous Ge_1−xMn_x thin films are ferromagnetic but the ferromagnetic magnetizations are changing gradually into paramagnetic as increasing temperature. Curie temperature and saturation magnetization vary with Mn concentration. Curie temperature of the deposited films is 80-160 K, and saturation magnetization is 35-100 emu/cc at 5 K. Hall effect measurement at room temperature shows the amorphous Ge_1−xMn_x thin films have p-type carrier and hole densities are in the range from 7×10¹⁷ to 2×10²² cm⁻³.     

An ab initio-based approach to phase diagram calculations for GaN(0 0 0 1) surfaces

Surface phase diagrams of GaN(0 0 0 1)-(2 × 2) and pseudo-(1 × 1) surfaces are systematically investigated by using our ab initio-based approach. The phase diagrams are obtained as functions of temperature T and Ga beam equivalent pressure p_Ga by comparing chemical potentials of Ga atom in the vapor phase with that on the surface. The calculated results imply that the (2 × 2) surface is stable in the temperature range of 700-1000 K at 10⁻⁸ Torr and 900-1400 K at 10⁻² Torr. This is consistent with experimental stable temperature range for the (2 × 2). On the other hand, the pseudo-(1 × 1) phase is stable in the temperature range less than 700 K at 10⁻⁸ Torr and less than 1000 K at 10⁻² Torr. Furthermore, the stable region of the pseudo-(1 × 1) phase almost coincides with that of the (2 × 2) with excess Ga adatom. This suggests that Ga adsorption or desorption during GaN MBE growth can easily change the pseudo-(1 × 1) to the (2 × 2) with Ga adatom and vice versa.     

Structural,magnetic and transport properties of half-metallic ferrimagnet Mn2VGa

Polycrystalline Mn₂VGa samples were synthesized using an arc furnace. X-ray diffraction (XRD) pattern was analyzed using General Structural Analysis System (GSAS) package and the refined lattice parameter was found to be 5.905 Å. We found magnetic ordering in the system below 783 K and the spontaneous magnetization was observed to be following the Bloch T^3/2 law below 80 K. The magnetic moment per formula unit at 5 K was observed to be 1.88 μ_B. The temperature variation of the electrical resistance was found to follow the relation R_n=R_0n+aT^α (α=1.616) and (R_n—normalized electrical resistance) in the temperature range of 25–300 K and we observed almost a temperature independent variation of the electrical resistance below 25 K indicating the absence of spin-flip scattering.     

Dielectric behavior and charge transport in polyaniline nanofiber reinforced PMMA composites

PAni nanofibers synthesized by interfacial polymerization were reinforced in the PMMA matrix in different weight ratios. Randomly oriented polyaniline nanofibers were observed in the TEM image with diameter ranging from 20 to 30 nm. The SEM revealed the microstructure of the fiber reinforced composites showing better connectivity. The XRD spectra of the composites showed peaks at 2θ=17.05°, 20.3°, 27.15° and 30.05° that were indexed in a pseudo-orthorhombic unit cell. The dielectric constant measured over a frequency range of 42 Hz-1 MHz and in the temperature range of 303-373 K showed dependence upon frequency, temperature and concentration of the conducting nanofibers in the composites. The ac conductivity (σ_ac) was interpreted as a power law of frequency. The frequency exponent s was found to lie in the range from 0.4 to 0.65 and decreased with the increase in temperature, which suggested that correlated barrier hopping (CBH) was the dominant charge transport mechanism. Existence of polarons as major charge carriers was confirmed by the low values of polaron binding energy (W_M). Decrease in the values of density of states N(E_F) with the increase in PAni nanofiber concentration indicated increased delocalization of electronic states in the band gap causing the increase in ac conductivity.