Narrow Results

We have investigated the effect of the partial substitution of Sb for Ru in Ru_{1 - x}Sb_xSr₂(Sm_0.7Ce_0.3)₂Cu₂O_{10 - δ}, (Ru:1222) prepared by conventional solid state reaction method, using X-ray diffraction analysis, electrical resistivity and DC-magnetic susceptibility measurements. Sb doping reduces the conductivity of the system and the onset superconducting transition temperature decreases from 42 K for x = 0 to 21 K for x = 0.06. This maybe due to a distortion of RuO₆ octahedral, which is responsible of the increase in hole localization. The corresponding magnetic moment in the investigated samples is about 0.1 μ_B for Ru. Furthermore we discuss the influence of phase composition on superconducting and magnetic order.

We investigated the origin of a non-linear temperature dependence of resistivity ρ_ab(T) in the a-b planes of YBCO, frequently observed in optimally doped twinned YBCO single crystals and thin films and in untwinned YBCO crystals along the chain direction. We performed the measurements of ρ_ab(T) in c-axis oriented YBCO thin films that were subjected to low temperature annealing at 120–140°C (400–420 K) in argon. Our results have shown that redistribution of oxygen at a fixed oxygen content in the chain layers of YBCO with a non-linear ρ_ab(T), leads to an increase of T_c, an increase of resistivity and a reduction of the non-linear component in ρ_ab(T).

The effect of partial substitution of Ca by Eu, Sm and Er on the structural, transport and magnetic properties of (Bi_1.6Pb_0.4)Sr₂(Ca_1-xRE_x)₂Cu₃O_y superconductors ceramics (0≤x≤0.025) have been investigated. The substitution of Eu, Sm and Er at the Ca site induced the decrease of the volume fraction for the 2223 phase and the increase of 2212 and 2201 phase volume fractions. The rate of depression for the critical temperature in x=0.015 samples found to be nearly the same for all of these substitutions. The nature of T_c suppression may be attributed to the increase of oxygen content in Bi₂O₂ double layers with increasing Eu, Sm and Er concentrations. The samples shows a linear dependence of intergranular temperature as a function of AC field amplitude, which is in agreement with Muller critical state model.

Polycrystalline samples of Ru_1-xSb_xSr₂GdCuO₈ doped with Sb, in which magnetic order and superconductivity coexist with T_magnetic≫T_c, were prepared by a solid state reaction technique. Lattice parameters, electrical resistivity, Hall and thermopower coefficients measurements on the same sintered ceramic samples are presented.

Both Hall effect and thermopower show anomalous decreases below T_magnetic which might be explained with a simple two-band model in the RuO₂ layer at T_magnetic. It was also observed that the Sb doping reduce the conductivity of the system and the transition temperature decreases with increasing Sb content. This may be due to a distortion of RuO₆ octahedra which is responsible for the increase of hole localization.

For high–T_c superconductor metal oxides (HTSC) it is shown that the dc resistivity and Hall effect temperature behaviour can be explained by the model of the paraelectric material close to the point of the Mott-Hubbard instability, in the ground state of which the current is carried by a liquid of boson-like pairs of carriers in upper and lower Hubbard bands. The Mott-Hubbard instability corresponds to the order-of-lattice-constant length of the mean free path and results in the temperature insensitivity of Drude conductivity. Nearly linear on T resistivity results from the Curie law via the local (acting) electric field. Fermion-like carriers, temperature excited over the energy of boson-like pair dissociation (pseudo gap), explain the temperature behaviour of Hall effect. Available data are compared with the model.

Ru_1-xSb_xSr₂(Eu_0.7Ce_0.3)₂Cu₂O_10-δ systems were prepared by conventional solid state reaction method. The structural analysis was investigated using X-ray diffraction. Electrical resistivity, Hall effect and magnetic susceptibility measurements on Ru:1222 doped with Sb are presented, together with results in the temperature range 5–300 K. Transition temperature decreases from 43 K for x = 0.00 to 20 K for x = 0.06 Sb. This may be due to a distortion of RuO₆ octahedral, which is responsible of the increase in hole localization. The Hall effect anomalous decreases below T_magnetic which may be explained within a simple two-band model by a transition from localized to more itinerant behavior in the RuO₂ layer at T_magnetic. The behavior of magnetic susceptibility is caused by an antisymmetric exchange coupling of the Dzyaloshinsky–Morya type between neighboring Ru moments.

A lanthanum-based mixed rare earth manganite system with general composition formula La_0.33Ln_0.34Sr_0.33MnO₃, (where Ln is a rare earth ion) has been prepared by the solid state reaction method. After usual characterization of these materials, a systematic study of the electrical resistivity both as a function of temperature (80–300 K) and magnetic field up to 7 Tesla was undertaken mainly to understand the conduction mechanism. On analyzing the experimental results, it has been concluded that the metallic (ferromagnetic) part of the resistivity (ρ) (below T_P) fits with the equation ρ(T)=ρ₀+ρ₂T²+ρ_4.5T^4.5, indicating the importance of grain/domain boundary effects (ρ₀), the electron-electron scattering process (ρ₂T²) and the two magnon scattering process (ρ_4.5T^4.5). On the other hand, the paramagnetic insulating regime may be explained by using adiabatic small polaron hopping and variable range hopping mechanisms, thereby indicating that polaron hopping might be responsible for the conduction mechanism.

The scaling behavior of the effective activation energy of high-quality epitaxial c-oriented Bi₂Sr₂Ca(Cu_1-xCo_x)₂O_d thin films with 0≤x ≤0.025 has been studied as a function of temperature and magnetic field. For all samples, the effective activation energy scales as U(T, μ_oH)=Uo(1-T/T_c)^mHⁿ with exponent m=1.25±0.03, n=-1/2 and the field scaling 1/μ_oH and -Uμ_oH for thick films and ultra thin films, respectively. The results are discussed taking into account of the influence of the Co substitution with a model in which U(T, H) arises from plastic deformations of the viscous flux liquid above the vortex-glass transition temperature.

Pure and Al doped nanocrystalline ZnO films have been prepared on Hydrogen terminated Si(100) substrates by nebulized spray pyrolysis. The dependence of the structural, compositional and electrical properties were investigated using XRD, EDX, AFM and spectrophotometer. The X-ray diffraction data coincide well with the pattern of ZnO reported with the Standard Database. Films annealed at higher temperatures show better orientation, as revealed from X-ray diffraction patterns. Annealing the films in air improved the electrical properties. From the I-V characteristics, the nonlinear coefficient α value has been estimated. Reflectance measurements show good reflectance in the IR region for pure ZnO films, and Al doping improved the reflectance values.

MgB₂ thin films were deposited at low temperature substrates, in situ, on c-plane sapphire substrates, with the aluminium nitride (AlN) buffer layers, using the multiple-target sputtering system. The magnetoresistivities were measured using dc-five probe method in applied magnetic field up to 9 Tesla. The upper critical field anisotropy, H_C2(T) and irreversibility field H_irr(T) versus temperature were determined. The Hall coefficients R_H are slightly temperature dependent and positive in the normal state. The critical temperature of 30–32 K and critical current density of 10⁶-10⁷A/cm² at 4.2 K were obtained. Using extracted data, the coherence length ξ_o, anisotropic coefficient γ and penetration depth λ_L were calculated.

This paper consists of two parts: (1) mechanism of formation of hole-pairs and quantum stripes in cuprate superconductors, and (2) resistivity versus temperature variation in YBa₂ Cu₃O_7-δ superconductor with different doping levels in the range 400–T_c. (1) On deoxygenation, CuO₂ plane of high-temperature cuprate superconductors are broken into small magnetically isolated fragments, predominantly Cu-tetramer (CuO)₄. Its electron paramagnetic resonance spectra show fine and hyperfine splittings. It was concluded that these splittings are due to FM coupling of the electronic spins of the four holes on the four Cu-ions in (CuO)₄ as well as the FM coupling of the nuclear spins of the four Cu-ions. A magnetic field is generated perpendicular to CuO₂ plane due to the spin magnetic moment of four holes and their orbiting around the (CuO)₄ frame. In the nondeoxygenated superconductors, this magnetic field leads to the formation of quantum stripes of free charge carriers and in special cases to the formation of preformed hole-pairs. This is the first model of a hole-pair, though its existence has been conjectured from various experimental results. (2) The electrical conduction in superconductors along c-axis seems to arise from the movement of free holes and along a-axis from the parallel combination of resistivities arising from the movement of free holes and paired holes, but no single formula connecting ρ and T is applicable to explain resistivities either along c- or a-axis in the entire temperature range. Nature of charge carriers seems to change continuously as the temperature is lowered. In the range 200–T_c, both along the c- and a-axes, the experimental resistivities depart significantly from those calculated by the formula applicable in the range 400–200 K. Plausible explanation has been suggested for the departure of experimental resistivities from the calculated ones in the low temperature region.

In this paper, dielectric properties of Ni_xMn_0.4-xZn_0.6Fe₂O₄ ferrites with x varying from 0.05 to 0.35 prepared by the citrate precursor method have been investigated as a function of frequency, temperature, composition, and sintering temperature. A decrease in the dielectric constant is observed with the increase in Ni concentration except for x = 0.3. This decrease in dielectric constant with the increase in Ni concentration is justified by inverse proportionality between resistivity and dielectric constant. Dispersion in the dielectric constant with frequency in the range of 75 Hz to 30 MHz is observed. Resonance peaks were observed in tan δ_ε versus frequency curve for all the samples. A shift in the resonance frequency toward higher frequency is observed with the increase in temperature. The peak height also increases with an increase in temperature. Phase change is confirmed by differential scanning calorimetry. Structural studies have been done by X-ray diffraction technique and scanning electron microscopy. Possible theories, models, and mechanisms contributing to these processes have been discussed.

Systematic investigation of structural, magnetic and transport properties of RuSr₂RE_2-xCe_xCu₂O₁₀ with RE being Sm and Eu has been performed. We present the experimental evidence that the synthesis of RuSr₂Eu_1.5Ce_0.5Cu₂O₁₀ under non-optimised heat treatment conditions may results in the formation of secondary RuSr₂EuCu₂O₈ phase. In contrast, similar preparation conditions for RuSr₂Sm_1.5Ce_0.5Cu₂O₁₀ samples lead to the pure phase. Consequently, RuSr₂Eu_1.5Ce_0.5Cu₂O₁₀ samples exhibit a small peak around 125 K in the temperature dependence of dc magnetization, pronounced irreversibility between zero-field cooled and field cooled curves at temperatures higher than the peak temperature, and the bell shaped behavior of the coercive field as the function of temperature above Curie's temperature. These anomalous features, which may arise due to the superposition effect of the main and secondary phases, which are both magnetic, are completely absent for RuSr₂Sm_1.5Ce_0.5Cu₂O₁₀ sample, which does not consist of the secondary phase.

The resistivities along c-axis ρ_c(H, T) of ErNi₂ B₂C have been measured with H_⊥ and H_‖c-axis for 2 < T < 300 K and the superconducting upper critical field H_c2(T) curves of ErNi₂B₂C were constructed for each magnetic fields. Our H_c2(T) curves have been compared and discussed with those from ρ_ab(H, T) measurements which explain the anisotropy and its temperature dependence of H_c2(T) are thought to arise from magnetic pair breaking and the anisotropic field dependence of Néel temperature T_N originated from Er⁺³ sublattice.

The temperature-dependent resistivity measurements of our Ag–Ni–Si silicide films with 51–343 nm thicknesses are studied as a function of temperature and film thickness over the temperature range of 100–900 K. The most striking behavior is that the variation of the resistivity of the Ag–Ni–Si silicide films with temperature exhibits an unusual temperature-dependent behavior with respect to those of the transition and untransition metals. Our measurements show that the total resistivity of the Ag–Ni– Si silicide films increases linearly with temperature up to a T_m temperature at which resistivity reaches a maximum thereafter T_m decreases rapidly and finally to zero at ~850 K. T_m temperature is found to decrease with decreasing film thickness. We have shown that in the temperature range of 100-T_m K, electron–phonon resistivity and grain boundary resistivity components responsible for the total resistivity increase. But the grain boundary scattering is dominant mechanism for the resistivity increase in our Ag–Ni–Si silicide films.

Indium Oxide quasi one-dimensional (1D) nanostructures known as nanowires and nanorods synthesis using the thermal evaporation method, has been articulated. To nucleate growth sites, substrate seeding promoted 1D nanostructures growth. The catalyst-mediated growth mechanism showed more favorable morphologies and physical properties in under vacuum conditions associated with bottom-up technique. Scanning electron microscopy (SEM) results showed that the Zn-doped 1D nanostructures had spherical caps. The X-ray diffraction (XRD) pattern and energy-dispersive X-ray (EDX) spectrum indicated that these caps intensively associated with ZnO. Therefore, it was reasonable that the vapor–liquid–solid mechanism (VLS) was responsible for the growth of the In₂O₃-ZnO heterostructure nanowires. This technique enhances optical and electrical properties in nanostructures. The photoluminescence (PL) analysis in Zn-doped In₂O₃ nanowires and nanorods shows that the intensity of the visible and UV-region emissions overwhelmingly increases and resistance measurement professes the improvement of linear conductance in VLS growth mechanism.

Polycrystalline cobalt substituted Ni–Zn ferrite with composition Ni_0.65-xCo_x Zn_0.35Fe₂O₄(x = 0.00–0.25 insteps of 0.05) have been prepared through the conventional solid state ceramic method. Calcination and sintering have been performed in air atmosphere at 950°C and 1250°C for 4 h and 2 h, respectively followed by natural cooling to room temperature. X-ray diffraction patterns of all samples indicated the formation of the single spinel structure and the accurate lattice parameter for each composition has been determined using the Nelson–Riley error function. The increase in lattice constant on cobalt substitution is attributed to the ionic radius difference between the displaced and the substituted ion. The variation in lattice constant on incorporation of Co²⁺ ion indicates its solubility into the spinel lattice and noticeable modification in structural properties have been observed. The observed increase in the saturation magnetization and Curie temperature with the increase in the Co²⁺ substitution is due to its higher magnetic moment compared to that of Ni²⁺, improvement in the A–B exchange interaction mechanism and large positive contribution to magnetic anisotropy due to presence of Co²⁺ when they are at the octahedral sites. The observed variation in the initial magnetic permeability and the magnetic loss factor with cobalt substitution measured at a low frequency of 1 KHz have been attributed to the modification in the density, porosity, grain size and anisotropy contributions. A nearly comparable variation is observed in the room temperature dc electrical resistivity and activation energy for conduction and is attributed to the modification in structure, role and nature of cobalt ions and the microstructure aspects like grain size and pore concentration. The activation energy values in the range of 0.28 to 0.36 eV suggest a possible electron hopping. The observed changes in the structural and the magnetic and electrical properties have all been discussed in the light of exiting understanding.

In this paper, we report on microstructural, optical and electrical properties of alternating multilayer of vanadium pentoxide (V₂O₅), 25 nm, and vanadium (V), 5 nm, thin films deposited at room temperature by radio frequency (RF) and DC magnetron sputtering, respectively. Raman and photoluminescence (PL) spectroscopy have been employed to investigate the effects of thermal annealing for 20, 30 and 40 min at 400${}^{\circ }$C in Nitrogen (N₂) atmosphere on the multiple phase formation and its impact on the film resistance and temperature coefficient of resistance (TCR). We demonstrate that the oxygen free annealing environment allows the formation of multiple phases including V₂O₅, V₆O${}_{13}$ and VO₂ through oxygen diffusion and consequent deficiency in V₂O₅ layer.

Ni${}_{0.65-x}$Zn${}_{0.35}$Mg${}_x$Fe₂O₄ ($0\le x\le 0.2$) with lower compositions of magnesium were synthesized by the sol–gel method and sintered at 1080${}^{\circ }$C for 4 h. X-ray diffraction (XRD) patterns suggest the single phase nature of the spinel-type ferrite and increase in lattice parameter for increasing magnesium content. Surface morphology reveals that grain size increases with increasing magnesium concentration. Magnesium substitution in Ni–Zn ferrites showed influence on the interatomic distance for tetrahedral A sites and octahedral B sites calculated using the standard formula. Conductivity along with dielectric measurements gives additional information on the transport mechanism for magnesium substitution in Ni–Zn ferrites calculated at lower frequencies. Both magnetic loss and dielectric loss were investigated at lower frequency to understand the losses associated with domain wall contribution for magnesium-substituted Ni–Zn ferrites.

In this paper we report influence of titanium substitution on the structural properties and d.c. conduction behavior of Li-Zn ferrites. The samples having the compositional formula Li_0.45+0.5tZn_0.1Ti_tFe_2.45-1.5tO₄ with t varying from 0 to 1.2 in steps of 0.2 were prepared by the conventional dry ceramic method. XRD analysis showed all the samples to be single phase spinel and the lattice parameter was found to increase with increasing t. D.C. resistivity increases with Ti concentration. The possible mechanisms involved are discussed.