Aluminium dopant segregation from AD-xAZO thin films on annealing is established by structural, optical and electrical characterizations.

The soft chemical route was used in the synthesis of undoped and 5% Mn doped ZnO nanocrystalline powders. XRD, TEM, TGA/DTA, FTIR, and superconducting quantum interference device techniques were used to study the structural, nano/microstructural, thermal decomposition and metastability aspects as a function of calcination temperatures (400-1100 degrees C) and magnetic properties. The evolution of the major wurtzite phase (ZnO) and minor non-stoichiometric nanocrystalline defect cubic spinel phase (ZnMnO3-) at various temperatures is clearly seen. The magnetic hysteresis loop is observed at room temperature in the undoped and doped samples calcined at 400 degrees C. Interestingly, the hysteresis loop parameters (M-s, H-c) are found to enhance dramatically as soon as the concentration of the minor phase is large enough up to the calcination temperature 700 degrees C. In contrast, the magnetic hysteresis loop vanishes slowly for the sample calcined at 1000 degrees C, it disappears completely. The room temperature ferromagnetic behavior at 400 degrees C is understood in terms of intrinsic cationic/anionic defects, extrinsic defects associated with the various species chemisorbed on the surface of the nanoparticles of undoped and Mn doped ZnO. During thermal annealing a nanocrysatllline seconadary phase of non-stoichiometric defect cubic spinel ZnMnO3- is formed, contributing to the enhancement of ferromagnetic behavior. All our experimental results are discussed in terms of model comparing various structural and localized electronic defects formed in the nanocrystalline powder. (C) 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

A molecular precursor barium zirconyl oxalate (BZO) was synthesized by exchange reaction between freshly generated water soluble sodium zirconyl oxalate and Ba-nitrate solutions at room temperature. The controlled pyrolysis of BZO in air at 800 °C/6 h resulted in the formation of single-phase cubic microcrystalline barium zirconate powder. These powders exhibit variable-shaped agglomerates with an average size ≍0.2 μm having a surface area ≍2.63 m2/gm.

Undoped and Ga doped ZnO thin films (1% GZO, 3% GZO and 5% GZO) were grown on c-Al2O3 substrates using the 1, 3 and 5at. wt.% Ga doped ZnO targets by pulsed laser deposition. X-ray diffraction studies revealed that highly c-axis oriented, single phase, undoped and Ga doped ZnO thin films with wurtzite structure were deposited. Micro-Raman scattering analysis showed that Ga doping introduces defects in the host lattice. The E2High mode of ZnO in Ga doped ZnO thin film was observed to shift to higher wavenumber indicating the presence of residual compressive stress. Appearance of the normally Raman inactive B1 modes (B1Low, 2B1Low and B1High) due to breaking of local translational symmetry, also indicated that defects were introduced into the host lattice due to Ga incorporation. Band gap of the Ga doped ZnO thin films was observed to shift to higher energy with the increase in doping concentration and is explicated by the Burstein-Moss effect. Electrical resistivity measurements of the undoped and GZO thin films in the temperature range 50 to 300K revealed the metal to semiconductor transition for 3 and 5% GZO thin films.

Pulsed laser deposition was used for growing undoped ZnO and Ga (3 and 5 at. wt.%) doped ZnO thin films on Si (100) substrates with native oxide. X-ray diffraction studies reveal highly c-axis oriented, single phase growth of wurtzite ZnO and Ga doped ZnO thin films. X-ray photoelectron spectroscopy study shows that gallium exists in the Ga3+ oxidation state replacing Zn2+ in the ZnO matrix. It, hence, acts as a donor, increasing the charge carrier density, which was confirmed by Hall measurement. Significant change in the surface morphology of the thin films, such as increased surface roughness, presence of nanostructures, was observed in the Ga doped ZnO thin films. Surface morphology was also influenced by the Ga doping concentration. Higher charge carrier density with the modified surface morphology vindicated its use for field emission studies. Reduction in the threshold field defined for obtaining an emission current density of 0.1 mu A/cm(2), increased field enhancement factor are features of Ga doped ZnO thin films. The stability of field emission current density of the undoped ZnO and Ga doped ZnO thin films was studied at the preset current density of 2.5 and 2.0 mu A/cm(2) respectively, for two and half hours. (C) 2011 Elsevier Masson SAS. All rights reserved.

Ultrafine/nanosize powders of iron oxide have been synthesized from a sintered alpha-Fe2O3 target by pulsed excimer laser ablation and cold condensation. The influence of target porosity and temperature of condensation on the formation of nanosize particles has been studied. The nanosize powders have been characterized by X-ray diffraction, Mossbauer spectroscopy, vibrating sample magnetometry and scanning electron microscopy. It is shown that the formation of ultrafine/nanosiae particles is strongly governed by the target porosity, whereas, the temperature of condensation, over a range from 27 degrees C to -130 degrees C, does not affect the magnetic properties of the nanoparticles.

A new varistor composition based on SnO 2 doped with Co, Al, and Nb has been successfully synthesized by a chemical route. It exhibits excellent nonlinear current-voltage ( I-V ) characteristics with α ≈ 72 and E 1mA ≈ 1820 V/mm.

Epsilon Iron Nitride has a wide range of nitrogen solubility in its lattice. While the stoichiometric epsilon-Fe3N has an ordered arrangement of nitrogen atoms in the octahedral voids of the hexagonal iron lattice, in non-stoichoimetric epsilon-FeyN (2<y<3) the nitrogen atoms occupy the voids in a disordered manner and the coordination of Fe with nitrogen varies with the number 'y' in the lattice. Mossbauer spectroscopy, which is a nuclear spectroscopic tool, can differentiate the different co-ordination of Fe through its characteristic isomer shift values and is used for identifying the different types of Iron sites. epsilon-FeyN (2<y<3) nanoparticles were prepared by the nitridation of hematite nanoparticles using ammonia at 550 degrees C. Mossbauer spectroscopy at ambient temperature shows a superparamagnetic doublet and a sextet. Spectra at 80 K and 5 K resolve the sextets nicely and thereby the different iron sites present in the epsilon-FeyN (2<y<3) nanoparticles.

Magnetic nanoparticles of iron oxide were synthesized by transferred arc plasma induced gas phase condensation method. Structural, morphological and magnetic studies of the as synthesized powder were carried out using X-ray diffraction, transmission electron microscopy and Mossbauer spectroscopy. These studies have revealed the simultaneous nucleation and condensation of different magnetic phases with a broad size distribution of the nanoparticles which is peaked at 30-50 nm and ranges from 10 nm to 80 nm. 57 Fe Mossbauer spectra recorded at various temperatures (5 K-300 K) in presence of external magnetic field (at 5 K) have suggested the presence of different phases of iron oxide with sizable amounts of gamma-Fe2O3 and alpha-Fe2O3 in addition to Fe3O4. The relative concentrations of these phases have been obtained by a self consistent spectral area analysis and were found to be 44:22:34 (%). (C) 2007 Elsevier Inc. All rights reserved.

The modifications in electrical and magnetic properties of polycrystalline bulk La0.7Ca0.3Mn1−xTxO3 (T=Fe, Ga) samples at relatively higher doping concentration (x=0.08–0.12) are investigated. All the synthesized, single phase samples were subjected to resistivity measurements in the temperature range 50–300K. No insulator–metal transition (TP) was observed for Fe doped samples with x=0.12. For all the other samples the transition temperature decreased with increase in doping concentration. The small polaron hoping energy was found to increase, rather slowly, with increase in doping concentration. The effect on magnetic properties is also prominently observed with respect to doping element and doping concentration. Interestingly, with the increase in doping concentration, the Curie temperature (TC) and TP separate out significantly indicating decoupling of electric and magnetic properties. Changes in these properties have been analyzed on the basis of magnetic disorder introduced in the system due to the magnetic and nonmagnetic nature of these ions rather than strong lattice effects which is insignificant due to similar ionic radii of Fe+3 and Ga+3 when compared to that of Mn+3.

Pulsed laser deposition technique was used for growing thin films of ZnO on Si (100) substrate held at different temperatures (Ts). All the as-deposited films have shown a preferential c-axis orientation associated with varying grain size as a function of Ts ranging from 100to600°C. Current-voltage (I-V) characteristics of these films show Ohmic behavior over the entire Ts range studied. These films were subjected to annealing at 800°C in air ambient for 4h. The grain size was observed to increase after the annealing process for all the films deposited at different Ts. Interestingly, these annealed films show nonlinear variation of current with applied voltage, very similar to the one observed in doped ZnO varistors. The nonlinear parameters such as the asymmetric behavior of change in current on the polarity reversal of voltage, the plateau region, and the break down voltage are observed to depend on Ts. This nonlinear behavior can perhaps be explained on the basis of electronic conduction model proposed for the bulk, doped ZnO varistors. The role of insulating intergranular layers between the disoriented microcrystallites is expected to be similar to that played by insulating intergranular layers in the doped ZnO varistors.

Microwave-hydrothermal (MH) route was employed to synthesize various iron oxide phases in ultra-fine crystalline powders by using ferrous sulphate and sodium hydroxide as starting chemicals. All chemical reactions were carried out under identical MH conditions, namely, at 190°C, 154 psi, 30 min, by varying the molar ratio (MR) of FeSO4/NaOH in the aqueous solutions. The variation of MR has a dramatic effect on the crystallization behavior of various phases of iron oxides under MH processing conditions. For example, spherical agglomerates of Fe3O4 powder were obtained if MR equal to 0.133 (pH>10 sample A). On the other hand non-stoichiometric Fe3O4 powders (Sample B) were obtained for all higher MR of FeSO4/NaOH between 0.133 and 4.00 (6.6<pH<10). However, when MR was equal to 4.0 (pH≅6.6) a varied distribution of shapes and sizes of agglomerates of α-Fe2O3 powders (sample C) were produced. Fe57 Mössbauer spectra were recorded for all the three sets of samples at room temperature. In the case of sample B, temperature dependent Mössbauer spectra were recorded in the range of 77–300 K to understand the non-stoichiometric nature of Fe3O4 powders. All these results are discussed in the present paper.

Nanosized powders of Fe3O4 and MgFe2O4 are synthesized by microwave-hydrothermal (MH) method. The ferrite powders thus obtained are characterized by powder X-ray diffraction, Mossbauer spectroscopy, scanning and transmission electron microscopies, vibrating sample magnetometer and AC magnetic susceptibility studies. Stoichiometric, nanosized (similar to 34 nm) Fe3O4 particles, having cubic symmetry with a(0) approximate to 8.39 angstrom, are obtained when molar ratio of Fe/NaOH used for the synthesis is 0.133. On the other hand, non-stoichiometric Fe3O4 is obtained when Fe/NaOH molar ratio is increased to 0.4. Though the powder XRD patterns of both stoichiometric and non-stoichiometric Fe3O4 are identical, Mossbauer spectroscopic studies showed different features in terms of asymmetry, intensity, isomer shift, etc. Saturation magnetization, M-s = 70 emu/g and coercivity, H-c = 178 Oe, at room temperature, are obtained for stoichiometric Fe3O4 powders. In the case of MgFe2O4, spherical, nanosized (similar to 3 nm) powders with cubic symmetry (a(0) approximate to 8.30 angstrom) and having very narrow size distribution are obtained under the MH conditions. The temperature and field dependent magnetic measurements on MgFe2O4 powders confirmed the existence of superparamagnetic state in this material with a superparamagnetic blocking temperature of 38 K below which typical magnetic hysteresis behaviour is observed.

Films of nanocrystalline γ-Fe2O3 were deposited on silicon substrates by using the technique of electrophoretic deposition. The precursor powder was nanocrystalline γ-Fe2O3, which was synthesized, using DC arc plasma in the oxygen ambient by vapour-vapour interaction in gas phase condensation; at a stabilized arc current of 40 A. This powder was characterized by X-ray diffraction, Transmission Electron Microscopy, Vibrating Sample Magnetometer and Mössbauer Spectroscopy. An increase in directional coercivity was observed in case of films deposited on silicon substrates, which is dramatically significant. Preferred orientation of almost similar sized nanocrystalline magnetic domains in deposited films is evident from the results of X-ray diffraction and Transmission Electron Microscopy results. The preferred alignment of the nanocrystallites seems to be responsible for the significant changes observed in magnetic properties of films.

The manganite system highlights wide range of remarkable properties on merely doping the host compound to change charge density, magnetic coupling, the nature of the ground state, and the effect of interfacial magnetic polarization. We have systematically studied the effect of transition metal ions (V, Ti, Cr, Fe, Co, Ni, Cu, Zn) doping (with fixed 5% concentration) in the Mn lattice sites in La0.7Ca0.3MnO3 (LCMO) which will modify the double exchange interaction, heart of transport mechanism in these systems and will give better understanding of underlying mechanisms. The variation in the metal-insulator transition temperature (Tp), the Curie temperature (Tc) and the maximum value of magrictoresistance (MR) with respect to lattice parameter and the ionic radii of dopants, was examined. The concentration dependence of these properties was probed by Fe doping (0 < x < 0.05) and isomorphous substitution of Al3+ ions (0 < x < 0.05) at Mn-site.

Magnetic multilayers have attracted much attention due to their potential applications in spintronic devices because of their ability to inject spin-polarized current. The bilayer of ferrimagnetic Fe3O4 and manganites La0.7Ca0.3MnO3 (LCMO) is an interesting system due to the (i) fascinating electrical and magnetic properties of the spinel and perovskite material and (ii) high-spin polarization character of Fe3O4. In this work, we report our results on pulsed laser deposited thin films of Fe3O4/LCMO bilayer on LaAlO3 (LAO) substrate. For comparison, LCMO and Fe3O4 films were also grown on LAO substrate separately. The films were characterized by techniques such as X-ray diffraction, four-probe electrical resistivity with and without magnetic field. It is shown that oriented Fe3O4/LCMO bilayer can be grown successfully on the c-axis oriented LAO substrate. The electrical transport properties suggest that at low temperature the junction shows considerable magnetoresistance.

Submicron-sized, spherical, stoichiometric strontium substituted barium titanate: Ba0.75Sr0.25TiO3 (BST) powders are prepared by microwave-hydrothermal (MH) route under the MH conditions of 200 °C, 200 psi, and 30 min in strongly alkaline conditions (pH>12) using potassium titanyl oxalate (KTO) and nitrates of Ba and Sr as the starting precursors and KOH as the mineralizer. The characterization studies by XRD, XRF and SEM indicated that stoichiometric cubic BST powders (a0=3.992 Å) were obtained by adjusting MH conditions mentioned earlier and keeping the (Ba+Sr)/Ti ratio in starting solution slightly in excess (∼1.06).

A simple one-step cation-exchange reaction between the stoichiometric solutions of ammonium titanyl oxalate (ATO) and barium hydroxide+strontium nitrate at room temperature (RT) is investigated successfully for the quantitative precipitation of barium–strontium titanyl oxalate (BSTO): Ba1−xSrxTiO(C2O4)2·4H2O (x=0.25) precursor powders with nearly theoretical yield (≥99%). The pyrolysis of BSTO at 730 °C/4 h in air produced barium–strontium titanate (Ba1−xSrxTiO3; BST) powders. The characterization studies on BSTO and BST powders by using various physico-chemical techniques: micro- and chemical analysis, differential thermal analysis (DTA)/thermo-gravimetric analysis (TGA), XRD, FTIR, X-ray fluorescence (XRF) and scanning electron microscopy (SEM) revealed that the powders formed are cubic, highly pure, stoichiometric and sub-micron-sized with nearly uniform size and shape distribution. The ceramic compacts obtained by sintering the BST pellets at 1300 °C/4 h showed density ∼95%, dielectric constant ɛ(Tc)∼9500, tan δ∼0.15% and TC∼32 °C.