Influence of spin-glass-like phase on magnetic and electrical transport properties of reactive sputtered Al1−xFexN films

Nanocrystalline Al_1−xFe_xN films were fabricated using the reactive sputtering method. A large amount of spin-glass-like moments are in the films. With the decrease of temperature, the films turn from the spin-glass-like behavior to ferromagnetism. At low temperatures, the saturation magnetization increases and the coercivity decreases with the increase of x. The coercivity increases significantly below 50 K due to the pinning effect of the frozen disordered spin-glass-like moments. All of the films are semiconducting. The low-temperature transport mechanism turns from tunneling to hopping as x increases. Magnetoresistance (MR) shows weak saturation trend with the applied field because of the hard alignment of the frozen moments. Meanwhile, MR follows the relation of log |MR|=a+bT−1, and the spin polarization satisfies P(T)=P₀e−βTα, related with the disordered spin-glass-like moments.     

Size distribution effect on the magnetization and magneto-resistance in ferromagnetic nanostructured manganites

We present experimental data of magnetization and magneto-resistance of nanostructured La_2/3B_1/3MnO₃ with B=Ca, Sr, which present difference between the coercive field in the magnetization loop with their corresponding maximum value in the magneto-resistance. This difference is described by a model that include, size distribution of magnetic particles, randomly oriented anisotropy axis and electronic transfer between the particles, which is mediated by spin-polarized tunneling process. Also, the model predicts that the maximum magneto-resistance can be, in the magnetic disorder state, two times larger than the experimental value. The model results can be used to estimate the size dispersion of nanoparticles in similar systems.     

Self-organization analysis for a nonlocal convective Fisher equation

Using both an analytical method and a numerical approach we have investigated pattern formation for a nonlocal convective Fisher equation with constant and spatial velocity fields. We analyze the limits of the influence function due to nonlocal interaction and we obtain the phase diagram of critical velocities vc as function of the width μ of the influence function, which characterize the self-organization of a finite system.     

Photoreflectance characterization of CdTe thin films grown by Molecular Beam Epitaxy on GaAs(100)

CdTe films have been grown on top of GaAs(100) by means of Molecular Beam Epitaxy (MBE) at 300 °C substrate temperature. Different procedures for the CdTe growth and for the preparation of the GaAs substrates resulted in diverse crystalline qualities of the CdTe films. We present the results obtained from PhotoReflectance (PR) measurements of these films employing HeNe and Ar-ion lasers as modulating excitation. For Ar excitation, the ratio of CdTe to GaAs signal strength for the E ₀ transition is enhanced, allowing a differentiation of the contributions from film and substrate. Both the PR line shape and intensity are correlated to the structural quality of the CdTe films. One of the samples presented a below-band-gap transition of the GaAs substrate around 30±5 meV from E ₀ which is attributed to donor states produced by Te atoms diffused in the interface; this result demonstrates the high sensitivity of the photoreflectance technique to the structural properties of interfaces.     

Synthesis and electronic behaviors of TiO2/carbon clusters/Cr2O3 composite materials

Nano-structured TiO₂/carbon clusters/Cr₂O₃ composite material has been successfully obtained by the microwave treatment of a TiO(acac)/Cr(acac)₃/epoxy resin complex. The compositions of the composite materials were determined using ICP, elemental analysis and surface characterization by SEM-EDX, TEM and XRD. ESR spectral examinations suggest the possibility of an electron transfer in the process of TiO₂ → carbon clusters → Cr₂O₃ with an oxidation site at TiO₂ particles and a reduction site at Cr₂O₃ particles. The preliminary experimental results show that the calcined materials could decompose methylene blue under visible-light irradiation.     

Temperature and magnetic field dependence of the voltagein GaAs films with superconducting Ga grains

We have studied granular films consisting of nanoscale Ga droplets formed on GaAs films via a method of vacuum annealing to promote As evaporation. For temperatures and magnetic fields below the bulk Ga critical parameters, the samples are very sensitive towards external microwave radiation when two point voltage measurements are performed. Together with the observation of an oscillating magnetic field dependence of the voltage, a scenario in which the samples consist of Josephson-coupled loops seems to be the most likely one for explaining the obtained results.     

Structure and thermomagnetic properties of powders produced from melt spun FeNbBCu ribbons

Amorphous ribbons of Fe₇₇Nb₇B₁₅Cu₁ prepared by melt-spinning and powders produced from them by ball-milling were characterized by means of calorimetry, X-ray diffraction, scanning electron microscopy and vibrating sample magnetometry. Upon thermal treatment the amorphous alloy experiences a primary crystallization that leads to bcc-Fe nanocrystals dispersed in an amorphous matrix. Magnetic measurements indicate that this alloy in the amorphous and nanocrystalline state is a good soft magnetic material. Values of saturation magnetization and coercivity are 120 Am²/kg and 5 A/m respectively, for the alloy in the nanocrystalline state. Pre-annealing, post-relaxation and nanocrystallization as well as various milling parameters were explored and the structural and magnetic changes induced have been studied. The analysis of the particle size distribution and morphology of the powders show that the brittleness resulting from pre-annealing of the ribbons is very effective in reducing the particle’s size. Recovery of the high coercitive field induced by milling is achieved by post-annealing to an extent that depends mostly on the milling conditions.     

Luminescence characteristics of Yb, La codoped yttrium oxide nanopowders

Luminescence characteristics of Yb³⁺, La³⁺ codoped yttrium oxide nanopowders were investigated. The grain size and the crystallinity of (Yb_0.05Y_0.90La_0.05)₂O₃ nanopowders increase with the increase of calcination temperature. The average grain size of the nanopowders calcined at 1100 °C is 66 nm and its cooperative up-conversion luminescence centered at 498 nm was detected due to nanometer size effect and perfect crystallinity. However, the cooperative up-conversion luminescence of (Yb_0.05Y_0.90La_0.05)₂O₃ transparent ceramics was not detected.     

Sol–gel preparation and photoluminescence properties of Ce-activated Y3Al5O12 nano-sized powders

Trivalent cerium-doped yttrium aluminum garnet (YAG:Ce³⁺) nanoparticles were successfully prepared by a facile sol–gel technique. X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and photoluminescence (PL) spectroscopy were used to characterize the as-prepared phosphors. Well-crystallized fine nanoparticles were obtained at 1000 °C. Single-crystal nanoparticles with irregular shapes were obtained, with crystallite sizes ranging between 20 and 60 nm. PL intensity of the particles increased monotonically with decreasing Ce doping concentration and showed the maximum value at 0.1 at%.     

Structural and optical property studies of CdSe crystalline nanorods synthesized by a solvothermal method

CdSe nanorods are synthesized via a simple solvothermal method at a moderate temperature of 180 °C. The influences of introducing hydrazine hydrate (N₂H₄·H₂O) as the reducing agent, and ammonia (NH₃·H₂O) as the complexing agent and also the reaction temperature, on the morphology and size of the obtained CdSe nanorods are investigated and reported. CdSe nanorods with a mean diameter and length of 25 and 82 nm, respectively, are synthesized and the problem of handling the stacking faults present in the long CdSe nanorods is analyzed. The use of increased quantity of hydrazine hydrate and also prolonged reaction time is found to reduce the stacking faults on the synthesized nanorods. The morphology, phase and the optical properties of CdSe nanoparticles are studied using powder X-ray diffraction, TEM and high-resolution transmission electron microscope (HRTEM), UV–visible absorption spectroscopy and photoluminescence (PL) spectroscopy. The low-resolution TEM images confirm the formation of CdSe nanorods, and also the agglomeration of nanoparticles and the presence of few spherical nanoparticles. The strong PL intensity from the CdSe nanorod at 702 nm confirms a blue shift of 14 nm, when compared with the bulk wurtzite CdSe.