High precision magnetic linear dichroism measurements in (Ga,Mn)As.

Investigation of magnetic materials using the first-order magneto-optical Kerr effects (MOKEs) is well established and is frequently used. On the other hand, the utilization of the second-order (or quadratic) magneto-optical (MO) effects for the material research is rather rare. This is due to the small magnitude of quadratic MO signals and the fact that the signals are even in magnetization (i.e., they do not change a sign when the magnetization orientation is reversed), which makes it difficult to separate second-order MO signals from various experimental artifacts. In 2005 a giant quadratic MO effect-magnetic linear dichroism (MLD)-was observed in the ferromagnetic semiconductor (Ga,Mn)As. This discovery not only provided a new experimental tool for the investigation of in-plane magnetization dynamics in (Ga,Mn)As using light at normal incidence, but it also motivated the development of experimental techniques for the measurement of second-order MO effects in general. In this paper we compare four different experimental techniques that can be used to measure MLD and to separate it from experimental artifacts. We show that the most reliable results are obtained when we monitor the polarization of reflected light while the magnetization of the sample is rotated by applying an external magnetic field. Using this technique we measure the MLD spectra of (Ga,Mn)As in a broad spectral range from 0.1 eV to 2.7 eV and we observe that MLD has a magnitude comparable to the polar MOKE signals in this material.

IR laser photodeposition of a-Fe/Si films developing nanograins of ferrisilicate, iron disilicide and rare hexagonal iron upon annealing.

IR laser-induced gas-phase photolysis of Fe(CO)(5)-SiH(4) mixtures occurs as SiH(4)-photosensitized decomposition of Fe(CO)(5) is accelerated by products of this decomposition and it results in deposition of amorphous Si/Fe nanocomposite films. Analyses of the deposited and subsequently annealed solid films were made by FTIR, Raman and X-ray photoelectron spectroscopy, X-ray diffraction and electron microscopy. The deposited films are amorphous, contain crystalline nanostructures of iron silicide FeSi(2) and undergo atmospheric oxidation in topmost layers to iron oxide and hydrogenated silicon oxide. Upon annealing they develop nanocrystalline structures of ferrisilicate, Fe(1.6)SiO(4), carbon-encaged iron disilicide, FeSi(2), and very rare hexagonal (high-pressure) Fe surviving at ambient conditions. The mechanism of formation of these nanostructures is discussed in terms of gas-phase and solid-phase reactions.

Magnesium incorporated bentonite clay for defluoridation of drinking water.

Low cost bentonite clay was chemically modified using magnesium chloride in order to enhance its fluoride removal capacity. The magnesium incorporated bentonite (MB) was characterized by using XRD and SEM techniques. Batch adsorption experiments were conducted to study and optimize various operational parameters such as adsorbent dose, contact time, pH, effect of co-ions and initial fluoride concentration. It was observed that the MB works effectively over wide range of pH and showed a maximum fluoride removal capacity of 2.26 mgg(-1) at an initial fluoride concentration of 5 mg L(-1), which is much better than the unmodified bentonite. The experimental data fitted well into Langmuir adsorption isotherm and follows pseudo-first-order kinetics. Thermodynamic study suggests that fluoride adsorption on MB is reasonably spontaneous and an endothermic process. MB showed significantly high fluoride removal in synthetic water as compared to field water. Desorption study of MB suggest that almost all the loaded fluoride was desorbed ( approximately 97%) using 1M NaOH solution however maximum fluoride removal decreases from 95.47 to 73 (%) after regeneration. From the experimental results, it may be inferred that chemical modification enhances the fluoride removal efficiency of bentonite and it works as an effective adsorbent for defluoridation of water.

Systematic study of Mn-doping trends in optical properties of (Ga,Mn)As.

We report on a systematic study of optical properties of (Ga,Mn)As epilayers spanning the wide range of accessible Mn(Ga) dopings. The material synthesis was optimized for each nominal Mn doping in order to obtain films which are as close as possible to uniform uncompensated (Ga,Mn)As mixed crystals. We observe a broad maximum in the mid-infrared absorption spectra whose position exhibits a prevailing blueshift for increasing Mn doping. In the visible range, a peak in the magnetic circular dichroism also shifts with increasing Mn doping. The results are consistent with the description of ferromagnetic (Ga,Mn)As based on the microscopic valence band theory. They also imply that opposite trends seen previously in the optical data on a limited number of samples are not generic and cannot serve as an experimental basis for postulating the impurity band model of ferromagnetic (Ga,Mn)As.

Study of the formation of perovskite type lanthanum ruthenates by heating their hydrous precursor.

Emanation thermal analysis (ETA), differential thermal analysis (DTA), thermogravimetry (TG), evolved gas analysis with mass spectrometric detection (EGA-MS), and X-ray diffraction (XRD) were used to investigate the formation of perovskite type lanthanum ruthenates on heating their hydroxide precursor in argon from 20 to 1200 degrees C. The co-precipitated lanthanum-ruthenium mixed hydroxide containing a small amount of carbonates was used as a precursor. The mass loss corresponding to the release of water and CO(2) from the precursor was determined by TG and EGA (MS), respectively. The ETA characterized the exposure of sample surface after release of water and CO(2), as well as microstructure development corresponding to the crystallization and structure ordering of LaRuO(3) and La(3.5)Ru(4.0)O(13) perovskite phases. The obtained information on formation of phases and their transformation is useful for optimizing their synthesis protocols for achieving the desired physical properties, and to estimate the thermal stability of these materials to be used as catalysts.

Some aspects of physico-chemical properties of TiO2 nanocolloids with respect to their age, size, and structure.

Aqueous colloidal solutions of quantum sized particles of titanium dioxide (Q-TiO2) undergo an aging process since the moment of their preparation. In course of time, the mean size of nanoparticles is gradually increasing and some of their physicochemical properties are changing as well. In the present study, the decrease of the blue spectral shift of the semiconductor absorption threshold was measured to determine the corresponding changes of band gap energy of the Q-TiO2 particles. In parallel, the decrease of the specific surface area of these particles was followed through their complexation with 2-coumaric acid. The formation kinetics and thermodynamic equilibrium of these surface charge-transfer complexes were investigated in detail by means of UV/vis absorption spectroscopy. Besides, the size and shape of the Q-TiO2 particles aged 2, 4, and 10 years were compared employing direct observation by means of high resolution transmission electron microscopy. On the basis of real particle images, a model of nanocrystalline anatase was developed. The specific surface areas estimated through complexation with 2-coumaric acid were confronted with the numbers of appropriate titanium atoms located on the particle surface that were calculated from a model of anatase nanocrystals of different sizes. The photocatalytic activity, which represents the most important quality of anatase from practical point of view, was repeatedly determined through photocatalytic degradation of 4-chlorophenol in a colloidal solution of Q-TiO2 particles during its aging at 4 degrees C for three years. The corresponding reaction rate was increasing rapidly in the first weeks; it almost tripled in 68 days, and afterward it approached a limiting value. On the whole, the initial value increased four times in three years. Voltammetry at hanging mercury drop as the last method for aging description did not show any significant change of voltammetric behavior in a short two months period. However, while compared to preceding results of a similar TiO2 system, new redox processes of Q-TiO2 colloids were observed in the negative potential range. Besides the reduction of surface protons reported previously, two new pairs of peaks appeared.

Oxidation behavior of Ru/TiO2 and metallic Ru fine particles on heating in air.

Thermogravimetry (TG) and differential thermal analysis (DTA) were used to investigate the oxidation behavior of Ru/TiO2 and metallic Ru fine particles during heating in air in the range 20-1000 degrees C. Temperature ranges of the oxidation for two samples of Ru/TiO2 with the compositions (92 wt% Ru, 8 wt% TiO2) and (75 wt% Ru, 25 wt% TiO2) and for pure metallic Ru fine particle agglomerates were determined. It was assumed that after the partial oxidation of Ru in the sample containing 75 wt% Ru and 25 wt% TiO2 and in the pure metallic Ru a diffusion barrier was formed, preventing further oxidation of Ru in Ru/RuO2 and Ru/RuO2/TiO2 matrices. XRD and TEM were used for the sample characterization.

Emanation thermal analysis study of the preparation of ruthenia-titania-based finely dispersed powders.

Emanation thermal analysis (ETA), thermogravimetry (TG), differential thermal analysis (DTA), X-ray diffraction analysis (XRD), and transmission electron microscopy (TEM) were used for the characterization of thermal behavior of hydrous oxides (RuO(2))(x)-(TiO(2))(1-x).nH(2)O and dehydrated oxides (RuO(2)) (x)-(TiO(2))(1-x), respectively (x=1, 0.9, and 0.7) on heating in argon and argon +10% hydrogen, respectively. The samples were heated in the range 20-600 degrees C at the constant heating rate 6 K/min. From the TG and DTA results the temperature intervals of dehydration and/or reduction of ruthenium dioxide into Ru metal were determined. ETA (based on the measurement of radon previously incorporated into the sample) brought about the information about surface area development and microstructure changes in the near surface layers up to 80 nm under in situ conditions of the heating. The temperature intervals of the annealing of near surface and structure defects serving as diffusion paths for radon were determined from the ETA results.