Mössbauer spectroscopic study of FeAl1−x Co x

We report the results of a Mössbauer study of the alloy sytem FeAl_1?x Co _x forx ≥ 0.3 at temperatures down to 83 K. Magnetic splitting is observed forx ≥ 0.35 at all temperatures. However, forx=0.3, no splitting is observed at room temperature, and superparamagnetic behavior occurs at LN₂ temperature. The magnetically split spectra are fitted each with a distribution of hyperfine fields and the average hyperfine field \(\bar B_{hf} \) as a function of temperature is obtained. The variation of \(\bar B_{hf} \) withT is explained using the model of magnetic clusters with collective magnetic excitations from which the saturation hyperfine field and the magnetic anisotropy energy for these clusters are obtained. Also, the results are discussed using the model of random atomic distributions, and the agreement between the calculated and the experimentally obtained distributions of hyperfine fields is found improve asx increases.     

LaAg x In1−x

The structures of LaAg _x In_1–x alloys withx=0.75, 0.89 are determined by neutron diffraction on powder samples. The space group isI4/mmm (D _4h ^/17 ). The lattice constants splitting, the order parameter and the mean square vibrational amplitudes of the atoms are given in the temperature range from 20 KT300 K.     

Low temperature properties in Ce(Cu x Al1−x )5, [0.60≦x≦1.00]

We report on the formation of a heavy fermion state in Ce(Cu _x Al_1–x )₅, with 0.60x1.00. This state evolves upon the exchange of Cu by Al in the hexagonal CeCu₅. Furthermore, this exchange causes a suppression of a common type of long range magnetic order. Both phenomena are driven by Kondo interaction.     

Characterisation of GaN–In x Ga1−x N quantum-well lasers

An efficient analytical model to calculate the optical gain of the quantum-well laser of the GaN–In _x Ga_1?x N material system is used in this paper. Based on the anisotropic effective mass theories, empirical formulas delineating the relations between optical gain, emission wavelength, well width and material compositions are obtained for such a quantum-well lasers.     

Calculation of the electronic structure and hyperfine fields for Fe1 − x Co x B and (Fe1 − x Co x )2B compounds by the Korringa-Kohn-Rostoker method

The magnetic properties of Fe_{1 ? x} Co _x B and (Fe_{1 ? x} Co _x )₂B disordered compounds were investigated using first-principles calculations of the electronic structure in the framework of the density functional theory with the Korringa-Kohn-Rostoker method. The concentration dependences of the magnetic moments and the electron density were calculated for the Fe_{1 ? x} Co _x B solid solutions. The results obtained were used to analyze in detail and to interpret the transition from a magnetic phase to a nonmagnetic phase, which was previously revealed from the experiments in the compounds under investigation. The performed analysis of the calculated hyperfine fields induced by the electronic shells at the iron and cobalt atoms in the (Fe_{1 ? x} Co _x )₂B borides made it possible to explain the experimentally observed magnetic anisotropy.     

Oxygen non-stoichiometry, redox thermodynamics, and structure of LaFe1 − x Co x O3 − δ

The oxygen non-stoichiometry and redox thermodynamic properties of the LaFe_{1 ? x} Co _x O_{3 ? δ} system (x?=?0.25 and 0.75) are studied. At low temperatures, the LaCoO₃ and LaFeO₃ systems show partial solid solubility. At 1,273 K (in air), both compounds are single phases and are orthorhombic and rhombohedral for x?=?0.25 and 0.75, respectively. Thermogravimetry has been used to measure the oxygen non-stoichiometry versus oxygen partial pressure at three temperatures, 1,223, 1,273, and 1,323 K. Redox thermodynamic quantities are extracted directly from the oxygen non-stoichiometry curves. The extracted enthalpies of oxidation do not vary significantly with stoichiometry, and for x?=?0.25 and 0.75, they are ?640?±?60 and ?440?±?60 kJ (mol O₂)^?1, respectively. Ideal solid solution thermodynamic models are used to analyze the redox mechanisms.     

Negatron effects in CdSe1 − x Te x and ZnS1−x Se x films

Various negatron effects in films of alloys of II–VI compounds deposited from solutions as a function of the deposition mode and heat treatment are studied. It is found that the negative photocapacitance effect, which was first discovered in ZnS_1?x Se _x films, and the slowly relaxing negative photoelectric effects, which are caused by the transition of electrons located in a nanoscale surface layer from the shallow energy levels of trapping centers to deeper levels with a lower polarizability and by the presence of nanoscale clusters in these materials, which play the role of a “reservoir” for minority charge carriers, occur according to a single mechanism. A model to explain the basic laws of negative photoconductivity in CdSe_{1 ? x} Te _x films deposited from a solution is proposed. Negative residual conductivity is explained in terms of double-barrier relief model, while negative differential photoconductivity is attributed to the presence of nanoscale electric domains.     

Chromium mask for plasma-chemical etching of Al x Ga1 − x N layers

The influence of the mask material used for formation of GaN mesa-structures using plasma-chemical etching in a BCl₃ : Ar : N₂ atmosphere is studied. It is shown that a bilayer SiO₂/Cr mask in which the thickness of the chromium layer is six to seven times smaller than a desired etch depth provides effective protection of the surface and allows the formation of structures with a flat surface for an etch depth of up to 2.5 μm. When the mask is produced by conventional lithography, the slope of the side walls is no more than 10° and decreases when liftoff lithography is applied.     

Morphology and magnetic properties of Fe x Co1−x /Co y Fe3−y O4 nanocomposites prepared by surfactants-assisted-hydrothermal process

Recently, we have demonstrated the successful synthesis of Fe _x Co_1−x /Co _y Fe_3−y O₄ nanocomposites with various alkaline solutions by using surfactants-assisted-hydrothermal (SAH) process. In this article, the synthesis of Fe _x Co_1−x /Co_yFe_3−y O₄ nanocomposites with their sizes varying between 20 nm and 2 μm was reported. X-ray powder diffraction (XRD) analyses showed that the surfactants, pH, precipitator, and temperature of the system play important roles in the nucleation and growth processes. The magnetic properties tested by vibrating sample magnetometer (VSM) at room temperature exhibit ferromagnetic behavior of the nanocomposites. These Fe _x Co_1−x /Co _y Fe_3−y O₄ nanocomposites may have a potential application as magnetic carriers for drug targeting because of their excellent soft-magnetic properties.     

The electronic structure of the mixed 5f system UC x N1−x

We present results of electronic structure calculations for UC _x N_1–x obtained with the relativistic Korringa-Kohn-Rostocker Greens function (RKKR-GF) method. While on the anion sites the disorder is treated within the Coherent Potential Approximation (CPA) the small change in the cation potential upon alloying is accounted for in the averaget-matrix approximation (ATA). The presence of strong local spin fluctuations forx0.6 restricts our treatment to the carbon rich phase, and even there we find that the observed linear specific heat coefficient is much enhanced over the bandstructure value. The computed X-ray photoemission spectra vary smoothly with composition the main change consisting in the melting away of the side peak at –3 eV binding energy seen in pure UC with the adjunction of nitrogen.     

Optical properties of GaAs1−x N x alloys grown by molecular beam epitaxy

Optical properties of GaAs_1?x N _x alloys grown by molecular beam epitaxy using GaAs (001) as the substrate have been studied. These include photoluminescence (PL), cathodoluminescence (CL), photocurrent and photomemory effects. The low-temperature (77?K) PL characteristics were measured on samples with 0–0.105% N content. The wide emission band indicates the defective nature of the materials. The widening of the band for materials with increasing nitrogen concentration also suggested that the concentration of defect states in these materials dramatically increased with increasing nitrogen content. The PL and CL spectra for GaAs_1?x N _x layer 1854 did not show identical characteristics. Some layers showed a very sharp fall in photocurrent at low temperatures, indicating a very sharp photoquenching and an interaction between antisite, interstitial and vacancy defects. The photomemory effect, which causes photoquenching and the transition from the EL2 to the EL2? metastable state, was strongly influenced by the optical exposure and thermal history of the sample.     

Characterization and magnetic properties of electrospun Co1−x Zn x Fe2O4 nanofibers

By the electrospinning and calcination techniques, we have prepared uniform nanofibers of Co_1−x Zn _x Fe₂O₄ (0.0≤x≤0.5) ferrites with diameters of 110–130 nm. The Co_1−x Zn _x Fe₂O₄ nanofibers are single-phase spinels and the lattice constant with Zn content deviates from the Vegard’s law for these Co_1−x Zn _x Fe₂O₄ nanofibers. The Co_1−x Zn _x Fe₂O₄ nanocrystal grains by which are built nanofibers increase with calcination temperature. Variations of coercivity and saturation magnetization with calcination temperature can be explained in terms of the grain-size (D) effect. The coercivity (H _c) of Co_0.5Zn_0.5Fe₂O₄ nanofibers varies as D ^0.65 and basically follows the predicted D ^2/3 dependence based on the random anisotropy model in a D range below the single-domain size around 40 nm. The saturation magnetization of Co_1−x Zn _x Fe₂O₄ nanofibers initially increases with increasing Zn content, reaches a maximum value at x=0.3 and then decreases with further increase of Zn content, while the coercivity exhibits a continuous reduction with the increase of Zn content.     

High-temperature background of internal friction in (Co45Fe45Zr10) x (Al2O3)100 − x , Co x (CaF2)100 − x , and Co x (PZT)100 − x nanocomposites

The elastic (G) and inelastic (Q ^?1) properties of (Co₄₅Fe₄₅Zr₁₀) _x (Al₂O₃)_{100 ? x} , Co _x (CaF₂)_{100 ? x} , and Co _x (PZT)_{100 ? x} (x = 23–76 at %) nanocomposites obtained by ion-beam sputtering are studied in the temperature range 300–900 K. A significant rise in the Q Q ^?1 (T) curve is observed at temperatures above 650 K, which is attributed to thermally activated migration of point defects under the conditions of confined geometry.     

Local atomic arrangement in mechanosynthesized Co x Fe1−x−y Ni y alloys studied by Mössbauer spectroscopy

Mechanosynthesized Co _x Fe_1?x?y Ni _y alloys were examined using X-ray diffraction (XRD) and Mössbauer spectroscopy. In order to explain the shape of hyperfine magnetic field (HMF) distributions for the alloys, a local environment model based on a multinomial distribution was proposed. The model was in agreement with the XRD data and confirmed that the studied alloys were disordered solid solutions. It was successfully applied to describe the samples with bcc and fcc crystalline lattice type within the relatively broad range of components concentration. The results showed that the change of the crystalline lattice type does not cause an abrupt change of the HMF value. Moreover, a mean number of unpaired spins for the first coordination sphere may be used as a parameter to describe the HMF value experienced by ⁵⁷Fe nucleus. Finally, a set of the most probable atomic configurations and their corresponding contributions to the HMF distribution were obtained.     

Martensitic transformation in La1−x Yb x Ag1−y In y

The structural phase transformation of La_1–x Yb _x Ag_1–y In _y has been studied on single crystals by low temperature Laue-technique. The martensitic transformation in this pseudobinary intermetallic alloy has to be characterized as a weak orthorhtombic distortion of a single I centered unit cell (c/a1.04;a/b1.006) and a collective slipping or twinning of these cells that gives a fixed orientation between the remaining cubic room temperature structure and the martensitic phase. Above room temperature exists an order-disorder transformation from the CsCl-B2 structure to an at room temperature metastable W-A2 structure. There is no dramatic change in the physical properties of this alloy by substituting La by Yb, so we may approximate our results to LaAg_1–y In _y .This paper is dedicated to Prof. Dr. S. Methfessel on the occasion of his 60^th birthday     

Magnetic properties of (GdxY1−x)Co2B2 compounds

Magnetic measurements were performed on the (Gd_xY_1−x)Co₂B₂ compounds, in the temperature range 2–800 K and fields up to 70 kOe. YCo₂B₂ is a paramagnet. The (Gd_xY_1−x)Co₂B₂ compounds with x≥0.2 shows a ferromagnetic type ordering. The saturation magnetization at 2 K coincides only with the contribution of gadolinium. The Curie temperatures are nearly linearly dependent on the composition. Above the Curie points, the thermal variations of the magnetic susceptibility can be described as a superposition of a temperature independent term ϰ₀ on a Curie-Weiss behavior. The Curie constants are determined by the contribution of Gd³⁺ ions only. The ϰ₀ values increase when the gadolinium content is greater. The observed properties are discussed in the wider framework of the magnetic behavior of cobalt in GdCo_xB_y compounds.     

Single crystal growth and superconductivity of Ca(Fe1−x Co x )2As2

We report the single crystal growth of Ca(Fe_1?x Co _x )₂As₂ (0?≤?x?≤?0.082) from Sn flux. The temperature–composition phase diagram is mapped out based on the magnetic susceptibility and electrical transport measurements. The phase diagram of Ca(Fe_1?x Co _x )₂As₂ is qualitatively different from those of Sr and Ba; this could be due to both the charge doping and structural tuning effects associated with Co substitution.     

Features of the Jahn-Teller transition in Ni1 − x Co x Cr2O4 solid solutions

The structure of Ni_{1 ? x} Co _x Cr₂O₄ solid solutions has been investigated by synchrotron X-ray powder diffraction in the temperature range of 90–350 K for compositions x = 0, 0.005, 0.01, 0.015, 0.02, 0.1, 0.5, and 0.8, and their structures have been refined using the Rietveld analysis. The T-x phase diagram for solid solutions in the studied range of temperatures and concentrations has been constructed based on the obtained data. The revealed structural phase transitions have been explained from the viewpoint of the crystal field theory and the Jahn-Teller cooperative effect. Distortions of polyhedra, which are the cause of the structural phase transition, have been evaluated. The applicability of the Landau phenomenological theory and the possibility of using the magnitude of spontaneous strains as the order parameter have been discussed.     

Structure and magnetic properties of the Co x Pt100−x nanowire arrays

The development of photocathodes materials has become an important task for X-ray free electron laser and new generation of particle accelerator. The choice of the optimum cathode type and its further improvement is a fundamental issue for the progress in radio-frequency photoinjectors. Metallic photocathodes offer several advantages over the semiconductor ones, e.g. long lifetime and prompt response time on the photoemission. This paper reviews the requirements and the current status of metallic photocathodes prepared by pulsed laser ablation deposition technique. Magnesium, yttrium and lead are proposed as good alternative to copper photocathode which is generally used in radio-frequency photoinjectors. Parametric studies of the irradiation conditions are demanded to optimize the metallic thin film deposition. The main achievements on the morphology and structure characterization as well as photoemission testing of metallic photocathodes are presented and discussed.     

Phase transitions in La1−x CaxMnO3−x/2 manganites

The crystal structure parameters and magnetic and electrical properties of La_1?x Ca_xMnO_3?x/2 reduced manganites with 0≤x≤0.5 are established. These investigations contribute to the understanding of magnetic interactions in manganites without Mn⁴⁺ ions. It is found that these manganites show a long-range antiferromagnetic order up to x=0.09 and transform into spin glasses at 0.09<x≤0.35. The compositions in the range 0.35<x≤0.5 show a strong increase in the spontaneous magnetization and critical point associated with the appearance of spontaneous magnetization and can therefore be viewed as inhomogenious ferromagnets. The magnetic and crystal structure peculiarities of La_0.5Ca_0.5MnO_2.75 are established by the neutron diffraction method. The strongly reduced samples show a large magnetoresistance below the point where the spontaneous magnetization develops. The magnetic phase diagram of La_1?x Ca_xMnO_3?x/2 is established by magnetization measurements. The magnetic behavior is interpreted assuming that the Mn³⁺-O-Mn³⁺ magnetic interaction is anisotropic (positive-negative) in the orbitally ordered phase and isotropic (positive) in the orbitally disordered phase. Introduction of the oxygen vacancies changes the magnetic interaction sign from positive to negative, thereby leading to a spin glass state in strongly reduced compounds. The results obtained reveal unusual features of strongly reduced manganites such as a large ferromagnetic component, a high magnetic ordering temperature, and a large magnetoresistance despite the absence of Mn³⁺-Mn⁴⁺ pairs. In order to explain these results, the oxygen vacancies are supposed to be ordered.