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Using a first-principles noncollinear wave-function-matching method, we studied the spin-transfer torques (STTs) in CoFe/Mg-B-O/CoFe(001) magnetic tunnel junctions (MTJs), where three different types of B-doped MgO in the spacer are considered, including B atoms replacing Mg atoms (Mg3BO4), B atoms replacing O atoms (Mg4BO3), and B atoms occupying interstitial positions (Mg4BO4) in MgO. A strong asymmetric angular dependence of STT can be obtained both in ballistic CoFe/Mg3BO4 and CoFe/Mg4BO4 based MTJs, whereas a nearly symmetric STT curve is observed in the junctions based on CoFe/Mg4BO3. Furthermore, the asymmetry of the angular dependence of STT can be suppressed significantly by the disorder of B distribution. Such skewness of STTs in the CoFe/Mg-B-O/CoFe MTJs could be attributed to the interfacial resonance states induced by the B diffusion into MgO spacer. [ABSTRACT FROM AUTHOR]

In this work, the state-of-the-art infrared variable angle spectroscopic ellipsometry (IR-VASE) and first-principles molecular dynamics (FPMD) method were combined to obtain the infrared dielectric functions of MgO crystal in the spectral range 300-1000 cm-1 and for temperatures up to 1950 K. The IR-VASE can measure the infrared dielectric functions of MgO crystal at temperatures ranging from 300 to 573 K and reproduce previous infrared-reflectivity experiments. As temperature increases, it demonstrates that the amplitude of dominant absorption peak centered around 400 cm-1 reduces, the width broadens, and the position shifts to longer wavelength. Besides ellipsometry study, the FPMD method was implemented, seeking to theoretically predict the infrared spectra of MgO crystal at elevated temperatures. Comparing with experimental measurements, the FPMD method can reproduce the essential feature of ellipsometry and previous infrared-reflectivity experiments even at elevated temperatures, though with some deviations in predicting the exact position and amplitude of dominant absorption peak. On the other hand, the FPMD method can predict the temperature effect on the infrared dielectric functions of MgO crystal, e.g., redshift and broadened absorption peak with increasing temperature. [ABSTRACT FROM AUTHOR]

High-pressure third-order elastic constants of materials have rarely been investigated experimentally and theoretically to date, so the predictive ability of the method of the volume-conserving, homogeneous deformations based on the first-principles total-energy calculations is tested for the first time in this work. Using this approach, the high-pressure third-order elastic constants C 111 − 3 C 112 + 2 C 123 ${C_{111}}-3{C_{112}}+2{C_{123}}$ , C 111 / 2 + 3 C 112 + C 123 ${C_{111}}/2+3{C_{112}}+{C_{123}}$ , C 144 − C 155 ${C_{144}}-{C_{155}}$ , and C456 of the MgO single crystal are obtained successfully. Meanwhile, the reliability of this method is also verified by comparing the calculated structural properties and high-pressure second-order elastic constants of the MgO single crystal with the available experimental results and other theoretical predications. Results not only indicate the accuracy of our calculations but also reveal the feasibility of the present theoretical method. It is hoped that the present theoretical method and predictions on the high-pressure third-order elastic constants of the MgO single crystal would serve as a valuable guidance or reference for further related investigations. [ABSTRACT FROM AUTHOR]

Oxide glasses generally have low thermal conductivity due to their disordered structures. Here, a fabrication method to provide them with high thermal conductivity while retaining their optical transparency is proposed. It consists of dispersing ∼20 μm MgO crystals in a glass matrix and matching their refractive indexes to prevent the decrease of the oxide glass optical transparency due to light scattering. The dispersion of the MgO crystals increased the thermal conductivity of the original glass by ∼50% and the new glass still exhibited optical transparency. Such MgO-dispersed glass could be successfully used in thermal management applications requiring both transparency and formability. [ABSTRACT FROM AUTHOR]

Summary: This study looks at investigating the influence of high surface area TiO2 and MgO‐doped TiO2 aerogel nanomaterials to improve the photovoltaic performance of monocrystalline silicon (mono‐Si) solar cells. TiO2 and MgO‐doped TiO2 anatase nanoaerogels were synthesized via a single‐step colloidal homogeneous precipitation sol‐gel method in a compact high‐pressure hydrogen reactor. TiO2‐based nanoparticles were encapsulated in ethylene vinyl acetate resins, and the obtained composite solutions were screen printed on the textured surface of the cells. The specific surface area, microstructural, composition, and optical properties of the nanoaerogels were characterized by Brunaur‐Emmett‐Teller, X‐ray powder diffractometer, energy‐dispersive X‐ray spectroscopy, field emission transmission electron microscope, field emission scanning electron microscope, and ultraviolet‐visible spectrophotometry. We observed that the MgO‐doped TiO2 (2% mol) nanoaerogel exhibited a much superior specific surface area (231 m2/g) compared with the undoped TiO2 (154 m2/g). Experimental results showed that the calculated relative power conversion efficiency increased by 4.6% for the MgO‐doped TiO2 coating and 3.4% for the undoped TiO2 under a simulated one‐sun illumination. [ABSTRACT FROM AUTHOR]

• The process of methane dissociation to produce methyl radical (CH 3 •) is studied in this work. • The electron-deficient state can be formed by Li doped, however it will be broken once the oxygen vacancy appears. • The active site with electronic-deficient state is the key to the efficient generation of CH 3 •. • A volcano relationship between the C-H activation energy and the H adsorption free energy exists on the Li doped MgO surfaces. Methyl radical (CH 3 •) is the critical precursor to generate C 2 hydrocarbons in oxidative coupling of methane (OCM) reaction. It is crucial to explore how to efficiently generate CH 3 • and reveal the intrinsic principles behind it. In this study, we select MgO with a relatively simple structure as the research object, and probe the effects of the electronic state of active site on the CH 3 • generation by the density functional theory (DFT) calculation. The results show that the pure MgO with the different crystal surfaces as the benchmark models exhibit poor catalytic performance for CH 3 • generation; while Li doped MgO surface or subsurface that have the electron-deficient of active sites, exhibit excellent catalytic performance for it, which is attributed to that the electron-deficient of active sites are easier to receive electron of H and thereby promote the efficient generation of CH 3 •; pure MgO surface or subsurface with oxygen vacancy as well as Li doped MgO surface or subsurface with oxygen vacancy as the contrast models that have the electron-rich of active sites, exhibit poor catalytic performance. It can be obtained that the active site with electronic-deficient state is the key to the efficient generation of CH 3 •. The study can provide some guidance for designing high efficient OCM catalysts. [Display omitted] [ABSTRACT FROM AUTHOR]

3000 Å Fe3O4 (magnetite) thin films were simultaneously grown on (001) MgO single crystal substrates with and without 30 Å buffer layers of Fe, Cr, Mo, and Nb. For all samples, the Verwey transition temperature (TV) occurs between 119 and 125 K, indicating good oxygen stoichiometry. We observe highly oriented (001) Fe3O4 with Mo and no buffer layer, reduced (001) texture with Nb and Fe, and polycrystalline growth with Cr. Mo, Cr, and unbuffered magnetite show typical magnetic behavior, whereas Nb and Fe buffers lead to anomalous magnetic properties that may be due to interfacial reactivity. [ABSTRACT FROM AUTHOR]

he infrared magnetorefractive effect (MRE) is used to compare the magnetoresistance (MR) in epitaxial thin films of Fe3O4 grown on MgO with (100) and (111) crystal orientations. The smaller MRE detected in the (111) film is shown to correlate with the smaller electrically measured MR, its behavior consistent with a lower density of antiphase boundaries in the (111) film. [ABSTRACT FROM AUTHOR]

Ferropericlase (Mg, Fe)O is an abundant mineral of Earth's lower mantle and the liquid phase of the material was an important component of the early magma ocean. Using quantum-mechanical, finite-temperature density-functional theory calculations, we compute the electronic component of the electrical and thermal conductivity of (Mg 0.75 , Fe 0.25 )O crystal and liquid over a wide range of planetary conditions: 0–200 GPa, 2000–4000 K for the crystal, and 0–300 GPa, 4000–10,000 K for the liquid. We find that the crystal and liquid are semi-metallic over the entire range studied: the crystal has an electrical conductivity exceeding 10 3 S/m, whereas that of the liquid exceeds 10 4 S/m. Our results on the crystal are in reasonable agreement with experimental measurements of the electrical conductivity of ferropericlase once we account for the dependence of conductivity on iron content. We find that a harzburgite-dominated mantle with ferropericlase in combination with Al-free bridgmanite agrees well with electromagnetic sounding observations, while a pyrolitic mantle with a ferric-iron rich bridgmanite composition yields a lower mantle that is too conductive. The electronic component of thermal conductivity of ferropericlase with X Fe = 0.19 is negligible (<1 W/m/K). The electrical conductivity of the crystal and liquid at conditions of the core-mantle boundary are similar to each other ( 3 × 10 4 S/m). A crystalline or liquid ferropericlase-rich layer of a few km thickness thus accounts for the high conductance that has been proposed to explain anomalies in Earth's nutation. The electrical conductivity of liquid ferropericlase exceeds that of liquid silica by more than an order of magnitude at conditions of a putative basal magma ocean, thus strengthening arguments that the basal magma ocean could have produced an ancient dynamo. [ABSTRACT FROM AUTHOR]

In this work, Algerian kaolinite, a naturally occurring clay mineral, was used as low-cost precursor for the synthesis of cordierite ceramics. The kaolinite was mixed with synthetic magnesia, and the mixture was ball milled and reaction sintered in the temperature range 900–1350 °C for 2 h. Thermogravimetry (TG), differential thermal analysis (DTA), dilatometry, high temperature x-ray powder diffraction (XRD), Raman spectroscopy, and scanning electron microscopy (SEM) complementary techniques were used to analyze sintering behavior, characterize phase transformations, and investigate crystallization kinetics. Milling the kaolinite and magnesia mixture for 10 h yielded a homogenous powder, decreased the average particle size, and improved the roundness of particles. Different crystalline phases were present in the samples sintered in the temperature range 900–1150 °C, the cordierite phase started to crystallize at 1200 °C, and the formation of highly dense cordierite (99%) was complete at 1250 °C. The activation energy values for cordierite formation calculated using Kissinger, Boswell, and Ozawa methods were found to be equal to 577, 589, and 573 kJ/mol, respectively. The kinetic parameters n and m had values close to 2. Bulk nucleation with a constant number of nuclei was the dominant mechanism in cordierite crystallization, followed by two-dimensional growth controlled by interface reaction. [ABSTRACT FROM AUTHOR]

A novel method for determination of total inorganic arsenic in water samples, based on magnesium oxide crystals quantitatively preconcentration and graphite furnace atomic absorption spectrometry (GFAAS) detection has been developed. As the MgO crystals could successively act as the adsorbent and matrix modifier, the method presents great potential in practical routine analysis of inorganic arsenic. With optimization of the experimental conditions, 1.5 mg of MgO was added into 4.0 mL water sample and ultrasonic dispersing 15 min for adsorbing the arsenic at first. Then, the mixture was centrifuged 5 min for separating the adsorbents and the water solution; the deposits were collected and dissolved with HNO 3 for arsenic determination. The enrichment factor of 13, limit of detection (LOD) of 0.087 μg L − 1 , relative standard deviation (RSD) of 4.5% ( n = 7, c = 2.0 μg L − 1 ) were achieved under the optimized conditions. The procedure was validated by analysis of certified water sample (GBW08605), which results in good agreement between the certified and the found values. The method was further demonstrated for analysis of lake water, snow water and tap water, and satisfactory spiked recoveries of 98.5% to 103.0% were achieved for these samples. These preliminary results indicated the present method was practical and accurate for analysis of trace-arsenic in natural water samples. [ABSTRACT FROM AUTHOR]

Compositions of 55SiO-10KO-(35- x)CaO- xMgO are prepared by melt and quench technique. Thermal parameters of the as-prepared glasses are studied using the differential thermal analyzer under non-isothermal conditions. Kissinger, Augis-Bennett and Lasocka models are employed to investigate the kinetics of crystallization and thermal stability of these glasses. Based on this, it is concluded that CM-15 glass exhibits highest thermal stability. Raman spectroscopy is used to reveal the structural units of the glasses. Dielectric properties are observed through impedance spectroscopy. All the glasses are phase separated. The ratio of CaO/MgO influences the thermal stability, which leads to affect the dielectric properties. The highest dielectric permittivity is observed ~22 at room temperature and 100 Hz for CM-15 glass, where CaO/MgO ratio is ~1.33. [ABSTRACT FROM AUTHOR]

Fresh Geobacillus thermoglucosidasius cells grown on soybean-casein digest nutrient agar were inoculated as a parent colony 1 cm in diameter on the surface of an agar gel containing acetate and calcium ions (calcite-promoting hydrogel) and incubated at 60 °C for 4 days, after which magnesium-calcite single crystals of 50-130 µm in size formed within the parent colony. Addition of EDTA, polyacrylic acid or N,N-dicyclohexylcarbodiimide to the calcite-forming hydrogel inhibited the parent colony from forming magnesium-calcite crystals. Inoculation of G. thermoglucosidasius on calcite-forming hydrogel containing 5 µM cadmium and 20 µM zinc resulted in a decrease in the sporulation rate from 55 to 7-8 %. Magnesium-calcite synthesis decreased relative to the sporulation rate. G. thermoglucosidasius exhibited higher adsorption/absorbance of calcium than other Geobacillus sp. that do not mediate calcite formation and higher levels of magnesium accumulation. Calcium ions contained in the calcite-promoting hydrogel and magnesium ions concentrated in G. thermoglucosidasius cells serve as the elements for magnesium-calcite synthesis. The observed decreases in sporulation rate and magnesium-calcite formation support the hypothesis that endospores act as nuclei for the synthesis of magnesium-calcite single crystals. [ABSTRACT FROM AUTHOR]

In this work, it was found that magnesium oxide nanoparticles (nano-MgO) catalyzed one-pot three-component reaction of hydroxylamine hydrochloride with aryl aldehydes (or heteroaryl aldehydes) and β-oxoesters to synthesize some biologically active isoxazole-5(4 H)-one-based heterocycles. The reactions were completed using 3 mol% catalyst loading in aqueous medium at room temperature. Nano-MgO was synthesized by precipitation and hydrothermal treatment of aqueous salt solution. The structure of the nano-MgO was characterized by X-ray diffraction (XRD) and scanning electron microscopy (SEM) analyses. The presented environmentally friendly heterocyclization offers some interesting merits, including safety, high product yield, mild conditions, low cost, minimal waste, good atom efficiency, catalyst recoverable, energy efficiency, avoidance of hazardous organic solvents, and easy workup. [ABSTRACT FROM AUTHOR]

The generation of optical parametric oscillator (OPO) at 4.1 μm in the periodically poled MgO doped congruent lithium niobate (MgOLN) single crystal was studied and tested. The stable domain structure with period 28.48 μm was produced in 2-mm-thick wafer by multi-pulse periodical poling at elevated temperature. The Nd:YAG laser was used as a pumping source. The OPO was realized by single resonant scheme with two-mirror cavity. The average power at the generated idler wavelength 70 mW was achieved for 5 W average pump power and pulse duration 10 ns. The produced OPO can be used for diagnostic equipment. [ABSTRACT FROM AUTHOR]

The kinetic pathway for magnesia-excess magnesium aluminate spinel (MgAl 2 O 4 ) formation is determined from the dilation behaviour of MgO–Al 2 O 3 (34:66 w/w) oxide compacts during reaction sintering under dynamic heating regime at different constant heating rates ( β ). It is calculated from expansion values at different β , analysed through model-free and model-based kinetic methods to arrive at the most suitable and unambiguous kinetic pathway, where spinelisation is best described ( r 2 =0.986) with Avrami model with exponent n =0.65 and activation energy ( E a ) 409 kJ mol −1 . To validate the kinetic model, an isothermal heat treatment of the oxide compact was done and % spinelisation was determined by Rietveld refinement of the XRD data. The results matched the predicted values very closely and validate our kinetic model. This method can be extended to several industrially important thermally activated solid state reactions which require thermal treatment in order to minimise the energy requirement by optimising the heating protocol. [ABSTRACT FROM AUTHOR]

We investigate the variation of the oscillation frequency of the Mg2+ and O2- ions in the magnesium oxide lattice due to the interactions of the surface with water monolayers by means of Low Energy Electron Diffraction. Our key result is a new technique to determine the adsorbate vibrations produced by the water monolayers on the surface lattice as a consequence of their change in the surface Debye temperature and its chemical shift. The latter was systematically investigated for different annealing times and for a constant external thermal perturbation in the range of 110-300K in order to accomplish adsorption or desorption of water monolayers in the surface lattice. [ABSTRACT FROM AUTHOR]

We report on the growth of cuprous oxide on MgO(001) by plasma-assisted molecular beam epitaxy. An increased copper to oxygen ratio enhances the cube-on-cube growth of the Cu 2 O crystallites compared to the Cu 2 O(110)||MgO(100) oriented crystallites. This correlates with the occurrence of a (3√2×√2)R45° surface reconstruction observed during growth. This reconstruction is known to be unstable under oxygen-rich conditions and stabilizes the Cu 2 O(001) surface compared to the Cu 2 O(110) surface for higher copper to oxygen ratios enhancing cube-on-cube growth. High resolution X-ray diffraction analysis of the resulting cube-on-cube Cu 2 O layers indicates partial relaxation by 39.7%. Investigations of the optical absorption and photoluminescence properties of the films reveal excitonic contributions comparable to those of single crystals. [ABSTRACT FROM AUTHOR]

FeMn/Co thin-films, a purely metallic exchange-bias system, were prepared on (100)-oriented MgO single crystals. The layers were grown by molecular beam epitaxy (MBE). The crystalline and magnetic properties could be tuned by using sputtered Pt buffer layers deposited at variable temperatures. A transition of the crystalline orientation from (111)-terminated layers at lower deposition temperatures to (100)-terminated layers at temperatures higher than 950 K was established and the impact on the magnetic sample properties was investigated. Here, the detailed sample preparation process is shown together with low energy electron diffraction (LEED), medium energy electron diffraction (MEED), atomic force microscopy (AFM), X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD) and transmission electron microscopy (TEM) measurements revealing the samples structure and composition. The crystalline orientation of Pt was imprinted to the magnetic layer stack and clearly influenced the exchange-bias (EB) and coercive field ( HC) as shown by superconducting quantum interference device (SQUID) and magneto-optical Kerr effect (MOKE) measurements. Furthermore, a correlation between the crystallite size and the temperature dependent magnetic properties could be identified. [ABSTRACT FROM AUTHOR]

The depth dependence of damage and modification of micromechanical properties in MgO single crystals irradiated with 150 MeV Kr and 24.5 MeV N ions (specific energy 1.75 MeV/u) at fluences up to 10 ions/cm has been studied. The effects of ion-induced increase in hardness and reduction in dislocation mobility, magnitude of which varies along the ion range, were observed. These effects are related to ion-induced dislocations which were revealed by chemical etching. The results confirm a joint contribution of electronic excitations and atomic displacements by elastic collisions in the structural damage of MgO. The excitation mechanism in hardening dominates in the incoming part of ion range (up to the Bragg's maximum), while the role of impact mechanism becomes dominant only at the end of ion range. [ABSTRACT FROM AUTHOR]

Nanocrystalline magnesium oxide (nano-MgO) has been used as an efficient catalyst for an improved synthesis of selective oxidation of benzoins to benzils inthe presence of acetonitrile as solvent in air atmosphere for the first time. Thepresent procedure has the following advantages: it is green, requires milder conditions and a shorter reaction time while providing higher yelds and selectiivity. [ABSTRACT FROM PUBLISHER]

Luminescent defect centers in wide bandgap materials such as MgO are of great interest for science and technology. Magnesium oxide nanocubes obtained by the self-combustion of Mg metal have long exhibited only a broad 2.9 eV cathodoluminescence band owing to oxygen vacancies (F centers). However, in this work, a room-temperature ultraviolet 4.8 eV cathodoluminescence band has been observed coincident with a 2.5 eV band of the same intensity from an unexplored intrinsic defect in MgO terraced nanocubes and nanospheres produced from Mg metal combustion in an H2/O2 flame. Synchrotron radiation excitation spectra reveal that the excitation energy at the onset of both bands is just above the bandgap energy of 7.7 eV, where electrons and holes are generated. We determine that a defect, responsible for both emission bands, creates proximal anion-cation vacancy pairs named P centers that may appear instead of F centers because of changes in the MgO nanoparticle growth conditions. [ABSTRACT FROM AUTHOR]

K and L X-ray emission spectra of Mg atoms in a MgO crystal are calculated. The wave functions and energies of the crystal needed for calculating the intensities of these spectra are obtained in calculations of crystalline clusters by the embedding potential method. In this method, a finite cluster is considered instead of an infinite crystal and the effect of the crystalline environment on the cluster is simulated by a potential that is usually called the 'embedding potential.' The electronic structure of clusters of different sizes and geometries in the embedding potential field is calculated by the Hartree-Fock and density functional methods. Qualitative agreement between calculated spectra and experimental data testifies that the embedding potential correctly describes the effect of the crystalline environment on the cluster. [ABSTRACT FROM AUTHOR]

Research on semiconductor nanowires underlies the development of the miniaturization of laser devices with low cost and low energy consumption. In general, nanowire lasers are made of direct band gap semiconductors, e.g., GaN, ZnO and CdS, and their band-edge emissions are used to achieve optically pumped laser emission. In addition to the existing class of nanowire lasers, we here show that air-annealed micrometer-sized MgO cubic crystals with well-defined facets exhibit room-temperature stimulated emission at 394 nm under pulsed laser pumping at ~350 nm. Surface midgap states are assumed to be responsible for the excitation and emission processes. The present findings will not only provide opportunities for the development of miniaturized lasers composed of insulating oxides, but will also open up functionality in various families of cubic crystalline materials. [ABSTRACT FROM AUTHOR]

A p-n heterojunction made of MgO and Ga4Se3S single crystal has been successfully produced. The current–voltage curve analysis has shown that the current conduction mechanism is mostly governed by the Richardson–Schottky mechanism. The width of the effective interface region of the p-n junction was found to be 3.72\times 10^-5 cm. The work function and the electron affinity of the Ga4Se3S crystals were also determined as 4.32 and 3.96 eV, respectively. On the other hand, the capacitance–voltage curve analysis, which was carried out in the power range that extends from Bluetooth to WLAN power outputs, reflected a built-in voltage of 0.48 eV and density of noncompensated carriers of 8.2\times 10^16 \mathrmcm^-3 . The device is observed to exhibit a wide range of negative resistance associated with the tunneling of charged particles at reverse biasing down to $\sim 1.28$ V. At that voltage, when exposed to a He–Ne laser beam of $\sim 3$ mW, the device reflected a responsivity of $\sim 80.$ The charge storability increased and the $I$ – $V$ characteristics are significantly shifted. These properties are promising because it indicates the applicability of these tunneling devices in optoelectronics. [ABSTRACT FROM PUBLISHER]

We have grown atomically smooth heteroepitaxial TiN thin films on (100) MgO single crystals by pulsed laser deposition (PLD). To generate optimal conditions for two-dimensional growth of TiN on MgO, the substrates were annealed in-situ using a CO2 laser heater. The MgO surface is smooth and uniformly stepped with a terrace height of half a unit cell; a value of 0.21 nm was measured by atomic force microscopy (AFM). In-situ RHEED oscillations during deposition of TiN indicate two-dimensional growth mode up to a film thickness of about 43 nm. This is confirmed by AFM. The TiN films show smooth, stepped surfaces with a uniform terrace height of 0.211 nm. X-ray diffraction measurements indicate a high film quality, as well. High resolution scans feature intensity fringes around the (200) TiN peak, indicating an abrupt interface between MgO and TiN. On asymmetric reciprocal space maps the peak broadening only in [ABSTRACT FROM AUTHOR]

The luminescence of nominally pure magnesium oxide crystals irradiated in a nuclear reactor is studied. It is shown that, in the entire fluence range (10-10 n/cm), the active luminescence centers are chromium impurity ions, which are initially (<10 n/cm) in the form of Cr ions and then (>10 n/cm), after the radiation-induced transformation Cr → Cr, are in the form of Cr ions. The role played by F centers is also shown. Pulsed stimulated emission is observed in crystals irradiated with a fluence of 10 n/cm. [ABSTRACT FROM AUTHOR]

The dispersion analysis of the dielectric spectra of the perovskite ceramics, CaTiO3, and perovskite-like monocrystalline thin films (Ba,Sr)TiO3deposited on a monocrystalline MgO substrate is carried out. The giant LO–TO splitting (∼700 cm−1) of the lowest-frequency absorption band (the soft mode) is found. Due to that splitting the LO and TO frequencies of other phonons are inverted. The result is in a good agreement with first-principle calculations. [ABSTRACT FROM PUBLISHER]

In this work, we experimentally study the temperature dependence of the perpendicular magnetic anisotropy (PMA) and of the voltage-controlled magnetic anisotropy (VCMA) in nanoscale MgO|CoFeB|Ta-based magnetic tunnel junctions. We demonstrate that the temperature dependences of both the PMA and the VCMA coefficient follow power laws of the saturation magnetization, but with different exponents. We also find that the linear dependence of the PMA on electric field is maintained over a wide temperature range, although the VCMA strength decreases faster as a function of temperature as compared to the PMA. Possible mechanisms leading to the different exponents are discussed. [ABSTRACT FROM AUTHOR]

The heterostructure thin films of yttria-stabilized zirconia (YSZ)/strontium titanate (STO)/YSZ with various thicknesses were deposited on MgO single crystal substrate by pulsed laser deposition (PLD) method. The structural and electrical properties of the YSZ/STO/YSZ heterostructure were studied through X-ray diffraction (XRD) and electrical conductivity measurements. The in-plane conductivities of the thin films were measured and compared with that of the bulk sample. The highest conductivities were reported for those samples with the thinnest YSZ (220) layers. The observed enhancement in the lateral ionic conductivity was probably caused by the combination of the misfit dislocation density and elastic strain in the interfaces. The enhanced ionic mobility was discussed in terms of the disorder introduced in the oxygen sublattice through the epitaxial strain at the interfaces. [ABSTRACT FROM AUTHOR]

Abstract: Lithium tantalate (LT) is an oxide material whose congruent composition differs from its stoichiometric composition in the Li2O–Ta2O5 binary system. Ionic species in the LT melt are segregated during crystal growth, even at the congruent composition, which leads to crystallization electromotive force (C-EMF) at the solid–liquid. On the other hand, when a compound has a stoichiometric structure at the congruent composition, such segregation does not occur. By employing a new concept of stoichiometry of a material where the activity of any constituent element could be unity, we developed a new lithium tantalate with MgO doping, cs-MgO:LT (Li2O:Ta2O5:MgO=40.8:50.0:9.2), that is stoichiometric and coincident with the congruent composition. Because of this coincidence, cs-MgO:LT does not yield C-EMF and does not show segregation during growth even for ionic species. A bulk crystal was grown from the melt of the cs-MgO:LT composition via the Czochralski (CZ) method, and it showed excellent compositional homogeneity, proved by the constant distribution of the Curie temperature throughout the crystal. [Copyright &y& Elsevier]

Local reactivity on periodically proton exchanged lithium niobate (PPE:LN) surfaces is a promising route for the fabrication of regularly spaced nanostructures. Here, using MgO-doped PPE:LN templates, we investigate the influence of the doping on the nanostructure formation as a function of the proton exchange (PE) depth. The deposition is found to occur preferentially along the boundary between MgO-doped LN and the PE region when the PE depth is at least 1.73 μm, however, for shallower depths, deposition occurs across the entire PE region. The results are found to be consistent with an increased photoconductivity of the MgO-doped LN. [ABSTRACT FROM AUTHOR]

Electron paramagnetic resonance spectroscopy is used to characterise Mo(V) centres present on the surface of MgO(100) films grown on Mo(100) single-crystal surfaces at 600 K. The Mo(V) centres exhibiting a molydenyl group to reduce the charge imbalance with respect to the MgO lattice substitute Mg cations on regular terrace sites within the MgO(100) islands as deduced from angular-dependent measurements and spectral simulations. The observed anisotropic Zeeman and hyperfine interaction is comparable with Mo(V) centres supported on different powdered oxide surfaces such as alumina or silica. Analysis of the films grown at 300 K and subsequently annealed to elevated temperatures to improve the long-range order shows that the presence of the Mo sites on the surface is a thermally activated process. Comparison of the defect structures of the two preparation methods suggests that Mo plays a role in the relaxation of the lattice misfit between the Mo substrate and the MgO film. [ABSTRACT FROM PUBLISHER]

MgO crystals have been exposed to vacuum ultraviolet (VUV) radiation from a synchrotron, with energies up to 9 eV, and the emitted light, at wavelengths above 200 nm, was observed. It is concluded that bulk excitons, play an important role in the diffusion of energy inside MgO crystals, resulting in 5.85 eV (212 nm) emission from the MgO terraces of large (0.2-2 μm) MgO: F crystals. In the case of aliovalent impurity doping, then the bulk exciton energy is also transferred to the Vk centres and 5.3 eV (235 nm) light is emitted. Both fluorine and silicon doping appear to promote UV surface emission, acting similarly to an ns2 ion inside MgO, while strong scandium doping is killing the surface emission completely. The 212 nm surface UV emission and the 235 nm bulk UV emission can be excited only at the bandgap edge. Broadband visible light, centred around 400 nm, is also emitted. Contrary to the UV emission, this is not generated when excited at the bandgap edge; instead, we find that it is only excited at sub-bandgap energies, with a maximum at the 5C surface excitation energy of 5.71 eV (217 nm) for the MgO terraces. [ABSTRACT FROM AUTHOR]

Abstract: Ni-MgO catalysts with varying Ni/Mg ratios were prepared by co-precipitation and tested in sorbitol hydrogenolysis. At 473K and 4MPaH2, the best catalyst with Ni/Mg ratio of 3:7 exhibited 67.8% conversion and 80.8% total selectivity of ethylene glycol, 1,2-propylene glycol and glycerol. These catalysts were characterized by XRD, CO2-TPD and H2 chemisorption, revealing that the activity depended strongly on the basicity and Ni surface area. The reaction conditions were optimized, which were relatively mild for this chemoselective conversion. [Copyright &y& Elsevier]

Abstract: Based on the technology of the non-collinear phase-matching about three-wave interaction in uniaxial crystal, the performance of nonlinear crystal MgO:LiNbO3(MgO:LN) including the phase-matching angles, effective nonlinear coefficient and THz-wave gain as well as its absorption coefficients are theoretically discussed under e-ee type phase-matching condition. And an external cavity is developed for terahertz-wave parametric oscillators pumped by picosecond laser (ps-TPOs). For optimizing the ps-TPOs performance, the focusing spot size in MgO:LN crystal, the geometric size of the MgO:LN crystal, the conversion efficiencies, and the stability of the THz-wave generation are designed and calculated. The calculated results provide a comprehensive theoretical basis for the ps-TPOs using difference-frequency generation (DFG) method and optimal performance to generate widely tunable terahertz waves in MgO:LN crystal. [Copyright &y& Elsevier]

MgZnO-based ultraviolet avalanche photodetectors (APDs) have been fabricated from Au/MgO/Mg0.44Zn0.56O/MgO/Au Schottky structures. The carrier avalanche multiplication is realized via an impact ionization process occurring in the MgO layer under relatively large electric field. The APDs exhibit an avalanche gain of 587 at 31 V bias, and the response speed of the APDs is in the order of microseconds. [ABSTRACT FROM AUTHOR]

Superconducting FeSe thin films have been deposited onto (001)-oriented single-crystalline MgO substrates by conventional radio-frequency magnetron co-sputtering of a FeSe target and a pure Se target. The composition of the films is within the homogeneity range of the tetragonal β phase close to stoichiometry. Films with a minimum residual resistivity and a maximum superconducting transition temperature can be prepared within a narrow substrate temperature range from 480 to 510 °C. In contrast to the equilibrium phase diagram the β phase surprisingly exists in thin films deposited at substrate temperatures as high as 680 °C. The polycrystalline tetragonal films grow with a preferred c-axis texture and a tiny admixture of misaligned grains. The temperature-dependent resistivity reveals a low-temperature linear part and a high-temperature part with a pronounced negative curvature. The origin of the non-Fermi liquid behaviour and the underlying electron scattering mechanisms are still unknown so far. [ABSTRACT FROM AUTHOR]

Oxygen polarity and interfacial structure of an Mg0.55Zn0.45O/Mg0.17Zn0.83O/MgO/sapphire heterostructure were determined by electron energy-loss spectroscopy and aberration-corrected scanning transmission electron microscopy. It was found that O-polarity occurs by an uncommon mechanism in the MgxZn1-xO film with a 0.5 nm-thick compressively strained rocksalt MgO buffer; and that the ~18% lattice mismatch between Mg0.17Zn0.83O and sapphire is mostly accommodated by misfit dislocations at the Mg0.17Zn0.83O/MgO interface. Density functional theory calculations indicate that the MgO buffer strongly tends to manifest a rocksalt structure rather than wurtzite. Zn substitution of the topmost Mg atoms in the MgO buffer is likely to induce such O-polarity [ABSTRACT FROM AUTHOR]

Abstract: The effect of calcination temperature and MgO crystallite sizes on the structure and catalytic performance of TiO2 supported nano-MgO catalyst for the transesterification of soybean oil has been studied. The catalyst has been prepared by deposition–precipitation method, characterised by XRD, TEM, XRF, BET and FTIR and tested in a batch autoclave at 225°C. The soybean oil conversion after 15min of the transesterification reaction increased when the calcination temperature was increased from 500 to 600°C and decreased with further increase in calcination temperature. Some glycerolysis activity was also detected on catalysts calcined at 600 and 700°C after 45min of reaction. The soybean oil conversion during the transesterification reaction increased with the decrease in MgO crystallite size for the first 30min. [Copyright &y& Elsevier]

Abstract: The effect of anisotropic strain induced by SrTiO3 (STO) substrate with different orientations on charge ordering (CO) behavior in epitaxial Sm0.5Ca0.5MnO3 (SCMO) films was investigated using cross-sectional transmission electron microscopy at 103K. Incommensurate modulated structure caused by CO transition was observed in the SCMO film grown on a [101]-oriented STO substrate while none were found for SCMO film grown on a [100]-oriented STO substrate. This distinctive orientation-dependent CO transition is correlated with anisotropic strain in the films induced by lattice mismatch, confirming the previous results for growth on different substrates. [Copyright &y& Elsevier]

Abstract: Bi2Sr2CaCu2O8 thin films have been deposited on MgO single crystal substrates by spin-coating a solution based on 2-ethylhexanoate precursors. Pyrolysis takes place between 200°C and 450°C and is accompanied by the release of 2-ethylhexanoic acid, CO2 and H2O vapour. Highly c-axis oriented Bi2Sr2CaCu2O8 films were obtained after heat treatment at 840°C in air. The highest Tc of 81K was measured in a 10-layer film. Subsequent post-annealing in Ar and pure O2 did not improve the superconducting properties of the films and resulted in the appearance of Bi2CaCuO5 or Bi2(Sr, Ca)2CuO6 impurities. [Copyright &y& Elsevier]

Abstract: The electronic energy levels of six-fold coordinated Ni2+ ion in magnesium oxide MgO were calculated using the exchange charge model of crystal field theory. The calculated energetic positions of the Ni2+ levels match well the experimental spectrum. Inclusion of the spin-orbit (SO) interaction is compulsory to account for the first excited 3T2g state fine structure; however, it does not explain why out of four levels arising from the 3T2g state, only two are seen in the experimental spectra. One possible explanation to this fact can be advanced by invoking the Jahn–Teller effect. [Copyright &y& Elsevier]

Abstract: The methodology of characterizing electronic structure in dielectric materials will be presented in detail. Energy distribution of the electrons emitted from dielectric materials by the Auger neutralization of ions is measured and rescaled for Auger self-convolution, which is restructured from the energy distribution of the emitted electrons. The Fourier transform is very effective for obtaining the density of states from the Auger self-convolution. The MgO layer is tested as an example of this new measurement scheme. The density of states in the valence band of the MgO layer is studied by measuring the energy distribution of the emitted electrons for MgO crystal with three different orientations of (111), (100) and (110). The characteristic energy of ɛ 0 corresponding to the peak density of the states in the band is determined, showing that the (111) orientation has a shallow characteristic energy ɛ 0 = 7.4 eV, whereas the (110) orientation has a deep characteristic energy ɛ 0 = 9.6 eV, consistent with the observed coefficient γ of the secondary electron emission for MgO crystal. Electronic structure in new functional nano-films spayed over MgO layer is also characterized. It is therefore demonstrated that secondary electron emission by the Auger neutralization of ions is highly instrumental for the determination of the density of states in the valence band of dielectric materials. This method simultaneously determines the valence band structure and the coefficient γ of the secondary electron emission, which plays the most important role in the electrical breakdown phenomena. [Copyright &y& Elsevier]

Abstract: MgO single crystals were implanted with 70keV C/N/O ions at room temperature with respective doses of 2×1016 and 2×1017 ions/cm2. All samples with high-dose implantation showed room temperature hysteresis in magnetization loops. Magnetization and slow positron annihilation measurements confirmed that room temperature ferromagnetism in O-implanted samples was attributed to the presence of Mg vacancies. Furthermore, the introduction of C or N played more effective role in ferromagnetic performance than Mg vacancies. Moreover, the magnetic moment possibly occurred from the localized wave function of unpaired electrons and the exchange interaction formed a long-range magnetic order. [Copyright &y& Elsevier]

Wetting of poly- and mono-crystalline MgO substrates by molten La was investigated at 1323 K in a high vacuum using a modified sessile drop method. The wettability seems to depend mildly on the substrate orientation but strongly on the surface roughness. The initial contact angles on the smooth (100), (110), and (111) surfaces are 63° ± 1°, 69° ± 1°, and 69° ± 1°, respectively, while on the rough polycrystalline surfaces they are much larger (104° ± 3°). The wetting behavior is dictated by the disruption of the oxide film covering the La surface, the extent of the interfacial reaction and the evolution of the reaction product. A thick layer of La2O3 phase formed at the interface and then enwrapped the liquid surface, leading to the recession and warping of the triple line and finally the deterioration in the wettability. On the other hand, magnesium was displaced by the reaction and its evaporation provided additional impetus for the movement of the triple line. Due to different reaction intensities, the wetting behavior of La on the different orientations of the MgO surfaces also showed some discrepancies. [ABSTRACT FROM AUTHOR]

We report results of high-resolution sputter depth profiling of an alternating MgO/ZnO nanolayer stack grown by atomic layer deposition (ALD) of 5.5 nm per layer. We used an improved dual beam time-of-flight secondary ion mass spectrometer to measure 24Mg+ and 64Zn+ intensities as a function of sample depth. Analysis of depth profiles by the mixing-roughness-information model yields a 1.5 nm nanolayer interfacial roughness within the MgO/ZnO multilayer. This finding was cross-validated using specular x-ray reflectivity. Such an analysis further suggested that the 1.5 nm roughness corresponds to native/jig-sawed interfacial roughness rather than interfacial interdiffusion during the ALD growth. [ABSTRACT FROM AUTHOR]

The surface impedance characteristics of epitaxial YBaCuO films of thickness d = 75, 150, 300, 600 nm, produced by magnetron thermal co-evaporation onto single crystal MgO substrates was studied using measurement technique based on Ka-band whispering gallery mode (WGM) dielectric resonator (DR) fabricated from single crystal sapphire. Characterization of the unpatterned films was carried out in temperature interval from 20 K to 90 K. It was shown that the effective surface resistance approaches the minimum value for d>300 nm. At the same time, intrinsic impedance properties are practically independent on d in the studied interval of d values. The temperature dependence of London penetration depth was estimated experimentally and approximated with the model expressions. Effect of reducing the surface resistance of approximately two times at low temperatures one year later after their manufacture was registered for all films(except the film of 75 nm thickness). The effect may be explained by changes of the film parameters in time after the film light overdoping. [ABSTRACT FROM AUTHOR]

L2,3-edge X-ray magnetic circular dichroism(XMCD) spectrahave been measured for the well-defined dilute NiIIandMnIIions doped into a MgO crystal, with sub-Kelvin dilutionrefrigerator cooling and 2 T magnetic field magnetization. A 30-elementGe array X-ray detector has been used to measure the XMCD for thesedilute ions, whose concentrations are 1400 ppm for NiIIand 10 000 ppm for MnII. Large XMCD effects havebeen observed for both NiIIand MnII, and multipletsimulation described the observed spectra. The fluorescence-detectedL-edge absorption spectrum and XMCD of NiIIin MgO arecomparable with both theoretical calculations and the total electronyield measured ions in similar chemical environments, at least qualitativelyvalidating the use of the sensitive fluorescence detection techniquefor studying XMCD for dilute 3d metal ions, such as various metalloproteins.Sum rule analyses on the XMCD spectra are also performed. In addition,these XMCD measurements have also been used to obtain the sample’smagnetization curve and the beamline’s X-ray helicity curve.This study also illustrated that bend magnet beamlines are still usefulin examining XMCD on dilute and paramagnetic metal sites. [ABSTRACT FROM AUTHOR]

A hierarchical multi-scale model was used to study the effect of high pressure on the critical shear stresses of MgO. The two main slip systems, ½⟨110⟩{110} and ½⟨110⟩{100}, were considered. Based on a generalised Peierls–Nabarro model, it is shown that the core structure of ½⟨110⟩ screw dislocations is strongly sensitive to pressure. Mostly planar and spread in {110} at ambient pressure, the core of screw dislocations tends to spread in {100} with increasing pressure. Subsequently, an inversion of the easiest slip systems is observed between 30 and 60 GPa. At high pressure, the plasticity of MgO single crystals is expected to be controlled by ½⟨110⟩{100} slip systems, except at high temperature where both slip systems become active. Pressure is also found to increase the critical resolved shear stresses and to shift the athermal temperature toward higher temperatures. Under high pressure, MgO is thus characterised by a significant lattice friction on both slip systems. [ABSTRACT FROM AUTHOR]