State Of MgO Research Articles

Prediction of Site Preference of Implanted Transition Metal Dopants in Rock-salt Oxides

Transition metals (TMs) implanted in oxides with rock-salt crystal structures (for example MgO and BaO) are assumed to substitute cations (Mg in case of MgO) from the lattice sites. We show that not all implanted TMs substitute cations but can be stable in interstitial sites as well. Stability of TM (Sc–Zn) dopants in various charge states in MgO and BaO has been investigated in the framework of density functional theory. We propose an effective way to calculate stability of implanted metals that let us predict site preference (interstitial or substitution) of the dopant in the host. We find that two factors govern the preference for an interstitial site: (i) relative ionic radius and (ii) relative oxygen affinity of cation and the TM dopants. If the radius of the cation is much larger than TM dopant, as in BaO, TM atoms always sit at interstitial sites. On the other hand, if the radius of the cation is comparable to that of the dopant TM, as in case of MgO, the transition of the preferred defect site, from substituting lattice Mg atom (Sc to Mn) to occupying interstitial site (Fe to Zn) is observed. This transition can be attributed to the change in the oxygen affinity of the TM atoms from Sc to Zn. Our results also explain experiments on Ni and Fe atoms implanted in MgO. TM dopants at interstitial sites could show substantially different and new properties from substitutionally doped stable compounds.

Consolidated picture of tunnelling spintronics across oxygen vacancy states in MgO

The field of tunnelling spintronics has flourished through the study of magnetic tunnel junctions (MTJs) with MgO barriers. The combination of high spintronic performance and low effective barrier heights has enabled new technologies, ranging from next-generation memories to bio-inspired computing. This combination is made possible by structural defects such as oxygen vacancies. So far, experiments have pegged an energy separation between these localized states and the Fermi level, while theory has predicted that these are in fact occupied states. To rationalize the defect-mediated potential tunnelling landscape, we have performed experiments in which we tune the MTJ’s Fermi level by altering one electrode’s work function. We find that switching the top electrode from FeCoB to FeB increases the amplitude of defect-mediated barrier heights. Ab initio theory attributes this increase to an increased energy separation between the localized states of single and double oxygen vacancies and the Fermi level. We thus extract a rationalized potential landscape of tunnelling across oxygen vacancies in MgO involving occupied states. In junctions with high R.A. product such as ours, this leads to a picture of hole tunnelling.

First principles calculations of optical properties for oxygen vacancies in binary metal oxides.

Using an advanced computational methodology implemented in CP2K, a non-local PBE0-TC-LRC density functional and the recently implemented linear response formulation of the Time-dependent Density Functional Theory equations, we test the interpretation of the optical absorption and photoluminescence signatures attributed by previous experimental and theoretical studies to O-vacancies in two widely used oxides-cubic MgO and monoclinic (m)-HfO2. The results obtained in large periodic cells including up to 1000 atoms emphasize the importance of accurate predictions of defect-induced lattice distortions. They confirm that optical transitions of O-vacancies in 0, +1, and +2 charge states in MgO all have energies close to 5 eV. We test the models of photoluminescence of O-vacancies proposed in the literature. The photoluminescence of VO +2 centers in m-HfO2 is predicted to peak at 3.7 eV and originate from radiative tunneling transition between a VO +1 center and a self-trapped hole created by the 5.2 eV excitation.

Photoactivity and Stability Co-Enhancement: When Localized Plasmons Meet Oxygen Vacancies in MgO.

Durability is still one of the key obstacles for the further development of photocatalytic energy-conversion systems, especially low-dimensional ones. Encouragingly, recent studies show that nanoinsulators such as SiO2 and MgO exhibit substantially enhanced photocatalytic durability than the typical semiconductor p25 TiO2 . Extending this knowledge, MgO-Au plasmonic defect nanosystems are developed that combine the stable photoactivity from MgO surface defects with energy-focusing plasmonics from Au nanoparticles (NPs), where Au NPs are anchored onto monodispersed MgO nanotemplates. Theoretical calculations reveal that the midgap defect (MGD) states in MgO are generated by oxygen vacancies, which provide the main avenues for upward electron transitions under photoexcitation. These electrons drive stable proton photoreduction to H2 gas via water splitting. A synergistic interaction between Au's localized plasmons and MgO's oxygen vacancies is observed here, which enhances MgO's photoactivity and stability simultaneously. Such co-enhancement is attributed to the stable longitudinal-plasmon-free Au NPs, which provide robust hot electrons capable of overcoming the interband transition barrier (≈1.8 eV) to reach proton reduction potential for H2 generation. The demonstrated plasmonic defect nanosystems are expected to open a new avenue for developing highly endurable photoredox systems for the integration of multifunctionalities in energy conversion, environmental decontamination, and climate change mitigation.

Defect states of transition metal-doped MgO for secondary electron emission of plasma display panel

High secondary electron emission (SEE) yield is one of the important properties of MgO for industrial applications such as alternating-current plasma display panel (AC-PDP), which reduces the firing voltage for discharge. SEE induced by low-energy Ne and Xe ions can be described by potential emission mechanism of Auger neutralization. Defect states in MgO can play an important role in enhancing the SEE. The electronic density of states of Ti-, Cr-, Fe-, Ni-, Zn-doped and undoped MgO was calculated using CASTEP. The occupied defect levels of the Ti-, Cr-, Fe-, Ni- and Zn-doped MgO are at 5.23, 3.90, 2.76, 1.22 and 0.06 eV above the valence band, respectively. Higher defect level appears to be advantageous for getting larger SEE yield. Considering the fact that Ti-doped MgO reveals n-type conductivity, Cr seems to be the proper dopant among the studied elements for the purpose of high SEE yield in AC-PDP.

Improving the Dispersity and DC Breakdown of MgO/LDPE Nanocomposite by Adding EVA

To improve the agglomerate phenomena of polarity nanosized MgO-filler in non-polar low density polyethylene (LDPE) matrix, in the composite (MgO/LDPE) master batch preparation process, adding a certain amount of ethylene-vinyl acetate copolymer (EVA), in order to improve the compatibility of nanoparticles and matrix materials. The dispersion states of MgO in the MgO/LDPE and EVA/MgO/LDPE composite material was evaluated by using scanning electron microscope (SEM), results shown that compared with MgO/LDPE nanocomposite, the dispersion of EVA modified MgO/LDPE is better and the agglomerate phenomena is not seen. DC breakdown tests shown that EVA modified MgO/LDPE compared with other several kinds of materials (LDPE; EVA/LDPE; MgO/LDPE and EVA/MgO/LDPE); the DC breakdown strength has improved significantly.

First Principle Study of the Structural, Electronic and Magnetic Properties of MgO Nanolayers on Fe Substrate

We have investigated the structural, electronic, and magnetic properties of MgO nanolayers with two different nanolayer thicknesses (1.5 nm and 1.75 nm) on a Fe substrate. The calculated results in this paper were obtained using the density functional theory (DFT) within the generalized gradient approximation (GGA). The total energies as a function of volume are calculated and thereby the lattice parameters, bulk moduli of MgO nanolayers with two different thicknesses have been calculated. The effects of surface atoms and Fe substrate atoms on physical properties of these nanolayers have been analyzed using the calculated total and partial electron density of states in its ferromagnetic phase. The spin-polarized density of states of MgO shows that this compound is an insulator in the nonmagnetic phase. MgO nanolayers on Fe substrate are metal in the ferromagnetic phase. The magnetic properties of surface atoms and Fe substrate atoms have been investigated and compared with bulk. Furthermore, the effect of hydrostatic pressure on the total and local magnetic moment of these nanolayers has been investigated.

Pressure–volume–temperature equations of state of Au and Pt up to 300 GPa and 3000 K: internally consistent pressure scales

By using a Mie–Grüneisen-type analysis method, the pressure–volume–temperature equations of state (P–V–T EOSs) of Au and Pt have been determined up to 300 GPa and 3000 K based on the experimental shock Hugoniot and thermodynamic data. The calculated results of Au and Pt show an excellent agreement with available experimental volume compression data over a wide range of pressures and temperatures. A comparison of our results with previous theoretical investigations has also been done. In addition, we have further examined the consistency of our results and the P–V–T EOS of MgO [K. Jin, X.Z. Li, Q. Wu, H.Y. Geng, L.C. Cai, X.M. Zhou, and F.Q. Jing, The pressure–volume–temperature equation of state of MgO derived from shock Hugoniot data and its application as a pressure scale, J. Appl. Phys. 107 (2010), pp. 113518] using simultaneous volume measurements of Au, Pt, and MgO at various temperatures. The good agreement among the P–V–T EOSs of Au, Pt, and MgO implies that these EOSs can be used as the reliable pressure scales in high pressure–temperature diamond anvil cell experiments.

37.1: Invited Paper: Surface Electronic States of MgO Induced by Auger Neutralization Process

AbstractThe surface electronic states of MgO are measured at Auger neutralization condition with an ion excitation source by using thermoluminescence (TL) spectroscopy. The advantage of this method is that the dynamic properties of excited states at the topmost MgO surface could be measured at the same conditions as γ measurement. Luminescence, activation energy, lifetime of trapped states are discussed.

Analysis of a new two-parameter equation of state for MgO

In this paper, a new two-parameter equation of state (EOS) is developed on the basis of lattice potential theory. The expressions are also obtained for isothermal bulk modulus and its pressure derivative. Numerical analysis is presented for MgO at high pressures (up to 260 GPa) and high temperatures (up to 2000 K). Results based on the new EOS are found to compare well with the corresponding values derived from a first principles approach.

Spin-dependent tunneling in epitaxial Fe/Cr/MgO/Fe magnetic tunnel junctions with an ultrathin Cr(001) spacer layer

We fabricated fully epitaxial Fe/Cr/MgO/Fe(001) magnetic tunnel junctions (MTJs) with an atomically flat ultrathin Cr(001) layer grown below the MgO barrier layer and studied the spin-dependent transport to clarify scattering process of tunneling electrons. Because Cr does not have Bloch states with ${\ensuremath{\Delta}}_{1}$ symmetry at the Fermi energy $({E}_{F})$, ${\ensuremath{\Delta}}_{1}$ evanescent states in MgO, which dominantly mediate the tunneling current, cannot couple with Cr Bloch states without a scattering process. The Fe/Cr/MgO/Fe(001) MTJs are therefore a model system for studying nonspecular scattering processes where the orbital symmetry of tunneling states is not conserved. The resistance-area (RA) product of the MTJs was found to not increase exponentially as a function of the Cr thickness $({t}_{\text{Cr}})$, indicating that the Cr layer does not act as a perfect tunnel barrier despite of the absence of ${\ensuremath{\Delta}}_{1}$ states at ${E}_{F}$. Moreover, the magnetoresistance ratio of the MTJs was seen to oscillate as a function of ${t}_{\text{Cr}}$ with a period of 2 monolayers, reflecting the layered antiferromagnetic structure of Cr(001). Surprisingly, the MR ratio showed local maxima at the odd numbers of Cr monolayers and local minima at the even numbers of Cr monolayers, indicating that the tunneling current is oppositely spin polarized with respect to the interface magnetization. These results suggest that nonspecular scatterings mediate the coupling between evanescent states in MgO and certain non-${\ensuremath{\Delta}}_{1}$ Bloch states in Cr that have negative spin polarization, thereby inducing nonspecular tunneling current even at a low temperature and a small bias voltage. We also investigated, as a reference sample, Fe/MgO/Cr/Fe MTJs with a less-oxidized Cr/MgO interface by growing the Cr(001) layer on the MgO barrier layer and found that their RA product increased much more rapidly with increasing ${t}_{\text{Cr}}$. This indicates that partial oxidation of interface Cr atoms in the Fe/Cr/MgO/Fe MTJs is one of the major origins of nonspecular scatterings. Both an increase in temperature and the application of bias voltage were found to greatly enhance the electron scattering that gives rise to tunneling conductance. While the temperature dependence of the antiparallel conductance due to magnon scatterings follows the Bloch ${T}^{3/2}$ law, the conductance due to nonspecular scatterings deviates from the Bloch ${T}^{3/2}$ law. The present experimental results give us important clues to a clearer understanding of the tunneling process in MgO-based MTJs.

Dependence on annealing temperatures of tunneling spectra in high-resistance CoFeB/MgO/CoFeB magnetic tunnel junctions

We investigated the annealing temperature dependence of differential tunneling conductance spectra ( d I / d V as a function of V ) in CoFeB/textured MgO(001)/CoFeB magnetic tunnel junctions (MTJs) that exhibit the giant tunneling magnetoresistance (TMR) effect at room temperature. The spectra were strongly affected by annealing the MTJs. A reduction in d I / d V at around ±300 mV was observed only in annealed MTJs in which the CoFeB electrodes were crystallized in a bcc(001) structure. Because the reduction in conductance was observed in both MTJs that have a 1.8-nm-thick MgO barrier and MTJs that have a 3.2-nm-thick MgO barrier, we concluded that Δ 5 and Δ 2 ′ evanescent states, which rapidly decay in the MgO tunneling barrier, are not the cause of the reduction in conductance. We believe the cause of the reduction is either the electronic structure of the interfaces between MgO(001)/bcc CoFeB(001) after annealing or a particular feature of Δ 1 states in MgO(001) or bcc CoFeB(001).

Application of shell model in molecular dynamics simulation to MgO

The P-V-T equation of state of MgO has been simulated under high pressure and elevated temperature using the molecular dynamics (MD) method with the breathing shell model (BSM). It is found that the MD simulation with BSM is very successful in reproducing accurately the measured molar volumes of MgO over a wide range of temperature and pressure. In addition, the MD simulation reproduces accurately the measured volume compression data of MgO up to 100GPa at 300K. It is demonstrated that the MD simulated P-V-T equation of state of MgO could be applied as a useful internal pressure calibration standard at elevated temperatures and high pressures.

Theoretical Calculations of Positron Lifetimes for Metal Oxides

Our recent positron lifetime measurements for metal oxides suggest that positron lifetimes of bulk state in metal oxides are shorter than previously reported values. We have performed theoretical calculations of positron lifetimes for bulk and vacancy states in MgO and ZnO using first-principles electronic structure calculations and discuss the validity of positron lifetime calculations for insulators. By comparing the calculated positron lifetimes to the experimental values, it was found that the semiconductor model well reproduces the experimental positron lifetime. The longer positron lifetime previously reported can be considered to arise from not only the bulk but also from the vacancy induced by impurities. In the case of cation vacancy, the calculated positron lifetime based on semiconductor model is shorter than the experimental value, which suggests that the inward relaxation occurs around the cation vacancy trapping the positron.

Nonlinear Microwave Response of Epitaxial YBaCuO Films of Varying Oxygen Content on MgO Substrates

We have investigated the nonlinear microwave properties of electron-beam coevaporated YBa2Cu3O7−x films on MgO, using stripline resonators at 2.3 GHz and temperatures 1.7–80 K. The oxygen content of the films ranged from strongly underdoped to overdoped. Above 20 K, the nonlinear response of the resonators was dominated by the superconductor. We could establish clear correlations between the nonlinear surface resistance, two-tone intermodulation (IMD), and oxygen content of the films, which indicate that the superconducting order parameter is important for the nonlinearities. A power-law representation of the nonlinear current-voltage relation would not be appropriate to explain our results phenomenologically. Below 20 K, the dielectric loss tangent of MgO dominated the nonlinear response of the resonators. With increasing power, the dissipation losses decreased markedly, accompanied by enhanced IMD. The surface reactance passed through a shallow minimum at about 5 K, independent of power. We attribute these effects to resonant absorption by impurity states in MgO.

Core exciton energies of bulk MgO,Al2O3,andSiO2from explicitly correlatedab initiocluster model calculations

Ab initio cluster model wave functions are used to predict the existence of localized excited states in MgO, ${\mathrm{Al}}_{2}{\mathrm{O}}_{3},$ and ${\mathrm{SiO}}_{2}$ arising from metal $2p$ core-level excitations. Theoretical values obtained at different levels of theory result in a quantitative agreement with experiment, and the use of different models permits us to quantify the different contributions to the final excitation energy. The most important contribution is atomic in nature; a meaningful zero-order approximation is that in MgO and ${\mathrm{Al}}_{2}{\mathrm{O}}_{3}$ the exciton can be assigned to a ${M(2p}^{6})\ensuremath{\rightarrow}{M(2p}^{5}{3s}^{1})$-like excitation, where $M=\mathrm{Mg}$ or Al. For the atomic models, the singlet-triplet exchange in the excited configuration is in good agreement with experiment. In addition, the solid-state effects on this exchange energy predicted by experiment are well reproduced by the cluster models representing MgO and ${\mathrm{SiO}}_{2},$ whereas a less clear situation appears in ${\mathrm{Al}}_{2}{\mathrm{O}}_{3}.$ The open-shell orbital in the final state has, however, important contributions from the ions near the atomic site where excitation occurs. Nevertheless, the final state appears to be localized in space without any a priori assumption, the localization following from the hole-particle interaction implicitly induced in the final-state wave function. The Madelung field reduces the excitation energy with respect to the atomic value; the effect of neighboring atoms, mainly Pauli repulsion, acts in the opposite way; and electronic correlation effects decrease it again. In agreement with the covalent nature of ${\mathrm{SiO}}_{2},$ the exciton cannot be simply understood as arising from a $\mathrm{Si}{(2p}^{6})\ensuremath{\rightarrow}\mathrm{Si}{(2p}^{5}{3s}^{1})$ in a fully oxidized Si cation.

The influence of the cluster models on the study of electronic structure of MgO/Ag interface

We performed local density calculations of energy eigenvalue, bond order and the density of states of MgO(100)/Ag(100) interface using discrete variation Xα method for different cluster models. Several cluster models are chosen for exploring the bond orders of the metal-ceramic interface. The bond order analyses suggest that the cluster model is important for the study of the electronic structure of the metal-ceramic interface using quantum chemical calculation methods. The larger cluster model is not definitely better than the smaller one. The results obtained from different cluster models show bonding essentially electrostatic at the (100)Ag/(100)MgO interfaces, and covalent effects are felt to be small.

Impurity-phonon interaction effect on the phonon density of states in MgO: V 2+

The effective phonon density of states of MgO: V 2+ is obtained from the emission vibronic structure of the V 2+ impurity ion under the assumption that the vibronic transition is mainly a single-phonon process. The validity for doing so is supported by the consistent fitting of the temperature dependences of the width and the shift of the zero-phonon line of MgO: V 2+ and the specific heat of pure MgO. The phonon softening and phonon stiffening is found by comparing the effective phonon density of states to the theoretically calculated one of the pure MgO lattice.

A MRCI PS and CASSCF study of the ground state MgO dissociation energy

Ab initio calculations at CASSCF and MRCI PS levels are used to determine the dissociation energy for the X 1Σ + state of MgO, which adiabatically dissociates to the ground state 1S g of magnesium and to the excited 1D g state of oxygen, as well as other spectroscopic parameters. Emphasis is placed upon the problem of properly selecting an adequate active space in CASSCF calculations and upon the improvements obtained in MRCI by selecting perturbatively the most important contributions to the total wavefunction and evaluating the remaining ones only by perturbational method. Through a procedure based on stabilizing the computed dissociation energy, values of 3.87 eV (MRCI PS) and 4.20 eV (CASSCF) are obtained. These values compare with the experimental value of 3.76±0.13 eV.

The infrared spectrum of magnesium oxide: a diode laser study using the discharge-enhanced reaction between hot magnesium vapor and N 2O

The high-resolution infrared spectrum of magnesium oxide is reported. The oxide was produced by a low-pressure discharge-enhanced reaction between hot magnesium vapor and nitrous oxide. Fifteen rovibrational transitions of the fundamental band and seven transitions of the first hot band of MgO(X 1Σ +) were detected with tunable diode lasers. The infrared transitions were combined with data from previous microwave and optical studies to yield precise spectroscopic constants for the X 1Σ + and B 1Σ + states of MgO.