MgO Layer Research Articles

Corrosion Characteristics of Plasma Electrolytic Oxidation Treated AZ31 Magnesium Alloy with an Increase of the Coating Thickness

The growth of the PEO coating with processing time on an AZ31B magnesium alloy in a dilute alkaline electrolyte containing silicate and the corrosion characteristics according to its thickness were investigated. The oxide film grew a porous outer layer mainly consisting of silicate with the spark discharge phenomenon from the beginning of the reaction and reached a certain limit thickness later in the reaction. This reaction was followed by the inner layer, mainly consisting of MgO, growing rapidly and densifying. Based on the results of the potentiodynamic polarization and chronoamperometric test, it was found that within 1 minute of the processing time, due to the contribution of the dielectric layer and MgF<sub>2</sub> phase layer formed at the interface between the oxide and the substrate, the corrosion resistance was relatively high, but its stability against corrosion was not enough. Moreover, when the thickness of the oxide coating was grown mainly with the silicate outer layer, the increase in the pore size, which acts as a channel of spark discharge, was the leading cause of the decrease in the corrosion resistance. The superior corrosion resistance and the stability of the PEO coating in the later reaction time of the processing are due to the growth and densification of the inner layer of MgO.

Calculation of the Energy Coefficients of Reflection and Transmission for the Layered Periodic Media

Currently, much attention is paid to the study of photonic crystals – materials with an ordered structure characterized by a strictly periodic change in the refractive index at scales comparable to the wavelengths of radiation in visible and near infrared ranges. This is a dynamically developing direction of modern materials science. It is connected with the possibility of creating LEDs with high efficiency, new types of lasers with low threshold generation, light waveguides, optical switches, filters, as well as digital computing devices based on Photonics. The aim of this work is to calculate the reflection and transmission of a polarized light wave from a layered system of nanostructures that form a periodic medium. The calculation is carried out by two methods: the method of characteristic matrices and the method based on the use of Chebyshev polynomials. The authors have created a basic component of the computer program for calculating the reflection coefficient and the transmittance of layered nanostructures. The paper calculates the spectra of reflection and transmission coefficients and presents the analysis of the results obtained. The basic element is chosen as a basic nanostructure: a layer of magnesium oxide MgO 100 nm thick, a diamond layer 160 nm thick, an arsenic layer AsBr3 tribromide 80 nm thick, a silicon layer 120 nm thick. The authors compare the two methods used: the results are almost the same, which makes it possible in practice for such structures to use a simpler method for the computational procedure based on Chebyshev polynomials.

Strained lateral structure and its relaxation in a Cu2O thin crystal epitaxially grown on MgO surface

We measured in-plane X-ray diffraction and investigated the strained lateral structure and its relaxation in a Cu2O thin crystal epitaxially grown in a small gap between paired MgO substrates. Although one side MgO was peeled off to irradiate Cu2O with X-ray, a thin MgO layer remained on Cu2O, and diffraction peaks of (400)-planes in both MgO and Cu2O were detected. The peak has four-fold symmetry on sample rotation, and the lattice constant estimated from the diffraction angle is 4.254 Å. The lattice constant is larger than that of MgO and smaller than that of strain-free Cu2O. Furthermore, it was found that the lattice strain in Cu2O relaxes at a depth of ≈0.15 μm from MgO on the basis of the estimated X-ray penetration depth under consideration of low incident angle, absorption and refraction of X-ray.

Direct joining of thermoplastic ABS to aluminium alloy 6061-T6 using friction lap welding

ABSTRACTA thermoplastic ABS and an aluminium alloy 6061-T6 were joined using friction lap welding (FLW). The joint characteristics were evaluated to investigate the effects of 6061-T6 surface and the welding parameters on the joint properties. ABS and 6061-T6 were joined via an interfacial MgO layer. The voids generated by thermal decomposition would affect the strength of the joint. Owing to the small temperature range between the melting point and the thermal decomposition point of ABS, controlling the welding temperature is the key to the success of the 6061-T6/ABS FLW process. The relationship between the 6061-T6 surface after welding and the adhesion properties of ABS and 6061-T6 under different processing parameters was investigated.

A highly efficient method to fabricate normally-off AlGaN/GaN HEMTs with low gate leakage via Mg diffusion

A method to achieve p-type doping gate by Mg diffusion is proposed to fabricate normally-off AlGaN/GaN high electron mobility transistors (HEMTs). The fabrication is completed via first slight etching to introduce defects into the gate region and then rapid annealing to diffuse Mg ions into the AlGaN barrier, thereby forming a p-type doping layer and positively shifted threshold voltage. In addition, the MgO layer formed by thermal oxidation could effectively passivate the surface traps that were caused in the previous etching procedure. The as-fabricated HEMTs demonstrate a low gate leakage of 2 × 10−7 mA/mm and a VTH of 1.4 V. This technique offers a simplified and highly effective method to fabricate high performance GaN power devices.

A Study on the Biocompatibility of MgO Coating Prepared by Anodic Oxidation Method on Magnesium Metal

Magnesium (Mg), is widely used for the bone repair in oral and orthopedic application due to excellent bioactivity, degradation and biocompatibility. However, the range of application is greatly limited because of the rapid degradation of Mg metal in the body. Surface modification is an effective method to enhance the corrosion resistance and reduce the degradation rate of Mg metal. In the present study, pure Mg metal (P-Mg) was subjected to alkali-heat treatment (AT-Mg) or anodic oxidation-heat treatment (AO-HT-Mg). Both AT-Mg and AO-HT-Mg had a layer of MgO on their surfaces after treatment. Then the effects of MgO coating on corrosion resistance, bioactivity, Mesenchymal Stem Cells’ (MSCs) proliferation, adhesion and osteogenic differentiation, and the bone repair capability of Mg metal were investigated. We found both AT-Mg and AO-HT-Mg had stronger corrosion resistance than P-Mg. MSCs on both AT-Mg and AO-HT-Mg had higher expression of proteins and genes of ALP, OCN, Col-I and Runx2 than those on P-Mg. They also showed better bone repair property than P-Mg in vivo. In general, MgO layer formed by anodic oxidation-heat treatment had better resistance and biocompatibility than that produced by alkali-heat treatment. This study indicated the MgO coating not only improved the corrosion resistance of Mg metal, but also promoted the osteogenic differentiation of MSCs and bone regeneration.

ZnO/MgO/ITO structured thin film transistor for ultraviolet photo detector application

ZnO based thin-film transistors have been fabricated by means of the gate dielectric of MgO using E beam deposition. In this work MgO and ZnO, bilayer of 150 nm each have been deposited on ITO glass substrate. MgO layer is supportive to reducing the gate leakage current due to a large band equalize the ZnO channel layer, and to attain high transparency in the visible light band due to a broad bandgap of 7.8 eV. The obtained UV Photodetector gives outstanding UV sensing properties. The improvement of the photoresponsivity of the ZnO UV detector was carried out by the surface treatment of the ZnO thin film. The results show a great extent facilitates the fabrication of significant metal-insulator-semiconductor at low cost and low temperatures and high improvements in the detector performance. The achieved field-effect mobility is 2.70 cm2 V−1 s−1 and on/off current ratio is around 1 × 104 which shows the ZnO-TFTs work in enhancement mode. The threshold voltage is achieved as 5.1 V. The electrical results of ZnO/MgO-TFTs are predictable to get better additional by optimizing expansion circumstances.

Interlayer exchange coupling and interface magnetic anisotropy with crossed in-plane and perpendicular magnetic anisotropies

We investigated interlayer exchange coupling (IEC) and interface magnetic anisotropy (Ki) between two ferromagnetic layers with crossed in-plane and perpendicular magnetic anisotropies separated by a non-magnetic spacer by using the anomalous Hall effect (AHE). The sample consisted of a Co0.75Fe2.25O4 layer with perpendicular magnetic anisotropy and a Fe layer with in-plane anisotropy, separated by a MgO layer with variable thickness. Since Co0.75Fe2.25O4 and MgO are insulators, the AHE signal only reflects the magnetization process of Fe. From this, we determined both IEC and Ki. A strong antiferromagnetic IEC was confirmed between Co0.75Fe2.25O4 and Fe. The strongest IEC of −1.1 mJ/m2 was observed for directly coupled Fe and Co0.75Fe2.25O4 for which Ki was −1.1 mJ/m2.

Ultra-stable metal nano-catalyst synthesis strategy: a perspective

Supported metal nanoparticles (NPs) as an important heterogeneous catalyst have been widely applied in various industrial processes. During the catalytic reaction, size of the particles plays an important role in determining their catalytic performance. Generally, the small particles exhibit superior catalytic activity in comparison with the larger particles because of an increase in low-coordinated metal atoms on the particle surface that work as active sites, such as edges and corner atoms. However, these small NPs are typically unstable and tend to migrate and coalescence to reduce their surface free energy during the real catalytic processes, particularly in high-temperature reactions. Therefore, a means to fabricate stable small metal NP catalysts with excellent sinter-resistant performance is necessary for maintaining their high catalytic activity. In this study, we have summarized recent advances in stabilizing metal NPs from two aspects including thermodynamic and kinetic strategies. The former mainly involve preparing uniform NPs (with an identical size and homogeneous distribution) in order to restrain Ostwald ripening to achieve stability, while the latter primarily involves fixing metal NPs in some special confinement materials (e.g., zeolites, mesoporous silica and mesoporous carbons), encapsulating NPs using an oxide-coating film (e.g., forming core–shell structures), or constructing strong metal–support interactions to improve stability. At the end of this review, we highlight our recent work on the preparation of high-stability metal catalysts via a unique interfacial plasma electrolytic oxidation technology, that is, metal NPs are well embedded in a porous MgO layer that has both high thermal stability and excellent catalytic activity.

Fabrication and luminescent properties of(In,Fe)Sb/GaAs/InGaAs diodes

The electrical and luminescent characteristics of InGaAs/GaAs spin light-emitting diodes with an injector based on (In,Fe)Sb were investigated and the circular polarization of the electroluminescence was obtained for such structure. It has been established that the deposition of (In,Fe)Sb layer does not introduce any additional defects into the region of near-surface quantum wells, but directly affects the (In,Fe)Sb/GaAs interface. It was found that the application of a thin protective layer of MgO between the ferromagnetic (In,Fe)Sb injector and the light-emitting structure minimizes this effect.

Flexible CoFeB/MgO-based magnetic tunnel junctions annealed at high temperature (≥350 °C)

This study investigates the effect of high-temperature (350–500 °C) annealing on CoFeB/MgO/CoFeB magnetic tunnel junctions (MTJs) directly formed on a flexible polyimide substrate, which has superior thermal tolerance. As the annealing temperature increases, the tunnel magnetoresistance (TMR) ratio enhances and reaches up to ∼200% at an annealing temperature of 450 °C. The annealing temperature dependence is similar to that of MTJs fabricated in the same way on a thermally oxidized silicon substrate. Images taken by a scanning transmission electron microscope confirm the improvement of the crystallization of the CoFeB and MgO layers, which can be an important factor in enhancing the TMR ratio. Furthermore, the endurance of the flexible MTJ against repeated stretching of its substrate is investigated. The TMR ratio shows no change during and after a 1000-cycle application of a tensile strain larger than 1%. The high TMR ratio and strain endurance demonstrated in this study suggest that the flexible MTJ structure is a promising candidate for a future strain-sensing device.

Microscopic nonequilibrium energy transfer dynamics in a photoexcited metal/insulator heterostructure

The element specificity of soft X-ray spectroscopy makes it an ideal tool for analyzing the microscopic origin of ultrafast dynamics induced by localized optical excitation in metal-insulator heterostructures. Using [Fe/MgO]$_n$ as a model system, we perform ultraviolet pump/soft X-ray probe experiments, which are sensitive to all constituents of these heterostructures, to probe both electronic and lattice excitations. Complementary ultrafast electron diffraction experiments independently analyze the lattice dynamics of the Fe constituent, and together with ab initio calculations yield comprehensive insight into the microscopic processes leading to local relaxation within a single constituent or non-local relaxation between two constituents. Besides electronic excitations in Fe, which are monitored at the Fe L$_3$ absorption edge and relax within 1 ps by electron-phonon coupling, soft X-ray analysis identifies a change at the oxygen K absorption edge of the MgO layers which occurs within 0.5 ps. This ultrafast energy transfer across the Fe-MgO interface is mediated by high-frequency, interface vibrational modes, which are excited by hot electrons in Fe and couple to vibrations in MgO in a mode-selective, non-thermal manner. A second, slower timescale is identified at the oxygen K pre-edge and the Fe L$_3$ edge. The slower process represents energy transfer by acoustic phonons and contributes to thermalization of the entire heterostructure. We thus find that the interfacial energy transfer is associated with non-equilibrium behavior in the phonon system. Because our experiments lack signatures of charge transfer across the interface, we conclude that phonon-mediated processes dominate the competition of electronic and lattice excitations in these non-local, non-equilibrium dynamics.

Corrosion behavior of Fe-37 wt% Ni in molten NaCl-MgCl2 and the effects of alloy element Sc on its corrosion resistance

As a solar energy storage medium or heat transfer medium at moderate-high temperature, the molten NaCl-MgCl2 has the advantages of high stability and low cost. But a pipe and container material with Cr will be corroded badly in a molten chloride. Based on the founded corrosion resistance phase (Fe0.64Ni0.36) in previous experiments, the corrosion behavior of a new alloy of Fe-37 wt% Ni with rare earth alloy element Sc in the molten eutectic NaCl-MgCl2 at 520 °C was studied. The corrosion rate of the Fe-37 wt% Ni alloy is lower than that of the researched Fe-based superalloys (GH1035 and GH1140). After corrosion for 15 h, a porous structure with Ni-rich and Fe-depletion is formed at the local location on the Fe-37 wt% Ni surface. For the specimens with Sc (0.3 and 0.5 wt %), a compact protective MgO layer covers on the porous structure, which prevents further corrosion in the molten salt. It is speculated that the Sc is oxidized to Sc2O3 directly or by the chlorination-oxidation reactions, which improves the adhesion strength between the MgO layer and the alloy matrix by the effects of inhomogeneous nucleation and pinning.

Array of symmetric nanohole dimers with high sensitivity for detection of changes in an STT-RAM ultrathin dielectric layer

A dimer nanohole array is proposed for in situ characterization of radiation damages in spin transfer torque random access memory (STT-RAM) multilayer structures. The structure supports a Fano resonance, which is about twice as sensitive to the refractive index changes in the surface layer, compared to the plasmonic mode in single nanohole arrays. A simplified STT-RAM multilayer of air/gold(5 nm)/MgO(10 nm)/gold(60 nm)/quartz substrate is considered as the platform for the dimer nanohole array. Our studies show that regarding the changes in the ultrathin MgO layer, the normalized figure of merit for this structure is up to more than 10 times larger than an array of single nanoholes. Furthermore, an improvement in bulk sensing performance over the array of nanohole heptamers is observed.

Interface engineering towards enhanced exchange interaction between Fe and FeO in Fe/MgO/FeO epitaxial heterostructures

In our study, we examined the chemical and magnetic properties of FeO in an epitaxial Fe/MgO(dMgO)/FeO trilayer for different MgO thicknesses, dMgO. Analysis of the chemical structure revealed a stoichiometry improvement in the FeO layer in Fe/MgO/FeO in comparison with a Fe/FeO bilayer. Furthermore, we showed that deposition of a subtle MgO layer at the Fe/FeO interface results in an enhanced exchange interaction between Fe and FeO. For dMgO = 1.4 Å, we noted a 200% enhancement in the exchange bias in Fe/MgO/FeO compared to that in Fe/FeO.

Stabilization of ε−Fe2O3 epitaxial layer on MgO(111)/GaN via an intermediate γ -phase

In the present study we have demonstrated epitaxial stabilization of the metastable magnetically-hard $\varepsilon$-Fe$_2$O$_3$ phase on top of a thin MgO(111) buffer layer grown onto the GaN (0001) surface. The primary purpose to introduce a 4\,nm-thick buffer layer of MgO in between Fe$_2$O$_3$ and GaN was to stop thermal migration of Ga into the iron oxide layer. Though such migration and successive formation of the orthorhombic GaFeO$_3$ was supposed earlier to be a potential trigger of the nucleation of the isostructural $\varepsilon$-Fe$_2$O$_3$, the present work demonstrates that the growth of single crystalline uniform films of epsilon ferrite by pulsed laser deposition is possible even on the MgO capped GaN. The structural properties of the 60\,nm thick Fe$_2$O$_3$ layer on MgO / GaN were probed by electron and x-ray diffraction, both suggesting that the growth of $\varepsilon$-Fe$_2$O$_3$ is preceded by formation of a thin layer of $\gamma$-Fe$_2$O$_3$. The presence of the magnetically hard epsilon ferrite was independently confirmed by temperature dependent magnetometry measurements. The depth-resolved x-ray and polarized neutron reflectometry reveal that the 10\,nm iron oxide layer at the interface has a lower density and a higher magnetization than the main volume of the $\varepsilon$-Fe$_2$O$_3$ film. The density and magnetic moment depth profiles derived from fitting the reflectometry data are in a good agreement with the presence of the magnetically degraded $\gamma$-Fe$_2$O$_3$ transition layer between MgO and $\varepsilon$-Fe$_2$O$_3$. The natural occurrence of the interface between magnetoelectric $\varepsilon$- and spin caloritronic $\gamma$- iron oxide phases can enable further opportunities to design novel all-oxide-on-semiconductor devices.

Switching Performance Comparison With Low Switching Energy Due to Initial Temperature Increment in CoFeB/MgO-Based Single and Double Barriers

Spin-transfer torque magnetic random-access memory (STT-MRAM) based on a single-barrier magnetic tunnel junction (SBMTJ) and a double-barrier magnetic tunnel junction (DBMTJ) has evolved along with a low switching current and low energy consumption to obtain a high areal density and a fast switching speed. The increment of initial temperature, ${T}_{{\text {in}}}$ , in STT-MRAM can achieve low switching energy, ${E}_{{\text {SW}}}$ . However, this leads to an unavoidable decrease in $\Delta $ and switching efficiency. In this paper, SBMTJ and DBMTJ were analyzed in terms of the switching efficiency factor with ${E}_{{\text {SW}}}$ reduction by increasing the ${T}_{{\text {in}}}$ . The switching temperature, ${T}_{{\text {SW}}}$ , was investigated using a finite-element method simulation. The results show that the DBMTJ(A) and SBMTJ with the same MgO layer thickness of 0.9 nm provide a higher switching current, ${I}_{{\text {C}}}$ , than the DBMTJ(B) with a MgO layer thickness of 1.3 nm. The ${T}_{{\text {SW}}}$ in a DBMTJ(A) is higher than that in an SBMTJ, while ${T}_{{\text {SW}}}$ for a DBMTJ(B) is the smallest because of its low ${I}_{{\text {C}}}$ . The DBMTJ(A) and DBMTJ(B) can be applied in a higher temperature range than the SBMTJ at a $\Delta $ of 40. In addition, the STT efficiency factor, $\Delta /{I}_{{\text {C0}}}$ , for a DBMTJ is better than the factor for an SBMTJ. Although the temperature increment would cause an undesirable reduction in $\Delta $ , it can reach a low ${E}_{{\text {SW}}}$ for the requirement of a fast write access time. Therefore, the control device for increasing the ${T}_{{\text {in}}}$ in the MTJs is attractive and should be promoted in the advancement of memory technology.

Study of Al interdiffusion in ultrathin β-Ta/Co2FeAl/MgO heterostructures for enhanced spin-orbit torque

In this work perpendicular magnetization of Co2FeAl (CFA) full-Heusler alloy films is achieved using rapid thermal annealing under N2/H2 gas flow in Si/β-Ta(6.5 nm)/CFA(0.6–2 nm)/MgO(1.15 nm)/Pd(1 nm) heterostructures. The Al interdiffusion into MgO layer plays an important role in determining perpendicular magnetic anisotropy(PMA) and increasing the damping constant (α) of ultrathin CFA samples. The time of flight SIMS depth profiling along with XPS is used to study interdiffusion of Al across CFA/MgO interface. XPS analysis of Al-2p and Mg-1s spectra indicate the formation of Al2O3 and MgAl2Ox oxide phases at CFA/MgO interface in addition to pure Al metal. The Slonczewski-like and the field-like SOT effective fields are quantified using second harmonics measurements. An effective spin hall angle of β-Ta (6.5 nm) is estimated in range −0.02 to −0.06 for CFA layer thickness varied from 0.8 nm to 1.5 nm.

Understanding the synergic roles of MgO coating on the cycling and rate performance of Na0.67Mn0.5Fe0.5O2 cathode

Fe and Mn-based P2-Na0.67Mn0.5Fe0.5O2 has attracted significant interest as cathode material for sodium ion batteries (SIBs). However, their poor cycling and rate performance are the main problems. In this study, we report an integrated strategy of MgO coating and Mg2+ doping to improve the electrochemical properties of Na0.67Mn0.5Fe0.5O2. The synergistic roles of MgO coating and Mg2+ doping in the cycling and rate performance were identified. Firstly, the MgO layer suppresses the dissolution of transition metal ions and the side reactions between the electrode and electrolyte. Secondly, the MgO coating layer reduces the interface resistance and enhances e−/Na+ migration. Thirdly, the slab thickness of the TMO2 layer was also expanded by a small fraction of Mg doping, which decreases Na+ migration barrier. Finally, operando X-ray diffraction indicates that the MgO coating alleviates the change in interlay spacing (d-spacing) induced by Na+ intercalation/extraction, and inhibits irreversible phase transformations in the charging-discharging cycles, which enhances the layered structural stability of the cathode materials. The capacity retention ratio of Na0.67Mn0.5Fe0.5O2 at 1 C increased from 20% to 72% by MgO coating after 100 cycles. Insights on the roles of MgO coating are also instructive for developing cathode materials with high performance.

Microstructure evolution of homoepitaxial MgO layer and the effect on the growth mode of CeO2 film

Using pulsed laser deposition technique, highly c-axis oriented CeO2 film with the in-plane full width of half maximum (FWHM) value of 2.78° was fabricated on the homoepitaxial MgO (homo-MgO) layer for the first time. Interestingly, the quality of the CeO2 film was inconsistent with the homo-MgO film. To explain the puzzling finding, the growth mechanism of CeO2 on the homo-MgO films had been illustrated. The potential barrier caused by large value of roughness was put forward to explain the appearance of the CeO2 (111) peak. In addition, by the interaction of MgO film's texture and surface roughness, the reason why a higher quality of CeO2 film obtained under the Ps of 70 W was clarified in detail. In the end, we presented compelling evidence to prove the reason why the c-axis oriented CeO2 film can grow on the IBAD-MgO substrate.