MgO Interface Research Articles

The Activation of Magnesium Sintering by Zinc Addition

Light alloys based on magnesium are widely used in most areas of science and technology. However, magnesium powder alloys are quite difficult to sinter due to the stable film of oxides that counteracts diffusion. Therefore, finding a method to activate magnesium sintering is urgent. This study examines the effect of adding 5 wt. % and 10 wt. % zinc to the sintering pattern of magnesium powders at 430 °C; a dwell of 30 min was used to homogenize at the densification’s temperature. Scanning electron microscopy (SEM) was used to characterize the alloy’s microstructure, while the phase composition was characterized using X-ray diffraction (XRD) and energy dispersion spectroscopy (EDS). The sintering densities of Mg–5Zn and Mg–10Zn were found to be 88% and 92%, respectively. The results show that after sintering, a heterophase structure of the alloy is formed based on a solid solution and phases MgZn and Mg50Zn21. To establish the sintering mechanism, the interaction at the MgO and Zn melt phase interface was analyzed using the sessile drop method. The minimum contact angle—65°—was discovered at 500 °C with a 20 min holding time. It was demonstrated that the sintering process in the Mg–Zn system proceeds through the following stages: (1) penetration of zinc into oxide-free surfaces; (2) crystallization of a solid solution, intermetallics; and (3) the removal of magnesium oxide from the particle surface, with oxide particles deposited on the surface of the sample.

Temperature and bias voltage dependences of magnetic tunnel junction with FeAlSi electrode

We fabricated magnetic tunnel junctions (MTJs) with FeAlSi free layers and investigated the tunnel magnetoresistance (TMR) properties. We found that the temperature and bias voltage dependences of the TMR effect in FeAlSi-MTJs were almost the same as MTJs with Fe free layers despite the low Curie temperature of FeAlSi. In the inelastic electron tunneling spectroscopy measured at low temperatures, the relatively large cutoff energy of magnon excitation at the FeAlSi and MgO interface was confirmed. In addition, we studied for the first time the exchange stiffness constant of FeAlSi films by Brillouin light scattering. The determined value of the stiffness constant of FeAlSi was 14.3 (pJ/m), which was similar to that of Fe. Both the large magnon cutoff at the interface and the stiffness constant of FeAlSi are considered to be the reason for the good temperature and voltage dependences of FeAlSi-MTJs.

Segregation of Yttrium at the Mg/MgO interface in an Mg-0.5Y Alloy

Interfacial segregation of selected elements can be exploited to manipulate the potency of solid substrates for heterogeneous nucleation, thus controlling the solidification process. As the native inclusions in Mg alloys, MgO acts as the nucleating substrate, but it has rarely been studied in terms of its interactions with alloying elements. In this work, investigations of yttrium (Y) segregation at interfaces between native MgO particles and Mg in an Mg-0.5Y alloy were carried out by state-of-the-art aberration-corrected scanning transmission electron microscopy (STEM) and associated spectroscopy. Experimental results show that native MgO particles in Mg-0.5Y possess two typical morphologies: truncated octahedron primarily faceted by {111}MgO and minorly by {100}MgO, and cubic shape with unique {100}MgO facets. Y atoms are found to segregate at both Mg/{111}MgO and Mg/{100}MgO interfaces, leading to the formation of two different 2-dimensional compounds (2DCs). The 2DC at the Mg/{111}MgO interface is identified as two atomic layers of a face-centered cubic Y2O3 phase in terms of crystal structure and chemistry, whilst it is an Mg(Y)-O monolayer at the Mg/{100}MgO interface, coherently matching with the terminating {100}MgO plane. Discussion is focused on the mechanisms underlying the formation of the 2DCs, their effects on the nucleation potency of MgO particles, and grain refinement. This work sheds light on how heterogeneous nucleation can be manipulated by altering the nucleation potency of a substrate through deliberately promoting elemental segregation of carefully chosen element(s).

First-Principles Studies on Spin-Dependent Transport of Magnetic Junctions with Half-Metallic Heusler Compounds

Two types of magnetoresistance (MR), i.e., tunnel magnetoresistance (TMR) and current perpendicular to plane giant magnetoresistance (CPP-GMR), are theoretically discussed for systems with half-metallic (HFM) Co-based full Heusler compounds. In TMR, the non-collinear spin structure at the Co2MnSi(CMS)/MgO(001) interface that results from thermal spin fluctuations significantly reduced the TMR ratio at room temperature (RT). Enhancement of the exchange stiffness of CMS/MgO was essential to suppress the reduction in TMR at RT. Furthermore, it is proposed that inserting of a B2-CoFe layer between CMS and MgO is a promising way to enhance the interface exchange stiffness constant. In CPP-GMR, the interface spin asymmetry γ in the Valet-Fert model was essential to enhance GMR at RT, because the temperature dependence of γ in experiments was much larger than that of the bulk spin asymmetry β. I showed that the experimental CPP-GMR ratio increases with the decrease in the theoretical resistance-area product RPA, which is roughly consistent with the RP dependence of γ. This means that matching of the conductive channel between HMF and a non-magnetic metallic spacer is a key parameter to enhance γ. These findings will be very important for room temperature spintronics devices applied in future artificial intelligence hardware.

Perpendicular Magnetic Anisotropy of an Ultrathin Fe Layer Grown on NiO(001)

The magnetic anisotropy and magnetic interactions at the interface between $\mathrm{Fe}$ and $\mathrm{Ni}\mathrm{O}$(001) were investigated. Depending on the growth conditions of the $\mathrm{Ni}\mathrm{O}$(001) layers and the postannealing temperature, the preferential magnetization direction of the ultrathin $\mathrm{Fe}$ layer grown on a $\mathrm{Ni}\mathrm{O}$(001) layer changed from the in-plane direction to a direction perpendicular to the film plane. The lattice constant of the $\mathrm{Ni}\mathrm{O}$(001) layers parallel to the growth direction increased with ${\mathrm{O}}_{2}$ flow rate, while that parallel to the in-plane direction were locked onto the MgO(001) substrate regardless of the growth conditions of the NiO layers. Moreover, perpendicular magnetization was observed only when the NiO layer was grown with ${\mathrm{O}}_{2}$ flow rates higher than 2.0 sccm corresponding to oxygen-rich NiO. X-ray magnetic circular dichroism measurements revealed an enhancement in anisotropic orbital magnetic moments similar to the origin of perpendicular magnetic anisotropy at the $\mathrm{Fe}$/MgO(001) interface. The interfacial magnetic anisotropy energies were 0.93 and $1.02\phantom{\rule{0.2em}{0ex}}{\mathrm{mJ}/\mathrm{m}}^{2}$ at room temperature and at 100 K, respectively, indicating less temperature dependence. In contrast, the coercivity ${H}_{c}$ exhibited a significant temperature dependence. Although no signature of exchange bias or unidirectional loop shift was observed, ${H}_{c}$ was strongly dependent on the NiO layer thickness, indicating that the exchange interaction at the interface between the ferromagnetic and antiferromagnetic layers was not negligible, despite the NiO(001) being a spin-compensated surface.

631% room temperature tunnel magnetoresistance with large oscillation effect in CoFe/MgO/CoFe(001) junctions

We demonstrate tunnel magnetoresistance (TMR) ratios of up to 631% at room temperature (RT) using CoFe/MgO/CoFe(001) epitaxial magnetic tunnel junctions (MTJs). The TMR ratio increased up to 1143% at 10 K. The large TMR ratios resulted from fine-tuning of atomic-scale structures of the MTJs, such as crystallographic orientations and MgO interface oxidation by interface insertion of ultrathin CoFe and Mg layers, which are expected to enhance the well-known Δ1 coherent tunneling transport. Interestingly, the TMR oscillation effect, which is not covered by the standard coherent tunneling theory, also became significant. A 0.32-nm period TMR oscillation with increasing MgO thickness dominates the transport in a wide range of MgO thicknesses; the peak-to-valley difference of the TMR oscillation exceeds 140% at RT, which is attributed to the appearance of large oscillatory components in the resistance area product.

Potassium as the best alkali metal promoter in boosting the hydrogenation activity of Ru/MgO for aromatic LOHC molecules by facilitated heterolytic H2 adsorption

Alkali metals (AM) are frequently used as promoters in a number of reactions. In aromatic hydrogenation over Ru/MgO, their basicity may promote heterolytic H2 adsorption, thereby boosting reaction kinetics. Herein, Ru/AM/MgO catalysts (AM: Na, K, and Cs) with different AM/Ru molar ratios are prepared and evaluated in the hydrogenation of toluene and benzyltoluene. The activity exhibits a volcano-type relationship with the AM/Ru molar ratio, where the optimal AM content decreases in the order Na+ > K+ > Cs+ and the AM showing the best activity is in the sequence Cs+ < Na+ < K+. These results originate from the heterolytic H2 adsorption induced by charge transfer from AM to proximate Ru atoms at the Ru–MgO interface. The degree of charge transfer depends on the AM’s ionic radius and electronegativity affecting surface mobility and charge donation ability. Consequently, K+ is the best AM promoter in Ru/MgO-catalyzed aromatic hydrogenation.

Observation of unidirectional spin Hall magnetoresistance in amorphous PtSn4/CoFeB bilayers

Unidirectional spin Hall magnetoresistance (USMR) is a magnetoresistance effect with potential applications to read two-terminal spin–orbit-torque (SOT) devices directly. In this work, we observed a large USMR value (up to 0.7 × 10−11 per A/cm2, 50% larger than reported values from heavy metals) in sputtered amorphous PtSn4/CoFeB bilayers. Ta/CoFeB bilayers with interfacial MgO insertion layers are deposited as control samples. The control experiments show that increasing the interfacial resistance can increase the USMR value, which is the case in PtSn4/CoFeB bilayers. The observation of a large USMR value in an amorphous spin–orbit-torque material has provided an alternative pathway for USMR application in two-terminal SOT devices.

Research on microstructure and interface bonding of high volume fraction SiCp/Al composite

Taking the high volume fraction SiCp/6061Al-based composite material prepared by the powder metallurgy method under different sintering temperatures and holding times as the research object, the microstructure and interface actions are investigated by X-ray diffractometer, scanning electron microscope, transmission electron microscope, etc. The results show that as the sintering temperature increases, the interface reaction between SiC and Al intensifies, producing brittle phase Al4C3 and MgAl2O4 and MgO interface products. The nucleation and growth of Al4C3 are controlled by the sintering temperature and time. When preparing large-size workpieces with high volume fraction SiCp/6061Al-based composite materials, the sintering temperature should be controlled below [Formula: see text]C, and the holding time should not exceed 24 h.

Cu nanoparticles supported on core–shell MgO-La2O3 catalyzed hydrogenolysis of furfuryl alcohol to pentanediol

Selective ring-opening hydrogenolysis of the furan ring is the key to the efficient conversion of furfuryl alcohol (FFA) to high-value pentanediol. In this work, highly dispersed Cu@MgO-La2O3 was designed and synthesized with a core–shell structure, and bifunctional metal and alkaline sites for the direct hydrolysis of FFA (94.9% conversion) to 1,2-pentanediol (1,2-PeD, with 67.1% selectivity) and 1,5-pentanediol (1,5-PeD, with 18.8% selectivity). Multiple characterization results indicated that at an Mg/La ratio of 1.9, the outer surface contained distributed La2O3-MgO species with uniformly dispersed Cu0 nanoparticles, and the inner layer of the catalyst possessed MgO, while some Cu0 were distributed at the La2O3 and MgO interface. The addition of MgO increased the specific surface area of the outer layer, improving the dispersion of Cu and facilitating the formation of Cu0. In-situ FTIR experiments showed that FFA was adsorbed on the La2O3 alkaline site of the carrier through the O atoms of the furan ring and CH2OH, which induced ring-opening to generate enol intermediates, which further hydrogenated to 1,2-PeD and 1,5-PeD over the Cu0 nanoparticles. Thus, this new core–shell structure combining with metal and alkaline sites provides new insight for developing effective and selective hydrogenolysis catalysts.

Advanced heterolytic H2 adsorption of K-added Ru/MgO catalysts for accelerating hydrogen storage into aromatic benzyltoluenes

Herein, we report a highly active K-added Ru/MgO catalyst for hydrogen storage into aromatic benzyltoluenes at low temperatures to advance liquid organic hydrogen carrier technology. The hydrogenation activity of Ru/K/MgO catalysts exhibits a volcano-shaped dependence on the K content at the maximum with 0.02 wt%. This is in good agreement with the strength and capacity of H2 adsorption derived from basicity, despite a gradual decrease in the textural property and the corresponding increase in the Ru particle size with increasing the K content. Density functional theory calculations show that heterolytic hydrogen adsorption properties (strength and polarization) are facilitated up to a specific density of K on the Ru–MgO interface and excessive K suppresses heterolytic H2 adsorption by direct interaction between K and hydrogen, assuring the hydrogenation activity and H2 adsorption capability of Ru/K/MgO catalysts. Hence, the Ru/K/MgO catalyst, when K is added in an optimal amount, is highly effective to accelerate hydrogen storage kinetics at low temperatures owing to the enhanced heterolytic H2 adsorption.

Monolithic Epitaxial Integration of β-Ga2O3 with 100 Si for Deep Ultraviolet Photodetectors

In this report, we demonstrate direct epitaxial integration of β-Ga2O3 on a (400) oriented silicon on insulator substrate toward deep-UV (DUV) optoelectronics. The 550 nm thick (400) epitaxial-β-Ga2O3 films are deposited onto Si(100) using a two-step buffer and a two-step epilayer scheme. The epitaxial orientation relation between β-Ga2O3, MgO, and silicon(100) is given by (400)β-Ga2O3||(100)MgO||(100)Si and ⟨010⟩β-Ga2O3||⟨011⟩MgO||⟨110⟩Si. The presence of rotational variants is confirmed by X-ray diffraction and transmission electron microscopy. Epitaxy was found to be mediated through a MgGa2O4 layer formed at the β-Ga2O3/MgO interface under oxygen-deficient conditions during pulsed layer deposition. The ω-scan symmetric and asymmetric full width at half-maximum values of β-Ga2O3 are 2.41 and 2.39°, respectively. Photodetectors realized in a conventional metal–semiconductor–metal geometry exhibit a maximum responsivity of 11.8 A/W at 246 nm at 40 V with a photo to dark current ratio of 2.5 × 102 and a UV-to-visible rejection ratio > 103. The detectors do not exhibit any persistent photoconductivity as is evident from the rise and fall times of 0.54 and 0.32 s, respectively. Such a monolithic integration of β-Ga2O3 on Si(100) opens up opportunities for the development of integrated DUV focal plane arrays on a SoC chip.

Electron Correlation Enhances Orbital Polarization at a Ferromagnetic Metal/Insulator Interface: Depth-Resolved X-ray Magnetic Circular Dichroism and First-Principles Study

The Fe(CoB)/MgO interface is vital to spintronics as it exhibits the tunneling magnetoresistance (TMR) effect and interfacial perpendicular magnetic anisotropy (PMA) simultaneously. To further enhance TMR and PMA for the development of high-density magnetoresistive random access memory, it is essential to clarify the behavior of Fe atoms interfaced with MgO. This study reveals that the spin and orbital magnetic moments of Fe are enhanced at the Fe/MgO interface. The enhancement in the orbital magnetic moment is much more significant than that predicted by the standard density functional theory. Theoretical calculations based on the orbital polarization correction reproduce this enhancement, the origin of which is attributed to the electron–electron correlation resulting from electron localization at the Fe/MgO interface. The present findings highlight the importance of electron–electron correlation at ferromagnet/oxide interfaces, which has often been disregarded in spintronics.

Standing wave hard X-ray photoemission study of the structure of the interfaces in Ta/Co2FeAl/MgO multilayer

Interfaces of the magnetic layer in the heterostructure Ta/Co2FeAl/MgO have been studied using hard X-ray photoelectron spectroscopy (HAXPES). Depth-selective information with sub-nanometer resolution is obtained using X-ray standing waves. It is found that the two magnetic species namely, Co and Fe behave very differently at the interfaces; 54% of Fe in the magnetic layer gets either oxidised or alloyed with Ta at the interfaces, as against 48% of Co. Oxidation state of both Co and Fe decreases, as one moves away from the MgO interface. On the average about 0.36 nm of Co2FeAl gets oxidised at MgO interface and 0.66 nm gets alloyed with Ta at the bottom interface. Thermal annealing at 200℃ results in further oxidation of Fe, accompanied by a partial reduction of Co. It is suggested that this opposite behaviour of Fe and Co at the MgO interface may be the cause of the thermal annealing induced shift of the centroid of Fe distribution towards MgO layer, as observed in some STEM studies in such heterostructures.

Effect of surface properties of MgO on its dissolution and spinel growth in CaO–SiO2–Al2O3 slag

The dissolution behavior of MgO in CaO–SiO2–Al2O3 ternary slag at the interface of single-crystal, dense poly-crystal, and porous poly-crystal MgO was investigated to evaluate the effect of the surface properties of the MgO. The experimental results revealed that a detached spinel layer formed at the MgO interface due to the change in thermodynamic condition of the slag, which was independent of the surface properties. On the other hand, it was also confirmed that the growth rate and morphology of the detached spinel layer strongly depended on the surface properties, such as porosity and curvature of MgO. During the formation of the spinel layer at the interface during MgO dissolution, a kinetic approach adopting parabolic relation theory was employed to determine the correlation between the surface properties and the spinel growth mechanism.

IrCrMnZ (Z = Al, Ga, Si, Ge) Heusler alloys as electrode materials for MgO-based magnetic tunneling junctions: a first-principles study

We study IrCrMnZ (Z = Al, Ga, Si, Ge) systems using first-principles calculations from the perspective of their application as electrode materials of MgO-based magnetic tunnel junctions (MTJs). These materials have highly spin-polarized conduction electrons with a partially occupied Δ1 band, which is important for coherent tunneling in a parallel magnetization configuration. The Curie temperatures of IrCrMnAl and IrCrMnGa are very high (above 1300 K), as predicted from mean-field-approximation. The stability of the ordered phase against various antisite disorders is investigated. We discuss here the effect of ‘spin-orbit-coupling’ on the electronic structure around the Fermi level. Further, we investigate the electronic structure of the IrCrMnZ/MgO heterojunction along the (001) direction. IrCrMnAl/MgO and IrCrMnGa/MgO maintain half-metallicity even at the MgO interface, with no interfacial states at/around the Fermi level in the minority-spin channel. Large majority-spin conductance of IrCrMnAl/MgO/IrCrMnAl and IrCrMnGa/MgO/IrCrMnGa is reported from the calculation of the ballistic spin-transport property for the parallel magnetization configuration. We propose IrCrMnAl/MgO/IrCrMnAl and IrCrMnGa/MgO/IrCrMnGa as promising MTJs with a weaker temperature dependence of tunneling magnetoresistance ratio, owing to their very high Curie temperatures.

N‐Doped and P‐Doped Graphene on MgO (111): A First‐Principles Study

Herein, first‐principles density functional theory is used to investigate the atomic structure and electrical properties of the contact between primitive graphene, N‐doped graphene, P‐doped graphene, and MgO (111) surface. It has been found that the energy band of the graphene/Mg‐terminated MgO (111) interface exhibits p‐type characteristics, and the energy band of the graphene/O‐terminated MgO (111) interface exhibits n‐type characteristics. N‐doped or P‐doped can control the energy band structure of the interface. The interaction between the graphene and Mg‐terminated MgO (111) interface is a weak physical interaction, and the interaction between the graphene and O‐terminated MgO (111) surface is a strong chemical interaction. N‐doped or P‐doped can promote the charge transfer between the interfaces, and further enhancing the interfacial coupling strength. This result is helpful to understand the interaction between graphene and MgO interface and provides theoretical guidance for the practical application of graphene/MgO interface‐based devices.

Atomic-layer stacking dependence of the magnetocrystalline anisotropy in Fe-Co multilayer thin films at MgO(001) interface

Magnetocrystalline anisotropy (MA) and electric-field-induced modification of the MA energy (E-field-induced MA modification) of Co-Fe multilayer thin films at MgO(001) interface with respect to atomic-layer stacking were investigated using first-principles calculations combined with the cluster expansion method. Although the magnetic quantities have very complicated dependence on atomic-layer stacking, we find that there are key short-range atomic-layer stackings. At the MgO interface, a double atomic-layer stacking of Fe on the MgO enhances an interfacial perpendicular MA while there is no role of the short-range stacking in the E-field-induced MA modification where a single Fe atomic-layer at the interface plays a role. The physical origins underlying these trends were subsequently elucidated by band structure calculations.

Irradiation Effects on Perpendicular Anisotropy Spin–Orbit Torque Magnetic Tunnel Junctions

We study the impact of irradiation on magnetic tunnel junction (MTJ) films with perpendicular magnetic anisotropy (PMA) and spin-orbit torque (SOT) switching using magneto-optical Kerr effect and transmission electron microscopy. Our results show that the thin-film stack is robust to gamma ionizing dose up to 1 Mrad(Si) and Ta <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1+</sup> ion irradiation fluences up to 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">12</sup> ions/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> , showing SOT PMA MTJs are radiation-hard. But, at very high Ta <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1+</sup> ion irradiation between 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">12</sup> and 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">14</sup> ions/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> , reduced coercivity and eventually greatly reduced PMA are observed, corresponding with an increase in intermixing of the CoFeB-MgO layers, particularly at the lower CoFeB-MgO interface. These results agree with displacement damage modeling that predicts higher damage in the layers closer to the bottom heavy metal and substrate. Compared to spin transfer torque and in-plane anisotropy MTJs, needing a top-pinned stack, a thicker heavy metal layer, and perpendicular anisotropy that is pinned out of plane by interfaces, all could make SOT PMA MTJs more susceptible to damage at high doses.

Brillouin light scattering investigations of films and magnetic tunnel junctions with perpendicular magnetic anisotropy at the CoFeB–MgO interface

Alternating gradient force magnetometry and Brillouin light scattering (BLS) experiments were exploited to investigate the perpendicular magnetic anisotropy (PMA) localized at the CoFeB–MgO interface, both in Ta/CoFeB/MgO heterostructures (bottom samples) and in the inverted ones MgO/CoFeB/Ta (top samples). Thermal treatment was found to induce a marked increase of the PMA with respect to the as grown state. Using linear spin wave theory, an analysis of the spin wave frequencies was performed which allowed to estimate the thickness dependence of the PMA in the single CoFeB film. Magnetic tunnel junctions with the Ta/CoFeB/MgO/CoFeB/Ta structure were also studied by BLS. The analysis of the experimental results, performed via the linear spin wave theory, revealed that there is an antiferromagnetic interlayer exchange coupling between the free and the fixed CoFeB layers which increases after the annealing procedure.