Diffusion-induced phase transformation of β-Ga2O3 to γ-phase Ga2O3-based spinels on (100) MgAl2O4
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House, Stephen David↗
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Structural and mechanical properties of magnesium aluminate nanoceramics under high pressure
Nanoceramics may have different structural and physical properties compared to their coarse-grained counterparts. Here, we report the high-pressure study of micro- and nano-crystalline MgAl2O4 in order to examine the effect of particle size on the structural stability. A reversible pressure-induced phase transition (cubic to tetragonal) is observed in MgAl2O4 nanocrystals under non-hydrostatic pressure at room temperature, in contrast to the previously reported structural transition of MgAl2O4 at high pressure and high temperature. It is also found that the compressed MgAl2O4 microcrystals do not fracture further below 60 nm, suggesting a plastic deformation mechanism transition. MgAl2O4 with a grain size above ∼60 nm exhibits normal cracking behaviors, but shows metal-like plastic deformation behaviors below this critical size. It is implied that combined ductility and strength can be achieved in nanoceramic MgAl2O4.
Xu, Jianing (ORCID:0000000325474344)↗
Interfacial magnetization enhancement and the perpendicularly magnetized component in Fe16N2 thin films
A detailed study of depth-resolved magnetization and microstructure of Fe16N2 thin films on MgO (001) and MgAl2O4 (001) substrates and Fe and Cr seed layers is presented. Two aspects of the magnetic properties of Fe16N2 thin films are discussed. First, magnetization enhancement at the interface is observed. Strain and nitrogen deficiency are discussed as possible interfacial mechanisms contributing to this enhancement. Second, the perpendicularly magnetized component (PMC) is identified in Fe16N2 thin films. Correlation with microstructural observations suggests that the PMC is associated with V-shaped grains that are not fully confined within the continuous Fe16N2 layer.
Hang, Xudong [University of Minnesota]↗
Short Range Order in Disordered Spinel and the Impact on Cation Vacancy Transport
Spinels are important complex oxides for use in radiation damage environments and resulting from the corrosion of steels. It is known that, in these environments, normal spinels exist with some concentration of antisite cation pairs known as inversion. In this work we show that even in highly disordered states characterized by high levels of inversion, spinel still shows some short range order (SRO) that manifests itself in antisite chains. The propensity to form these antisite chains is confirmed through Monte Carlo simulations which find that the length of chains which can form depend on the spinel chemistry. We also consider the effect of antisite chains on the diffusivity of cation vacancies and find the effect strongly depends on the spinel chemistry. At the extremes, chains in FeCr2O4 significantly increase vacancy transport but in MgAl2O4 chains have the inverse effect of drastically reducing vacancy mobility. The explanation of these dramatically different effects results from the assessment of the thermodynamic stability of the antisite chains and contrasting attractive/repulsive interactions of vacancies with the chains.
37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
The role of iron in magnetic damping of Mg(Al,Fe) 2 O 4 spinel ferrite thin films
© 2020 Author(s). We have investigated magnesium aluminum ferrite thin films with a range of iron concentrations and identified the optimal iron content to obtain high crystalline quality thin films with the low magnetic damping required for spin current-based applications. Epitaxial MgAl 2-x FexO4 films with 0.8 < x < 2.0 were grown by pulsed laser deposition on single crystal MgAl2O4 substrates and were characterized structurally and magnetically. We find that the x = 1.5 composition minimizes the room-temperature magnetic damping with a typical Gilbert damping parameter of α eff = 1.8 × 10-3. This minimized damping is governed by a competition between the more robust magnetic ordering with increased iron content, x, and the more defective structure due to larger film-substrate lattice mismatch with increased iron content. The temperature-dependent magnetization curves indicate that Tc is suppressed below room temperature for iron content x ≤ 1.2 and eventually suppressed entirely for x = 0.8. X-ray magnetic circular dichroism results indicate that for all x the magnetic moment is dominated by Fe 3 + cations distributed in a 60:40 ratio on the octahedral and tetrahedral sites, with minimal contribution from Fe 2 + cations. Films with x = 1.4-1.6 exhibit very strong ferromagnetic resonance and low Gilbert damping with α eff = (1.8-6) × 10-3, making them ideal candidates for microwave and spintronic applications.
36 MATERIALS SCIENCE↗
Anomalous Hall effect and perpendicular magnetic anisotropy in ultrathin ferrimagnetic NiCo2O4 films
The inverse spinel ferrimagnetic NiCo2O4 possesses high magnetic Curie temperature TC, high spin polarization, and strain-tunable magnetic anisotropy. Understanding the thickness scaling limit of these intriguing magnetic properties in NiCo2O4 thin films is critical for their implementation in nanoscale spintronic applications. In this work, we report the unconventional magnetotransport properties of epitaxial (001) NiCo2O4 films on MgAl2O4 substrates in the ultrathin limit. Anomalous Hall effect measurements reveal strong perpendicular magnetic anisotropy for films down to 1.5 unit cell (1.2 nm), while TC for 3 unit cell and thicker films remains above 300 K. The sign change in the anomalous Hall conductivity (σxy) and its scaling relation with the longitudinal conductivity (σxx) can be attributed to the competing effects between impurity scattering and band intrinsic Berry curvature, with the latter vanishing upon the thickness driven metal–insulator transition. Our study reveals the critical role of film thickness in tuning the relative strength of charge correlation, Berry phase effect, spin–orbit interaction, and impurity scattering, providing important material information for designing scalable epitaxial magnetic tunnel junctions and sensing devices using NiCo2O4.
Chen, Xuegang↗
Site and Structural Requirements for the Dehydra-Decyclization of Cyclic Ethers on ZrO2
In this study, we examined the site and structural requirements for the dehydra-decyclization of cyclic ethers, tetrahydrofuran, and tetrahydropyran to produce conjugated dienes over ZrO2-based catalysts, a reaction that could be an important step in the use of biomass-derived sugars as a starting material to produce monomers for the plastics industry. To help identify the active sites for this reaction, studies were conducted in which ZrO2 surfaces were decorated with Na. These studies showed that Na was effective at poisoning the activity for the ring opening of cyclic ethers, but much less so for the dehydration of the resulting adsorbed alkoxides. The studies of the activity of different types of ZrO2 for the dehydra-decyclization reaction, including single crystals and ultra-thin films supported on MgAl2O4 and silica, also showed that the reaction was dependent on the local structure of the ZrO2 surface. The insights these results provide for identifying the active sites on the ZrO2 surface are discussed.
37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
A Thermodynamic Investigation of Ni on Thin-Film Titanates (ATiO3)
Thin, ~1-nm films of CaTiO3, SrTiO3, and BaTiO3 were deposited onto MgAl2O4 by Atomic Layer Deposition (ALD) and then studied as catalyst supports for ~5 wt % of Ni that was added to the perovskite thin films by Atomic Layer Deposition. Scanning Transmission Electron Microscopy demonstrated that both the Ni and the perovskites uniformly covered the surface of the support following oxidation at 1073 K, even after redox cycling, but large Ni particles formed following a reduction at 1073 K. When compared to Ni/MgAl2O4, the perovskite-containing catalysts required significantly higher temperatures for Ni reduction. Equilibrium constants for Ni oxidation, as determined from Coulometric Titration, indicated that the oxidation of Ni shifted to lower PO2 on the perovskite-containing materials. Based on Ni equilibrium constants, Ni interactions are strongest with CaTiO3, followed by SrTiO3 and BaTiO3. The shift in the equilibrium constant was shown to cause reversible deactivation of the Ni/CaTiO3/MgAl2O4 catalyst for CO2 reforming of CH4 at high CO2 pressures, due to the oxidation of the Ni.
37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗