Search Results (4)

Thin (~50 nm thick) BaM hexaferrite (BaFe₁₂O₁₉) films were grown on (1–102) and (0001) cut α-Al₂O₃ (sapphire) substrates via laser molecular beam epitaxy using a one- or two-stage growth protocol. The advantages of a two-stage protocol are shown. The surface morphology, structural and magnetic properties of films were studied using atomic force microscopy, reflected high-energy electron diffraction, three-dimensional X-ray diffraction reciprocal space mapping, powder X-ray diffraction, magneto-optical, and magnetometric methods. Annealed BaFe₁₂O₁₉/Al₂O₃ (1–102) structures consist of close-packed islands epitaxially bonded to the substrate. The hexagonal crystallographic axis and the easy axis (EA) of the magnetization of the films are deflected from the normal to the film by an angle of φ~60°. The films exhibit magnetic hysteresis loops for both in-plane H_in-plane and out-of-plane H_out-of-plane magnetic fields. The shape of M_out-of-plane(H_in-plane) and M_in-plane(H_in-plane) hysteresis loops strongly depends on the azimuth θ of the H_{in plane}, confirming the tilted orientation of the EA. The M_out-of-plane(H_out-of-plane) magnetization curves are caused by the reversible rotation of magnetization and irreversible magnetization jumps associated with the appearance and motion of domain walls. In the absence of a magnetic field, the magnetization is oriented at an angle close to φ. Full article

Thin films of lithium spinel ferrite, LiFe₅O₈, have attracted much scientific attention because of their potential for efficient excitation, the manipulation and propagation of spin currents due to their insulating character, high-saturation magnetization, and Curie temperature, as well as their ultra-low damping value. In addition, LiFe₅O₈ is currently one of the most interesting materials in terms of developing spintronic devices based on the ionic control of magnetism, for which it is crucial to control the lithium’s atomic content. In this work, we demonstrate that dual ion beam sputtering is a suitable technique to tailor the lithium content of thin films of lithium ferrite (LFO) by using the different energies of the assisting ion beam formed by Ar⁺ and O₂⁺ ions during the growth process. Without assistance, a disordered rock-salt LFO phase (i.e., LiFeO₂) can be identified as the principal phase. Under beam assistance, highly out-of-plane-oriented (111) thin LFO films have been obtained on (0001) Al₂O₃ substrates with a disordered spinel structure as the main phase and with lithium concentrations higher and lower than the stoichiometric spinel phase, i.e., LiFe₅O₈. After post-annealing of the films at 1025 K, a highly ordered ferromagnetic spinel LFO phase was found when the lithium concentration was higher than the stoichiometric value. With lower lithium contents, the antiferromagnetic hematite (α-Fe₂O₃) phase emerged and coexisted in films with the ferromagnetic Li_xFe_6-xO₈. These results open up the possibility of controlling the properties of thin lithium ferrite-based films to enable their use in advanced spintronic devices. Full article

Hydrogen embrittlement (HE) of steel pipelines in high-pressure gaseous environments is a potential threat to the pipeline integrity. The occurrence of gaseous HE is subjected to associative adsorption of hydrogen molecules (H₂) at specific “active sites”, such as grain boundaries and dislocations on the steel surface, to generate hydrogen atoms (H). Non-metallic inclusions are another type of metallurgical defect potentially serving as “active sites” to cause the dissociative adsorption of H₂. Al₂O₃ is a common inclusion contained in pipeline steels. In this work, the dissociative adsorption of hydrogen at the $\mathsf{\alpha }\text{-}{\mathrm{A}\mathrm{l}}_2{\mathrm{O}}_3\left(0001\right)/\mathsf{\alpha }\text{-}\mathrm{F}\mathrm{e}\left(111\right)$ interface on the $\mathrm{F}\mathrm{e}\left(01\overline{1}\right)$ plane was studied by density functional theory calculations. The impact of gas components of O₂ and CH₄ on the dissociative adsorption of hydrogen was determined. The occurrence of dissociative adsorption of hydrogen at the Al₂O₃ inclusion/Fe interface is favored under conditions relevant to pipeline operation. Thermodynamic feasibility was observed for Fe and O atoms, but not for Al atoms. H atoms can form more stable adsorption configurations on the Fe side of the interface, while it is less likely for H atoms to adsorb on the Al₂O₃ side. There is a greater tendency for the occurrence of dissociative adsorption of O₂ and CH₄ than of H₂, due to the more favorable energetics of the former. In particular, the dissociative adsorption of O₂ is preferential over that of CH₄. The Al-terminated interface exhibits a higher H binding energy compared to the O-terminated interface, indicating a preference for hydrogen accumulation at the Al-terminated interface. Full article

First-principles calculations based on density functional theory (DFT) were carried out to study the energetic stability and electronic properties of a bimetallic-doped α-Fe₂O₃ photoanode surface with (Zn, Ti) and (Zn, Zr) pairs for enhanced PEC water splitting. The doped systems showed negative formation energies under both O-rich and Fe-rich conditions which make them thermodynamically stable and possible to be synthesised. It is found that in a bimetallic (Zn, Ti)-doped system, at a doping concentration of 4.20% of Ti, the bandgap decreases from 2.1 eV to 1.80 eV without the formation of impurity states in the bandgap. This is favourable for increased photon absorption and efficient movement of charges from the valance band maximum (VBM) to the conduction band minimum (CBM). In addition, the CBM becomes wavy and delocalised, suggesting a decrease in the charge carrier mass, enabling electron–holes to successfully diffuse to the surface, where they are needed for water oxidation. Interestingly, with single doping of Zr at the third layer (L3) of Fe atoms of the {0001} α-Fe₂O₃ surface, impurity levels do not appear in the bandgap, at both concentrations of 2.10% and 4.20%. Furthermore, at 2.10% doping concentration of α-Fe₂O₃ with Zr, CBM becomes delocalised, suggesting improved carrier mobility, while the bandgap is altered from 2.1 eV to 1.73 eV, allowing more light absorption in the visible region. Moreover, the photocatalytic activities of Zr-doped hematite could be improved further by codoping it with Zn because Zr is capable of increasing the conductivity of hematite by the substitution of Fe³⁺ with Zr⁴⁺, while Zn can foster the surface reaction and reduce quick recombination of the electron–hole pairs. Full article