Magnetic oxygen stored in quasi-1D form within BaAl2O4 lattice

Magnetic properties of Fe-doped NiO nanoparticles

Undoped and Fe-doped NiO nanoparticles were successfully synthesized using a lyophilization method and systematically characterized through magnetization techniques over a wide temperature range, with varying intensity and frequency of the applied magnetic fields. The Ni1-xFexO nanoparticles can be described by a core-shell model, which reveals that Fe doping enhances exchange interactions in correlation with nanoparticle size reduction. The nanoparticles exhibit a superparamagnetic blocking transition, primarily attributed to their cores, at temperatures ranging from above room temperature to low temperatures, depending on the Fe-doping level and sample synthesis temperature. The nanoparticle shells also exhibit a transition at low temperatures, in this case to a cluster-glass-like state, caused by the dipolar magnetic interactions between the net magnetic moments of the clusters. Their freezing temperature shifts to higher temperatures as the Fe-doping level increases. The existence of an exchange bias interaction was observed, thus validating the core-shell model proposed.

Structural Behavior and Spin-State Features of BaAl2O4 Scaled through Tuned Co3+ Doping

Pure and Co³⁺-doped BaAl₂O₄ [Ba(Al_1-xCo_x)₂O₄, x = 0, 0.0077, 0.0379] powder samples were prepared by a facile hydrothermal route. Elemental analyses by static secondary ion mass spectrometry (SIMS), X-ray absorption spectroscopy (XAS) measurements at the Co K-edge, and X-ray diffraction studies were fully correlated, thus addressing a complete description of the structural complexity of Co³⁺-doped BaAl₂O₄ powder. Powder X-ray diffraction (PXRD) patterns indicated that prepared samples were nanocrystalline with a hexagonal P6₃ symmetry. The X-ray absorption near-edge structure (XANES) measurements revealed the presence of cobalt in a +3 oxidation state, while the rarely documented, tetrahedral symmetry around Co³⁺ was extracted from the extended X-ray absorption fine structure (EXAFS) oscillation patterns. Rietveld structure refinements showed that Co³⁺ preferentially substitutes Al³⁺ at tetrahedral Al3 sites of the BaAl₂O₄ host lattice, whereas the (Al3)O₄ tetrahedra remain rather regular with Co³⁺-O distances ranging from 1.73(9) to 1.74(9) Å. The underlying magneto-structural features were unraveled through axial and rhombic zero-field splitting (ZFS) terms. The increased substitution of Al³⁺ by Co³⁺ at Al3 sites leads to an increase of the axial ZFS terms in Co³⁺-doped BaAl₂O₄ powder from 10.8 to 26.3 K, whereas the rhombic ZFS parameters across the series change in the range from 2.7 to 10.4 K, showing a considerable increase of anisotropy together with the values of the anisotropic g-tensor components flowing from 1.7 to 2.5. We defined the line between the Co³⁺ doping limit and influenced magneto-structural characteristics, thus enabling the design of strategy to control the ZFS terms' contributions to magnetic anisotropy within Co³⁺-doped BaAl₂O₄ powder.