Defect Structure Guided Room Temperature Ferromagnetism of Y‑Doped CeO₂ Nanoparticles

In this study, the defect structure of Y doped CeO₂ nanoparticles (NPs) was investigated systematically by using spectroscopy and microscopy. The doping level of Y ranges from 0% to 15%. It is demonstrated that Y³⁺ substitutes Ce and governs the formation of oxygen vacancy. At low doping level, Y³⁺ randomly distributed throughout the particle. However, as doping level increased above 9%, Y³⁺ aggregates at the surface and forms Y-rich clusters. Room temperature ferromagnetism (FM) was observed in these Y-doped CeO₂ NPs. It is found that the value of saturation magnetization (M_s) increases until Y reaches 9%, then it decreases. Raman, X-ray absorption near edge spectroscopy and X-ray magnetic circular dichroism (XMCD) analysis has provided several aspects on the electronic properties of theses nanoparticles. A charge delocalization occurs upon Y doping on the Ce­(Y)-O­(V_O)-Ce­(Y) orbitals. The magnetism is evidenced by XMCD spectroscopy only on Ce orbitals, and the magnetism intensity is mainly related to the amount of Ce³⁺ at the surface. These features plead for the presence of a defect band at the surface, related to the Ce³⁺–Y interaction, as the origin of the ferromagnetism

Enhanced Magnetic Anisotropy via Quasi-Molecular Magnet at Organic-Ferromagnetic Contact

To realize the origin of efficient spin injection at organic-ferromagnetic contact in organic spintronics, we have implemented the formation of quasi-molecular magnet via surface restructuring of a strong organic acceptor, tetrafluoro-tetracyano-quinodimethane (F4-TCNQ), in contact with ferromagnetic cobalt. Our results demonstrate a spin-polarized F4-TCNQ layer and a remarkably enhanced magnetic anisotropy of the Co film. The novel magnetic properties are contributed from strong magnetic coupling caused by the molecular restructuring that displays an angular anchoring conformation of CN and upwardly protruding fluorine atoms. We conclude that the π bonds of CN, instead of the lone-pair electrons of N atoms, contribute to the hybridization-induced magnetic coupling between CN and Co and generate a superior magnetic order on the surface

Three Oxidation States of Manganese in the Barium Hexaferrite BaFe_{12–<i>x</i>}Mn_<i>x</i>O₁₉

The coexistence of three valence states of Mn ions, namely, +2, +3, and +4, in substituted magnetoplumbite-type BaFe_{12–<i>x</i>}Mn_<i>x</i>O₁₉ was observed by soft X-ray absorption spectroscopy at the Mn-L_2,3 edge. We infer that the occurrence of multiple valence states of Mn situated in the pristine purely iron­(III) compound BaFe₁₂O₁₉ is made possible by the fact that the charge disproportionation of Mn³⁺ into Mn²⁺ and Mn⁴⁺ requires less energy than that of Fe³⁺ into Fe²⁺ and Fe⁴⁺, related to the smaller effective Coulomb interaction of Mn³⁺ (d⁴) compared to Fe³⁺ (d⁵). The different chemical environments determine the location of the differently charged ions: with Mn³⁺ occupying positions with (distorted) octahedral local symmetry, Mn⁴⁺ ions prefer octahedrally coordinated sites in order to optimize their covalent bonding. Larger and more ionic bonded Mn²⁺ ions with a spherical charge distribution accumulate at tetrahedrally coordinated sites. Simulations of the experimental Mn-L_2,3 XAS spectra of two different samples with <i>x</i> = 1.5 and <i>x</i> = 1.7 led to Mn²⁺:Mn³⁺:Mn⁴⁺ atomic ratios of 0.16:0.51:0.33 and 0.19:0.57:0.24

Understanding and Tuning Electronic Structure in Modified Ceria Nanocrystals by Defect Engineering

This study investigates the effect of Fe³⁺ on the electronic structure of nanocrystalline ceria. Systematic synchrotron X-ray absorption spectroscopy coupled with scanning transmission electron microscopy/electron energy loss spectroscopy was utilized. The oxygen vacancies can be engineered and their number varied with the degree of iron doping. Comparing the local electronic structure around Ce sites with that around Fe sites reveals two stages of defect engineering. The concentration of Ce³⁺ and the distribution of defects differ between lower and higher degrees of doping. Charge is transferred between Ce and Fe when the doping level is less than 5%, but this effect is not significant at a doping level of over 5%. This transfer of charge is verified by energy loss spectroscopy. These Fe-modified ceria nanoparticles exhibit core–shell-like structures at low doping levels and this finding is consistent with the results of scanning transmission electron microscopy/electron energy loss spectroscopy. More Fe is distributed at the surface for doping levels less than 5%, whereas the homogeneity of Fe in the system increases for doping levels higher than 5%. X-ray magnetic circular dichroism spectroscopy reveals that Ce, rather than Fe, is responsible for the ferromagnetism. Interestingly, Ce³⁺ is not essential for producing the ferromagnetism. The oxygen vacancies and the defect structure are suggested to be the main causes of the ferromagnetism. The charge transfer and defect structure Fe³⁺-Vo-Ce³⁺ and Fe³⁺-Vo-Fe³⁺ are critical for the magnetism, and the change in saturated magnetization can be understood as being caused by the competition between interactions that originate from magnetic polarons and from paired ions

LaMn₃Ni₂Mn₂O₁₂: An A- and B‑Site Ordered Quadruple Perovskite with A‑Site Tuning Orthogonal Spin Ordering

A new oxide, LaMn₃Ni₂Mn₂O₁₂, was prepared by high-pressure and high-temperature synthesis methods. The compound crystallizes in an AA′₃B₂B′₂O₁₂-type A-site and B-site ordered quadruple perovskite structure. The charge combination is confirmed to be LaMn³⁺₃Ni²⁺₂Mn⁴⁺₂O₁₂, where La and Mn³⁺ are 1:3 ordered at the A and A′ sites and the Ni²⁺ and Mn⁴⁺ are also distributed at the B and B′ sites in an orderly fashion in a rocksalt-type manner, respectively. A G-type antiferromagnetic ordering originating from the A′-site Mn³⁺ sublattice is found to occur at <i>T</i>_N ≈ 46 K. Subsequently, the spin coupling between the B-site Ni²⁺ and B′-site Mn⁴⁺ sublattices leads to an orthogonally ordered spin alignment with a net ferromagnetic component near <i>T</i>_C ≈ 34 K. First-principles calculations demonstrate that the A′-site Mn³⁺ spins play a crucial role in determining the spin structure of the B and B′ sites. This LaMn₃Ni₂Mn₂O₁₂ provides a rare example that shows orthogonal spin ordering in the B and B′ sites assisted by ordered A-site magnetic ions in perovskite systems

Magnetic Mesocrystal-Assisted Magnetoresistance in Manganite

Mesocrystal, a new class of crystals as compared to conventional and well-known single crystals and polycrystalline systems, has captured significant attention in the past decade. Recent studies have been focused on the advance of synthesis mechanisms as well as the potential on device applications. In order to create further opportunities upon functional mesocrystals, we fabricated a self-assembled nanocomposite composed of magnetic CoFe₂O₄ mesocrystal in Sr-doped manganites. This combination exhibits intriguing structural and magnetic tunabilities. Furthermore, the antiferromagnetic coupling of the mesocrystal and matrix has induced an additional magnetic perturbation to spin-polarized electrons, resulting in a significantly enhanced magnetoresistance in the nanocomposite. Our work demonstrates a new thought toward the enhancement of intrinsic functionalities assisted by mesocrystals and advanced design of novel mesocrystal-embedded nanocomposites

A Complete High-to-Low spin state Transition of Trivalent Cobalt Ion in Octahedral Symmetry in SrCo_0.5Ru_0.5O_3‑δ

The complex metal oxide SrCo_0.5Ru_0.5O_3‑δ possesses a slightly distorted perovskite crystal structure. Its insulating nature infers a well-defined charge distribution, and the six-fold coordinated transition metals have the oxidation states +5 for ruthenium and +3 for cobalt as observed by X-ray spectroscopy. We have discovered that Co³⁺ ion is purely high-spin at room temperature, which is unique for a Co³⁺ in an octahedral oxygen surrounding. We attribute this to the crystal field interaction being weaker than the Hund’s-rule exchange due to a relatively large mean Co–O distances of 1.98(2) Å, as obtained by EXAFS and X-ray diffraction experiments. A gradual high-to-low spin state transition is completed by applying high hydrostatic pressure of up to 40 GPa. Across this spin state transition, the Co Kβ emission spectra can be fully explained by a weighted sum of the high-spin and low-spin spectra. Thereby is the much debated intermediate spin state of Co³⁺ absent in this material. These results allow us to draw an energy diagram depicting relative stabilities of the high-, intermediate-, and low-spin states as functions of the metal–oxygen bond length for a Co³⁺ ion in an octahedral coordination

Synthesis, Structure, and Properties of the Layered Oxyselenide Ba₂CuO₂Cu₂Se₂

A new layered oxyselenide, Ba₂CuO₂Cu₂Se₂, was synthesized under high-pressure and high-temperature conditions and was characterized via structural, magnetic, and transport measurements. It crystallizes into space group <i>I</i>4/<i>mmm</i> and consists of a square lattice of [CuO₂] planes and antifluorite-type [Cu₂Se₂] layers, which are alternately stacked along the <i>c</i> axis. The lattice parameters are obtained as <i>a</i> = <i>b</i> = 4.0885 Å and <i>c</i> = 19.6887 Å. The Cu–O bond length is given by half of the lattice constant <i>a</i>, i.e., 2.0443 Å. Ba₂CuO₂Cu₂Se₂ is a semiconductor with a resistivity of ∼18 mΩ·cm at room temperature. No magnetic transition was found in the measured temperature range, and the Curie–Weiss temperature was obtained as −0.2 K, suggesting a very weak exchange interaction. The DFT+<i>U</i>_eff calculation demonstrates that the band gap is about 0.2 eV for the supposed antiferromagnetic order, and the density of state near the top of the valence band is mainly contributed from the Se 4p electrons