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

Intermediate-Valence Ytterbium Compound Yb₄Ga₂₄Pt₉: Synthesis, Crystal Structure, and Physical Properties

The title compound was synthesized by a reaction of the elemental educts in a corundum crucible at 1200 °C under an Ar atmosphere. The excess of Ga used in the initial mixture served as a flux for the subsequent crystal growth at 600 °C. The crystal structure of Yb₄Ga₂₄Pt₉ was determined from single-crystal X-ray diffraction data: new prototype of crystal structure, space group <i>C</i>2<i>/m</i>, Pearson symbol <i>mS</i>74, <i>a</i> = 7.4809(1) Å, <i>b</i> = 12.9546(2) Å, <i>c</i> = 13.2479(2) Å, β = 100.879(1)°, <i>V</i> = 1260.82(6) ų, <i>R</i>_<i>F</i> = 0.039 for 1781 observed reflections and 107 variable parameters. The structure is described as an <i>ABABB</i> stacking of two slabs with trigonal symmetry and compositions Yb₄Ga₆ (<i>A</i>) and Ga₁₂Pt₆ (<i>B</i>). The hard X-ray photoelectron spectrum (HAXPES) of Yb₄Ga₂₄Pt₉ shows both Yb²⁺ and Yb³⁺ contributions as evidence of an intermediate valence state of ytterbium. The evaluated Yb valence of ∼2.5 is in good agreement with the results obtained from the magnetic susceptibility measurements. The compound is a bad metallic conductor

Canted Antiferromagnetism on Rectangular Layers of Fe²⁺ in Polymorphic CaFeSeO

From stoichiometric amounts of CaO, Fe, and Se, pure powders and single crystals of quaternary Ca[FeSe2/2O2/2]∞2 can be obtained by solid-state reaction and self-flux growth, respectively. The as-synthesized compound exhibits a polymorphic crystal structure, where the two modifications have different stacking sequences of [FeSe2/2O2/2]2−∞2 layers. The two polymorphs have similar unit cells but different crystal symmetries (<i>Cmc</i>2₁ and <i>Pnma</i>), of which the former is non-centrosymmetric. Fe is divalent (d⁶) and high-spin, as proven by X-ray spectroscopy, Mössbauer spectroscopy, and powder neutron diffraction data. The latter two, in combination with magnetic susceptibility and specific heat data, reveal a long-range antiferromagnetic spin order (<i>T</i>_N = 160 K) with a minor spin canting. CaFeSeO is an electronic insulator, as confirmed by resistivity measurements and density functional theory calculations. The latter also suggest a relatively small energy difference between the two polymorphs, explaining their intimate intergrowth

Heteroepitaxy of Fe₃O₄/Muscovite: A New Perspective for Flexible Spintronics

Spintronics has captured a lot of attention since it was proposed. It has been triggering numerous research groups to make their efforts on pursuing spin-related electronic devices. Recently, flexible and wearable devices are in a high demand due to their outstanding potential in practical applications. In order to introduce spintronics into the realm of flexible devices, we demonstrate that it is feasible to grow epitaxial Fe₃O₄ film, a promising candidate for realizing spintronic devices based on tunneling magnetoresistance, on flexible muscovite. In this study, the heteroepitaxy of Fe₃O₄/muscovite is characterized by X-ray diffraction, high-resolution transmission electron microscopy, and Raman spectroscopy. The chemical composition and magnetic feature are investigated by a combination of X-ray photoelectron spectroscopy and X-ray magnetic circular dichroism. The electrical and magnetic properties are examined to show the preservation of the primitive properties of Fe₃O₄. Furthermore, various bending tests are performed to show the tunability of functionalities and to confirm that the heterostructures retain the physical properties under repeated cycles. These results illustrate that the Fe₃O₄/muscovite heterostructure can be a potential candidate for the applications in flexible spintronics

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