Structure and magnetic properties of the cubic oxide fluoride BaFeO2F

Fluorination of the parent oxide, BaFeO_3−δ, with polyvinylidine fluoride gives rise to a cubic compound with a=4.0603(4) Å at 298 K. ⁵⁷Fe Mössbauer spectra confirmed that all the iron is present as Fe³⁺. Neutron diffraction data showed complete occupancy of the anion sites, indicating a composition BaFeO₂F, with a large displacement of the iron off-site. The magnetic ordering temperature was determined as T_N=645±5 K. Neutron diffraction data at 4.2 K established G-type antiferromagnetism with a magnetic moment per Fe³⁺ ion of 3.95 μ_B. However, magnetisation measurements indicated the presence of a weak ferromagnetic moment that is assigned to the canting of the antiferromagnetic structure. ⁵⁷Fe Mössbauer spectra in the temperature range 10-300 K were fitted with a model of fluoride ion distribution that retains charge neutrality of the perovskite unit cell.     

Iron-57 Mössbauer Spectroscopic Studies of the High Temperature Properties of Metal-Doped Iron Oxides

⁵⁷Fe Mössbauer spectra recorded in situ in vacuo from chromium- and gallium-doped Fe₃O₄ at elevated temperatures show the dopant ions to depress the Curie temperature of Fe₃O₄ by ca. 70 K. Spectra recorded from aluminium-doped -Fe₂O₃ show the onset of conversion to aluminium-doped -Fe₂O₃ to begin at ca. 750 K, which is ca. 100 K above the conversion temperature for undoped -Fe₂O₃. The variation in magnetic hyperfine fields at temperatures exceeding ca. 750 K is similar to that recorded from -Fe₂O₃ when doped with zinc or magnesium. At temperatures exceeding 900 K the zinc doped -Fe₂O₃ partially converts to spinel-related ZnFe₂O₄.     

Eigenspaces of the Laplacian; integral representations and irreducibility

An integral representation is given for eigenfunctions of the Laplacian on a noncompact two-point homogeneous space. For the flat case an irreducibility criterion for the eigenspace representations is proved, complementing an earlier result [6a, p. 143] for the nonflat cases.     

Mössbauer spectroscopic properties of tin-doped iron oxides

The ⁵⁷Fe Mössbauer spectra recorded in situ from tin-doped Fe₃O₄ at elevated temperature in vacuo shows the Curie temperature to decrease with increasing concentrations of the dopant. Thermal treatment under oxidising conditions results in the initial formation of tin-doped γ-Fe₂O₃ which subsequently undergoes a phase transformation to tin-doped α-Fe₂O₃. ⁵⁷Fe Mössbauer spectroscopy at elevated temperatures shows the Néel temperature for tin-doped γ-Fe₂O₃ to be lower than that of pure γ-Fe₂O₃. The ¹¹⁹Sn Mössbauer spectra recorded from all the tin-doped iron oxides show the presence of a hyperfine magnetic field at the Sn⁴⁺ site which is more complex in the spectra recorded from tin-doped γ-Fe₂O₃ and α-Fe₂O₃.     

A generalization of Polyak's convergence result for subgradient optimization

This paper generalizes a practical convergence result first presented by Polyak. This new result presents a theoretical justification for the step size which has been successfully used in several specialized algorithms which incorporate the subgradient optimization approach.     

Disordered chromite in the Martian meteorite Allan Hills 84001

The magnetic structure of NiFe₂O₄ nanoparticles has been investigated by means of Mössbauer spectra at T?=?4.2 K in applied fields up to 12 T. Four samples were studied, with mean particle diameters ranging from 4.3 to 8.9 nm. All spectra could be decomposed into three sextets, two corresponding to the ferrimagnetic sublattices of Fe ions in the spinel structure (core) and the third one to randomly frozen spins near the particle surface (shell). The shell thickness, calculated from the fraction of disordered spins, was found to be about one-third of the particle radius at H _app?=?0 and to decrease with the applied field toward a common limit of ~0.4 nm. The mean canting angle relative to the field was also found to decrease for increasing fields, at a rate inversely correlated to the particle size.