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Abstract: We demonstrate a sensitivity of the Mn 3d valence states to strain in the ferromagnetic semiconductors (Ga, Mn)As and (Al, Ga, Mn)As using x-ray magnetic circular dichroism (XMCD). The spectral shape of the Mn L(2,3) XMCD is dependent on the orientation of the magnetization, and features with cubic and uniaxial dependence are distinguished. Reversing the strain reverses the sign of the uniaxial anisotropy of the Mn L(3) prepeak which is ascribed to transitions from the Mn 2p core level to p-d hybridized valence band hole states. With increasing carrier localization, the L(3) prepeak intensity increases, indicating an increasing 3d character of the hybridized holes.

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Abstract: Using X-ray absorption spectroscopy (XAS) at the Cu L(2,3) and Co L(2,3) edges, we demonstrate that the valence state of copper in the thiospinel carrollite (CuCo(2)S(4)) is Cu(+) with a d count of Cu d(9.8). The Co has a d count of d(6.4), so that the highly covalent mineral has an electronic formula of Cu(1.2+)(Co(2.4+))(2)(S(1.5-))(4). There is half a hole per atom in the S 3p band (per unit formula); the Co L(2,3) XAS indicates a covalent low-spin electronic structure. These data dispel the notion of the presence of Cu(2+) in carrollite.

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Abstract: The Fe(III)-reducing bacterium Geobacter sulfurreducens produces nanometer-sized magnetite by transferring electrons from organic matter or hydrogen to ferrihydrite, resulting in the reductive transformation of the ferrihydrite to magnetite, and the generation of ATP for growth. Electron transfer can occur by direct contact between the cell surface and the mineral or via a soluble “electron shuttle,” for example a quinone-containing humic species. The minerals produced at different stages of ferrihydrite reduction during two experiments, one with and one without the humic analog anthraquinone-2, 6-disulphonate (AQDS), were measured using high-resolution synchrotron powder X-ray diffraction. Amorphous 2-line ferrihydrite converts to goethite, then to a mixture of goethite and magnetite, and finally to magnetite. Samples with and without AQDS showed the same general mineralogical trends, and the rate of reaction was faster in the presence of AQDS. In addition, two transient minerals structurally similar to goethite and magnetite were observed to form as intermediates between ferrihydrite and goethite and goethite and magnetite, but only in samples produced in the absence of the electron shuttle. These transient minerals were named proto-goethite and proto-magnetite.

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Abstract: Ferromagnetism above room temperature has been observed in Mn/Si 3 N 4 multilayered films for the first time. Characterization of the structural and electronic properties was performed to study the ferromagnetic behavior of this system. X-ray absorption spectroscopy at the Mn?K and L edges, as well as x-ray magnetic circular dichroism, evidences the presence of divalent Mn in the films.X-ray absorption near edge structure measurements, which are compared to calculations, confirms the presence of a slightly distorted Mn 3 N 2 phase that is proposed to be the origin of the ferromagnetism in this system.

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Abstract: We present the catalyst-free growth of binary Bi2Te3 topological insulator nanostructures on c-plane sapphire substrates by molecular beam epitaxy. Dense arrays of single-crystalline nanostructures, growing along the [110] direction, are obtained for substrate temperatures ranging from ∼180 °C to 260 °C. The growth rate and shape of the nanostructures are highly temperature-dependent. The microscopic study of the nanostructures and their relationship to the underlying Bi2Te3 thin film gives an insight into the growth mechanism.