Energy Focus: Photovoltaic device made only of nanocarbon
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“Spin bag” model proposed for room-temperature ferromagnetism in Sr3YCo4O10+δ
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ransition-metal perovskite oxides are used in many devices ranging from piezoelectric transducers to thermoelectric coolers and novel magnetic memories. Despite their abundance, many questions remain about fundamental magnetic ordering in these systems. Researchers Jerry Bettis Jr. and Myung-Hwan Whangbo at North Carolina State University, in collaboration with Hongjun Xiang at Fudan University in China, have now used density functional theory (DFT) to explore the origin of the room-temperature ferromagnetism in Sr3YCo4O10+δ (0.5 < δ < 1.0) (SYCO). They propose a novel kind of ferromagnetism in this oxygen-deficient perovskite, namely, the formation of ferromagnetic “spin bags.” Reporting their results in the August 28 issue of Chemistry of Materials (DOI: 10.1021/cm302007q; p. 3117), the researchers explain the origin of ferromagnetism in SYCO, whose structure comprises two kinds of perovskite layers that alternate along the c-axis direction. Oxygen vacancies are absent in the oxygen-rich perovskite layers (R layers),
Energy Focus Photovoltaic device made only of nanocarbon
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major goal of the solar photovoltaic research community is to make solar cells entirely from nanocarbon materials, and therefore free from the polymer binders that currently limit long-term stability. Michael S. Strano and colleagues from the Massachusetts Institute of Technology have now achieved just this, fabricating a solar cell containing only single-walled carbon nanotubes (SWNTs) and C60 as the active photovoltaic material, as reported in the August 22 issue of Advanced Materials (DOI: 10.1002/adma.201202088; p. 4436). “It wasn’t clear that you could get a
Schematic of the different perovskite layers present in the room-temperature ferromagnet Sr3YCo4O10+δ (0.5 < δ < 1.0). Panel (a) shows an oxygen-rich layer with no oxygen vacancies, while (b) and (c) demonstrate the oxygen-deficient layers with ordered oxygen vacancies. Each circled CoO6 octahedron of the R layer contains a low-spin Co3+ ion, and forms a ferromagnetic spin bag with the surrounding four adjacent CoO6 octahedra containing high-spin Co3+ ions. Reproduced with permission from Chem. Mater. 24 (2012), DOI: 10.1021/cm302007q; p. 3117. © 2012 American Chemical Society.
but ordered oxygen vacancies are present in the oxygen-deficient perovskite layers (D1 and D2 layers). The research group carried out DFT calculations using a supercell of stacked D1–R–D2–R layers for three situations. A first case has both the crystal structure and the G-type antiferromagnetic structure kept frozen, while a second case has the crystal structure frozen but the magnetic structure relaxed. The third case has both the crystal and magnetic structures relaxed. The researchers find that the last two cases give rise to ferromagnetism with a Co magnetic moment close to that
measured experimentally (~0.25 μB/Co). Moreover, they find that the Co3+ ions in the R layers form isolated ferromagnetic “spin bags,” wi
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