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- Azad, A.K., et al.
(author)
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Spin-glass transition in a La-doped Sr2MnWO6 double perovskite
- 2008
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In: PHYSICAL REVIEW B. ; 77:6, s. 064418-
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Journal article (peer-reviewed)abstract
- Polycrystalline Sr2-xLaxMnWO6 (x=0.25 and 0.5) materials have been prepared by a traditional solid state sintering method and studied by neutron powder diffraction (NPD) and magnetization measurements. Rietveld analysis of the temperature dependent NPD data shows that the compounds crystallize in monoclinic symmetry (space group P2(1)/n). The unit cell volume decreases with increasing La3+ concentration at the A site. The crystal structure contains alternating MnO6 and WO6 octahedra, considerably tilted due to the relative small size of the cations that occupy the A sublattice of the perovskite. ac and dc magnetization measurements show a destruction of the low temperature antiferromagnetic phase by electron doping through substitution of Sr2+ for La3+. The electron doping creates a mixed tungsten valence (W6+/W5+) resulting in a low temperature spin-glass state.
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- Azad, A. K., et al.
(author)
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Structural, magnetic and electrochemical characterization of La(0.83)A(0.17)Fe(0.5)Cr(0.5)O(3-delta) (A = Ba, Ca) perovskites
- 2009
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In: Materials Research Bulletin. - : Elsevier BV. - 0025-5408. ; 44:7, s. 1451-1457
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Journal article (peer-reviewed)abstract
- The structural, magnetic and electrical properties of the perovskite-type compound La(0.83)A(0.17)Fe(0.5)Cr(0.5)O(3-delta) (A = Ba, Ca) have been investigated by neutron diffraction, magnetization measurements and conductivity measurements. Rietveld refinement of X-ray and neutron diffraction data shows that the compound adopts an orthorhombic crystal structure with Pbnm symmetry with a random positioning of the iron and chromium cations on the B sublattice. The magnetic structures at 10 K are collinear antiferromagnetic with the magnetic moment per site being equal to 2.91(2)mu(B) (for Ba) and 3.05(2)mu(B) (for Ca). Magnetization measurements confirm the overall antiferromagnetic behavior. The magnetic structure is based Oil a unit cell related to that of the nuclear structure and the magnetic cell can be considered the same as nuclear cell. Barium doped samples show lower oxygen deficiency and higher conductivity than calcium doped samples. At low oxygen pressure, both compounds show p-type electronic conduction. (c) 2009 Elsevier Ltd. All rights reserved.
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| 6. |
- Gyllström, Mikael, et al.
(author)
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The role of climate in shaping zooplankton communities of shallow lakes
- 2005
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In: Limnology and Oceanography. - 1939-5590. ; 50:6, s. 2008-2021
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Journal article (peer-reviewed)abstract
- We analyzed data from 81 shallow European lakes, which were sampled with standardized methods, for combined effects of climatic, physical, and chemical features of food-web interactions, with a specific focus on zooplankton biomass and community structure. Multiple-regression analysis showed that total phosphorus (TP) generally was the most important predictor of zooplankton biomass and community structure. Climate was the next most important predictor and acted mainly through its effect on pelagic zooplankton taxa. Benthic and plant-associated taxa (typically almost half the total zooplankton biomass) were, however, affected mainly by macrophyte coverage. Neither climate nor TP affected the relation between small and large taxa, and we found only a weak trend with increasing TP of increasing mean crustacean body mass. Dividing the data set into three climate zones revealed a pronounced difference in response to lake productivity between cold lakes, with long periods of ice cover, and the two warmer lake types. These ‘‘ice’’ lakes differed from the others with respect to the effect of TP on chlorophyll a, the zooplankton : chlorophyll a ratio, the chlorophyll a :TP ratio, and the proportion of cyclopoids in the copepod community. Our data suggest that bottom-up forces, such as nutrient concentration, are the most important predictors of zooplankton biomass. In addition, climate contributes significantly—possibly by affecting top-down regulation by fish—and may interact with productivity in determining the zooplankton standing biomass and community composition. Hence, the present study suggests that food-web dynamics are closely linked to climatic features.
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