| 1. |
- Brok, Erik, et al.
(författare)
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Polarized neutron powder diffraction studies of antiferromagnetic order in bulk and nanoparticle NiO
- 2015
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Ingår i: Physical Review B (Condensed Matter and Materials Physics). - 1098-0121. ; 91:1
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Tidskriftsartikel (refereegranskat)abstract
- In many materials it remains a challenge to reveal the nature of magnetic correlations, including antiferromagnetism and spin disorder. Revealing the spin structure in magnetic nanoparticles is further complicated by the large incoherent neutron scattering cross section from water adsorbed at the particle surfaces and by the broadening of diffraction peaks due to the finite crystallite size. Moreover, the spin structure in magnetic nanoparticles may deviate significantly from that of the corresponding bulk material because of the low-symmetry surroundings of surface atoms and the large relative surface contribution to the magnetic anisotropy. Here we explore the potential use of polarized neutron diffraction to reveal the magnetic structure in NiO bulk and nanoparticle powders by applying the XYZ-polarization analysis method. Our investigations address in particular the spin orientation in bulk NiO and platelet-shaped NiO nanoparticles with thickness from greater than 200 nm down to 2.0 nm. The advantage of the applied method is that it is able to clearly separate the structural, the magnetic, and the spin-incoherent scattering signals for all particle sizes. For platelet-shaped particles with thickness from greater than 200 nm down to 2.2 nm we find that the spin orientation deviates about 16 degrees from the primary (111) plane of the platelet-shaped particles. In the smallest particles (2.0 nm thick) we find the spins are oriented with a 30 degrees. average angle to the primary (111) plane of the particles. The results show that polarization analyzed neutron powder diffraction is a viable method to investigate magnetic order in powders of antiferromagnetic nanoparticles.
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| 2. |
- Gavilán, Helena, et al.
(författare)
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Colloidal Flower-Shaped Iron Oxide Nanoparticles : Synthesis Strategies and Coatings
- 2017
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Ingår i: Particle & particle systems characterization. - : Wiley. - 0934-0866 .- 1521-4117. ; 34:7
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Tidskriftsartikel (refereegranskat)abstract
- The assembly of magnetic cores into regular structures may notably influence the properties displayed by a magnetic colloid. Here, key synthesis parameters driving the self-assembly process capable of organizing colloidal magnetic cores into highly regular and reproducible multi-core nanoparticles are determined. In addition, a self-consistent picture that explains the collective magnetic properties exhibited by these complex assemblies is achieved through structural, colloidal, and magnetic means. For this purpose, different strategies to obtain flower-shaped iron oxide assemblies in the size range 25–100 nm are examined. The routes are based on the partial oxidation of Fe(OH)2, polyol-mediated synthesis or the reduction of iron acetylacetonate. The nanoparticles are functionalized either with dextran, citric acid, or alternatively embedded in polystyrene and their long-term stability is assessed. The core size is measured, calculated, and modeled using both structural and magnetic means, while the Debye model and multi-core extended model are used to study interparticle interactions. This is the first step toward standardized protocols of synthesis and characterization of flower-shaped nanoparticles.
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| 3. |
- Yu, Changxun, 1983-, et al.
(författare)
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Iron speciation and valence in the upper 1 km of fractured crystalline bedrock on the Baltic shield
- 2017
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Ingår i: Goldschmidt2017 Abstracts.
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Konferensbidrag (refereegranskat)abstract
- The widespread networks of open water-conducting fractures in crystalline bedrock are covered by a variety of Fe-bearing minerals. Quantitative information on Fe mineralogy and valence of these minerals is of great importance not only in constraining the biogeochemical cycle of Fe and other related elements in this largely unexplored space, but also in evaluating the mineralogical capacity to reduce oxygen which is one of the key issues in the risk assessment of nuclear waste repositories. Here, we studied Fe mineralogy and valence in fracture coatings, fresh rocks and altered rocks in the upper 1 km of fractured crystalline bedrock at two sites (Laxemar and Forsmark areas, Sweden) on the Baltic shield. Fe3+/∑Fe ratios in these materials were quantified based on the centroid position of the pre-edge feature on Fe XANES spectra, while the speciation of Fe was predicted by reconstructing the sample EXFAS spectra using a linear combination of a large dataset of reference spectra collected previously[1] and in this study. The results were compared with Mössbauer spectra. The fresh and altered rocks showed no systematic difference in Fe3+/∑Fe ratio, indicating that past hydrothermal activities (red-staining on fracture wall-rock) did not lead to a reduction in reducing capacity within the fracture networks. The fracture coatings from the Forsmark area are of clear hydrothermal character (as indicated by an abundance of hematite, hornblende and muscovite) and have not experienced the same degree of low-T oxidative weathering as the samples from the Laxemar area having frequent and abundant illite and ferrihydrite. However, Fe3+/∑Fe ratios of the fracture coatings from the two areas showed similar features, including no depth trend and a similar variability from 0.24-0.85 and 0.12-0.71 which are overall larger than the fresh and altered rocks. This suggests that regional geological events can have a significant impact on the speciation of Fe, but not Fe valence.
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