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Anomalous Magnetic Properties of Nanoparticles Arising from Defect Structures: Topotaxial Oxidation of Fe1-xO|Fe3-δO4 Core|Shell Nanocubes to Single-Phase Particles
Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK).
Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK).
Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK).
Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK).
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2013 (English)In: ACS Nano, ISSN 1936-0851, E-ISSN 1936-086X, Vol. 7, no 8, 7132-7144 p.Article in journal (Refereed) Published
Abstract [en]

Here we demonstrate that the anomalous magnetic properties of iron oxide nanoparticles are correlated with defects in their interior. We studied the evolution of microstructure and magnetic properties of biphasic core|shell Fe1–xO|Fe3−δO4 nanoparticles synthesized by thermal decomposition during their topotaxial oxidation to single-phase nanoparticles. Geometric phase analysis of high-resolution electron microscopy images reveals a large interfacial strain at the core|shell interface and the development of antiphase boundaries. Dark-field transmission electron microscopy and powder X-ray diffraction concur that, as the oxidation proceeds, the interfacial strain is released as the Fe1–xO core is removed but that the antiphase boundaries remain. The antiphase boundaries result in anomalous magnetic behavior, that is, a reduced saturation magnetization and exchange bias effects in single-phase nanoparticles. Our results indicate that internal defects play an important role in dictating the magnetic properties of iron oxide nanoparticles.

Place, publisher, year, edition, pages
2013. Vol. 7, no 8, 7132-7144 p.
Keyword [en]
iron oxide, core|shell, nanoparticles, defects, antiphase boundary, electron microscopy, exchange bias
National Category
Materials Chemistry Nano Technology Chemical Sciences
Research subject
Materials Chemistry
Identifiers
URN: urn:nbn:se:su:diva-93790DOI: 10.1021/nn402487qISI: 000323810600076OAI: oai:DiVA.org:su-93790DiVA: diva2:648776
Note

AuthorCount: 5;

Funding Agencie:

Swedish Research Council (VR);  Knut and Alice Wallenberg (KAW) Foundation  3DEM-NATUR 

Available from: 2013-09-16 Created: 2013-09-16 Last updated: 2017-12-06Bibliographically approved
In thesis
1. Building crystals out of crystals: Synthesis, structure and magnetic properties of iron oxide nanoparticles and self-assembled mesocrystals
Open this publication in new window or tab >>Building crystals out of crystals: Synthesis, structure and magnetic properties of iron oxide nanoparticles and self-assembled mesocrystals
2013 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

This thesis is focused on the fabrication and characterization of self-assembled arrays of magnetic iron oxide (Fe3O4, γ-Fe2O3 and Fe1-xO) nanoparticles. The synthesis of spherical and cubic iron oxide nanocrystals, with sizes between 5 and 30 nm and narrow size distributions, is demonstrated, along with a rigorous morphological characterization of the cubic nanoparticles. The transformation of core|shell Fe1-xO|Fe3-δO4 particles into single-phase Fe3-δO4 particles is studied in detail. It is found that anti-phase boundaries in the particles result in the emergence of anomalous magnetic properties i.e. exchange bias, and a reduced saturation magnetization compared to that of bulk Fe3O4. Cubic nanocrystals are assembled into arrays possessing an exceptionally high degree of translational ordering and a high degree of crystallographic alignment. A combination of electron microscopy and small-angle X-ray scattering is used in the characterization of the 3D nanostructures. The directional (anisotropic) interactions in the 3D structures are modeled in an attempt to find a link between the nanocrystal morphology and the corresponding mesostructure. Here, the cohesive van der Waals energy is estimated for a system of nanocubes with a variable truncation. The assembly of nanocubes in magnetic fields of various strengths is systematically investigated. A perturbed mesocrystal growth habit is observed at intermediate fields, whereas at high field strengths, the assembly is dominated by ferrohydrodynamic instabilities. Last, magnetometry is used to study the collective magnetic properties of self-assembled nanocrystals. The magnetic susceptibility in a weak magnetic field is studied as a function of film thickness and particle size. An increase in the tendency to form ferromagnetic couplings  with decreasing film thickness can be established. This 2D to 3D crossover of the magnetic properties of the nanoparticle arrays can be related to a change in the magnetic vortex states.

Place, publisher, year, edition, pages
Stockholm: Department of Materials and Environmental Chemistry (MMK), Stockholm University, 2013. 112 p.
Keyword
iron oxide, nanoparticle, synthesis, self-assembly, characterization, electron microscopy, scattering, magnetic properties
National Category
Materials Chemistry
Research subject
Materials Chemistry
Identifiers
urn:nbn:se:su:diva-93791 (URN)978-91-7447-768-9 (ISBN)
Public defence
2013-10-18, Ahlmannsalen, Geovetenskapens hus, Svante Arrhenius väg 12, Stockholm, 10:00 (English)
Opponent
Supervisors
Available from: 2013-09-26 Created: 2013-09-16 Last updated: 2013-10-08Bibliographically approved

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Wetterskog, ErikTai, Cheuk-WaiGrins, JekabsBergström, LennartSalazar-Alvarez, German
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