Dirba, Imants (2017)
Fe8Nx Thin Films and Nanoparticles: from Intrinsic Properties Towards Magnetic Applications.
Technische Universität Darmstadt
Ph.D. Thesis, Primary publication
Abstract
Iron nitride Fe8Nx could potentially provide an environmentally friendly and resource-efficient functional magnetic material in the areas of permanent magnets, magnetic recording as well as biomedical applications. Despite the amount of research within the last decades, questions remain on whether or not the intrinsic magnetic properties are sufficient and if they can, by sustainable means, be engineered into the useful extrinsic properties. Another key issue is the phase stability in different environments which needs a thorough investigation. In this thesis, the Fe8Nx material synthesis, an analysis of structure and the corresponding magnetic properties, particularly in thin films and nanoparticles, are presented. The focus lies first on the fabrication of buffer-free, phase-pure α'-Fe8Nx and α''-Fe16N2 samples in order to converge towards an unambiguous interpretation of the observed physical phenomena. The main aim of this work is to study the magnetic properties, the thermal stability and consequently feasibility for the proposed applications, by performing advanced synthesis and in-depth characterization of high-quality α'-Fe8Nx and α''-Fe16N2 samples. α'-Fe8Nx thin films are deposited in the full range of 0 ≤ x ≤ 1. The nitrogen incorporation leads to a gradually induced tetragonal unit cell expansion of the compounds which is accompanied by an increase in the magnetic moment, reaching 2.50 ± 0.09μB per Fe atom at 10 K. The origin of the increased magnetic moment is solely the lattice expansion. The uniaxial anisotropy constant increases with c/a ratio (or resp. nitrogen content) reaching a value of 0.54MJm3 for c/a ≈1.1. The interstitial N atoms play a decisive role in stabilizing the enhanced perpendicular magnetocrystalline anisotropy. These findings can be generalized to other nitrogen containing interstitial Fe alloys. The second major activity is the development of a novel route with a high-pressure hydrogen reduction step for the synthesis of α''-Fe16N2 nanoparticles. With this route, phase-pure α''-Fe16N2 nanoparticles are successfully synthesized and characterized. The Ms(0) for α''-Fe16N2 nanoparticles is found to be 215Am2kg-1 and coercivity μ0Hc = 0.22T. Fe-O shells form around the particles when exposed to atmosphere which leads to a reduced magnetization. Overall the Fe8Nx alloys are shown to possess semi-hard magnetic properties as well as relatively poor phase stability, which has direct consequences on applications, such as bulk permanent magnets, nanocomposites and magnetic nanoparticle hyperthermia.
Item Type: | Ph.D. Thesis | ||||
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Erschienen: | 2017 | ||||
Creators: | Dirba, Imants | ||||
Type of entry: | Primary publication | ||||
Title: | Fe8Nx Thin Films and Nanoparticles: from Intrinsic Properties Towards Magnetic Applications | ||||
Language: | English | ||||
Referees: | Gutfleisch, Prof. Dr. Oliver ; Niewa, Prof. Dr. Rainer | ||||
Date: | 2017 | ||||
Place of Publication: | Darmstadt | ||||
Refereed: | 22 May 2017 | ||||
URL / URN: | http://tuprints.ulb.tu-darmstadt.de/6259 | ||||
Abstract: | Iron nitride Fe8Nx could potentially provide an environmentally friendly and resource-efficient functional magnetic material in the areas of permanent magnets, magnetic recording as well as biomedical applications. Despite the amount of research within the last decades, questions remain on whether or not the intrinsic magnetic properties are sufficient and if they can, by sustainable means, be engineered into the useful extrinsic properties. Another key issue is the phase stability in different environments which needs a thorough investigation. In this thesis, the Fe8Nx material synthesis, an analysis of structure and the corresponding magnetic properties, particularly in thin films and nanoparticles, are presented. The focus lies first on the fabrication of buffer-free, phase-pure α'-Fe8Nx and α''-Fe16N2 samples in order to converge towards an unambiguous interpretation of the observed physical phenomena. The main aim of this work is to study the magnetic properties, the thermal stability and consequently feasibility for the proposed applications, by performing advanced synthesis and in-depth characterization of high-quality α'-Fe8Nx and α''-Fe16N2 samples. α'-Fe8Nx thin films are deposited in the full range of 0 ≤ x ≤ 1. The nitrogen incorporation leads to a gradually induced tetragonal unit cell expansion of the compounds which is accompanied by an increase in the magnetic moment, reaching 2.50 ± 0.09μB per Fe atom at 10 K. The origin of the increased magnetic moment is solely the lattice expansion. The uniaxial anisotropy constant increases with c/a ratio (or resp. nitrogen content) reaching a value of 0.54MJm3 for c/a ≈1.1. The interstitial N atoms play a decisive role in stabilizing the enhanced perpendicular magnetocrystalline anisotropy. These findings can be generalized to other nitrogen containing interstitial Fe alloys. The second major activity is the development of a novel route with a high-pressure hydrogen reduction step for the synthesis of α''-Fe16N2 nanoparticles. With this route, phase-pure α''-Fe16N2 nanoparticles are successfully synthesized and characterized. The Ms(0) for α''-Fe16N2 nanoparticles is found to be 215Am2kg-1 and coercivity μ0Hc = 0.22T. Fe-O shells form around the particles when exposed to atmosphere which leads to a reduced magnetization. Overall the Fe8Nx alloys are shown to possess semi-hard magnetic properties as well as relatively poor phase stability, which has direct consequences on applications, such as bulk permanent magnets, nanocomposites and magnetic nanoparticle hyperthermia. |
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URN: | urn:nbn:de:tuda-tuprints-62591 | ||||
Classification DDC: | 500 Science and mathematics > 500 Science | ||||
Divisions: | 11 Department of Materials and Earth Sciences > Material Science > Functional Materials 11 Department of Materials and Earth Sciences > Material Science 11 Department of Materials and Earth Sciences |
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Date Deposited: | 04 Jun 2017 19:55 | ||||
Last Modified: | 04 Jun 2017 19:55 | ||||
PPN: | |||||
Referees: | Gutfleisch, Prof. Dr. Oliver ; Niewa, Prof. Dr. Rainer | ||||
Refereed / Verteidigung / mdl. Prüfung: | 22 May 2017 | ||||
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