Magnetotransport properties of nanocomposites close to the percolation threshold
Fadeev, Egor (2022-05-02)
Väitöskirja
02.05.2022
Lappeenranta-Lahti University of Technology LUT
Acta Universitatis Lappeenrantaensis
School of Engineering Science
School of Engineering Science, Laskennallinen tekniikka
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In reference to IEEE copyrighted material which is used with permission in this thesis, the IEEE does not endorse any of Lappeenranta-Lahti University of Technology LUT's products or services. Internal or personal use of this material is permitted. If interested in reprinting/republishing IEEE copyrighted material for advertising or promotional purposes or for creating new collective works for resale or redistribution, please go to http://www.ieee.org/publications_ standards/publications/rights/rights_link.html to learn how to obtain a License from RightsLink.
Julkaisun pysyvä osoite on
https://urn.fi/URN:ISBN:978-952-335-818-8
https://urn.fi/URN:ISBN:978-952-335-818-8
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Tiivistelmä
Magnetic semiconductors and granular metal-insulator systems are the key materials for broadly developing spintronics and for many other fields of solid-state physics. The idea of this work was to investigate magnetotransport and magnetic properties of multilayered ZnO/C magnetic semiconductors and nanogranular metal-insulator composites that were fabricated using an ion-beam sputtering method. The focus of the investigation was on determining conduction mechanisms and finding positive magnetoresistance in both systems, as well as on establishing the role of dispersed magnetic atoms in metal-insulator nanocomposites.
The conduction mechanism of ZnO/C heterostructures changes from the 2D Mott’s variable range hopping to the hopping to the nearest neighbors depending on film thickness, the amount of bilayers and temperature. The peculiar low-temperature magnetoresistance behavior was observed for ZnO/C heterostructures – magnetoresistance changes its sign twice in the magnetic field range of 20 T. The three mechanisms were proposed to explain this unusual behavior: the Fermi level shift under the influence of a magnetic field, scattering on the magnetic disorder, and wave function shrinkage due to the effect of magnetic blockade. ZnO/C heterostructures have shown the presence of ferromagnetic ordering in T ⩽ 120 K.
The study of metal-insulator nanocomposites has shown that an increase of concentration of dispersed magnetic atoms shifts the metal-insulator transition to lower values, widening the concentration range where temperature dependence of electrical conductivity follows ρ ∝ ln T. Low-temperature linear positive magnetoresistance in the range of 10−3–10−2% T−1 was observed for nanocomposites with concentrations close to the percolation threshold. The magnetoresistance results were quantitatively explained by the modified expression based on the Inoue-Meakawa model. The observed magnetoresistance anisotropy of one of the nanocomposites was associated with the formation of the columnar structure at the initial stage of growth of the nanocomposite.
The conduction mechanism of ZnO/C heterostructures changes from the 2D Mott’s variable range hopping to the hopping to the nearest neighbors depending on film thickness, the amount of bilayers and temperature. The peculiar low-temperature magnetoresistance behavior was observed for ZnO/C heterostructures – magnetoresistance changes its sign twice in the magnetic field range of 20 T. The three mechanisms were proposed to explain this unusual behavior: the Fermi level shift under the influence of a magnetic field, scattering on the magnetic disorder, and wave function shrinkage due to the effect of magnetic blockade. ZnO/C heterostructures have shown the presence of ferromagnetic ordering in T ⩽ 120 K.
The study of metal-insulator nanocomposites has shown that an increase of concentration of dispersed magnetic atoms shifts the metal-insulator transition to lower values, widening the concentration range where temperature dependence of electrical conductivity follows ρ ∝ ln T. Low-temperature linear positive magnetoresistance in the range of 10−3–10−2% T−1 was observed for nanocomposites with concentrations close to the percolation threshold. The magnetoresistance results were quantitatively explained by the modified expression based on the Inoue-Meakawa model. The observed magnetoresistance anisotropy of one of the nanocomposites was associated with the formation of the columnar structure at the initial stage of growth of the nanocomposite.
Kokoelmat
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