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Hyperfine interactions and electronic structure of magnetite
Řezníček, Richard ; Štěpánková, Helena (advisor) ; Máca, František (referee) ; Schneeweiss, Oldřich (referee)
The present work focuses on analyses of the experimental 57 Fe nuclear magnetic resonance data and the results of ab initio calculations in order to achieve deeper insight into the hyperfine and electronic structure of magnetite both below and above the Verwey phase transition. Parameters of hyperfine interactions and electronic structure data of the Cc phase of magnetite were extracted from the results of ab initio calculations based on the recently reported crystal structure. Published experimental dependences of the 57 Fe nuclear magnetic resonance frequencies on the external magnetic field direction were quantitatively reanalyzed, yielding hyperfine anisotropy data, which were compared to the analogous hyperfine field parameters from the ab initio calculations. The findings were interpreted in the context of up-to-date charge ordering models for magnetite. Attention was also paid to the impact of various cationic substitutions and defects on the electronic structure and hyperfine interactions in magnetite manifested in the 57 Fe nuclear magnetic resonance spectra: below the Verwey transition, signal intensity shifts in the spectra were analyzed; above the Verwey transition, the mean field model was adapted for an analysis of temperature dependence of satellite signal frequencies, while the anomalous...
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Hyperfine interactions in maghemite and magnetite particles
Křišťan, Petr
Thesis is aimed at studying of magnetic iron oxide particles of submicron and nanoscale dimensions by means of nuclear magnetic resonance (NMR). 57 Fe NMR inves- tigations were carried out in composite bentonite/maghemite with respect to tempera- ture of calcination (Tcalc) during the sample preparation and in magnetite submicron powders with respect to various range of the particles size. One of the main findings is that increasing Tcalc improves resolution in the NMR spectra, which is most likely connected with higher degree of atomic ordering in the spinel structure. Evaluating the integral intensities of NMR spectra allowed us to determine the relative content of maghemite phase in particular samples of the series: the content rapidly grows for Tcalc up to ∼420 deg. An approach to distinguish signal from tetrahedral and octahedral irons was developed and tested on pure maghemite sample. Analysis based on vacancy- distribution models was performed in the spinel structure and the results were compared to the experiment. 57 Fe NMR spectra in submicron magnetite samples were found to differ markedly from spectrum of a single crystal. It was concluded that the investigated powders possess high amount of defects in the crystal structure or contain additional phase (probably closely related to the maghemite phase).
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Influence of cationic substitution on hyperfine interactions in magnetite
Řezníček, Richard
The subject matter of the present work is a study of a series of single crystal samples of magnetite with substitutions of zinc and titanium by means of the NMR method. Ions of the zinc substitution Zn2+ replace a part of ferric ions at tetrahedral (A) sites, while the titanium ions Ti4+ occupy octahedral (B) sites replacing iron ions Fe2.5+ . Hyperfine interactions and local electronic structure are sensitive to the presence of substitution. The case when the valence of the substitution ion is different from that of the replaced ion is of a particular interest. Resonance frequencies of nuclei in the neighbourhood of the substitution are shifted due to the modified hyperfine field, thus satellite lines can be observed in NMR spectra. Temperature dependences of spectra above the Verwey transition were measured in a zero external magnetic field. Additionally, NMR spectra were also acquired at the temperature of 4.2 K. Temperature dependences of frequencies of main lines and satellite signals in the spectra above the Verwey transition were constructed and compared to the data for pure magnetite and magnetite with other substitutions and with cationic vacancies. Furthermore, variations of widths of A lines against the temperature above the Verwey transition were found and discussed.
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Hyperfine Interactions in Ferrites with Spinel Structure
Chlan, Vojtěch
Title: Hyperfine Interactions in Ferrites with Spinel Structure Author: Vojtěch Chlan Faculty of Mathematics and Physics, Charles University in Prague Supervisor: Prof. RNDr. Helena Štěpánková, CSc. Abstract: Ferrite systems with spinel structure, manganese ferrite, lithium ferrite and magnetite, are studied experimentally by nuclear magnetic resonance (NMR) spectroscopy and from the first principles by electron structure calculations based on density functional theory (DFT). Manganese ferrites with various degrees of manganese-iron inversion and with different manganese contents are investigated, with respect to magnetic structure and cationic arrangement. An approach for NMR spectra interpretation is elaborated on ordered lithium ferrite, consisting in matching the calculated hyperfine field anisotropy with experimental results. This method is then applied to low temperature structure of magnetite, showing a promising path towards solving its complicated structure, orbital and charge ordering. The switching of magnetic easy axis induced by external magnetic field is observed in magnetite and the accompanying structural changes are studied by means of magnetic and NMR measurements. Magnetite with cationic vacancies is investigated by analyzing satellite structure induced by the vacancies; it is shown that...
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Measuring magnetic properties of environmental samples
Zárubová, Alžběta ; Petrovský, Eduard (advisor) ; Kapička, Aleš (referee)
A. Zárubová: Měření magnetických vlastností environmentálních vzorků 1 ENGLISH ABSTRACT This bachelor's thesis presents briefly principles and examples of applications of magnetic measurements in environmental studies. Environmental magnetism is based on determination of iron oxides, their content, concentration and grain size. This information can be interpreted in terms of particle origin, transport or diagenesis. The results can provide information on, e.g., climatic changes in the past, short-term events influencing our environment (e.g., volcanic eruptions) or about contemporary problems associated with human activity (environmental pollution). In the last case, environmental magnetism does not intend to replace geochemical methods, but the main aim is to provide fast and easily available additional information, which helps us to be more effective in mapping contaminated areas, monitoring progression of pollution or modeling transport processes of contaminants. Magnetometry is currently intensively used for mapping contamination of atmosphere, soils and sediments; it can be used in defining long-term levels of contamination, individual pollution sources and their contribution to the overall pollution. This thesis provides a brief overview of the physical basics of environmental magnetism and examples...
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Low-field excitations in magnetite
Švindrych, Zdeněk ; Janů, Zdeněk (advisor) ; Novák, Pavel (referee) ; Svoboda, Pavel (referee)
We have performed detailed measurements of magnetic and dielectric properties of high quality magnetite (Fe3O4) single crystals in weak magnetic and electric fields. These measurements can reveal details of phase transitions and other features that are not yet fully understood. We focused not only at the Verwey transition - a well known phase transition taking place at about 120 K in stoichiometric samples - but we also explored and described new relaxation effects in magnetite at low temperatures. The low-temperature properties were also found to be exceptionally sensitive to sample quality, stoichiometry and homogeneity. The results presented in this thesis were acquired on sensitive non-commercial SQUID magnetometer complemented by four-probe dielectric spectroscopy and dc conductivity measurements.
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Study of hyperfine interactions in nanoparticles present in biological systems
Kubániová, Denisa ; Kohout, Jaroslav (advisor) ; Závěta, Karel (referee)
In the submitted bachelor thesis we studied iron oxides by zero-field and in-field Mössbauer spectroscopy at room and liquid helium temperature. The relative composition in synthe- tically prepared nanoparticles of ferric oxide in SiO2 matrix is investigated in dependence on ferric oxide concentration and heating temperature and further the phase composi- tion and structure of biological nanoparticles produced by Geoalkalibacter ferrihydriticus bacteries in dependence on concentration of quinone in the cultivating solution. By data analysis the chemical and phase composition of the samples and hyperfine parametres of 57 Fe cations in nonequivalent crystallographic positions are determined. 1
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Hyperfine interactions in maghemite and magnetite particles
Křišťan, Petr ; Štěpánková, Helena (advisor) ; Procházka, Ivan (referee)
Thesis is aimed at studying of magnetic iron oxide particles of submicron and nanoscale dimensions by means of nuclear magnetic resonance (NMR). 57 Fe NMR inves- tigations were carried out in composite bentonite/maghemite with respect to tempera- ture of calcination (Tcalc) during the sample preparation and in magnetite submicron powders with respect to various range of the particles size. One of the main findings is that increasing Tcalc improves resolution in the NMR spectra, which is most likely connected with higher degree of atomic ordering in the spinel structure. Evaluating the integral intensities of NMR spectra allowed us to determine the relative content of maghemite phase in particular samples of the series: the content rapidly grows for Tcalc up to ∼420 deg. An approach to distinguish signal from tetrahedral and octahedral irons was developed and tested on pure maghemite sample. Analysis based on vacancy- distribution models was performed in the spinel structure and the results were compared to the experiment. 57 Fe NMR spectra in submicron magnetite samples were found to differ markedly from spectrum of a single crystal. It was concluded that the investigated powders possess high amount of defects in the crystal structure or contain additional phase (probably closely related to the maghemite phase).
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Synthesis and photocatalytic applications of titanium dioxide
Solný, Tomáš ; Drbohlavová, Jana (referee) ; Vlček,, Jozef (referee) ; Ptáček, Petr (advisor)
V práci je zkoumán vliv podmínek na průběh hydrolýzy alkoxidů titanu a vlastností připravovaných nanočástic oxidu titaničitého s důrazem na teplotu a množství vody přítomné v systému. Připravované hydrolyzáty alkoxidů titanu a nanočástice oxidu titaničitého připravené z hydrolyzátů jsou studovány metodami XRD, DTA – TGA, SEM – EDS, BET a PCCS. Nanočástice magnetitu byly syntetizovány pomocí precipitační reakce z roztoku Mohrovy soli a jejich krystalová struktura, velikost a povrchové vlastnosti byly sledovány s vyhodnocením vlivu teploty a při modifikaci povrchu polykarboxyletherovým superplastifikátorem. Pro upevnění TiO2 na povrch magnetitu byla použita kombinovaná metoda aplikace vybraných nanočástic TiO2 s hydrolýzou TiO2 pomocí alkoxidů titanu za účelem přípravy fotokatalyticky aktivního core-shell práškového katalyzátoru s vylepšenými vlastnostmi adsorpce na povrchu. Studovány byly možnosti aplikace TiO2 na povrch Mn-Zn feritu, kdy byl studován vliv depozice tenkých vrstev C a Au na morfologii povrchu. Fotokatalytická aktivita vybraných připravených materiálů byla studována pomocí dekompozice methylenové modři v roztoku a par isopropanolu a ethanolu rozkládaných pomocí Mn-Zn feritu v experimentálním chemickém reaktoru s magnetickým polem stabilizovaným ložem nosiče katalyzátoru.
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Single Atomic Catalysts
Závodný, Adam ; Švec,, Martin (referee) ; Čechal, Jan (advisor)
Single atom catalysts are prospective class of materials, which holds promises to reach the ultimate limit of improvement in catalyst performance, selectivity, lifetime and cost reduction. The ability to efficiently capture the adsorbates at the active sites is the key prerequisite for catalytic transformation to the products. In this respect, our experimental study aims to describe the interaction of gas molecules (H2O, CO, O2 and NO) with single metal atoms (Rh, Ir, Cu) on the magnetite surface employing scanning tunneling microscopy.
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