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Phillips, Peter John. "Conduction processes in spinel ferrites." Thesis, University of Warwick, 1991. http://wrap.warwick.ac.uk/108772/.
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An apparatus has been designed, constructed and tested for measurements of resistivity and thermopower, having the following novel features:- I) buffer amplifiers with driven shields to make measurements on highly resistive specimens, II) “in house” software and hardware developed to automate the measurements and, III) a new method of temperature measurement for thermopower studies. Measurements have been carried out on single crystal ZnxFe3_x04 and MnxFe3_x04 with 0 < X < 1 in the temperature range 4.2K-300K. The results strongly suggest that substitutional disorder plays an important role in the electrical transport properties of ferrites. The electrical conductivity for high X near 300K is interpreted in terms of nearest-neighbour hopping. The temperature dependence of G(a7’)=dLn(< 77’)/d( 1/T) between 300K and 100K and the concentration dependence of G(crT) at 100K provides evidence for the formation of a Coulomb Gap at low X. At low temperatures, variable-range and many-electron hopping for high and low X respectively is observed. An estimate of the effective radius of the carrier wavefunction is obtained from the conductivity measurements, which at ~0.4Á is comparable to that previously deduced for nickel ferrite. For all X a peak in the thermopower at about 50K was observed. The peak for high X has been attributed to the high temperature limit of variable- range hopping, while for low X interpreted as the transition from activation across a Coulomb Gap to many-electron hopping. In the high temperature range, the temperature variation is shown to be consistent with degenerate statistics for low X and non-degenerate statistics for high X. In the latter case it is possible to extract the dopant concentrations, which compare favourably with the values found by Electron Probe Micro-Analysis (EPMA). The high temperature thermopower and resistivity behaviour have been used to deduce the variation of the energy width of the conduction states, or bandwidth, with X and this is shown to be consistent with the expected behaviour. A polaron energy of ~0.04eV is calculated at 300K for high values of X, which compares favourably with previous work.
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Price, Sean. "Critical and magnetic behaviour of some spinel ferrites." Thesis, University of Portsmouth, 1991. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.304578.
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Vestal, Christy Riann. "Magnetic couplings and superparamagnetic properties of spinel ferrite nanoparticles." Diss., Available online, Georgia Institute of Technology, 2004:, 2004. http://etd.gatech.edu/theses/available/etd-06072004-131405/unrestricted/vestal%5Fchristy%5Fr%5F200405%5Fphd.pdf.
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Song, Qing. "Size and Shape Controlled Synthesis and Superparamagnetic Properties of Spinel Ferrites Nanocrystals." Diss., Georgia Institute of Technology, 2005. http://hdl.handle.net/1853/7645.
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Size and Shape Controlled Synthesis and Superparamagnetic Properties of Spinel Ferrites Nanocrystals
Qing Song
216 pages
Directed by Dr. Z. John Zhang
The correlationship between magnetic properties and magnetic couplings is established through the investigations of various cubic spinel ferrite nanocrystals. The results of this thesis contribute to the knowledge of size and shape controlled synthesis of various spinel ferrites and core shell architectured nanocrystals as well as the nanomagnetism in spinel ferrites by systematically investigating the effects of spin orbital coupling, magnetocrystalline anisotropy, exchange coupling, shape and surface anisotropy upon superparamagnetic properties of spinel ferrite nanocrystals. A general synthetic method is developed for size and shape control of metal oxide nanocrystals. The size and shape dependent superparamagnetic properties are discussed. The relationship between spin orbital coupling and magnetocrystalline anisotropy is studied comparatively on variable sizes of spherical CoFe2O4 and Fe3O4 nanocrystals. It also addresses the effect of exchange coupling between magnetic hard phase and soft phase upon magnetic properties in core shell structured spinel ferrite nanocrystals. The role of anisotropic shapes of nanocrystals upon self assembled orientation ordered superstructures are investigated. The effect of thermal stability of molecular precursors upon size controlled synthesis of MnFe2O4 nanocrystals and the size dependent superparamagnetic properties are described.
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Bonholzer, Michael. "Magnetic Tunnel Junctions based on spinel ZnxFe3-xO4." Doctoral thesis, Universitätsbibliothek Leipzig, 2016. http://nbn-resolving.de/urn:nbn:de:bsz:15-qucosa-212756.
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Die vorliegende Arbeit befasst sich mit magnetischen Tunnelkontakten (magnetic tunnel junctions, MTJs) auf Basis des Oxids Zinkferrit (ZnxFe3-xO4).
Dabei soll das Potential dieses Materials durch die Demonstration des Tunnelmagnetowiderstandes (tunnel magnetoresistance, TMR) in zinkferritbasierten Tunnelkontakten gezeigt werden. Dazu wurde ein Probendesign für MTJs auf Basis der „pseudo spin valve“-Geometrie entwickelt. Die Basis für dieseStrukturen ist ein Dünnfilmstapel aus MgO (Substrat) / TiN / ZnxFe3-xO4 / MgO / Co. Dieser ist mittels gepulster Laserabscheidung (pulsed laser deposition, PLD) hergestellt. Im Rahmen dieser Arbeit wurden die strukturellen, elektrischen und magnetischen Eigenschaften der Dünnfilme untersucht. Des weiteren wurden die fertig prozessierten MTJ-Bauelemente an einem im Rahmen
dieser Arbeit entwickeltem und aufgebautem TMR-Messplatz vermessen. Dabei ist es gelungen einen TMR-Effekt von 0.5% in ZnxFe3-xO4-basierten MTJs nachzuweisen.
Das erste Kapitel der Arbeit gibt eine Einführung in die spintronischen Effekte Riesenmagnetowiderstand (giant magnetoresistance, GMR) und Tunnelmagnetowiderstand (TMR). Deren technologische Anwendungen sowie die grundlegenden physikalischen Effekte und Modelle werden diskutiert. Das zweite Kapitel gibt eine Übersicht über die Materialklasse der spinellartigen Ferrite. Der Fokus liegt auf den Materialien Magnetit (Fe3O4) sowie Zinkferrit (ZnxFe3-xO4). Die physikalischen Modelle zur Beschreibung der strukturellen, magnetischen und elektrischen Eigenschaften dieser Materialien werden dargelegt sowie ein Literaturüberblick über experimentelle und theoretische Arbeiten gegeben. Im dritten Kapitel werden die im Rahmen dieser Arbeit verwendeten Probenpräparations- und Charakterisierungsmethoden vorgestellt und technische Details sowie physikalische Grundlagen erläutert. Die Entwicklung eines neuen Probendesigns zum Nachweis des TMR-Effekts in ZnxFe3-xO4-basierten MTJs ist Gegenstand des vierten Kapitels. Die Entwicklung des Probenaufbaus sowie die daraus resultierende Probenprozessierung werden beschrieben. Die beiden letzten Kapitel befassen sich mit der strukturellen, elektrischen und magnetischen Charakterisierung der mittels PLD abgeschiedenen Dünnfilme sowie der Tunnelkontaktstrukturen.
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Han, Man Huon. "Development of synthesis method for spinel ferrite magnetic nanoparticle and its superparamagnetic properties." Diss., Atlanta, Ga. : Georgia Institute of Technology, 2008. http://hdl.handle.net/1853/26465.
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Thesis (Ph. D.)--Chemistry and Biochemistry, Georgia Institute of Technology, 2009.Committee Chair: Z. John Zhang; Committee Member: Angus Wilkinson; Committee Member: C P Wong; Committee Member: E. Kent Barefield; Committee Member: Mostafa El-Sayed. Part of the SMARTech Electronic Thesis and Dissertation Collection.
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Mercier, Adrien. "Contribution à l'étude et à la réalisation de composants magnétiques monolithiques réalisés par PECS/SPS et à leurs applications en électronique de puissance." Thesis, Université Paris-Saclay (ComUE), 2016. http://www.theses.fr/2016SACLN057/document.
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L’augmentation des fréquences de commutation des alimentations induit de nouvelles problématiques pour les composants. Que ces composants soient actifs ou passifs, il est nécessaire de contrôler les pertes afin que les rendements restent acceptables. La thèse se propose d’étudier et de fabriquer une nouvelle structure de transformateur destiné à s’insérer dans une alimentation à découpage. Ces transformateurs sont produits à l’aide du procédé PECS/SPS, qui est une technologie de frittage. Cette technologie permet de fritter des ferrites tout autour des enroulements primaires et secondaires. Il en résulte que les composants sont monolithiques.Une première partie présente l’état de l’art, ainsi que le magnétisme dans la matière. Il s’ensuit un chapitre dédié à la fabrication des matériaux magnétiques utilisés durant la thèse : les ferrites.Une deuxième partie concerne les ferrites fabriqués par le procédé PECS/SPS. Il est question dans un premier temps d’étudier l’anisotropie magnétocristalline de ces matériaux, et il est possible de la diminuer en jouant sur la composition chimique. Dans un second temps, d’autres grandeurs telles que la perméabilité ou la polarisation sont mesurées, toujours en fonction de la composition chimique. Les principaux résultats montrent que le frittage par le procédé PECS/SPS est plus réducteur que le frittage classique, ce qui dégrade certaines propriétés comme la résistivité des ferrites. Un dernier chapitre est dédié à la réalisation des transformateurs monolithiques. Un protocole détaillé de la fabrication est alors présenté.Une troisième partie illustre le fonctionnement des transformateurs réalisés. Les mesures usuelles permettent d’identifier les inductances propres, mutuelles et de fuite. Les valeurs de ces inductances montrent qu’il est plus judicieux d’utiliser les composants fabriqués non pas en tant que transformateur, mais en tant que coupleur. Enfin un convertisseur basé sur une structure de type VRM est réalisé. La fréquence de découpage est de 2 MHz, le rendement est supérieur à 90 %, et la densité de puissance est de 15 kW/litreThe increase in switching frequency of power supply induces new problems for the components. These components are active or passive, it is necessary to control the losses so that efficiency remains acceptable. The thesis deals with the study and production of a new transformer structure intended to be part of a switching power supply. These transformers are produced using PECS/SPS method, which is a sintering technology. This technology can be sintered ferrite around the primary and secondary windings. It follows that the components are monolithic.A first part presents the state of the art, and magnetism in the matter. It follows a chapter dedicated to the manufacture of magnetic materials used in the thesis : the ferrites.A second part concerns the ferrites produced by the PECS / SPS process. Firstly, the magnetocrystalline anisotropy of these materials is studied, and it is possible to decrease it by varying the chemical composition. In a second stage, other variables such as the permeability or the polarization are measured, always depending on the chemical composition. The main results show that the sintering by PECS / SPS method is more reducing than conventional sintering, which degrades certain properties such as the resistivity of the ferrites. The last chapter is dedicated to the realization of monolithic processors. A detailed manufacturing protocol is presented.A third part shows the operation of the realized transformers. The usual measurements allow identifying self, mutual and leakage inductances. The values of these inductances show that it is better to use components made not as a transformer, but as a coupler. Finally a converter based on a VRM structure is realized. The switching frequency is 2 MHz, the efficiency is greater than 90%, and the power density is 15 kW / liter
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Moura, Alysson Elson Galv?o de. "S?ntese, sinteriza??o e caracteriza??o de ferritas ? base de Ni-Zn." Universidade Federal do Rio Grande do Norte, 2008. http://repositorio.ufrn.br:8080/jspui/handle/123456789/17599.
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Were synthesized different ferrites NixZn1-xFe2O4 (0,4 ≤ x ≤ 0,6) compositions by using citrate precursor method. Initially, the precursors citrates of iron, nickel and zinc were mixed and homogenized. The stoichiometric compositions were calcined at 350?C without atmosphere control and the calcined powders were pressed in pellets and toroids. The pressed material was sintered from 1100? up to 1200?C in argon atmosphere. The calcined powders were characterized by XRD, TGA/DTG, FTIR, SEM and vibrating sample magnetometer (VSM). All sintered samples were characterized using XRD, SEM, VSM and measurements of magnetic permeability and loss factor were obtained. It was formed pure ferromagnetic phase at all used temperatures. The Rietveld analyses allowed to calculate the cations level occupation and the crystallite size. The analyses obtained nanometric crystals (12-20 nm) to the calcined powder. By SEM, the sintered samples shows grains sizes from 1 to 10 μm. Sintered densities (ρ) were measured by the Archimedes method and with increasing Zn content, the bulk density decrease. The better magnetization results (105-110 emu/g) were obtained for x=0,6 at all sintering temperatures. The hysteresis shows characteristics of soft magnetic material. Two magnetization processes were considered, superparamagnetism at low temperature and the magnetic domains formation at high temperatures. The sintered toroids presents relative magnetic permeability (μr) from 7 to 32 and loss factor (tanδ) of about 1. The frequency response of toroids range from 0,3 kHz to 0,2 GHz. The composition x=0,5 presents both greater μr and tanδ values and x=0,6 the most broad range of frequency response. Various microstructural factors show influence on the behavior of μr and tanδ, such as: grain size, porosity across grain boundary and inside the grain, grain boundary content and domain walls movement during the process of magnetization at high frequency studies (0,3kKz 0,2 GHz)
Foram sintetizadas diferentes composi??es da ferrita Ni1-xZnxFe2O4 com 0,4 ≤ x ≤ 0,6 pelo uso do m?todo dos citratos precursores. Para se obter a fase estequiom?trica do sistema Ni1-xZnxFe2O4 foram misturados e homogeneizados os citratos precursores de ferro, n?quel e zinco. As composi??es estequiom?tricas foram calcinadas em atmosfera ambiente na temperatura de 350?C e depois prensadas em pastilhas e tor?ides. As amostras prensadas foram sinterizadas nas temperaturas de 1100?, 1150? e 1200?C em atmosfera de arg?nio. Os p?s calcinados foram caracterizados por DRX, TGA/DTG, FTIR, MEV e magnetometria de amostra vibrante (MAV) e as amostras sinterizadas por DRX, MEV, MAV, massa espec?fica e medidas de permeabilidade e perdas magn?ticas. Observou-se a forma??o de fase pura ferrimagn?tica em todas as temperaturas aplicadas. A an?lise pelo m?todo de Rietveld calculou o n?vel de ocupa??o dos c?tions e o tamanho de cristalito. Foram obtidos tamanhos de cristais nanom?tricos, de 12 a 20 nm para os p?s calcinados. Por MEV, as amostras sinterizadas apresentam tamanhos de gr?os na faixa de 1 a 10 μm. A massa espec?fica (ρ) do material sinterizado apresenta uma tend?ncia de diminui??o com a adi??o de Zn. Os melhores resultados de magnetiza??o foram obtidos para x=0,6 nas tr?s temperaturas de sinteriza??o, variando de 105 a 110 emu/g. As histereses mostram um perfil de materiais magn?ticos moles. Dois processos de magnetiza??o foram considerados, o superparamagnetismo a baixa temperatura (350?C) e a forma??o de dom?nios magn?ticos em altas temperaturas. Os materiais sinterizados apresentam permeabilidade (μ) de algumas unidades, de 7 a 30, e perdas magn?ticas (tanδ) por volta de 1. A resposta em freq??ncia dos n?cleos toroidais est? na faixa de 0,3 kHz a 0,2 GHz. Os maiores valores de μ e tanδ s?o para x=0,5 e a maior faixa de resposta em freq??ncia ? para x=0,6. V?rios fatores da microestrutura contribuem para o comportamento das grandezas μ e tanδ, tais como: os tamanhos dos gr?os, porosidade inter e intragranular, quantidade de contornos de gr?os e os aspectos da din?mica das paredes de dom?nios quando excitadas magneticamente sob alta freq??ncia
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Scarberry, Kenneth Edward. "Biomedical applications of cobalt-spinel ferrite nanoparticles for cancer cell extraction and drug delivery." Diss., Georgia Institute of Technology, 2009. http://hdl.handle.net/1853/33951.
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In this presentation it is demonstrated that the unique magnetic properties of superparamagnetic cobalt-spinel ferrite nanoparticles can be employed in several novel applications. A method to selectively capture and remove pathogens from infected organisms to improve longevity is presented. Evidence is provided to show that automated methods using modified forms of hemofiltration or peritoneal dialysis could be used to eliminate the particle/pathogen or particle/infected cell conjugates from the organism postoperatively. It is shown that disparately functionalized nanoparticles can be used in concert as drug carrier and release mechanisms. Lastly, we provide preliminary evidence to support the use of magnetic nanoparticles for controlling reaction kinetics.
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Marija, Milanović. "Sinteza i karakterizacija nanočestičnih prahova na bazi cink-ferita." Phd thesis, Univerzitet u Novom Sadu, Tehnološki fakultet Novi Sad, 2010. https://www.cris.uns.ac.rs/record.jsf?recordId=82019&source=NDLTD&language=en.
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U ovom radu prikazani su rezultati ispitivanja strukturnih i magnetnih osobina čistih cink- ferita, ZnFe2O4 i cink-ferita sa dodatkom indijuma Zn1-xInxFe2O4 i itrijuma, ZnYxFe2-xO4, gde je 0 ≤ x ≤ 0,6. Prahovi na bazi cink-ferita su sintetisani koristeći nisko temperaturnu metodu sinteze iz tečne faze – metodu koprecipitacije. Osnovni cilj ove doktorske disertacije je bio da se utvrdi veza između uslova sinteze, uticaja različitih katjona, strukture i osobina čistih cink-feritnih nanočestica, kao i cink-ferita sa dodatkom indijuma i itrijuma. Radi praćenja uticaja veličine čestica dobijenog praha na strukturu i osobine ovih materijala, sintetisani čist cink-ferit je kalcinisan na različitim temperaturama. Posebno je razmatran i uticaj dodatka različitih jona na distribuciju i preraspodelu katjona u spinelnoj strukturi. Pored toga ispitivan je uticaj tako pripremljenih prahova na njihove magnetne osobine. Rentgenostrukturna i TEM analize potvrdili su da ispitivani uzorci spadaju u klasu nanomaterijala spinelne strukture. Analiza Raman i Mössbauer spektara je ukazala na moguću raspodelu katjona između tetraedarskih i oktaedarskih mesta, te formiranje delimično inverznog spinela. Ispitivanja magnetnih osobina su pokazala da histerezisne petlje ne pokazuju saturaciju u prisustvu jakog magnetnog polja, što je potvrdilo superparamagnetnu i jednodomensku prirodu čestica. Pokazano je da pored uticaja veličine čestica, dodatak različitih katjona (u ovom slučaju itrijum i indijum) ima veliki uticaj na uređenje strukture, a posledično i na magnetno ponašanje ispitivanih nanočestičnih sistema.This thesis presents the results of the investigation of the structural and magnetic properties of nanostructured zinc ferrites, ZnFe2O4 and zinc ferrites supstituted with different amount of indium and yttrium, Zn1-xInxFe2O4 and ZnYxFe2-xO4 (0 ≤ x ≤ 0,6). Powders based on zinc ferrites were synthesised by a low temperature wet-chemical method – coprecipitation. The main purpose of this thesis was to establish the relationship between the synthesis, dopants, structure and properties of zinc ferrite based materials. Nanoparticles of ZnFe2O4 were calcined at different temperatures in order to elucidate the influence of the particle size on the magnetic properties of the obtained nanoparticles. In addition, we have investigated the effect of dopant addition on cation distribution in spinel structure, in order to modify the magnetic properties and to obtain the magnetic ceramics with improved properties compared to the bulk-counterparts. The results of X-ray and TEM analyses confirmed the nanosized nature and spinel type structure of the investigated samples. Raman and Mössbauer spectroscopy studies implied on the possible cation distribution between the tetrahedral and octahedral sites and formation of the partially inversed spinel. The study of the magnetic properties showed that hysteresis loops do not saturate even in the presence of high magnetic fields, which confirmed the superparamagnetic and single domain nature of the samples. These observations imply that, besides the particle size, doping (e.g. yttrium and indium) causes significant structural rearrangements which in turn induce changes in magnetic behavior of the investigated nanoparticulate systems.
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Bezerra, Maria Jos? Oliveira da Silva. "S?ntese e caracteriza??o da ferrita de MnZn obtida pelo m?todo dos citratos precursores." Universidade Federal do Rio Grande do Norte, 2007. http://repositorio.ufrn.br:8080/jspui/handle/123456789/17778.
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It was synthesized MnZn ferrite with general formulae Mn1-xZnxFe2O4 (mol%), 0,3 ≤ x ≤ 0,7 by using the citrate precursor method. The precursors decomposition was studied by thermogravimetric analysis (TGA), differential thermogravimetric analysis (DTG), differential thermal analysis (DTA) and Fourier transform infrared (FTIR) of powder calcined at 350?C/3,5h. X-ray diffraction pattern (XRD) of samples was done from 350 to 1200?C/2h using various atmospheres. The power calcined at 350?C/3,5h formed spinel phase. It is necessary atmosphere control to avoid secondary phase such as hematite. From 900 to 1200?C was obtained 90,66 and 100% of MnZn spinel ferrite phase, respectively. Analysis by dispersive energy scanning (EDS) at 350?C shows high Mn and Zn dispersion, indicating that the diffusion process was homogeneous. Semi-quantitative analysis by EDS verified that despite the atmosphere control during calcinations at high temperatures (< 800?C) occurred ZnO evaporation causing stoichiometric deviation. Vibrating sample magnetometer (VSM) measures show soft ferrite material characteristics with Hc from 6,5 x 10-3 to 11,1 x 10-2 T. Saturation magnetization (Ms) and initial permeability (?i) of MnZn spinel phase obtained, respectively, from 14,3 to 83,8 Am2/kg and 14,1 to 62,7 (Am2/kg)T
Foi sintetizada ferrita de MnZn com f?rmula geral Mn1-xZnxFe2O4 (mol%) para 0,3 ≤ x ≤ 0,6 com uso do m?todo dos citratos precursores. A decomposi??o dos precursores foi estudada por an?lise termogravim?trica (TGA), termogravim?trica diferencial (DTG), an?lise t?rmica derivada (DTA) e espectroscopia na regi?o do infravermelho (FTIR) dos p?s calcinados em 350?C/3,5h. Foram feitas difra??es de raios X (DRX) entre 350 e 1200?C/2h usando v?rias atmosferas. O p? calcinado a 350?C/3,5h formou fase espin?lio. ? necess?rio o controle da atmosfera para evitar fases secund?rias como a hematita. Em 900 e 1200?C foram obtidos respectivamente, 90,7 e 100% de fase ferrita espin?lio. An?lises por espectroscopia de energia dispersiva (EDS) em 350?C mostrou alta dispers?o de Mn e Zn, indicando que o processo de difus?o foi homog?neo. A an?lise semi-quantitativa por EDS verificou que apesar do controle da atmosfera durante a calcina??o em altas temperaturas (> 800?C), ocorreu vaporiza??o de ZnO causando desvio estequiom?trico. Medidas por magnet?metro de amostra vibrante (MAV) mostraram caracter?sticas de materiais magneticamente macios com Hc de 6,5 x 10-3 at? 11,1 x 10-2 T. A magnetiza??o de satura??o e a permeabilidade inicial da fase espin?lio de MnZn foi obtida entre 14,3 a 83,8 Am2/kg e 14,1 a 62,7 (Am2/kg)T, respectivamente
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Silva, Jos? Eves Mendes da. "S?ntese an?lise das propriedades magn?ticas da ferrita de NiMg e caracter?sticas de absor??o de radia??o." Universidade Federal do Rio Grande do Norte, 2008. http://repositorio.ufrn.br:8080/jspui/handle/123456789/17600.
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It was synthesized different Ni1-xMgxFe2O4 (0,2 ≤ x ≤ 0,7) compositions by use of citrate precursor method. Initially, the precursory citrates of iron, nickel and magnesium were mixed and homogenized. The stoichiometric compositions were calcined from 350?C to 1200?C at ambient atmosphere or in argon atmosphere. The calcined powders were characterized by XRD, TGA/DTG, FTIR, magnetic measures and reflectivity using the wave guide method. I was observed pure magnetic phase formation between 350?C and 500?C, with formation of ferrite and hematite after 600?C at ambient atmosphere. The calcined powder at argon atmosphere formed pure ferromagnetic phase at 1100?C and 1200?C. The Rietveld analyses calculated the cations level occupation and the crystallite size. The analyses obtained nanometric crystals (11-66 nm), that at 900?C/3h presents micrometric sizes (0,45 - 0,70 Om). The better magnetization results were 54 Am2/Kg for x= 0,2 composition, calcined at 350?C/3h and 30 min, and 55,6 Am2/Kg for x= 0,2 1200?C, calcined in argon. The hysteresis shows characteristics of soft magnetic material. Two magnetization processes were considered, superparamagnetism at low temperature and the magnetic domains formation at high temperatures. The materials presented absorption less or equal the 50 % in ranges specific frequency. As for the 2,0 and 3,0 thickness (in 11,0 - 11,8 GHz), the reflectivity of the x= 0,3, 0,5 and 0,4 compositions, all calcined at 900?C/3h showed agreement with MS and O. Various factors contribute for the final radiation absortion effect, such as, the particle size, the magnetization and the polymer characteristics in the MARE composition. The samples that presented better magnetization does not obtaining high radiation absorption. It is not clear the interrelaction between the magnetization and the radiation absorption in the strip of frequencies studied (8,2 - 12,4 GHz)
Foram sintetizadas diferentes composi??es da ferrita Ni1-xMgxFe2O4 com (0,2 ≤ x ≤ 0,7) pelo uso do m?todo dos citratos precursores. Para se obter a fase estequiom?trica Ni1-xMgxFe2O4, inicialmente foram misturados e homogeneizados os citratos precursores de ferro, n?quel e magn?sio. As composi??es estequiom?tricas foram calcinadas entre as temperaturas de 350?C e 1200?C, em atmosfera ambiente ou de arg?nio. Os p?s calcinados foram caracterizados por DRX, TGA/DTG, FTIR, medidas magn?ticas e refletividade pelo m?todo de guia de ondas. Foi observada a forma??o de fase pura ferrimagn?tica entre 350?C e 500?C, formando ferrita e hematita ap?s 600?C, em atmosfera ambiente. O p? calcinado em atmosfera de arg?nio formou fase ferrimagn?tica pura em 1100?C e 1200?C. A an?lise pelo m?todo de Rietveld calculou o n?vel de ocupa??o dos c?tions e o tamanho de cristalito. A an?lise obteve tamanhos de cristais nanom?tricos, (11 - 66 nm), que a 900?C/3h apresentam tamanhos microm?tricos (0,45 0,70 Om). Os melhores resultados de magnetiza??o foram 54 Am2/Kg para a composi??o x= 0,2, calcinada a 350?C/3h e 30 min, e 55,6 Am2/Kg para x= 0,2 a 1200?C, calcinada em arg?nio. As histereses mostraram um perfil de materiais magn?ticos moles. Dois processos de magnetiza??o foram considerados, o superparamagnetismo a baixa temperatura e a forma??o de dom?nios magn?ticos em altas temperaturas. Os materiais apresentaram absor??o igual ou inferior a 50 % em faixas de freq??ncias espec?ficas. Para as espessuras 3,0 e 2,0 mm (em 11-11,8 GHz), as refletividades das composi??es x= 0,3, x= 0,5 e x= 0,4, todas a 900?C/3h demonstraram concord?ncia com MS e O. V?rios fatores contribuem para o efeito final de absor??o de radia??o, tais como, o tamanho de part?culas, a magnetiza??o, e as caracter?sticas do pol?mero na composi??o do MARE. As amostras que apresentaram maiores magnetiza??es n?o atingiram alta absor??o de radia??o. N?o ficou esclarecido a interrela??o entre a magnetiza??o e a absor??o de radia??o na faixa de freq??ncia estudada (8,2 12,4 GHz)
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Sabo, Daniel E. "Novel synthesis of metal oxide nanoparticles via the aminolytic method and the investigation of their magnetic properties." Diss., Georgia Institute of Technology, 2012. http://hdl.handle.net/1853/50122.
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Metal oxide nanoparticles, both magnetic and nonmagnetic, have a multitude of applications in gas sensors, catalysts and catalyst supports, airborne trapping agents, biomedicines and drug delivery systems, fuel cells, laser diodes, and magnetic microwaves. Over the past decade, an inexpensive, simple, recyclable, and environmentally friendly large, scale synthesis method for the synthesis of these metal oxide nanoparticles has been sought. Many of the current techniques in use today, while good on the small, laboratory bench scale, suffer from drawbacks that make them unsuitable for the industrial scale. The aminolytic method, developed by Dr. Man Han while working for Dr. Zhang, fits industrial scale-up requirements. The aminolytic method involves a reaction between metal carboxylate(s) and oleylamine in a non-coordinating solvent. This system was shown to produce a range of spinel ferrites. Dr. Lisa Vaughan showed that this method can be recycled multiple times without degrading the quality of the produced nanoparticles. The purpose of this thesis is to test the versatility of the aminolytic method in the production of a wide range of metal oxides as well as various core/shell systems. Chapter 2 explores the effect of precursor carboxylates chain length on the aminolytic synthesis of cobalt ferrite, and manganese ferrite nanoparticles. In Chapter 3, a series of CuxMn1-xFe₂O₄, (x ranges from 0.0 to 0.2), nanoparticles were synthesized via the aminolytic method. This series allows for the investigation of the effects of orbital Jahn-Teller distortion as well as orbital angular momentum on the magnetic properties of this ferrite. The quantum couplings of magnetic ions in spinel ferrites govern their magnetic properties and responses. An understanding of the couplings between these metal ions allows for tailoring magnetic properties to obtain the desired response needed for various applications. Chapter 4 investigates the synthesis of MnO and Mn₃O₄ nanoparticles in pure single phase with high monodispersity. To the best of our knowledge, the range of sizes produced for MnO and Mn₃O₄ is the most extensive, and therefore a magnetic study of these systems shows some intriguing size dependent properties. The final part of this chapter investigates the applicability of the aminolytic method for building a MnO shell on a CoFe₂O₄ core. Chapter 5 explores the synthesis of another metal oxide, ZrO₂ in both the cubic and monoclinic phases with no impurities. The use of the aminolytic method here removes the need for dangerous/expensive precursors or equipment and eliminates the need for extensive high temperature heat treatments that destroy monodispersity which is required for most techniques. The creation of a core/shell system between CoFe₂O₄ and ZrO₂ using the aminolytic method was also tested. This core/shell system adds magnetic manipulation which is especially useful for the recovery of zirconia based photocatalyst. Chapter 6 studies the application of the aminolytic method in the synthesis of yttrium iron garnet (YIG) and yttrium iron perovskite (YIP) nanoparticles. Current synthesis techniques used to produce YIG and YIP nanoparticles often requires high temperatures, sensitive to contamination, which could be eliminated through the use of our method
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Abdul, Latiff Hawa Alima Binti. "Magnetic anisotropy and coercivity of tetragonally distorted spinel ferrite particles via the Jahn-Teller distortion and the magnetoelastic coupling." Thesis, Université Grenoble Alpes (ComUE), 2019. http://www.theses.fr/2019GREAY005.
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Cette étude propose l'idée des aimants dits de ferrite tétragonale en rendant la symétrie cristalline des ferrites de spinelle cubique afin d'améliorer l'anisotropie magnétique (et donc, d'améliorer la coercivité). Pour concrétiser cette idée, nous avons synthétisé des particules (Cu, Co) -ferrite à distorsion tétragonale et caractérisé systématiquement les propriétés magnétiques en conséquence avec leurs distorsions de réseau. Les facteurs intrinsèques et extrinsèques contribuant à la coercivité ont été étudiés. Pour élucider l'anisotropie magnétique, nous avons démontré un modèle de couplage physique de l'effet Jahn-Teller (JT) et de l'effet magnétoélastique (ME) au sein de la théorie phénoménologique. Ensuite, nous avons effectué une analyse de coercivité dans deux modèles généraux de coercivité afin de clarifier les paramètres de la microstructure contribuant au mécanisme d'inversion de la magnétisation. À partir de l'analyse du modèle magnétoélastique, nous avons démontré l'expression linéaire de l'anisotropie magnétique en utilisant le paramètre tétragonal obtenu à partir de la distorsion JT. Les valeurs du coefficient magnétoélastique pour Cu (B1Cu = 2 MJ / m3) et Co (B1Co = 40 MJ / m3) déduites de la courbe expérimentale étaient acceptables avec la valeur calculée pour le ferrite de cuivre en vrac (B1Cu en vrac = 4 MJ / m3) et le cobalt. ferrite (masse B1Co = 55 MJ / m3). Les résultats suggèrent que l’anisotropie magnétique peut être attribuée au couplage de la distorsion JT avec l’effet magnétoélastique de Co. Au lieu d’une augmentation indéfinie avec x, l’anisotropie magnétique Ku tend à atteindre une valeur de saturation en raison de la concurrence entre les effet magnétoélastique de Co et le JT de Cu. Entre le x tétragonal x = 0,1 et le x cubique = 0,2, les valeurs de Ku constantes d'anisotropie magnétique intrinsèque ne varient pas de manière aussi significative que la différence entre les champs de coercivité et d'anisotropie. La réduction des champs d'anisotropie supérieurs à x = 0,1 peut alors être attribuée à l'augmentation de l'aimantation spontanée. L'analyse de la coercivité au sein du modèle micromagnétique a révélé une contribution importante à la coercivité de la microstructure et de l'effet démagnétisant local. Le paramètre de microstructure αMM = 0,25 obtenu était une valeur classique de l'analyse micromagnétique, suggérant le départ du champ d'anisotropie avec ce facteur de réduction. Les facteurs démagnétisants locaux effectifs NeffMM d’environ 1,4 obtenus étaient plutôt importants, ce qui suggère un effet démagnétisant significatif. Dans l'analyse du modèle global (GM), les valeurs de NeffGM obtenues étaient were 0,38 pour l'échantillon x = 0,1. La valeur négative suggère la présence d'une interaction d'échange agissant efficacement en opposition à l'interaction dipolaire. En deçà de 100 K, une différence dans le modèle suggère l’idée d’un réchauffement local consécutif à l’activation thermique due au changement d’énergie Zeeman et à une dissipation de chaleur inefficace. Cet événement peut avoir conduit à la réduction du champ coercitif à une température suffisamment basse dans l'échantillon x = 0.1 en supposant que les grains sont fortement couplés en échangeThis study proposes the idea of the so-called tetragonal ferrite magnets by rendering the crystal symmetry of the cubic spinel ferrites to enhance the magnetic anisotropy (and hence, enhance the coercivity). To realize this idea, we synthesized tetragonally distorted (Cu,Co)-ferrite particles and systematically characterized the magnetic properties accordingly with their lattice distortions. The intrinsic and extrinsic factors contributing to coercivity were investigated. To elucidate the magnetic anisotropy, we demonstrated a physical coupling model of the Jahn-Teller (JT) effect and the magnetoelastic (ME) effect within the phenomenological theory. Then, we performed coercivity analysis within two general models of coercivity to clarify the microstructure parameters contributing to the magnetization reversal mechanism. From the magnetoelastic model analysis, we demonstrated the linear expression of the magnetic anisotropy using the tetragonal parameter obtained from the JT distortion. The magnetoelastic coefficient values for Cu (B1Cu = 2 MJ/m3) and Co (B1Co = 40 MJ/m3) deduced from the experimental curve were agreeable with the value calculated for bulk copper ferrite (B1Cu bulk= 4 MJ/m3) and cobalt ferrite (B1Co bulk= 55 MJ/m3). The results suggests that the source of magnetic anisotropy can be attributed to the coupling of the JT distortion with the magnetoelastic effect of Co. Instead of an indefinite increase with x, the magnetic anisotropy Ku tends to reach a saturation value due to the competition between the magnetoelastic effect of Co and the JT effect of Cu. Between the tetragonal x = 0.1 and the cubic x = 0.2 samples, the intrinsic magnetic anisotropy constant Ku values do not vary as significantly compared to the difference in the coercivity and the anisotropy fields. The reduction of anisotropy fields above x = 0.1 then can be attributed to the increase in the spontaneous magnetization.The coercivity analysis within the micromagnetic model revealed significant contribution to the coercivity by the microstructure and the local demagnetizing effect. The microstructure parameter αMM = 0.25 obtained was a classical value in the micromagnetic analysis, suggesting the departure of anisotropy field with this reduction factor. The effective local demagnetizing factor NeffMM of about 1.4 obtained were rather large suggesting a significant demagnetizing effect. Within the global model (GM) analysis, the values of NeffGM obtained were －0.38 for the x = 0.1 sample. The negative value suggests the presence of an exchange interaction acting effectively in opposition to the dipolar interaction. Below 100 K, discrepancy in the GM suggests the idea of a local heating event following the thermal activation due to the change in Zeeman energy and ineffective heat dissipation. This event may have led to the reduction of coercive field at sufficiently low temperature in the x = 0.1 sample assuming the grains are strongly exchange-coupled
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Dondero, Russell A. "Silica coating of spinel ferrite nanoparticles." Thesis, Georgia Institute of Technology, 2000. http://hdl.handle.net/1853/27375.
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Brachwitz, Kerstin. "Defekt-induzierte Leitungsmechanismen und magnetische Eigenschaften spinellartiger Ferrite." Doctoral thesis, Universitätsbibliothek Leipzig, 2014. http://nbn-resolving.de/urn:nbn:de:bsz:15-qucosa-141251.
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Im Rahmen dieser Arbeit wurde der Einfluss von Defekten auf die Eigenschaften von Ferrit-Dünnfilmen untersucht. Die Dünnfilme wurden mit Hilfe von gepulster Laserabscheidung bei verschiedenen Züchtungsparametern hergestellt. Durch Variation der Substrattemperatur und des Sauerstoffpartialdrucks wurden Dünnfilme verschiedener kristalliner Qualität gezüchtet. Diese wurden hinsichtlich ihrer chemischen Komposition mit Hilfe von energie-dispersiver Röntgenspektroskopie und Röntgenphotoelektronenspektroskopie untersucht. Durch Korrelation der Ergebnisse mit Messungen zum zirkularen magnetischen Röntgendichroismus, konnte eine partielle Inversion der Spinellstruktur nachgewiesen werden. Der Grad der Inversion ist höher für geringe Abscheidetemperaturen. Für diese defektreichen Dünnfilme zeigen Röntgenbeugungsuntersuchungen eine geringere kristalline Ordnung der Dünnfilme. Die strukturellen Defekte haben einen maßgeblichen Einfluss auf die elektrischen und magnetischen Eigenschaften der Ferrit-Dünnfilme. So zeigen die Ferrit-Dünnfilme für geringe Züchtungstemperaturen eine erhöhte elektrische Leitfähigkeit, während Dünnfilme, die bei hohen Substrattemperaturen gezüchtet wurden, isolierend sind. Die Temperaturabhängigkeit der elektrischen Leitfähigkeit kann auf thermisch aktivierte Hopping-Leitung oder die Leitung zwischen Clustern, die in einer Matrix eingebettet sind, zurückgeführt werden.
Die magnetischen Eigenschaften von Zinkferrit-Dünnfilmen werden maßgeblich durch Defekte in der Spinellstruktur bestimmt, da es nominell in der normalen Spinellstruktur kristallisiert und daher antiferromagnetisch ist. Die partielle Inversion der Eisen- und Zinkionen führt zu Ferrimagnetismus in den Zinkferrit-Dünnfilmen, der mit Hilfe von SQUID-Messungen in dieser Arbeit eingehend untersucht wurde.
Durch Korrelation der Ergebnisse der verschiedenen Untersuchungsmethoden konnten Rückschlüsse auf die dominierenden Defekte in den Ferrit-Dünnfilmen geschlossen werden. So sind zum einen Defekte auf atomarer Skala, wie Antisite-Defekte und divalenten Fe-Ionen für die erhöhte elektrische Leitfähigkeit und die größere Magnetisierung der defektreichen Dünnfilme verantwortlich. Zum anderen können ausgedehnte Defekte, im Speziellen Cluster, die in einer amorphen Matrix eingebettet sind, nicht ausgeschlossen werden.
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Anderson, Richard M. "Magneto-optical properties of superparamagnetic spinel ferrite nanoparticles." Diss., Georgia Institute of Technology, 2002. http://hdl.handle.net/1853/30027.
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Rondinone, Adam Justin. "Superparamagnetic relaxation dynamics of magnetic spinel ferrite nanoparticles." Diss., Georgia Institute of Technology, 2001. http://hdl.handle.net/1853/30958.
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Vaughan, Lisa Ann. "Enviromentally benign synthesis and application of some spinel ferrite nanopartilces." Diss., Georgia Institute of Technology, 2011. http://hdl.handle.net/1853/44876.
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In this thesis, the commercial viability of the aminolytic synthesis method is explored through robustness, versatility, and waste reduction studies. We report the preparation of metal precursors and the development of a synthetic approach using an aminolytic reaction of metal carboxylates in oleylamine and non-coordinating solvent. Manganese doping in the cobalt ferrites allows for the investigation of the couplings. All the compositions in the series Co1-xMnxFe2O4, 0.0 x 1.0 were synthesized via the aminolytic reaction. The coercivity decreases with increasing Mn2+ concentration due to reducing of high magnetic anisotropy ion (Co2+) content. To our knowledge, this work is the first completed series of Co1-xMnxFe2O4. The method is used to synthesize manganese ferrites dope with chromium. This allows for the investigation of the effects of orbital momentum quantum coupling. All the compositions of MnFe2-xCrxO4, x= 0.0, 0.05, 0.13, 0.25, 0.43, 0.62, and 0.85, were synthesized via the In-situ aminolytic method. Chromium concentration weakens the couplings resulting in the decrease in overall magnetic moment. All by-products can be recycled for re-utilization. The "mother" solution can be used for multiple batches without treatment. Our trials have shown that the reaction could undergo ten reactions using the same solution without scarifying the quality or yield of the product. Finally, an environmental application is explored through the use of iron oxides. Samples of goethite, maghemite, magnetite, and hematite were synthesized and characterized. These nanoparticles were exposed to arsenic and chromium solutions to measure the percent uptake of contaminant by each phase. Adsorption isotherms were plotted to obtain Freundlich parameters. The adsorption constant (K) averages over a 400% increase on literature values. We synthesized hematite and maghemite core-shell particles and exposed them to arsenite and maghemite core-shell particles have the higher removal affinity due to their smaller size.
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Heintz, Eva Liang-Huang. "Surface Biological Modification and Cellular Interactions of Magnetic Spinel Ferrite Nanoparticles." Diss., Georgia Institute of Technology, 2004. http://hdl.handle.net/1853/4944.
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Surface Biological Modification and Cellular Interactions of Magnetic Spinel Nanoparticles
Eva Liang-Huang Heintz
191 Pages
Directed by Dr. Z. John Zhang
The interest in magnetic nanoparticles is multi-dimensional. Fundamentally, it is important to be able to control their magnetic properties and to correlate to specific applications. In biology, magnetic nanoparticles offer promising potential as magnetic carriers or chaperones for magnetic localization and manipulation of therapeutic reagents.
The synthesis of superparamagnetic CoFe2-xSmxO4 nanoparticles and the tunability of their magnetic properties by size and composition variations are discussed. An increase in size of CoSm0.19Fe1.81O4 nanoparticles produced an increase in blocking temperature and saturation magnetization, but a non-linear coercitivity response was observed with change in size. By varying the composition, the saturation magnetization of CoFe2-xSmxO4 decreased dramatically while the coercitivity increased when compared to native cobalt spinel ferrite (CoFe2O4) nanoparticles. These results demonstrate how the magnetic properties of cobalt spinel ferrite nanoparticles can be tailored to specific applications.
Surface modifications of cobalt spinel ferrite nanoparticles facilitated the conjugation of oligonucleotides. Using a transfection reagent, CoFe2O4 ??igonucleotide conjugates were delivered into mammalian cells. Post transfection, synchronized movement of cells in response to an external magnetic field was observed. This demonstrated the possibility of magnetic manipulation and localization of therapeutic reagents coupled to CoFe2O4 magnetic nanoparticles.
Results from this thesis demonstrate the potential role of magnetic spinel nanoparticles in cell biology and will facilitate the progress towards in vivo testing.
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Cripps, Chala Ann. "Synthesis and characterization of cobalt ferrite spinel nanoparticles doped with erbium." Thesis, Georgia Institute of Technology, 2003. http://hdl.handle.net/1853/30855.
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Liu, Chao. "Correlation between superparamagnetic properties and chemical composition in some spinel ferrite nanoparticles." Diss., Georgia Institute of Technology, 2001. http://hdl.handle.net/1853/30391.
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Silva, Maya Dayana Penha da. "SÍNTESE E CARACTERIZAÇÃO DO SISTEMA ESPINÉLIOS CO1-XCUXFE2O4 (X = 1,0, 0,75,0,50, 0,25, 0,0) PELO MÉTODO DOS PRECURSORES POLIMÉRICOS." Universidade Federal do Maranhão, 2012. http://tedebc.ufma.br:8080/jspui/handle/tede/938.
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Ferrites are complex oxides, thermally stable, that presents crystalline structures equivalent to the spinel mineral.These solids alsoretain significant dielectric, magneticand catalytic properties. Thus, this work aims to obtain Co1-xCuxFe2O4spinel nanoparticles (x = 0.00, 0.25, 0.50, 0.75, 1.00) by the polymeric precursor method as well as evaluates the effect of its thermal treatment (700 - 1200 °C) over the crystalline structure. Therefore, all spinel structures were characterized by means of Raman and infrared vibrational spectroscopies and X-ray powder diffraction (XRPD). According to the X-ray diffraction patters, all the primary CoFe2O4 adopts an ideal inverse spinel structure [(Fd3 ̅m (O_h^7))], whereas the CuFe2O4 materials preferentially assume tetragonal distorted spinel structure [(I41/amd(D_4h^19))], except the 700 and 800 oC annealed CuFe2O4, that was indeed hematite. Besides, such case the FWHM analysis has indicates that the ternary thermal treatment leads to a higher organization of the lattice, as well as, increases the crystallite sizes. In general, the average crystallite sizes for CuFe2O4 varies between 27 35 nm; whereas, for the CoFe2O4 and Co-substituted CuF2O4 are respectively 28 37 nm and 33.3 52.6 nm. The crystal microdeformation oscillates between 0.17 and 0.32%, reducing, in most cases, with the thermal evolution. Raman spectra for CuFe2O4 samples shows vibrational modes at about 166 (F2g), 456 (F2g), 519 (F2g), 217 (Eg), e 686(A1g) cm-1. In other hand, CoFe2O4 samples present vibrational modes at 195, 457 and 587 cm-1 (all F2g symmetries), 298 cm-1 (A1g) and 671 cm-1(Eg). Similar optical-active modes were detected for the Co-substituted ferrites. Conversely, FTIR spectra for all CuFe2O4 shows characteristic stretching at about 531.79 559.45 cm-1 (Fe-O)A, 605.18 672.12 cm-1 (Cu-O)Aand 474.69 397.33 cm-1 (Fe-O)B. However, the CoFe2O4 samples exhibits strong stretching modes at 581.70 578.51 cm-1 (Co-O)A. Similar modes were also observed for the intermediary ferrites. Finally, the UV-vis spectra for CuFe2O4 shows 6A1g → 4Eg (G) (Fe3+ )B, and 6A1g → 4Eg (G) (Cu2+)Atransitions between 2.14 2.72 eV and 1.76 1.80 eV, respectively. However, the CoFe2O4 ferrites presents6A_1g → 2A_2g(P)(Co2+)A electronic transitions 2.11 and 2.39 eV.
As ferritas são óxidos compostos, termicamente estáveis, que apresentam estruturas cristalinas equivalentes as do mineral espinélio. Estas retêm propriedades dielétricas, magnéticas e catalíticas específicas. Neste sentido, este trabalho objetiva avaliar a síntese de estruturas espinélios Co1-xCuxFe2O4 (x = 1,0, 0,75, 0,25, 0,0) pelo método do precursores poliméricos (MPP), em diferentes temperaturas. Para tanto, tais sistemas foram caracterizados por difração de raios X (DRX) e espectroscópicas na região do infravermelho com transformada de Fourier (FTIR), Raman e UV-visível. Os resultados de DRX sugerem a formação de estruturas monofásicas CuFe2O4[(I41/amd(D_4h^19))] e CoFe2O4[(Fd3 ̅m (O_h^7))], a exceção das amostras CuFe2O4 700 e 800 oC.O parâmetro FWHM indica que o aumento da temperatura de calcinação ternária gera uma melhor organização estrutural a longo alcance. Ademias, o tamanho médio dos cristalitos aumenta com a temperatura de calcinação ternária devido a maior difusão dos íons na rede cristalina. Os tamanhos médios dos cristalitos variam entre 27 35 nm (CuFe2O4), 28 37 nm (CoFe2O4) e 33,3 - 52,6 nm(CuFe2O4 substituídas). As microdeformações da rede cristalina variaram entre 0,17 e 0,32%, diminuindo, na maioria dos casos, com a evolução da temperatura de tratamento. Os espectros Raman das amostras CuFe2O4exibem modos vibracionais próximos a 166 (F2g), 456 (F2g) , 519 (F2g), 217 (Eg), e 686(A1g) cm-1. Por outro lado, as estruturas CoFe2O4 demonstraram modos vibracionais F2g (195, 457 e 587 cm-1), A1g(298 cm-1) e Eg(671 cm-1). Modos vibracionais equivalentes foram também detectados nas estruturas intermediárias. Os espectros FTIR das CuFe2O4 exibiram estiramentos vibracionais nas regiões de 531,79 - 559,45 cm-1 (Fe-O)A, 605,18 - 672,12 cm-1(Cu-O)Ae 474,69 - 397,33 cm-1(Fe-O)B. Já as CoFe2O4 demonstraram uma forte absorção entre 581,70 - 578,51 cm-1(Co-O)A. Tais modos vibracionais também foram observados nas ferritas intermediarias. Os espectros UV-vis das CuFe2O4 exibiram transições 6A1g → 4Eg (G) (Fe3+ )B, entre 2,14 2,72 eV, e 6A1g → 4Eg (G) (Cu2+)A, entre 1,76 1,80 eV. Já as CoFe2O4 apresentam transições 6A_1g → 2A_2g(P)(Co2+)A entre 94, 2,11 e 2,39 eV.
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Samia, Anna Cristina S. "Design and control of the superparamagnetic properties of cobalt-based spinel ferrite nanoparticles." Diss., Georgia Institute of Technology, 2002. http://hdl.handle.net/1853/31048.
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Haffer, Stefanie [Verfasser]. "Mesoporous spinel-type cobalt oxide, cobalt ferrite and alumina by nanocasting / Stefanie Haffer." Paderborn : Universitätsbibliothek, 2014. http://d-nb.info/1046651994/34.
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Assis, Raimison Bezerra de. "S?ntese e caracteriza??o de ferrita ni-zn pelo m?todo dos precursores polim?ricos calcinada por energia de microondas." Universidade Federal do Rio Grande do Norte, 2013. http://repositorio.ufrn.br:8080/jspui/handle/123456789/12816.
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Magnetic ceramics have been widely investigated, especially with respect to intrinsic and extrinsic characteristics of these materials. Among the magnetic ceramic materials of technological interest, there are the ferrites. On the other hand, the thermal treatment of ceramic materials by microwave energy has offered various advantages such as: optimization of production processes, high heat control, low consumption of time and energy among others. In this work were synthesized powders of Ni-Zn ferrite with compositions Ni1- xZnxFe2O4 (0.25 ≤ x ≤ 0.75 mols) by the polymeric precursor route in two heat treatment conditions, conventional oven and microwave energy at 500, 650, 800 and 950?C and its structural, and morphological imaging. The materials were characterized by thermal analysis (TG/ DSC), X-ray diffraction (XRD), absorption spectroscopy in the infrared (FTIR), scanning electron microscopy (SEM), X-ray spectroscopy and energy dispersive (EDS) and vibrating sample magnetometry (VSM). The results of X-ray diffraction confirmed the formation of ferrite with spinel-type cubic structure. The extrinsic characteristics of the powders obtained by microwave calcination and influence significantly the magnetic behavior of ferrites, showing particles ferrimagn?ticas characterized as soft magnetic materials (soft), is of great technological interest. The results obtained led the potential application of microwave energy for calcining powders of Ni-Zn ferrite
Cer?micas magn?ticas t?m sido amplamente investigadas, principalmente com rela??o as caracter?sticas intr?nsecas e extr?nsecas destes materiais. Dentre os materiais cer?micos magn?ticos de interesse tecnol?gico, destacam-se as ferritas. Por outro lado, o tratamento t?rmico de materiais cer?micos por energia de microondas tem oferecido diversas vantagens, tais como: otimiza??o dos processos de obten??o, alto controle de aquecimento, menor consumo de tempo e energia entre outras. Neste trabalho, foram sintetizados p?s de ferritas Ni-Zn com composi??es Ni1-xZnxFe2O4 (0,25 ≤ x ≤ 0,75 mols) pela rota dos precursores polim?ricos em duas condi??es de tratamento t?rmico, em forno convencional e por energia de microondas a 500, 650, 800 e 950?C, e sua caracteriza??o estrutural, morfol?gica e magn?tica. Os materiais obtidos foram caracterizados por an?lises t?rmicas (TG/DSC), difra??o de raios x (DRX), espectroscopia de absor??o na regi?o do infravermelho (FTIR), microscopia eletr?nica de varredura (MEV), espectroscopia de raios X por dispers?o de energia (EDS) e magnetometria de amostra vibrante (VSM). Os resultados da difra??o de raios X confirmaram a forma??o da ferrita com estrutura c?bica do tipo espin?lio. As caracter?sticas extr?nsecas dos p?s obtidos e a calcina??o por microondas influenciaram de forma significativa o comportamento magn?tico das ferritas, apresentando part?culas ferrimagn?ticas, caracterizadas como materiais magn?ticos moles (macias), sendo de grande interesse tecnol?gico. Os resultados obtidos induziram o potencial de aplica??o de energia de microondas para a calcina??o de p?s de ferritas Ni-Zn
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Sorrentino, Neto Antônio. "Síntese, caracterização e propriedades magnéticas de Ferritas de níquel – cobre." Universidade Federal da Paraíba, 2016. http://tede.biblioteca.ufpb.br:8080/handle/tede/9163.
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Coordenação de Aperfeiçoamento de Pessoal de Nível Superior - CAPES
The ceramic oxides that exhibit ferromagnetic behavior are important commercial products for electronics industries and are commonly known as ferrites. In this work, we synthesized different compositions of CuxNi1-xFe2O4 ferrite with x = 0,0, 0,2, 0,4, 0,6, 0,8 and 1,0 by using the combustion method starting from the iron nitrate, nickel and copper and urea as a reducing agent. The powders were characterized by determination of specific surface area by the method (BET), X-ray diffraction (XRD), refinement of XRD data by the Rietveld method and vibrating sample magnetometry (MAV). The obtained copper nickel ferrites, referring x = 0,0, 0,2, 0,4, 0,6 synthesized had the same number of peaks corresponding to the single phase-forming powders that have been indexed to the spinel ferrite phase, forming a cubic phase and x = 0,8 to 1,0 tetragonal phase formed enhancing the efficiency of the applied method of synthesis. The refinement by the Rietveld method resulted in a determination of cell parameter of the samples, the cubic phase the value 0.8359 to 0.8340 nm and the crystallite size whose values occur between 239.62 to 187.24 nm. While in the tetragonal phase the cell parameter a = b was 5.8253 to 5,8230nm c = 8.5693 to 8.6941 nm. The hysteresis curves showed magnetic materials profile. The nickel ferrite - synthesized copper proved as materials with high potential for applicability in different areas of high-frequency devices such as phones and components for computers.
Os óxidos cerâmicos que exibem comportamento ferrimagnético representam importantes produtos comerciais para indústrias de eletrônica e são comumente conhecidos por ferritas. Neste trabalho, foram sintetizadas diferentes composições da ferrita CuxNi1-xFe2O4 com X = 0,0, 0,2, 0,4, 0,6, 0,8 e 1,0 pelo uso do método de combustão partindo-se dos nitratos de ferro, níquel e cobre e ureia como agente redutor. Os pós obtidos foram caracterizados por determinação de área superficial específica pelo método (BET), difração de raio X (DRX), refinamento dos dados de DRX pelo método Rietveld e magnetometria de amostra vibrante (MAV). As ferritas de níquel cobre obtidas, referentes x = 0,0, 0,2, 0,4, 0,6 sintetizadas apresentaram o mesmo conjunto de picos correspondentes à formação de pós monofásicos que foram indexados à fase espinélio de ferrita, formando uma fase cubica e para x = 0,8, 1,0 formou fase tetragonal predominante reforçando a eficiência do método de síntese aplicado. O refinamento pelo método de Rietveld resultou na determinação de parâmetro de cela das amostras, da fase cúbica o valor de 0,8359 a 0,8340 nm e do tamanho de cristalito cujos valores ocorrem entre 239,62 a 187,24 nm. Enquanto na fase tetragonal o parâmetro de cela a=b foi 5,8253 a 5,8230nm e c= 8,5693 a 8,6941 nm. As curvas de histerese mostraram perfil de materiais magnéticos. As ferritas de níquel - cobre sintetizadas se mostraram como materiais com alto potencial de aplicabilidade em diferentes áreas de equipamentos de alta frequência como telefonia e de componentes para de computadores.
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Myndyk, Maxym, Mohammad Javad Nasr Isfahani, Dirk Menzel, Armin Feldhoff, Jamshid Amighian, Klaus-Dieter Becker, and Vladimír Šepelák. "Local structure and magnetic behavior of nanocrystalline MnZn spinel ferrite prepared by a one-step mechanochemical route." Universitätsbibliothek Leipzig, 2015. http://nbn-resolving.de/urn:nbn:de:bsz:15-qucosa-188008.
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Myndyk, Maxym, Mohammad Javad Nasr Isfahani, Dirk Menzel, Armin Feldhoff, Jamshid Amighian, Klaus-Dieter Becker, and Vladimír Šepelák. "Local structure and magnetic behavior of nanocrystalline MnZn spinel ferrite prepared by a one-step mechanochemical route." Diffusion fundamentals 12 (2010) 73, 2010. https://ul.qucosa.de/id/qucosa%3A13902.
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Lüders, Ulrike Anne. "Development and integration of oxide spinel thin films into heterostructures for spintronics." Doctoral thesis, Universitat Autònoma de Barcelona, 2005. http://hdl.handle.net/10803/3373.
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En esta memoria se describe el crecimiento, mediante pulverización catódica rf, de capas delgadas de NiFe2O4 y CoCr2O4 sobre distintos substratos y la subsiguiente caracterización magnética y eléctrica. El objetivo es integrar dichas capas en dispositivos magnetoelectrónicos tales como uniones túnel o filtros de spin.Hemos descubierto que el crecimiento epitaxial permite estabilizar fases nuevas del óxido NiFe2O4, fases que no existen en la forma másiva, y que tienen propiedades remarcablemente distintas. Como por ejemplo: un aumento dramático de la magnetización o la posibilidad de modificar drásticamente sus propiedades de transporte, pudiéndose obtener capas aislantes -como es en forma cerámica- o conductivas. Se ha realizado un estudio sistemático de los efectos del espesor de la capa y de las condiciones de crecimiento sobre las propiedades de magnetotransporte y los mecanismos de crecimiento.
Argumentamos que el aumento de la magnetización es debido a la estabilización de una fase NiFe2O4 espinela que es parcialmente inversa, en la que los iones Ni2+ están distribuidos entre las dos posiciones disponibles (tetraédrica y octaédrica) de la estructura. En la forma masiva del material los iones Ni solo se encuentran en los sitios octaédricos. La introducción adicional de vacantes de oxígeno es probablemente la causa de la existencia de una configuración electrónica mixta Fe2+/3+ en la subred octaédrica y de la alta conductividad de las capas.
Hemos aprovechado la capacidad de obtener epitaxias de NiFe2O4 ferrimagnéticas conductoras o aislantes para integrarlas en dos distintos dispositivos magnetoelectrónicos: una unión túnel magnética y un filtro de spin.
Las capas conductoras de NiFe2O4 se han empleado como electrodos ferrimagnéticos-metálicos en uniones túnel. El otro electrodo magnético es (La,Sr)MnO3 y la barrera túnel SrTiO3. Se ha podido medir una magnetoresistencia túnel importante hasta temperaturas tan altas como 280K. Los valores de magnetoresistencia corresponden a una polarización de spin del NiFe2O4 de aproximadamente un 40%, que es prácticamente independiente de la temperatura. Estos resultados sugieren que la nueva fase conductora que hemos estabilizado es un candidato interesante como fuente de corriente polarizada en spin.
Por otra parte, el NiFe2O4 aislante se ha implementado, por primera vez, como barrera túnel en una heteroestructura de filtro de spin. El electrodo magnético es (La,Sr)MnO3 y el electrodo no magnético Au. Hemos observado una magnetoresistencia túnel que alcanza valores de hasta un 50%. A partir de estas medidas, hemos deducido detalles relevantes de la estructura electrónica de la fase parcialmente inversa de NiFe2O4.
Hemos crecido el óxido CoCr2O4 sobre distintos substratos, tales como MgO(001) y MgAl2O4(001). Hemos podido comprobar que este óxido presenta una pronunciada tendencia a un crecimiento 3D. Por esta razón, las superficies de la capa no son nunca suficientemente planas y no se pueden usar en heteroestructuras túnel.
Sin embargo hemos aprovechado esta característica para controlar el crecimiento de estas estructuras 3D y hemos conseguido la formación de objetos submicrónicos, autoorganizados con formas piramidales muy bien definidas. El estudio detallado del efecto de los parámetros de crecimiento nos ha permitido por una parte, dilucidar cuales son los mecanismos que llevan a una autoorganización tan perfecta y por otra determinar que, en las condiciones adecuadas, se pueden obtener templates totalmente faceteados con múltiples posibilidades para futuras aplicaciones.
In this thesis the growth of thin films of NiFe2O4 and CoCr2O4 by RF sputtering on different oxide substrates and the characterization of their magnetic and electric properties is reported. The aim is to integrate the films into spintronic devices namely magnetic tunnel junctions and spin filter.
It was found that the epitaxial growth of these films permits to stabilize new phases of NiFe2O4, which are not found for the bulk material and which show remarkably distinct properties. A strong enhancement of the saturation magnetization was found as well as the possibility to tune the electric behaviour of the films from insulating - like in bulk NiFe2O4 - to conducting. A systematic study of the influence of the film thickness and growth parameters on the properties of the films was carried out.
The enhancement of the saturation magnetization can be explained by a partially inversed spinel structure, where the Ni2+ ions are distributed over both available sites (octahedral and tetrahedral) of the structure, whereas in bulk NiFe2O4 the Ni2+ ions are only located on the octahedral sites of the structure. An additional introduction of oxygen vacancies causes the formation of mixed valence Fe2+/3+ chains on the octahedral sites and thus a hopping conductivity.
We have taken advantage of our ability to obtain epitaxial ferromagnetic NiFe2O4 films of insulating or conducting character to integrate them in two different spintronic devices: the magnetic tunnel junction and the spin filter.
The conducting NiFe2O4 was integrated in a magnetic tunnel junction as a magnetic electrode, with a (La,Sr)MnO3 counterelectrode and a SrTiO3 barrier. A magnetoresistance was measured up to a temperature of 280K. The values of the magnetoresistance correspond to a spin-polarization of 40%, which is basically constant in temperature. This results show that the conductive phase of NiFe2O4 is an interesting candidate for the application as a source of highly spin-polarized current.
On the other hand the insulating NiFe2O4 has been integrated into a spin filter as the magnetic barrier. The magnetic electrode was again (La,Sr)MnO3 and the counter electrode Au. A magnetoresistance up to 50% was observed. It was possible to deduce the band structure of NiFe2O4 from these measurements.
Thin films of CoCr2O4 were grown on different substrates like MgO(001) or MgAl2O4(001). It was found that the material shows a pronounced tendency to grow in a three dimensional manner. Thus the surface of these films is not sufficiently smooth to integrate them into tunnel contacts.
However, we were able to control the growth and morphology of the three dimensional structures leading to the formation of submicron self-organized pyramids with a square or elongated base. By a detailed study of the influence of the growth parameters it was possible to elucidate the underlying growth mechanisms and to obtain a fully faceted surface, which can be used in different applications.
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Rodrigues, ?dila Priscilla Gomes. "S?ntese e caracteriza??o de ferritas de n?quel dopadas com cobalto e efeito da substitui??o nas suas propriedades magn?ticas." Universidade Federal do Rio Grande do Norte, 2014. http://repositorio.ufrn.br:8080/jspui/handle/123456789/17766.
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Previous issue date: 2014-12-11Coordena??o de Aperfei?oamento de Pessoal de N?vel Superior
The ferrite composition Ni1 - xCoxFe2O4 (0 ≤ x ≤ 0.75) were obtained by the method of microwave assisted synthesis and had their structural and magnetic properties evaluated due to the effect of the substitution of Ni by Co. The compounds were prepared: according to the concept of chemical propellants and heated in the microwave oven with power 7000kw. The synthesized material was characterized by absorption spectroscopy in the infrared (FTIR), Xray diffraction (XRD) using the Rietveld refinement, specific surface area (BET) , scanning electron microscopy (SEM) with aid of energy dispersive analysis (EDS) and magnetic measurements (MAV). The results obtained from these techniques confirmed the feasibility of the method of synthesis employed to obtain the desired spinel structure, the ferrite, nickel ferrite as for nickel doped with cobalt. The results from XRD refinement ally showed the formation of secondary phases concerning stages α - Fe2O3, FeO, (FeCo)O e Ni0. On the other hand, there is an increase in crystallite size with the increase of cobalt in systems, resulting in an increased crystallinity. The results showed that the BET systems showed a reduction in specific surface area with the increase of cobalt and from the SEM, the formation of irregular porous blocks and that the concentration of cobalt decreased the agglomerative state of the system. The magnetic ferrites studied showed different characteristics according to the amount of dopant used, ranging from a very soft magnetic material (easy magnetization and demagnetization ) - for the system without cobalt - a magnetic material with a little stiffer behavior - for systems containing cobalt. The values of the coercive field increased with the increasing growth of cobalt, and the values of saturation magnetization and remanence increased up to x = 0,25 and then reduced. The different magnetic characteristics presented by the systems according to the amount of dopant used, allows the use of these materials as intermediates magnetic
As ferritas de composi??o Ni1−xCoxFe2O4 (0≤x≤0,75) foram obtidas atrav?s do m?todo de s?ntese assistida por microondas e tiveram suas propriedades estruturais e magn?ticas avaliadas em fun??o do efeito da substitui??o do Ni pelo Co. Os compostos foram preparados de acordo com o conceito da qu?mica dos propelentes e aquecidos em forno micro-ondas com pot?ncia 7000kw. O material sintetizado foi caracterizado por espectroscopia de absor??o na regi?o do infravermelho (FTIR), difra??o de raios X (DRX), com o uso do refinamento pelo m?todo de Rietveld, ?rea superficial espec?fica (BET), microscopia eletr?nica de varredura (MEV) com aux?lio de an?lise por dispers?o de energia (EDS) e medidas magn?ticas (MAV). Os resultados obtidos, a partir destas t?cnicas confirmaram a viabilidade do m?todo de s?ntese empregado para a obten??o da estrutura espin?lio desejada, tanto para a ferrita de n?quel quanto para as ferritas de n?quel dopadas com cobalto. Os resultados do DRX, aliado ao refinamento, mostraram a forma??o de fases secund?rias tais como as fases α - Fe2O3, FeO, (FeCo)O e Ni0. Por outro lado, observou-se um aumento no tamanho do cristalito com o incremento do cobalto nos sistemas, implicando em um aumento da cristalinidade. Os resultados do BET mostraram que os sistemas apresentaram uma redu??o da ?rea superficial espec?fica com o incremento do cobalto e a partir do MEV observou-se a forma??o de blocos porosos irregulares e que o aumento da concentra??o de cobalto dimunuiu o estado de aglomera??o dos sistemas. As ferritas estudadas apresentaram caracter?sticas magn?ticas diferenciadas de acordo com a quantidade do dopante utilizado, variando de um material magn?tico bastante mole (f?cil magnetiza??o e desmagnetiza??o) - para o sistema sem cobalto - a um material magn?tico com comportamento um pouco mais duro - para os sistemas contendo cobalto. Os valores do campo coercitivo aumentaram com o crescente incremento do cobalto, e os valores de magnetiza??o de satura??o e reman?ncia aumentaram at? x=0,25 e depois reduziram. As caracter?sticas magn?ticas diferenciadas apresentadas pelos sistemas, de acordo com a quantidade de dopante utilizado, permite o uso desses materiais como magn?ticos intermedi?rios
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Silva, Jos? Eves Mendes da. "S?ntese, caracteriza??o de ferritas espin?lio com propriedades magn?ticas e absorvedoras de microondas." Universidade Federal do Rio Grande do Norte, 2013. http://repositorio.ufrn.br:8080/jspui/handle/123456789/17768.
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Were synthesized spinel-type ferrites with general formula Ni0,8Mg0.2-xMxFe2O4, where M represents the doping Mn, Co or Mn + Co simultaneously, x ranges for the values 0.02, 0.05 and 0.1. The value of x was divided by 2 in cases where M equals Mn and Co conjugates. We used the citrate precursor method and heat treatment to obtain the phases at 1100?C. The materials were characterized by XRD, TGA/ DTGA, SEM, MAV and reflectivity measurements by the method of waveguide. Powders to 350?C/3.5 h were crystalline and nanosized. According to the results this temperature all powders have a percentage of ferrite phase over 90%. The composition had the addition of Mn and Co simultaneously showed a higher percentage of secondary phase NiO, 5.8%. The TGA/DTGA curves indicate that this sample reached phase (s) crystalline (s) at lowest temperatures. The X-ray diffractograms of the samples calcined at 350?C and 1100?C were treated with the Rietveld refinament technique. The powders calcined at 1100 ?C/3h in air show to be 100% except spinel phase composition with 0.02 doping. The micrographs show clusters of particles with sizes smaller than 1 μm in calcination temperature of 1100?C which agreed with the result of Rietveld refinement. In the compositions doped with Mn were higher values of magnetization (45.90 and 53.20 Am2/kg), which did not cause high microwave absorption. The theoretical calculation of magnetization (MT) was consistent with the results, considering that there was agreement between the increase of magnetization experimental and theoretical. It was observed that there was the interrelation of the final effect of absorption with the thickness of MARE, the composition of ferrimagnetic materials and in particular the specific values of frequency. The analysis shows that the reflectivity increases in the concentration of cobalt increased the frequency range and also for absorption 10.17 GHz and 84%, respectively. The best result of chemical homogeneity and the value of 2.96 x 10-2 tesla coercive field were crucial for high performance ferrite absorber with 0.1 cobalt. The Cobalt has high magnetocrystalline anisotropy, it is associated with an increased coercive field, Hc. Therefore, this property improves the results of reflectivity of spinel ferrites
Foram sintetizadas ferritas do tipo espin?lio com f?rmula geral Ni0,8Mg0,2-xMxFe2O4, onde M representa os dopantes Mn, Co ou Mn + Co simultaneamente, x varia para os valores 0,02; 0,05 e 0,1. O valor de x foi dividido por 2 nos casos em que M equivale a Mn com Co conjugados. Foi utilizado o m?todo dos citratos precursores e tratamento t?rmico na obten??o das fases a 1100?C. Os materiais foram caracterizados por DRX, TGA/DTGA, MEV, MAV e medidas de refletividade pelo m?todo de guia de ondas. Os p?s a 350?C/3,5h foram cristalinos e nanom?tricos. De acordo com os resultados nesta temperatura todos os p?s apresentam um percentual de fase ferrita superior a 90%. A composi??o que teve a adi??o de Mn e Co simultaneamente apresentou maior percentual de fase secund?ria NiO, 5,8%. As curvas TGA/DTGA indicam que essa amostra alcan?ou fase(s) cristalina(s) em temperaturas mais baixas. Os difratogramas de raios X das amostras calcinadas a 350?C e 1100?C foram tratados pela t?cnica de refinamento Rietveld. Os p?s calcinados a 1100?C/3h em atmosfera ambiente mostram ser 100% de fase espin?lio exceto a composi??o com 0,02 de dopantes. As micrografias apresentam aglomerados de part?culas com tamanhos menores que 1 μm na temperatura de calcina??o de 1100?C, o que concordou com o resultado de refinamento Rietveld. Nas composi??es dopadas com Mn ocorreram os maiores valores de magnetiza??o (45,90 e 53,20 Am2/kg), o que n?o acarretou alta absor??o de microondas. O c?lculo te?rico da magnetiza??o (MT) foi coerente com os resultados, considerando-se que houve concord?ncia entre o aumento das magnetiza??es experimental e te?rica. Observa-se que houve interrela??o do efeito final de absor??o com a espessura do MARE, a composi??o do material ferrimagn?tico em particular e os valores espec?ficos de freq??ncia. A an?lise de refletividade indica que o aumento na concentra??o de cobalto aumentou a faixa de freq??ncia e a absor??o para 10,17 GHz e 84%, respectivamente. O resultado indica que a melhor homogeneidade qu?mica e o valor 2,96 x 10-2 tesla de campo coercitivo foram determinantes para o alto desempenho absorvedor da ferrita com 0,1 de cobalto. O Cobalto tem alta anisotropia magnetocristalina, isto est? associado ao aumento do campo coercitivo, Hc. Portanto, essa propriedade melhora os resultados de refletividade das ferritas espin?lio
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Abbali, Zineb. "Etude de la cristallisation de ferrites spinelles dans des verres borates." Grenoble 2 : ANRT, 1988. http://catalogue.bnf.fr/ark:/12148/cb376110731.
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Tailhades, Philippe. "Elaboration, caracterisation et optimisation des proprietes morphologiques et magnetiques de ferrites lacunaires destines a l'enregistrement haute densite." Toulouse 3, 1988. http://www.theses.fr/1988TOU30185.
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Cette etude presente la mise au point de particules ferro ou ferrimagnetiques submicroniques dans le but d'ameliorer les performances a l'enregistrement magnetique haute densite (numerique). Tout d'abord, on a etudie les conditions de precipitations des precurseurs oxaliques a base de fer permettant d'obtenir des particules aciculaires dont les dimensions moyennes sont 0,2 mu m pour la longueur et 0,05 mu m pour le diametre. Par traitement thermique, on transforme ces precurseurs en ferrites lacunaires, en cherchant a obtenir des particules monocristallines monodomaines. En jouant sur l'anisotropie magnetocristalline grace a la substitution des ions fe**(3+) et des lacunes de gamma -fe::(2)o::(3) par des ions cobalt et du manganese, on a reussi a regler les proprietes magnetiques des particules (champ coercitif, aimantation remanente, aimantation saturation) de facon a les rendre compatibles avec les exigences de l'enregistrement magnetique haute densite
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EL, HARFAOUI MOHAMMED. "Etude de l'etat magnetique localement cante et de sa dynamique dans le ferrite de lithium-titane dilue." Paris 6, 1988. http://www.theses.fr/1988PA066650.
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L'etude experimentale de l'etat incline (canted) et de sa dynamique dans la ferrite de lithium-titane est faite a partir des experiences moessbauer avec et sans champ applique, des mesures d'aimantation a champs faibles et eleves et des mesures de thermoremanence. Un etat ferrimagnetique perturbe est observe ainsi qu'un etat desordonne a basses temperatures. Toutes les proprietes particulieres et complexes observees resultent de l'existence d'un etat localement incline (canted) ou la composante transverse relaxe entre des directions preferentielles
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Bui, Thi Mai Anh. "Etude de la décomposition spinodale de cobaltite de fer sur couches minces." Thesis, Toulouse 3, 2015. http://www.theses.fr/2015TOU30028/document.
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Ces travaux de thèse avaient pour objectif d'étudier les effets du phénomène de décomposition spinodale sur l'évolution structurale et microstructurale de couches minces de cobaltites dont la composition se trouve dans la lacune de miscibilité du système de CoFe2O4 - Co3O4. Dans un premier temps, nous avons élaboré les couches minces par pulvérisation cathodique radiofréquence en configuration magnétron à partir d'une cible céramique de composition moyenne Co1.73Fe1.27O4. Les dépôts à forte pression d'argon ou à puissance RF élevée favorisent la formation de couches contenant une phase monoxyde associée à une phase spinelle. L'obtention de la phase monoxyde dans ces couches est probablement due en partie, à la réduction de la surface de la cible en raison d'un fort bombardement, sans pour autant exclure la présence d'un phénomène physique lié à la thermalisation des atomes d'oxygène. Nous avons optimisé une condition de dépôt à 0.5 Pa - 20 W ce qui permet un compromis alliant une faible quantité de phase monoxyde dans la couche et des vitesses de dépôt qui demeurent acceptables. La décomposition spinodale a été mise en évidence sur les couches déposées à cette condition, puis traitées à 600 °C pendant différentes durées. Les caractérisations par diffraction des rayons X, spectroscopie Raman et mesures magnétiques VSM, ont confirmé la formation progressive d'un système de deux phases, une riche en cobalt et l'autre riche en fer. Néanmoins, la formation des zones périodiques correspondant à ces deux phases n'a pas été observée sur les couches décomposées. Ces deux phases ne présentent en effet qu'une différence très faible au niveau de leur structure. Enfin, les études sur les couches traitées à plus basses températures (par exemple à 450 °C) montrent des évolutions similaires à celles observées lors de traitements à 600 °C. La transformation spinodale semble s'initier par une migration des cations divalents vers les sites tétraédriques et des cations trivalents vers les sites octaédriques. La structure spinelle tend donc à devenir normale avant que la séparation en deux phases spinelles s'effectue selon un mécanisme de transformation spinodaleThis work aimed to study the effects of spinodal decomposition process on structural and microstructural evolution of cobaltite thin films whose composition is in the miscibility gap of CoFe2O4 - Co3O4 system. At the first time, thin films were elaborated by RF magnetron sputtering from a ceramic target with average composition of Co1.73Fe1.27O4. Deposits at high argon pressure or high RF power favor the formation of thin films containing an oxide phase associated with a spinel phase. The oxide phase obtained in these films is probably in part due to the reduction of the target surface owing to a strong bombardment, without excluding the presence of a physical phenomenon related to the thermalization of oxygen atoms. Deposition conditions were optimized at 0.5 Pa - 20 W. That allows a compromise between a small amount of oxide phase in the thin films and an acceptable deposition rate. Spinodal decomposition has been demonstrated on the thin films deposited in this condition and post-annealed at 600 ° C for various times. The characterizations by X-ray diffraction, Raman spectroscopy and VSM magnetic measurements, have confirmed the gradual formation of a two-phase system made of a cobalt-rich phase and an iron-rich phase. However, the formation of periodic zones, corresponding to these two phases, was not observed on the decomposed thin films. These two phases exhibit in fact a very small difference in their structure. Finally, the studies on thin films annealed at lower temperatures (for example at 450 ° C) showed evolutions similar to those observed during treatment at 600 ° C. Spinodal transformation seems to be initiated by a migration of divalent cations into the tetrahedral sites and trivalent cations into octahedral sites. The spinel structure thus tends to turn into a normal structure, before the separation into two spinel phases, due to the spinodal transformation
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Ammar, Mehdi. "Étude du magnétisme de composites métal-oxyde et métal-diélectrique nanostructurés pour composants passifs intégrés." Phd thesis, École normale supérieure de Cachan - ENS Cachan, 2007. http://tel.archives-ouvertes.fr/tel-00371362.
Full textAbstract:
Ce travail s'inscrit dans le cadre du développement de matériaux composites nanostructurés à propriétés électriques et magnétiques inédites. Afin de répondre à des besoins technologiques pour, l'électronique de puissance intégrée : le stockage ou la transmission de l'énergie, les télécommunications (antenne intégrée...), le stockage de l'information par enregistrement magnétique et le marquage biologique, le composite doit présenter globalement une polarisation magnétique élevée ainsi qu'un comportement isolant permettant de pousser les limites fréquentielles, minimiser les pertes dynamiques et découpler les grains entre eux. Les matériaux composites élaborés sont constitués d'une matrice d'accueil - magnétique (ferrite spinelle) ou non-magnétique (diélectrique = silice) - dans laquelle sont dispersées des particules métalliques (Fer-Nickel ou Cobalt). Ces matériaux sont novateurs dans la mesure où le matériau final peut bénéficier d'un couplage des propriétés magnétiques des deux phases constitutives. L'holographie électronique en transmission a mis en évidence une ocnfiguration de spins de type « vortex » dans les nanoparticules de Fe30Ni70. Les mesures holographiques ont été comparées au profil de l'aimantation, dans un vortex, modélisé par une approche micromagnétique. Des analyses physico-chimiques approfondies nous ont permis de confirmer les topologies visées : pour le composite métal-diélectrique, l'épaisseur de la couche d'enrobage a pu être contrôlée à l'échelle nanonométrique. Pour le composite métal-oxyde obtenu par croissance directe du ferrite sur la phase métallique, on a démontré une bonne dispersion des particules métalliques. Les propriétés magnétiques et structurales des différents composites, en poudre ou compactés par SPS (compactage-frittage flash), ont été caractérisées et discutées. Les propriétés fonctionnelles ont été aussi étudiées et sont très prometteuses pour les applications visées. L'enrobage des nanoparticules par la silice a permis la préparation de leur surface dans la perspective d'une fonctionnalisation par des entités biologiques.
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Holec, Petr. "Magnetické nanočástice a nanokompozitní materiály se spinelovou strukturou, jejich příprava a charakterizace." Doctoral thesis, 2012. http://www.nusl.cz/ntk/nusl-326583.
Full textAbstract:
This work presents the preparation and physical properties of spinel nanoparticles and nanocomposites. All nanocomposites in diamagnetic matrix like chromites CoCr2O4, CuCr2O4, NiCr2O4, ZnCr2O4 and ferrite MgFe2O4 were prepared using sol-gel method. On the other hand, isolated nanoparticles such as MgCr2O4, MnCr2O4, CuCr2O4, NiCr2O4, and FeCr2O4, were prepared using autocombustion a co-precipitation methods. CoFe2O4 and MgFe2O4 were prepared by microemulsion alkoxide method. This microemulsion method was used for the spinel nanoparticles preparation for the first time. This work describes the influence of heat treatment temperature on the final particle size and influence of particle size on physical properties of material. The study of the influence of twovalent cationt in the spinel structure on the magnetic properties of chromites was also carried out. The final samples were characterized by XRD powder diffraction, Mössbauer spectroscopy, infrared and Raman spectroscopy, and HRTEM. The dependence of magnetization on applied magnetic field at constant temperature and ZFC (zero-field cooling) - FC (field cooling) measurement was carried out on the prepared sample.
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Zviagin, Vitaly. "Ellipsometric Determination of Cation Disorder in Magnetically Ordered Spinel Ferrite Thin Films." 2019. https://ul.qucosa.de/id/qucosa%3A35443.
Full textAbstract:
In this investigation, the cation distribution in ferrites of spinel-type structure is spectroscopically investigated with respect to the observed magnetic behavior. The ferrite thin films were fabricated by pulsed laser deposition and consequently annealed at different temperatures as well as atmospheres. Structural properties were determined with various methods and the crystalline quality was examined. The dielectric function line-shape was parametrized based on empirical evidence and was found to be dominated by electronic transitions between d orbitals of Fe2+ cations as well as transitions from O 2p to 3d and 4s orbitals of iron and zinc cations. The strongest magneto-optical response was observed for transitions involving cations, which correspond to lattice disorder and inversion within the normal spinel structure.
With the decrease in the substrate temperature during fabrication, a decrease in the magnetic response was observed. The diminishing ferrimagnetic order was directly correlated to the decrease in strength of the transitions, involving Fe3+ on tetrahedral lattice sites. After thermal treatment in argon atmosphere and at a temperature below the deposition temperature, the increase in the magnetic response was explained through the facilitation of oxygen vacancies. With the increase in treatment temperature, a decrease in ferrimagnetic order was related to the recrystallization of the disordered spinel structure toward a more stable normal configuration, evident in the dielectric function spectra.
The cationic configuration distribution in the surface as well as the bulk region, as a function of Zn concentration, was determined from approximation of the XPS and the dielectric function spectra, respectively. The difference in the cation configuration distribution, in films of predominantly inverse configuration, was related to the weak magnetic response, as opposed to films of predominantly normal spinel configuration. Our results demonstrate that a defect-rich surface region could serve as a possible explanation for the ferrimagnetic order in a nominally non-magnetic normal spinel ZnFe2O4. In combination with structural property determination, the net magnetic behavior is explained through the local cationic disorder, determined from the parametrization of the dielectric function spectra in a wide spectral range.:1 Introduction
2 Theoretical background and fundamental considerations
2.1 Spinel ferrite crystal structure
2.2 Crystal field stabilization energy
2.3 Band structure description
2.4 Verwey transition
2.5 Magnetic exchange interactions
3 Sample preparation and modification
3.1 Macroscopic spinel film formation
3.2 Pulsed laser deposition
3.3 Thermal treatment
3.4 Sample overview
4 Methods and general properties
4.1 Structure characterization techniques
4.1.1 X-ray diffraction
4.1.2 X-ray reflectivity
4.1.3 Energy dispersive X-ray spectroscopy
4.1.4 Focused ion beam and scanning electron microscopy
4.1.5 Raman spectroscopy
4.2 Surface properties
4.2.1 Atomic force microscopy
4.2.2 X-ray photoelectron spectroscopy
4.3 Dielectric tensor properties
4.4 Spectroscopic ellipsometry
4.5 Magneto-optical Kerr effect
4.6 Magneto-static properties
5 Results and discussion
5.1 Magnetic and optical properties of Fe3O4 thin film and single crystal
5.2 Magneto-optical properties of ZnxFe3−xO4 thin films
5.3 Fabrication temperature dependent ferrimagnetic order
5.4 Thermally induced structural stabilization
5.5 Cation configuration in dependence on the Zn concentration
5.5.1 Structural property determination
5.5.2 Composition characterization
5.5.3 Magneto-static behavior
5.5.4 Section summary and discussion
6 Summary and outlook
Bibliography
List of article contributions
Selbstständigkeitserklärung
Acknowledgments
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江偉菁. "Study on the magnetic properties of spinel ferrites MFe2O4 (M=Mn, Fe, Co, Ni, Zn and Mg) nanoparticles." Thesis, 2006. http://ndltd.ncl.edu.tw/handle/83098216779658062033.
Full textAbstract:
碩士國立彰化師範大學
物理學系
94
The magnetic properties of ferrites with spinel structure have been researched widely at 1950’s, but most research was carried out for bulk samples. In resent years, nanoparticles of ferrites have attracted a lot of attention because of the need of magnetic recording material. In this thesis, we have investigated the magnetic properties of MFe2O4 nanoparticles with different doping metals which was prepared by sol-gel method, where M = Mn、Fe、Co、Ni、Zn、Mg. The structure and particle size were determined by X-ray powder diffractometer. Magnetic properties were measured by electron spin resonance spectrometer ( ESR ) , superconducting quantum interference device ( SQUID ) , and alternating gradient magnetometer ( AGM ) . We find that different preparing condition can change the magnetic properties of samples. For example, the saturated magnetization of MnFe2O4 prepared in a pH = 3 solution is 3.16 times that prepared in a pH = 5 solution at 6 K. As to size effect, the nanoparticles show different characteristics from bulk samples on both the rate of change with temperature and the magnitude of magnetic properties. In the samples we studied, the magnetic-doping-metal sample has stronger magnetic nature, while the nonmagnetic-doping-metal sample shows paramagnetic-like behavior at high temperature. The cation distribution in ferrites influence the magnetic properties of ferrites obviously.
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Brachwitz, Kerstin. "Defekt-induzierte Leitungsmechanismen und magnetische Eigenschaften spinellartiger Ferrite." Doctoral thesis, 2013. https://ul.qucosa.de/id/qucosa%3A12425.
Full textAbstract:
Im Rahmen dieser Arbeit wurde der Einfluss von Defekten auf die Eigenschaften von Ferrit-Dünnfilmen untersucht. Die Dünnfilme wurden mit Hilfe von gepulster Laserabscheidung bei verschiedenen Züchtungsparametern hergestellt. Durch Variation der Substrattemperatur und des Sauerstoffpartialdrucks wurden Dünnfilme verschiedener kristalliner Qualität gezüchtet. Diese wurden hinsichtlich ihrer chemischen Komposition mit Hilfe von energie-dispersiver Röntgenspektroskopie und Röntgenphotoelektronenspektroskopie untersucht. Durch Korrelation der Ergebnisse mit Messungen zum zirkularen magnetischen Röntgendichroismus, konnte eine partielle Inversion der Spinellstruktur nachgewiesen werden. Der Grad der Inversion ist höher für geringe Abscheidetemperaturen. Für diese defektreichen Dünnfilme zeigen Röntgenbeugungsuntersuchungen eine geringere kristalline Ordnung der Dünnfilme. Die strukturellen Defekte haben einen maßgeblichen Einfluss auf die elektrischen und magnetischen Eigenschaften der Ferrit-Dünnfilme. So zeigen die Ferrit-Dünnfilme für geringe Züchtungstemperaturen eine erhöhte elektrische Leitfähigkeit, während Dünnfilme, die bei hohen Substrattemperaturen gezüchtet wurden, isolierend sind. Die Temperaturabhängigkeit der elektrischen Leitfähigkeit kann auf thermisch aktivierte Hopping-Leitung oder die Leitung zwischen Clustern, die in einer Matrix eingebettet sind, zurückgeführt werden.
Die magnetischen Eigenschaften von Zinkferrit-Dünnfilmen werden maßgeblich durch Defekte in der Spinellstruktur bestimmt, da es nominell in der normalen Spinellstruktur kristallisiert und daher antiferromagnetisch ist. Die partielle Inversion der Eisen- und Zinkionen führt zu Ferrimagnetismus in den Zinkferrit-Dünnfilmen, der mit Hilfe von SQUID-Messungen in dieser Arbeit eingehend untersucht wurde.
Durch Korrelation der Ergebnisse der verschiedenen Untersuchungsmethoden konnten Rückschlüsse auf die dominierenden Defekte in den Ferrit-Dünnfilmen geschlossen werden. So sind zum einen Defekte auf atomarer Skala, wie Antisite-Defekte und divalenten Fe-Ionen für die erhöhte elektrische Leitfähigkeit und die größere Magnetisierung der defektreichen Dünnfilme verantwortlich. Zum anderen können ausgedehnte Defekte, im Speziellen Cluster, die in einer amorphen Matrix eingebettet sind, nicht ausgeschlossen werden.
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Hsieh, Chang-Tsun, and 謝長村. "Domain Wall and Spin Resonance Studies of Spinel Ferrite Nanoparticles and Nickel thin films." Thesis, 2003. http://ndltd.ncl.edu.tw/handle/12020619175987222020.
Full textAbstract:
博士國立清華大學
物理學系
92
Domain wall and spin resonance of magnetic thin films and nanoparticles are investigated in this dissertation. Two main thrusts are: (1) electron spin resonance studies of quantum phase transition in magnetic nanoparticles, and (2) magnetic force microscopic studies of domain walls on thin nickel films. Sol-gel glass embedded with iron nanoparticles provides fascinating features inheriting with paramagnetic, ferromagnetic, and superparamagnetic resonance properties under various compositional weight ratios and annealing conditions. Two spectra arising from paramagnetic Fe3+ ions and ferrimagnetic Fe2O3 particles both centered at ge =2.0 compete with intensities as the annealing temperature TA increases. The asymmetric and rather broad line shape can be elucidated by the ferromagnetic resonance of the single domain Fe2O3 nanoparticles. The half linewidth simulation agrees well with the super paramagnetic resonance theory of particle size ~10.5 nm. The classical, thermally driven transition from ferrimagnets to superparamagnets in Fe3O4 nanoparticles can be converted into another quantum phase by a strong internal anisotropic field. The field induced by crystal anisotropy, which is perpendicular to the Ising axis can destroy the magnetic long-range order to quantum paramagnets by exceeding some critical values. Electron spin resonance (ESR) spectrometer, a very sensitive instrument with fast detecting window to explore quantum phase transition for magnetic nanoparticles, was exploited to study the fascinating interplay between thermal and quantum fluctuation in the vicinity of a quantum critical point. We have determined the static spin susceptibility and critical exponent g, which is power-law dependent spanning the quantum critical point and investigated the effects of various microwave fields, particle sizes, and temperature dependence on the magnetic states of single domain spinel ferrite nanoparticles. As temperature decreases, the spectrum behaves from superparamagnetic (SPR) to ferromagnetic resonance (FMR) and retrieves to quantum superparamagnetic as the temperature lowers down further. On account of the highly anisotropy field of Co-ferrite, the quantum critical behavior of CoFe2O4 nanoparticles is prominently observed. We have combined magnetization measurement and electron spin resonance to study the fascinating quantum tunneling of magnetization in Co-ferrite nanoparticles at low temperatures. ESR spectra are shifted to low fields (1600G) at 35 K accompanying with the intensity increasing and linewidth narrowing. The increasing of magnetic moment at low-temperature (below 11 K) and the extraordinarily small coercive field (1.3 kOe) at 2K of Co-ferrite NPs are investigated by magnetization measurements. A new approach, Heisenberg model with strong easy-plane anisotropy, is proposed to attribute these to the conversion of ferrimagnets to quantum superparamagnets (short-range ordered). The quantum superparamagnetic size is estimated about~1.5nm corresponding to the short-range ferromagnetically correlated volumes existing in MNPs. Magnetic force microscopy images with a resolution as high as 3 nm are examined for thin nickel films deposited by argon ion sputtering method. Samples grown at 100∘C with grain sizes of near 60nm exhibits coexistence of Bloch and Neel walls whereas samples grown at room temperature with grain size of ~10nm display an alternative Bloch line with a cap switch. The relationship between domain size and film thickness follows a law (where d is the film thickness) as predicted by Kittel.
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FitriWidhiastuti and 魏巧玲. "Remediation of boron from aqueous solution by electrochemically co-precipitated spinel ferrite in electrocoagulation process." Thesis, 2017. http://ndltd.ncl.edu.tw/handle/2tk447.
Full textAbstract:
碩士國立成功大學
化學工程學系
105
Electrocoagulation process using transition metals as electrolytes and sacrificial anodes was studied for the removal of boron through formation spinel ferrites. The spinel ferrite (MFe2O4, M = Fe, Cu, Ni, and Co) evolved during electrocoagulation could be promising adsorbents for boron. The parameters such as pH (4 – 12), current density (1.25 – 5 mA/cm2), boron initial concentration (10 – 100 mg/l), and combination pairs of electrodes were studied. The efficiency of boron removal could achieve 95% as electrocoagulation was optimized at conditions: initial B = 10 mg/l, reaction pH 8 ± 0.5, current density = 3.75 mA/cm2, and 60 min reaction. X-ray diffraction (XRD) patterns revealed that several spinel type such as nickel ferrites, cobalt ferrites and copper ferrites were obtained by electrocoagulation process. The adsorption isotherms of spinel type materials fitted by Langmuir and Freundlich models indicated that qmax of nickel ferrites was 28.90 mg/g with pHpzc 10.25. Most of the spinel ferrites were granular crystallites with the crystalline size of around 26 – 30 nm. The highest saturation magnetization was nickel ferrites 50.30 emu/g by vibrating sample magnetometer (VSM) analysis. Magnetic separable spinel ferrites offers easier solid - liquid separation after electrocoagulation process.
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Ou, Jeng-Lung, and 歐政隴. "The study of microwave absorbing properties for composites β-SiC/spinel Mn-Zn ferrite/carbon black." Thesis, 2011. http://ndltd.ncl.edu.tw/handle/30673496864709810958.
Full textAbstract:
碩士國防大學中正理工學院
應用化學碩士班
99
Nano MnZn cubic ferrite powders (Mn1-XZnXFe2O4) were prepared by sol-gel aqueous combustion synthesis (ACS) method in this study. The starting materials of Fe, Mn, and Zn nitrates were used as the sources of metallic ions and oxidizing agents, while glycine was applied as the reducing agent as well as the chelating reagent. Accordingly, the metallic nitrates combined with glycine in aqueous solution forming complex ions. Spinel structure of the final product was driven by the self-propagation combustion of the sol-gel solution. X-ray diffraction pattern of the as-burnt product indicated a cubic or spinel ferrites crystal structure. Scanning electron microscopy (SEM) images revealed that the grain sized of the as-burnt powders ranged in 60~100 nm. However, the average diameter of the powder after a heat treatment at 1200 ℃ were found at ~1.0 μm. For a composition of Mn0.8Zn0.2Fe2O4, the powders were obtained to have the optimal magnetic properties by using VSM instrument. The saturated magnetization (Ms), remanence magnetization (Mr), and the coercivity (Hc) of Mn0.8Zn0.2Fe2O4 are 109.80 emu/g, 11.712 emu/g and 81.398 Oe, respectively. Because of the only magnetic lose properties of the Mn-Zn spinel ferrites can have, therefore the conductive carbon black and dielectric material of β-SiC were mixted with ferrites and organic polymers to form the high absorbability and low density of microwave absorbing composite materials. In which Agilent Technologies E8362B Analyzer were used to characterize the microwave absorption properties of the materials.
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"Synthesis and Characterization of Nanocrystalline Nickel-Zinc Spinel Ferrite Thin Films Using the Spin-Spray Deposition Method." Doctoral diss., 2013. http://hdl.handle.net/2286/R.I.17906.
Full textAbstract:
abstract: The overall objective of this project is to optimize the development of magnetic ferrite thin films targeted for enabling low-loss broadband communication devices, miniaturized low-microwave inductors and electromagnetic noise suppressors. The focus of this objective is to design and build a reactor and improve the spin-spray process. Each film is then characterized and optimized to have a high permeability and high frequency in the range of 500 MHz - 3 GHz. Films produced by the µ-droplet deposition regime yields a higher Snoek's product than the continuous liquid layer regime. The highest Snoek's product occurs when it is deposited at an oxidant pH of 8.28. The Ni-Zn-Co ferrite magnetic domains were imaged using the Lorentz TEM in which multi-grain domains are experimentally observed for the first time.Dissertation/Thesis
Ph.D. Materials Science and Engineering 2013
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Hsu, Wei-mau, and 徐維懋. "Materials Characteristics of Spinel Nickel Ferrite Oxygen Carrier and It’s Selection of Inert Supports for Chemical Looping Combustion Process." Thesis, 2012. http://ndltd.ncl.edu.tw/handle/01088156383690048443.
Full textAbstract:
碩士國立臺灣科技大學
機械工程系
100
Chemical looping combustion (CLC) process is a promising technology of CO2 capture and sequestration (CCS) with high energy efficiency and low emission of greenhouse gases. For the practical use, the performance of oxygen carriers is a key issue for the application on CLC process. Unfortunately, the formation of inactive compounds by the interaction between the ordinary metal oxygen carriers and inner supported were evidently encountered in a CLC process, which subsequently reduced the performance of CLC process. In our preliminary results, after 5 successive cycles, NiFe2O4 powder with a single phase of spinel structure represented a higher redox cycling behavior and stability as compared to the standard of NiO and Fe2O3. To identify the initial reduction mechanism of NiFe2O4, we obtained the activation energy (Ea) of the reduction behavior using the reduction rate equation and Arrhenius equation. However, at long redox cycles (10 cycles), the performance of NiFe2O4 oxygen carriers was dramatically reduced due to the agglomeration of oxygen carriers. Thus, the addition of inner materials as the supports to oxygen carriers is a necessary process to prepare a superior oxygen carrier system in the operation of long redox cycles in CLC process. This study is to investigate the effects various inner supports (Al2O3, ZrO2 and YSZ) of in NiFe2O4 oxygen carrier system and the performance of CLC process by TGA system. Mingled various inner supports shift the reduction kinetics of oxygen carriers, NiFe2O4/Al2O3 system decayed the reduction kinetics but both NiFe2O4/ZrO2 and NiFe2O4/YSZ system promoted the reduction kinetics. To identify the initial reduction mechanism and activation energy, NiFe2O4/Al2O3 system’s activation energy is 83.4 kJ/mol reduction mechanism controlled by solid-state diffusion. For the systems of NiFe2O4/ZrO2 and NiFe2O4/YSZ, both reduction mechanisms are mainly dominated by gas diffusion with the activation energy values of 9.0 kJ/mol and 10.1 kJ/mol, respectively. Our results also demonstrated the higher redox cycling behavior of using the proposed preparation of NiFe2O4 as an oxygen carrier with Al2O3 support in a reversible chemical looping process (CLP).
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Hsu, Chia-Shuo, and 許家碩. "Systematic investigation of spinel-type ferrite MFe2O4 (M = Mn, Fe, Co, Ni, Zn): in-situ and ex-situ X-ray approaches to oxygen evolution reaction." Thesis, 2016. http://ndltd.ncl.edu.tw/handle/11900529667026818271.
Full textAbstract:
碩士國立臺灣大學
化學研究所
104
Metal oxide of spinel family has performed great potential to replace RuO2 or IrO2 toward oxygen evolution reaction (OER), but the fundamental mechanism of spinel oxides is still far from a complete understanding especially for the role of metal ions. Owing to various coordinated sites of divalent/trivalent metals ions and surface condition (mor-phology and defects), it’s a great challenge to have a fair assessment of electrocatalytic performance of spinel system. Herein, we demonstrated a series of MFe2O4 (M = Mn, Fe, Co, Ni, Zn) family with a well-controlled morphology to achieve comprehensive study of electrocatalytic activity toward OER, in which most of the divalent metals (M2+) of MFe2O4 were located at octahedral sites and well-defined single-crystal nanocubes were synthesized to minimize the influence caused by crystallographic sites/morphology of variables for each samples. The effects of M2+ ion in MFe2O4 can be realized through investigating the OER behaviors of MFe2O4 and that of γ-Fe2O3 as a reference to have in-depth understanding of the roles of divalent/trivalent metal ions. In-situ X-ray diffraction and ex-situ X-ray absorption spec-troscopies were employed to provide direct evidence that the superior OER activities of CoFe2O4 and NiFe2O4 samples could be attributed to a remarkable phase transformation forward metal oxyhydroxide, while the rest of compounds remain unchanged during oxygen evolution reaction.