Orientador: Edison Zacarias da Silva / Tese (doutorado) - Universidade Estadual de Campinas, Instituto de Física Gleb Wataghin / Made available in DSpace on 2018-08-21T01:33:25Z (GMT). No. of bitstreams: 1 Faccin_GiovaniManzeppi_D.pdf: 8582909 bytes, checksum: f53f27084aaa92a73ab76984290715ff (MD5) Previous issue date: 2012 / Resumo: Esta tese apresenta um estudo baseado em simulação computacional de propriedades físicas de nanopartículas de ferro, cobalto, ítrio e compostos híbridos destes elementos visando mapear as estruturas mais estáveis destas nanopartículas e seu comportamento magnético. Para este fim, empregamos cálculos de primeiros princípios baseados no formalismo da teoria do funcional da densidade aliados a uma estratégia heurística visando mapear possíveis isômeros de cada nanopartícula e, posteriormente, caracterizar suas propriedades magnéticas. Os principais resultados encontrados são o mapa estrutural dos isômeros mais prováveis e seu o mapa de magnetização, os quais podem servir de guia para a síntese direcionada de novos materiais com propriedades físicas e químicas de interesse / Abstract: This thesis presents a computational study regarding physical properties of iron, cobalt and yttrium nanoparticles as well as hybrid composites of these elements aiming to map their most stable structures and their magnetic behavior. To this end, we have used first principles calculations based on the density functional theory formalism coupled with a heuristic strategy aiming to map possible isomers of each nanoparticle and also characterize their magnetic properties. The main results found are a map of the structures of the most likely isomers and also their magnetization map, which can be used as a guide for the directed synthesis of new materials that have physical and chemical properties of interest / Doutorado / Física / Doutor em Ciências

<p>U ovom radu prikazani su rezultati ispitivanja strukturnih i magnetnih osobina čistih cink- ferita, ZnFe₂O₄ i cink-ferita sa dodatkom indijuma Zn_1-xIn_xFe₂O₄ i itrijuma, ZnY_xFe_2-xO₄, gde je 0 &le; x &le; 0,6. Prahovi na bazi cink-ferita su sintetisani koristeći nisko temperaturnu metodu sinteze iz tečne faze &ndash; 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 M&ouml;ssbauer 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, &scaron;to 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 pona&scaron;anje ispitivanih nanočestičnih sistema.</p> / <p> 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 &le; x &le; 0,6). Powders based on zinc ferrites were synthesised by a low temperature wet-chemical method &ndash; 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 M&ouml;ssbauer 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.</p>

In this project, I investigate the photoconductivenature of photochromic rare-earth metal oxy hydrides (REMHO). Such materials have received increasingscientific attention since they show a color-neutralphotochromic effect that can be applied, e.g., in smartwindows or chromogenic devices. Photochromicmaterials reversibly turn opaque from transparentunder illumination with light of optical wavelength. Inrecent studies it was found that these materials alsoshow an instant decrease in resistivity whenilluminated which can be used in optical sensors. Tounderstand the nature of this photoconductive effect,I grew yttrium oxy hydride thin films by reactivemagnetron sputtering. I measured the resistivity forillumination from front and substrate side, opticaltransmission and compositions of the samples andrelated the results to photoconductivity. I show thatphotoconductivity is a bulk effect and not directlyrelated to photochromism. Samples that almost lostphotochromism due to aging, still show strongphotoconductivity. Moreover, it was observed that theresistance increased faster during bleaching for frontillumination than for back illumination.

Pyrochlore iridates have received considerable attention for the past few years as they possess strong electron correlations and spin orbit coupling, giving rise to a finite temperature metal-insulator transition (MIT). The nature of this MIT transition is related to the magnetic order of the Ir atoms which also experience frustration as they are part of a pyrochore structure. The aim of this study is to elucidate the magnetic configuration of the magnetic iridium ions by doping Y2Ir2O7 with Bi. Here we present a study on the magnetic properties of the Y2−xBixIr2O7 (x = 0, 0.2, 0.4, 0.9, 1.3, 2) system using μSR and DC susceptibility. Our results show that pure Y2Ir2O7 has a magnetic transition to a long- range ordered state. Substituting Bi by Y results in a lower temperature transition with increasing amount of bismuth. When the system goes into the ordered state a weak ferromagnetic moment is seen. This is in agreement with the belief that the system orders in an canted antiferromagnetic fashion with all-in all-out spins in the tetrahedron of the pyrochlore structure. However the addition of bismuth into the sample does not change the internal magnetic field measured at low temperatures with μSR, but only changes the transition temperature and the ordered volume fraction. For the Bi2Ir2O7 system two magnetic transitions had been measured previously counter to the belief that this material did not order magnetically. In this work, our μSR measurements show no evidence of such a transition. In the transverse field μSR set up a small Knight shift is measured due to the local susceptibility of Bi2Ir2O7. / Thesis / Master of Science (MSc)

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Thesis (PhD (Electrical and Electronic Engineering))--University of Stellenbosch, 2010. / Dissertation presented for the degree of Doctor of Philosophy in Engineering at the University of Stellenbosch / ENGLISH ABSTRACT: Planar weak link structures, such as micro-bridges, variable thickness bridges and nanobridges, have always attracted a lot of attention. Their potential to behave as real Josephson elements make them useful devices, with numerous applications. Powerful techniques, such as focused ion-beam and electron-beam lithography, were successfully used and are well understood in planar weak link structure fabrication. In this dissertation the results of an experimental study on planar weak link structures are presented. For the first time these structures have been successfully fabricated using AFM nanolithography on hard high-temperature superconducting YBCO tracks, where diamond coated silicon tips were used as a ploughing tool. Superconducting YBCO thin films were deposited on different substrates, using inverted cylindrical magnetron sputtering. The films were used to fabricate micro-bridges, variable thickness bridges and nano-bridges, by using conventional photolithography, argon ion-beam milling and AFM nanolithography. The measured I-V characteristics of the fabricated micro-bridges (width down to 1.9 µm), variable thickness bridges (thickness down to 15 nm) and nano-bridge (width down to 490 nm) showed well defined DC and AC Josephson effect characteristics. For better understanding of the behaviour of these types of weak links, critical current versus temperature measurements, and magnetic field modulation of the critical current measurements, were also performed, with the results and discussions given inside the chapters. The major challenges that were experienced in the laboratory during the fabrication processes and the operation of the fabricated devices are also discussed, with the solutions given where appropriate. / AFRIKAANSE OPSOMMING: Swak-skakel vlakstrukture, soos mikrobr.ue, br.ue met veranderlike dikte en nanobr.ue, het nog altyd baie aandag getrek. Hul het die potensiaal om soos werklike Josephson-elemente te kan funksioneer en is, as gevolg hiervan, nuttige toestelle met veelvuldige toepassings. Kragtige tegnieke, soos gefokuste ioonstraal- en elektronstraal litografie, is suksesvol gebruik en word goed verstaan in die vervaardiging van swak-skakel vlakstrukture. In hierdie proefskrif word die resultate van ¡¦n eksperimentele studie van swak-skakel vlakstrukture voorgel.e. Vir die eerste keer is hierdie strukture suksesvol vervaardig, deur gebruik te maak AFMnanolitografie op harde, ho¡Le-temperatuur supergeleier YBCO (Yttrium Barium Koperoksied) spore, waar diamantbedekte silikonpunte gebruik is as ploeginstrument. ¡¦n Dun lagie van supergeleidende YBCO is op verskillende substrate gedeponeer, deur gebruik te maak van omgekeerde silindriese magnetron verstuiwing. Die dun lagies is gebruik in die vervaardiging van mikrobr.ue, br.ue met veranderlike dikte en nanobr.ue, deur die gebruik van gewone fotolitografie, argon-ioonstraal frees en AFM nanolitografie. Die gemete I-V eienskappe van die vervaardigde mikrobr.ue (met breedte so laag as 1.9 µm), veranderlike-dikte br.ue (dikte tot 15 nm) en nanobr.ue (breedte so min as 490 nm) toon goed gedefinieerde GS en WS eienskappe van die Josephson-effek. Ten einde die gedrag van hierdie tipes swak-skakels beter te kan verstaan, is metings gedoen van kritieke stroom teenoor temperatuur, asook magnetiese veld modulasie van die kritieke stroom. Hierdie resultate en besprekings daarvan word binne die toepaslike hoofstukke aangebied. Die grootste uitdagings wat in die laboratorium, sowel as met die toetsing van die vervaardigde toestelle ondervind is, word ook bespreek. Waar moontlik, word toepaslike oplossings voorgestel.

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Os estudos visando o aproveitamento dos minerais de interesse econômico associados ao Granito Madeira, Pitinga, AM, foram subdivididos em três etapas. A primeira etapa compreendeu a realização de estudos de caracterização tecnológica, com um detalhado estudo mineralógico enfocando a forma de ocorrência e granulometria de liberação dos minerais de interesse. Foi identificado pela primeira vez a presença de criolita, mineral raro, cujo valor agregado supera o conjunto de todos os demais minerais de interesse econômico. Verificou-se que a granulometria de liberação dos minerais pesados era muito distinta; valores de 80% são observados para a zirconita acumulada abaixo de 0,595 mm, para a cassiterita abaixo de 0,297 mm e para os minerais de nióbio abaixo de 0,149 mm. A segunda etapa compreendeu, essencialmente, a realização de estudos de desenvolvimento de processo de concentração dos minerais pesados, incluindo ensaios de concentração densitária e separações eletrostática e magnética. Foram estabelecidas as características dos concentrados passíveis de serem obtidos e suas respectivas recuperações metalúrgicas, bem como definido um fluxograma de beneficiamento a ser confirmado em escala piloto. A terceira etapa teve por enfoque principal o desenvolvimento de uma rota de processo para a concentração da criolita, a qual, embora ainda não otimizada, é sumariada a seguir: - deslamagem dos rejeitos da concentração densitária, seguido de flotação aniônica direta, mediante a utilização de \"tal oil\" como coletor e amido de milho e como processor; - remoagem do concentrado \"rougher\" a menos 0,074 mm, em circuito fechado com classificação, seguido de flotação reversa do concentrado \"rougher\" remoído, com coletor catiônico, Armacflote MIC-66 (marca Akzo); - descoleta do concentrado da flotação catiônica, seguido de flotação aniônica com sulfonato de petróleo, Aero 840 (marca Cyanamid), para a separação entre a criolita e os demais minerais de flúor portadores de cálcio; - separação magnética de alta intensidade via úmida, \"WHIMS\", do concentrado da última etapa de flotação (com 0,5% a 0,7% de Fe203), visando minimizar os teores de ferro; - lixiviação clorídrica do produto não-magnético, com a redução suplementar dos teores de ferro, atingindo valores de 0,16 a 0,20% de Fe203. / Studies at the Pitinga primary ore envolved three stages. The first one consisted of detailed mineralogical studies emphasizing the occurrence patterns and the degree of liberation of the minerals in the granitic ground ore. Cryolite in the Pitinga ore was then identified for the first time. The liberation sizes for heavy minerals are quite distinct; a liberation degree of 80% was observed for zircon accumulated under 0,595 mm for cassiterite under 0,297 mm and for niobium minerals under 0,149 mm. The second stage comprised essentially mineral dressing studies to concentrate the heavy minerals. Discontinuous gravity concentration, electrostatic and magnetic separation tests were performed to establish the characteristics of the mineral concentrates and their recoveries, as well as to define a plant flowsheet to be confirmed in continuous pilot plant tests. The definition of a mineral processing route for cryolite concentration was the main object of the last stage. The following sequence of procedures, to be further investigated, was established: - disliming of gravity concentration taillings, followed by direct anionic flotation of fluorine minerals, using tall oil as collector and starch as depressant; - regrinding of the rougher concentrate to minus 0,074 mm, in a closed circuit with classification, followed by reverse cationic flotation to remove silicates, with Armacflote MIC-66 (Akzo trade mark); - remotion of the cationic collector and subsequence anionic flotation with petroleum sulfonate as collector (Aero 840, Cyanamid trade mark), to separate cryolite from the other fluorine minerals and small amounts of silicates; - wet high intensity magnetic separation (WHIMS) on the obtained flotation concentrate; - dilute hydrochloric acid leaching of the non-magnetic product from WHIMS to reduce the iron contamination levels to around 0,16% to 0,25% Fe203.