Tesis: "Magnetism"

1

Barrera, Angela Dayana Barra. "Estudo do acoplamento de troca no sistema NiFe/FeMn e efeitos da irradiação iônica". Universidade de São Paulo, 2005. http://www.teses.usp.br/teses/disponiveis/43/43134/tde-12112013-095231/.

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O objetivo do presente trabalho foi produzir e estudar magnética e estruturalmente filmes finos de Ni IND. 81 Fe IND. 19 Mn IND. 50. Os filmes foram produzidos através da técnica de DC magnetron sputtering. Iniciamos nosso estudo em filmes de Si/buffer/ Ni IND. 81 Fe IND. 19 (30nm)/Fe IND 50 Mn IND. 50 (15nm)/Ta(5nm), utilizando-se substratos de Si(100), Si(111) e buffer de Cu e Ta, a fim de determinar as melhores condições para a obtenção de bom acoplamento de exchange entre as bicamadas magnéticas. Em seguida estudamos a propriedades magnéticas de filmes em função da espessura das camadas magnéticas. As amostras produzidas para este estudo foram filmes de Si(100)/Cu(20nm)/ Ni IND. 81 Fe IND. 19 (t IND. FM nm)/ Fe IND. 50 Mn IND. 50(t IND AFM nm)/Ta(3nm), variando-se t IND. FM entre 5 e 53nm e mantendo fixa t IND. AFM em 10 nm e vice-versa. Finalmente estudamos o efeito das irradiações iônicas de He e Ne no acoplamento de exchange. Os filmes foram caracterizados utilizando-se as técnicas de magnetrometria de amostra vibrante, difração de raios-x, refletometria de raios-x, absorção de raios-x na borda do Mn e microscopia óptica de campo próximo (SNOM). Os resultados da caracterização magnética de todos os filmes de Si/buffer/NiFe/FeMn/Ta mostram que eles apresentam acoplamento de exchange logo após a sua obtenção por sputtering. Porém o acoplamento via troca precisa ser induzido através do procedimento field cooling nos filmes de Si/buffer/FeMn/NiFe//Ta. Os resultados das medidas de difração de raios x mostram que os filmes apresentam estrutura cristalina FCC e texturas cristalográficas (111) e (200) quando as camadas magnéticas são depositadas sobre Cu e uma única textura cristalográfica, a textura (111) quando elas são depositadas sobre Ta. Por outro lado, os resultados da caracterização magnética nos indicam que as amostras com buffer de Cu apresentam as melhores propriedades magnéticas alto valor de H IND. exc e baixa coercividade. A análise de EXAFS dos filmes com diferentes buffers nos indica que as amostras depositadas sobre Ta e diretamente sobre silício apresentam maior desordem na sua estrutura fina em relação às amostras depositadas sobre Cu. A caracterização magnética dos filmes irradiados ionicamente mostrou que o valor de Hexc dos filmes irradiados com diferentes doses de He a temperatura ambiente aumentou em comparação ao valor do campo de exchange das amostras antes das irradiações iônicas. Já nas amostras irradiadas com Ne o campo de exchange diminuiu drasticamente. A partir das medidas realizadas no microscópio óptico de varredura em campo próximo (SNOM), em diferentes regiões de amostras antes e depois de serem irradiadas ionicamente observamos que não há variações no valor do campo de exchange.
The aim of the present work was to produce and to study magnetic and structural properties of Ni81Fe19/Fe50Mn50 thin films. These films were produced by DC magnetron sputtering. We began our study on Si/buffer/Ni81Fe19(30nm)/Fe50Mn50(15nm)/Ta(5nm) films, using Si(100) or Si(111) substrates and Cu or Ta buffer layer, in order to determine the best conditions for obtaining a good exchange coupling between magnetic bilayers. After that, we studied the magnetic properties of these films relative to magnetics layers thickness. The samples studied were Si(100)/Cu (20mn)/Ni81Fe19 (tFM mn)/Fe50Mn50 (tAFM nm)/Ta(3nm), with tFM varying between 5 to 53run, with fixed tAFM of 10mn, and vice versa, namely with fixed tFM and varying tAFM. Finally, we studied the effect of He and Ne ionic irradiation on the exchange-coupling interaction. The thin films were characterized by vibrating sample magnetometry, X-ray diffraction, X-ray reflectometry, X-ray absorption spectroscopy at the Mn K edge, and near field optical microscopy (SNOM). The magnetic characterization of all Si/buffer/NiFe/FeMn/Ta films shows that pristine films present exchange coupling. However, the exchange-coupling interaction needs to be inducing through field cooling procedure for Si/buffer/FeMn/NiFe/Ta films. X ray diffraction measurements show that these films presents FCC crystal structure, plus (111) and (200) crystal textures when the magnetic layers are deposited on Cu buffer layer, and only the (111) crystal texture when the layers are deposited on Ta. On other hand, the magnetic measurement point out that the Cu buffer samples presents the best magnetic properties with high values for Hexc and low values of coercive field. The EXAFS analysis of films with different buffers points out that samples deposited on Ta and directly on silicon present a larger local disorder than samples deposited on Cu. The magnetic measurements of the ionic irradiated films showed an increase of Hexc values for samples irradiated at room temperature with different He rates, compared with the same samples before irradiation. On the other hand, for Ne irradiated samples the exchange field decreased drastically. We observed irradiation no changes on the exchange field values from near field optical microscopy (SNOM) measurements at different areas of samples before and after ionic. These results point out that, even if we are characterizing very small areas of the samples, the exchange field represents the average behavior of the exchange interaction in these areas.

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2

Moya, Álvarez Carlos. "Structure versus Magnetism in Magnetic Nanoparticles". Doctoral thesis, Universitat de Barcelona, 2015. http://hdl.handle.net/10803/384539.

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From the fundamental point of view, NPs formed by MFe2O4 with (M= Co, Fe) are ideal system models to study the new magnetic phenomena associated with the so-called particle-like behaviour, which emerges from the size reduction towards the nanometre scale and contrasts with the well-established magnetic properties of their bulk-counterparts. It is well known that most of the particle-like behaviour and in general the large variability of the magnetic properties observed in this kind of nanomaterials are related to structural features of the NPs rather than being originated from intrinsic finite-size or surface effects, at least for NPs bigger than a few nanometers. These structural features, such as crystallographic defects, polycrystalline nature of the NPs, lack of crystallinity at the particle surface, etc., have strong influence on their magnetic properties and can be modified at will through the synthesis method. Therefore, whenever this particle-like behaviour is unwanted for applications with highly demanding requirements, the choice of a suitable synthesis method is of key importance to obtain NPs of high-crystalline quality. On the contrary, particle-like behaviour controlled by the crystalline nature of the NPs could be useful to tailor their magnetic properties for specific applications. Among the common synthesis methods, high-temperature decomposition of metal-organic precursors results the best alternative due to the remarkable final properties of the obtained NPs, such as narrow size distribution, high crystallinity and relatively simple tuning of their size and shape. So this will be the chemical route chosen in this work to study the capabilities of this synthesis method to control the final properties of the NPs through their nanostructure. In addition, to get a deeper insight in the magnetic and structural properties of those materials and to shed light on relevant issues that are still under discussion (dynamic response, magnetic frustration or inter¬particle interactions) it could be useful to combine experimental techniques enabling the characterization of the system from macroscopic scales towards single-particle structures. Within this framework, we present this work that is divided into three main parts. First, it is studied the effect of the concentration of two common reactants, involved in the thermal decomposition method, on the final properties of magnetic NPs based on iron oxides aiming at optimizing the synthesis procedure and getting a good control of the structure of the final product. In the second part, those NPs obtained in the former way are applied to demonstrate the crucial role of the nanostructure on the physical properties of nanoparticulate systems; specially, the strong interplay existent between structure and both magnetic frustration and interparticle interactions. Finally, in the third part, MFM experiments with an external applied magnetic field have been performed to directly observe the reversal of the magnetization of isolated particles and the dynamic behaviour of small aggregates.

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3

Bode, Peter Jan. "Ultrathin magnetic structures and interface magnetism". Thesis, University of Cambridge, 2006. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.614245.

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4

Barbosa, Andreia Guedes Santiago. "Estudo de microestruturas magnéticas por microscopia de força magnética". CNEN - Centro de Desenvolvimento da Tecnologia Nuclear, Belo Horizonte, 2010. http://www.bdtd.cdtn.br//tde_busca/arquivo.php?codArquivo=132.

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Coordenação de Aperfeiçoamento de Pessoal de Nível Superior
A manipulação e o controle das propriedades magnéticas de materiais com pequenas dimensões tem atraído interesse crescente nos últimos anos. Para sistemas magnéticos micrométricos ou submicrométricos, diferentes configurações magnéticas são energeticamente acessíveis. Vórtices magnéticos merecem destaque entre essas configurações e figuram em um grande número de pesquisas tecnológicas que vão desde o armazenamento magnético (VMRAM) até a biofuncionalização de estruturas para o tratamento do câncer. Em uma configuração de vórtice magnético, a energia magnetostática é minimizada por uma configuração de caminho fechado no plano do filme e uma região central com magnetização perpendicular à superfície. A quiralidade (sentido de rotação da magnetização no plano) e a polarização (direção da magnetização na região central) são os dois principais parâmetros que caracterizam um vórtice magnético. Apesar do esforço recente, ainda não se alcançou um entendimento detalhado que permita a manipulação controlada dessas características. Um aspecto importante para a aplicação tecnológica das estruturas de vórtice magnético é a uniformidade e a reprodutibilidade do comportamento de inversão de magnetização da partícula. O tamanho do núcleo do vórtice e o valor da magnetização, fatores que dependem fortemente da anisotropia do sistema, são aspectos relevantes a serem considerados para que as aplicações destas estruturas magnéticas se tornem realidade. Neste trabalho, arranjos regulares de discos multicamadas Co/Pt com diâmetro de 1 e 2 μm e pemalloy com diâmetro na faixa de 5 a 17 μm, ambos com espessura nanométrica, foram investigados por Microscopia de força magnética (MFM) e magnetometria (VSM e PPMS). Um dos objetivos foi investigar a correlação entre a anisotropia magnética nas multicamadas e o tamanho do núcleo do vórtice magnético. Os resultados obtidos demonstraram a presença de estados de vórtice magnético em algumas das amostras estudadas, em função do diâmetro do disco. Além disso, foram estudadas propriedades magnéticas da configuração de vórtices magnéticos desde a nucleação à aniquilação e efeitos de variação de dimensões de disco (diâmetro e espessura) e anisotropia magnética (multicamadas Co/Pt).
The manipulation and control of magnetic properties in size reduced materials have attracted a great interest in the last years. For micrometric or submicron magnetic structures different magnetic configurations are energetically accessible. Magnetic vortex noteworthy belongs to those configurations, and often represents the lowest energy configuration. Nowadays, it appears in a number of technological research ranging from the magnetic storage (VRAM) to the biofunctionalized microdisks for cancer treatment. In a magnetic vortex configuration, magnetostatic energy is minimized by in-plane closed flux domain structure and this curling magnetization turns out of the plane at the centre of the vortex structure. The chirality (direction of rotation of the in-plane magnetization) and polarization (up or down direction of the vortex core) are two topological features that characterize a magnetic vortex. In spite of the great effort on this matter, a controlled manipulation of magnetic vortex features was not reached. A critical aspect for the technological application of magnetic vortex structures is the uniformity and reproducibility of the reversal behavior of the particle magnetization. The vortex core size and the related value of its overall magnetization are also very relevant for the use of such magnetic structures. It is usually considered that the size of the vortex core depends on parameters such as anisotropy, thickness and diameter of the magnetic disk. In this work, regular arrays of Co/Pt multilayers disks with diameter of 1 and 2 μm and pemalloy disks with diameter in the range 5 -17 μm, both nanometer-thick, were investigated by Magnetic Force Microscopy (MFM) and magnetization measurements (VSM and PPMS). The results show the existence of magnetic vortex states for the samples, depending on the disk diameter. Furthermore, it was investigated the magnetic properties of the magnetic vortex, since the nucleation to annihilation, and the effect of variation of disk dimensions (diameter and thickness) and magnetic anisotropy (Co/Pt multilayers).

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5

Zhang, Wenxu. "Magnetism, Structure and their Interactions". Doctoral thesis, Saechsische Landesbibliothek- Staats- und Universitaetsbibliothek Dresden, 2008. http://nbn-resolving.de/urn:nbn:de:bsz:14-ds-1215950409324-02446.

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In my work, magnetic and structural transitions of three categories of compounds are investigated by density functional calculations under local spin density approximation (LSDA). The first compound is Rh2MnGe with full Heusler structure at ambient condition. However, the structure is unstable at T=0 according to our calculations. A more stable structure we found is tetragonal one with either extension or compression along c-axis. The electronic reason of this distortion is the band Jahn-Teller effect where the Jahn-Teller active states are 4d states of Rh which is accidently put at the Fermi level by spin splitting. Then, magnetic moment behavior under pressure in itinerant compounds is investigated in four cubic Laves phase compounds (YFe2, ZrFe2, HfFe2, and LuFe2). The magnetic spin moment is decreased under pressure. A magnetic collapse where the spin moment vanishes is predicted under pressure around 20 GPa for Zr and Hf compounds, 40 GPa for Y and Lu compounds. The behavior of the magnetic moment is the result of competition between magnetic exchange interactions and kinetic energy during the compression of the volume, as described by the Stoner model. The last material investigated is CoO using LDSA+U in order to describe the strong Coulombic interaction of the transition metal ion. The pressure induced a magnetic transition, which was discovered in experiments, is explained by the competition between ligand field splitting and exchange energy. The ligand field splitting is increased under pressure, and suppresses the intraatomic exchange. As a result, the spin state changes from high spin to low spin, and at last to nonmagnetic state.

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6

Pojar, Mariana. "Estudo das propriedades magnéticas de um objeto microestruturado através do SNOM-MO". Universidade de São Paulo, 2008. http://www.teses.usp.br/teses/disponiveis/43/43134/tde-30032009-110159/.

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Nanociência e nanotecnologia se tornaram palavras-chaves para o desenvolvimento científico da maioria das áreas de pesquisa, inclusive a de magnetismo. Por esta razão, há forte demanda por instrumentos capazes de caracterizar sistemas em escalas nanométricas. O Microscópio Ótico de Varredura em Campo Próximo no Modo Magnetoótico (SNOM-MO) se insere neste contexto por ser uma técnica de microscopia com boa resolução espacial e alta sensibilidade magnética, estimada em DM= 2 x 10-12 emu. Ao contrário da microscopia ótica tradicional, os SNOMs detetam a radiação eletromagnética evanescente e conseqüentemente, a resolução não é limitada pelo critério de Rayleigh. O SNOM-MO demonstrou ser uma poderosa ferramenta para obter informações magnéticas locais através de medidas de susceptibilidade diferencial e de ciclos de histerese locais. Utilizando essa segunda ferramenta, foi feito um mapeamento micromagnético experimental do vetor magnetização sobre um objeto quadrado de 2m de CoFeSiBNb amorfo. As medidas obtidas forneceram informações sobre as duas quiralidades existentes em uma estrutura de domínios magnéticos de fechamento, cujo comportamento é determinado pela anisotropia de forma. O estudo também evidenciou que pinnings gerados por defeitos na superfície do objeto exercem grande influência sobre a evolução dos vetores magnetização. Devido à grande quantidade de informações locais este estudo se torna uma potencial base de dados para o desenvolvimento de modelos teóricos mais precisos e completos. Os resultados experimentais obtidos apresentam resolução melhor que 125 nm. O mapeamento nos possibilitou acesso a comportamentos magnéticos intrinsecamente locais que motivaram uma interessante discussão sobre informações de pinnings magnéticos, rotações da magnetização, campos de reversão, processos de reversibilidade da magnetização e anisotropia local. Além disso, atenção especial foi dada para a otimização instrumental dessa técnica com o objetivo de tornar o SNOM-MO um instrumento de medida com resolução nanométrica. Entre esses esforços destacamos a produção de pontas pela técnica do FIB e a introdução de um novo sistema ótico que contribuiu de forma significativa para um melhor controle da polarização da luz.
Nanoscience and nanotechnology have become keywords for scientific development in most areas of research, including magnetism. For this reason, there is a strong demand for tools devoted to nano-scale characterization. The Magnetooptical Scanning Near Field Optical Microscope (SNOM-MO) falls in this context due to be a technique of microscopy with high spatial resolution and magnetic sensitivity, estimated to be DM = 2 x 10-12 emu. In contrast to traditional optical microscopes, SNOM deals with evanescent electromagnetic radiation and, consequently, the resolution is no longer limited by the Rayleigh criterion. The SNOM-MO is a powerful tool to obtain local magnetic information through differential susceptibility and local hysteresis loops. Using this last technique, an experimental micromagnetic mapping was made for the magnetization vector on a square amorphous CoFeSiBNb object. The experimental results obtained provided information about the two chiralities existing in its closure magnetic domain structure, whose behavior is determined mainly by the shape anisotropy. The study also showed that pinnings generated by defects on surface´s object exerted great influence on the dynamic of the magnetization vectors. Due to the large amount of local magnetic information, this kind of study becomes a potential background for the development of more accurate and complete theoretical models. The experimental results demonstrate resolution better than 125 nm. This study has allowed us to access intrinsic magnetic behaviors that motivated an interesting discussion about magnetic pinnings, rotation of magnetization, reversal magnetic fields and local anisotropy. In addition, also special attention was given to the optimization of instrumental technique in order to make the SNOM-MO a measurement instrument with nanometer resolution. Among these efforts we emphasize the production of tips by FIB technique and the introduction of a new optical system which has significantly contributed to a better control of polarization of light.

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7

Nascimento, Ulisses Magalhães. "Preparação, caracterização e testes catalíticos de um fotocatalisador magnético (Fe3O4/TiO2) na degradação de um poluente-modelo: acid blue 9". Universidade de São Paulo, 2013. http://www.teses.usp.br/teses/disponiveis/75/75132/tde-23042013-112144/.

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A aplicação de semicondutores no tratamento de água e efluentes líquidos é uma tecnologia de remediação ambiental promissora, em especial para poluentes orgânicos. Entre os vários semicondutores que também são fotocatalisadores, o TiO2 é amplamente usado em aplicações ambientais, por ser inerte biológica e quimicamente, ter elevado potencial de oxidação, baixo custo e estabilidade frente à corrosão. Entretanto, o TiO2 também tem algumas desvantagens, tais como: ele é excitado apenas por luz UV e requer uma operação unitária adicional (por exemplo, filtração ou centrifugação) para o reuso do catalisador. Para contornar estas limitações, usou-se um procedimento simples para a síntese de um fotocatalisador magnético (Fe3O4/TiO2) com alta área superficial específica e atividade catalítica, quando comparado com o TiO2 P25 da Evonik. O fotocatalisador foi sintetizado através de um procedimento em três etapas: (1) Partículas α-Fe2O3 foram obtidas por precipitação de uma solução de FeCl3.6H2O 0.01 mol L-1, que foi submetida a uma hidrólise forçada à 100°C por 48 h; (2) Partículas de α-Fe2O3/TiO2 foram obtidas por heterocoagulação de oxi-hidróxidos de Ti(IV) sobre as partículas de α-Fe2O3, as quais foram calcinada a 500°C por 2 h; e (3) As partículas \"casaca/caroço\" do fotocatalisador foram obtidas por calcinação a 400°C por 1 h sob atmosfera redutora (H2). A atividade fotocatalítica do material sintetizado foi avaliada aplicando-o no descoramento de uma solução do corante Azul Ácido 9 (C.I. 42090). Os efeitos do pH e da concentração de catalisador foram estimados por meio de um planejamento fatorial 22. Foi obtido um fotocatalisador com área superficial específica de 202 m2 g-1, facilmente separável do meio reacional em aproximadamente 2 min com o auxílio de um ímã. O fotocatalisador apresentou absorção em toda a região do visível. A maior remoção de cor (54%) foi obtida com pH 3,0, 1,0 g L-1 de catalisador e 2 horas de reação.
The use of semiconductors for treating polluted waters and wastewaters is a promising environmental remediation technology, especially for organic pollutants. Among the several semiconductors that are also photocatalysts, TiO2 is extensively used for environmental application, due to its biological and chemical inertness, high oxidation power, low cost, and stability regarding corrosion. However, TiO2 also has some disadvantages, such as: it is only UV-excited and requires an additional unit operation (e.g. filtration or centrifugation) for reuse purposes. In order to work around those limitations, a simple procedure for synthesizing a magnetic photocatalyst (Fe3O4/TiO2), with high specific surface area and good photocatalytic activity when compared to Evonik\'s TiO2 P25, was used. The photocatalyst was synthesized in a three-step procedure: (1) α-Fe2O3 particles were obtained, by precipitation, from FeCl3.6H2O 0.01 mol L-1, which underwent a forced acid hydrolysis at 100°C for 48 h; (2) α-Fe2O3/TiO2 particles were obtained, by heterocoagulation, of Ti(IV) oxide species on the α-Fe2O3, followed by calcination at 500°C for 2 h; and (3) The core/shell photocatalyst particles were obtained by calcination the α-Fe2O3/TiO2 particles at 400°C for 1 h under reducing atmosphere (H2). The photocatalytic activity of the synthesized material was assessed by the color removal of an Acid Blue 9 (C.I. 42090) dye solution. pH and catalyst dosage effects were estimated by a 22 factorial design. Fe3O4/TiO2 core/shell particles with specific surface area of 202 m2 g-1were obtained. They were easily separated from the reaction medium, in approximately 2 min, with the aid of a magnet. The photocatalyst absorbed radiation throughout the visible spectrum. The greatest color removal (54%) was achieved with pH 3.0, 1.0 g L-1 of photocatalyst, and 2 h of reaction.

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8

Jaqueto, Plinio Francisco. "Magnetism of a speleothem from Midwest Brazil and paleoclimatic implications". Universidade de São Paulo, 2016. http://www.teses.usp.br/teses/disponiveis/14/14132/tde-31052017-165938/.

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This thesis provides a detailed study of environmental magnetism in a speleothem. It focuses on a stalagmite from Pau DAlho cave (15°1220S, 56°4841W) located in Rosário dOeste, Mato Grosso State, Brazil. This speleothem grew during the past 1355 years, with average growth rate of ~168 mm/ka and encompasses two key events in the South American Summer Monsoon (SASM), the Medieval Climate Anomaly (MCA) and the Little Ice Age (LIA), major dry and wet, respectively. The rock magnetic experiments conducted, include isothermal remanent magnetization (IRM) and anhysteretic remanent magnetization (ARM) acquisition curves, thermal demagnetization of three-axis IRM acquisition, hysteresis loops, first order reversal curves (FORC) and low-temperature SQUID magnetometry experiments. The main magnetic remanence carriers in the stalagmite are magnetite and goethite, with a nearly constant relative proportion. Magnetite has remanent coercivities between 14-17 mT, and its magnetic properties are similar to those produced by pedogenic processes. Magnetic remanence is broadly correlated with carbon and oxygen isotope data throughout most of the speleothem, suggesting that precipitation and soil dynamics above the cave exert a strong control on the input of magnetic minerals into the Pau dAlho cave system. Dry periods like the MCA are associated with less stable soils that result in higher mineral fluxes carried into karst systems via groundwater, while conversely, colder and wetter periods like the LIA are associated with soils topped by denser vegetation that are more capable of retaining micrometer-scale pedogenic minerals, and thus reduce mineral fluxes into karst environments.
Esta tese fornece um estudo detalhado do magnetismo ambiental de espeleotemas. Este estudo é feito em uma estalagmite da caverna Pau D\'Alho (15 ° 12\'20 \"S, 56 ° 48\'41\" W), localizado em Rosário d\'Oeste, Mato Grosso, Brasil. Este espeleotema cresceu durante os últimos 1355 anos, com taxa média de crescimento de ~ 168 mm/ka e engloba dois eventos climáticos do Sistema de Monção Sul-americano (SMSA), a Anomalia Climática do Medieval (ACM) e a Pequena Idade do Gelo (PIG), eventos secos e molhados, respectivamente. Os experimentos de magnetismo de rocha incluem: magnetização remanecte isotermal (MRI), ciclos de histerese, magnetização remanente anisterética (MRA), desmagnetização térmica em três eixos, first order reversal curves (FORC) e experimentos de baixa temperatura. Os principais portadore magnéticos na estalagmite são magnetita e goethita, com uma proporção relativa quase constante. A magnetita tem coercividades entre 14-17 mT, e as suas propriedades magnéticas são semelhantes às produzidas por processos pedogênicos. As remanências magnéticas são amplamente correlacionadas com dados de isótopos de carbono e oxigênio durante o registro, sugerindo que a precipitação e a dinâmica do solo acima da caverna exerce um forte controle na entrada de minerais magnéticos no sistema de cavernas Pau d\'Alho. Períodos secos como o ACM estão associados a solos menos estáveis, que resultam em maiores fluxos de minerais detríticos carreados para o sistema de cavernas, ao passo que, inversamente, os períodos frios e chuvosos como a LIA estão associados a solos cobertos pela vegetação mais densa que são mais capazes de reter minerais pedogênicos de escala micrométrica, e, assim, diminuir os fluxos de minerais detríticos para o sistema de cavernas.

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Ghannadzadeh, Saman. "Investigating magnetism and superconductivity using high magnetic fields". Thesis, University of Oxford, 2014. http://ora.ox.ac.uk/objects/uuid:4b78618e-89a3-424e-a673-59d363a2605d.

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This thesis investigates a number of transition-metal coordination polymers and iron-pnictide superconductors through the use of high magnetic fields, low temperatures, and on occasion, high pressures. The thesis will begin by describing my development of the proximity detector dynamic susceptometer, a novel technique that can be used for magnetometery and transport measurements in high magnetic fields. This technique is highly compact and has no moving parts, making it suitable for use in pressure cells, hence opening the way for a variety of new experiments. Through high-field magnetometery and other measurements, I will demonstrate that the pressure can be used to directly control the magnetic properties of the polymeric magnet CuF₂(H₂O)₂(pyrazine). In particular, I observe a transition from quasi-two-dimensional to quasi-one-dimensional antiferromagnetism at 9~kbar, driven by the rotation of the Jahn-Teller axis. I will then present a series of measurements on two coordination polymers, showing how a small chemical difference can lead to drastically different magnetic properties. I show that [Cu(pyrazine)H₂O(glycine)₂]ClO₄ is an excellent spin-chain, while the sister compound [Cu(pyrazine)(glycine)]ClO₄ is a dimerised material that shows a spin-gap and is disordered down to very low temperatures, but then undergoes a field-induced phase transition to an ordered phase. I will also describe a series of pulsed-field measurements of the upper critical field of the iron-based superconductors NaFe_1-xCo_xAs across the whole of the doping phase diagram. It is shown that paramagnetic pair-breaking effects dominate the critical field when the field is parallel to the crystal planes. In the parent compound the paramagnetic limit is equal to that expected from BCS theory, but becomes significantly enhanced above the BCS limit upon doping. It is shown that the multi-band nature of the superconductivity leads to a convex curvature in the evolution of the critical field as the temperature is reduced.

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10

Fan, Yichun. "Optical Characterization of Magnetism in Magnetic/Nonmagnetic Heterostructures". W&M ScholarWorks, 2013. https://scholarworks.wm.edu/etd/1539623362.

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This thesis focuses on the study of the static and dynamic magnetic interactions in ferromagnetic/nonmagnetic heterostructures using interface-specific and time-resolved optical techniques. The goal of this thesis is to elucidate the interface exchange coupling, magnetic anisotropy, and coherent spin dynamics in these advanced material systems, crucial to the realization of high performance spintronic devices.;First, a pronounced exchange bias (EB) phenomenon is observed in Fe/MgO (001) by magnetic second harmonic generation. The bulk magnetization does not, in marked contrast to typical systems where EB is manifested only in the net magnetization. The magnitude of the exchange bias varies with interface oxygen concentration, suggesting that the pinning layer originates from local FeO nanoclusters formation. Temperature and strain dependent studies show that the lattice mismatch between MgO and Fe enhances the FeO nanoclusters blocking temperature above room temperature. Our results have broad implications for understanding ferromagnet/oxide heterostructures, and provide new insights into the interface spin system and exchange bias.;Second, the magnetization reversal process within the first two iron layers at the Fe/GaAs(001) interface is found to be different and independent from the Fe bulk, as measured by magnetic second-harmonic generation and magneto-optical Kerr effect, respectively. The interface magnetization is largely noncollinear from the bulk with an abrupt magnetic boundary and an anisotropic exchange coupling stiffness, weak inter-layer coupling but relatively strong intra-layer stiffness. In contrast, Fe/GaAs(110) exhibits a rigid coupling between interface and bulk magnetization suggesting that the interfacial bonding structure can dramatically change the nature of the exchange coupling. These results are consistent with the observation of noncollinear alignment of interface and bulk magnetization in Fe/MgO(001), and also relevant to other magnetic/non-magnetic interfaces with abrupt chemical bond structures.;Last, the relaxation mechanism of coherent spin precession is investigated in single crystalline Fe/CoO/MgO(001) heterostructure by time-resolved magneto-optical Kerr effect. at 78K, the intrinsic damping property is enhanced by AF spins in CoO layer for thicknesses of 2.5 nm and 4 nm. In contrast, for thicknesses of 1 nm and 1.5 nm or at room temperature, the damping process is dominated by a dephasing effect caused by disordered AF spin clusters.

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11

Zhang, Wenxu. "Magnetism, Structure and their Interactions". Doctoral thesis, Technische Universität Dresden, 2007. https://tud.qucosa.de/id/qucosa%3A23720.

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In my work, magnetic and structural transitions of three categories of compounds are investigated by density functional calculations under local spin density approximation (LSDA). The first compound is Rh2MnGe with full Heusler structure at ambient condition. However, the structure is unstable at T=0 according to our calculations. A more stable structure we found is tetragonal one with either extension or compression along c-axis. The electronic reason of this distortion is the band Jahn-Teller effect where the Jahn-Teller active states are 4d states of Rh which is accidently put at the Fermi level by spin splitting. Then, magnetic moment behavior under pressure in itinerant compounds is investigated in four cubic Laves phase compounds (YFe2, ZrFe2, HfFe2, and LuFe2). The magnetic spin moment is decreased under pressure. A magnetic collapse where the spin moment vanishes is predicted under pressure around 20 GPa for Zr and Hf compounds, 40 GPa for Y and Lu compounds. The behavior of the magnetic moment is the result of competition between magnetic exchange interactions and kinetic energy during the compression of the volume, as described by the Stoner model. The last material investigated is CoO using LDSA+U in order to describe the strong Coulombic interaction of the transition metal ion. The pressure induced a magnetic transition, which was discovered in experiments, is explained by the competition between ligand field splitting and exchange energy. The ligand field splitting is increased under pressure, and suppresses the intraatomic exchange. As a result, the spin state changes from high spin to low spin, and at last to nonmagnetic state.

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12

Poole, A. L. "Magnetism in frustrated magnets revealed by neutron polarimetry". Thesis, University College London (University of London), 2011. http://discovery.ucl.ac.uk/1302402/.

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The work in this thesis uses neutron polarimetry and representational analysis to illuminate complex magnetic structures. The combination of these techniques is particularly suited to examining magnetic materials that have frustrated magnetic order and domain structures. The materials that are investigated in depth are Er2Ti2O7 and MnWO4. Er2Ti2O7 is a member of the pyrochlore family of materials and exhibits the classic signs of geometric frustration. The material has been proposed as an XY antiferromagnet that selects a specic basis vector ground state due to an order-by-disorder transition. The previous experimental work could not fully determine the precise details of the ground state and hence was not able to fully conrm the proposed theory. The structure was examined using neutron polarimetry and representational analysis to try and determine the magnetic order at low temperature. MnWO4 is an example of a magnetic material with complex order and frustration that arises due to competing exchange interactions. The material has a cycloidal magnetic structure that breaks the inversion symmetry and gives rise to dierent k-domains. The population of these dierent domains is intrinsically linked to the electronic polarization of the material, such that when one domain is populated MnWO4 has a spontaneous electric polarization and is belongs to the multiferroic family of materials. By using representation analysis the number of parameters that is required to describe the magnetic structure is greatly reduced and the link between inversion symmetry breaking and multiferroicity may be better understood. This thesis aims to identify the structures of both Er2Ti2O7 and MnWO4 as well as develop the interpretation of the polarimetry techniques.

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13

Riley, Melissa Alessandra. "The use of magnets in biomedical applications". Thesis, University of Birmingham, 2000. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.364498.

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14

Radovanovic, Pavle V. "Synthesis, spectroscopy, and magnetism of diluted magnetic semiconductor nanocrystals /". Thesis, Connect to this title online; UW restricted, 2004. http://hdl.handle.net/1773/8494.

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15

Raanaei, Hossein. "Tailoring Properties of Materials at the Nanoscale". Doctoral thesis, Uppsala : Uppsala University, 2009. http://urn.kb.se/resolve?urn=urn:nbn:se:uu:diva-107425.

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16

Felton, Solveig. "Tunable Magnetic Properties of Transition Metal Compounds". Doctoral thesis, Uppsala : Acta Universitatis Upsaliensis : Univ.-bibl. [distributör], 2005. http://urn.kb.se/resolve?urn=urn:nbn:se:uu:diva-5939.

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17

Henderson, John. "SPIN QUANTUM DYNAMICS IN MOLECULAR MAGNETS". Doctoral diss., University of Central Florida, 2009. http://digital.library.ucf.edu/cdm/ref/collection/ETD/id/3535.

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Molecular magnets are ideal systems to probe the realm that borders quantum and classical physics, as well as to study decoherence phenomena in nanoscale systems. The control of the quantum behavior of these materials and their structural characteristics requires synthesis of new complexes with desirable properties which will allow probing of the fundamental aspects of nanoscale physics and quantum information processing. Of particular interest among the magnetic molecular materials are single-molecule magnets (SMMs) and antiferromagnetic (AFM) molecular wheels in which the spin state of the molecule is known to behave quantum mechanically at low temperatures. In previous experiments the dynamics of the magnetic moment of the molecules is governed by incoherent quantum tunneling. Short decoherence times are mainly due to interactions between molecular magnets within the crystal and interactions of the electronic spin with the nuclear spin of neighboring ions within the molecule. This decoherence problem has imposed a limit to the understanding of the molecular spin dynamics and the sources of decoherence in condensed matter systems. Particularly, intermolecular dipolar interactions within the crystal, which shorten the coherence times in concentrated samples, have stymied progress in this direction. Several recent works have reported a direct measurement of the decoherence time in molecular magnets. This has been done by eliminating the dephasing created by dipolar interactions between neighboring molecules. This has been achieved by a) a dilution of the molecules in a liquid solution to decrease the dipolar interaction by separating the molecules, and b) by polarizing the spin bath by applying a high magnetic field at low temperatures. Unfortunately, both approaches restrict the experimental studies of quantum dynamics. For example, the dilution of molecular magnets in liquid solution causes a dispersion of the molecular spin orientation and anisotropy axes, while the large fields required to polarize the spin bath overcome the anisotropy of the molecular spin. In this thesis I have explored two methods to overcome dipolar interactions in molecular magnets: a) studying the dynamics of molecular magnets in single crystals where the separation between magnetic molecules is obtained by chemical doping or where the high crystalline quality allows observations intrinsic to the quantum mechanical nature of the tunneling of the spin, and b) studying the electronic transport through an individual magnetic molecule which has been carefully placed in a single-electron transistor device. I have used EPR microstrip resonators to measure Fe17Ga molecular wheels within single crystals of Fe18 AFM wheels, as well as demonstrating, for the first time in a Single Molecule Magnet, the complete suppression of a Quantum Tunneling of the Magnetization transition forbidden by molecular symmetry.
Ph.D.
Department of Physics
Sciences
Physics PhD

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18

DAMENTO, MICHAEL ANTHONY. "DETECTION OF MAGNETIZATION REVERSAL IN A NEODYMIUM-IRON-BORON MAGNET USING A HALL-EFFECT MICROPROBE". Diss., The University of Arizona, 1986. http://hdl.handle.net/10150/183945.

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Magnetization processes in a sintered Nd-Fe-B permanent magnet (NEOMAX-35) were examined on a small scale using a Hall-effect microprobe with an active area 75 μm on a side. Probes were made by evaporating bismuth through a stencil mask onto glass slides. Experiments were performed by placing a probe onto the polished pole face of a Nd-Fe-B magnet and inserting the probe-magnet assembly into an electromagnet. Barkhausen steps, indicating rapid domain wall motion, were observed (superimposed upon the blank probe signal) in the demagnetization of a fully magnetized magnet. Magnetization traces for a thermally demagnetized Nd-Fe-B magnet did not exhibit measurable Barkhausen steps until a field of approximately 1.2 T was applied. The following observations were made for two thermally demagnetized samples which were cycled through minor hysteresis loops (maximum applied field of approximately 2 T): (1) virgin magnetization traces did not contain measurable Barkhausen steps, however all other forward and reverse magnetization traces did; (2) the initial reverse magnetization trace exhibited more and larger Barkhausen steps than subsequent traces; and (3) some Barkhausen steps were repeatable, that is, occurring at approximately the same field on each subsequent forward or reverse trace. Hall voltage signals were on the order of millivolts for probe currents of 10 mA.

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19

López, Mir Laura. "Transport phenomena and magnetism in nanostructures of lanthanum manganitebased oxide thin films". Doctoral thesis, Universitat Autònoma de Barcelona, 2018. http://hdl.handle.net/10803/462765.

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Els òxids basats en manganites de lantà formen una extensa família de compostos derivats del LaMnO3: una perovskita de la forma general ABO3. Les propietats físiques d’aquesta es poden modi car substancialment per substitució catiònica. En aquesta tesi estudiem dos compostos obtinguts per substitució dels cations A i B. D’una banda, el La2Co0.8Mn1.2O6 (LCMO), obtingut per substitució del Mn per Co (catió B), que conforma una estructura doble perovkita amb propietats aïllants i ferromagnètiques. D’altra banda, el La0.7Sr0.3MnO3 (LSMO) format per substitució parcial de La per Sr (catió A) i que resulta en un material metàl.lic i ferromagnètic. El desenvolupament de tècniques de creixement de materials òxids en forma de capes primes els dóna un gran potencial per ser integrats en la tecnologia de semiconductors i per dissenyar dispositius micro i nanomètrics per a aplicacions en spintrònica i memòries no-volàtils. Actualment hi ha gran interès en l’estudi de capes primes de perovskites dobles amb propietats ferromagnètiques i aïllants, ja que tenen una temperatura de transició relativament alta i són fàcils d’integrar sobre altres òxids. La implementació d’aïllants ferromagnètics com a substituts dels dispositius ferromagnètics basats en metalls pot conduir a una e ciència de commutació molt més gran i un transport de corrent menys dissipatiu. Assolir capes ultra primes d’aquests materials permetria obtenir una barrera túnel activa que selecciona l’espín, per tant actua com a ltre d’aquest. Així, l’objectiu principal de la primera part d’aquesta tesi és comprendre les propietats físiques del LCMO i obrir una via per futures aplicacions potencials. En primer lloc, es presenta un estudi detallat del creixement i la caracterització estructural i magnètica de capes primes de LCMO. Demostrem que podem obtenir capes d’alta qualitat, amb temperatures de transició de ns a 230 K i amb un bon ordre catiònic tot i petites desviacions de l’estequiometria. A continuació, analitzem els efectes d’anisotropia magnètica centrats en els aspectes següents: 1) l’estudi de l’aparició d’una forta anisotropia magnètica perpendicular (PMA) amb un alt camp coercitiu i d’anisotropia, i 2) la relació del signe i la força de l’anisotropia magnètica amb els desajustos estructurals amb el substrat. En particular, la PMA apareix en el cas que la tensió sigui expansiva mentre que la tensió compressiva produeix un eix fàcil d’imantació dins el pla. Per trobar una explicació a l’origen de l’anisotropia magnètica desenvolupem un model atòmic simple basat en càlculs de primer ordre de teoria de pertorbacions. A més, relacionem les prediccions extretes amb experiments de dicroisme magnètic circular de raigs X (XMCD tot demostrant que l’anisotropia magnètica en l’LCMO té un origen magnetocristal.lí a causa del fort acoblament d’espín-òrbita del Co2+. Amb l’objectiu d’integrar les propietats aïllants ferromagnètiques i la PMA en un dispositiu, hem fabricat unions túnel utilitzant el LCMO com a barrera magnèticament activa i hem explorat les seves funcionalitats d’espín. Hem trobat que el dispositiu proporciona una alta e ciència de ltratge d’espín (gairebé el 100% de polarització) i funcionalitats de sensor i memòria anisotròpica. És a dir, que la forta PMA induïa per la tensió epitaxial provoca una gran diferència entre les corbes de magnetoresistència mesurades amb el camp magnètic aplicat perpendicular o parallelament; aquesta és l’anomenada magnetoresistència túnel anisotròpica (TAMR). S’han trobat valors de TAMR de 20-30%. Finalment, s’ha provat que el dispositiu pot funcionar com una memòria magnètica ja que podem detectar dos estat no volàtils de resistència ben diferenciats que s’escriuen canviant la direcció del camp magnètic aplicat. En la darrera part de la tesi, s’han presentat resultats basats en l’altre sistema de manganita, el LSMO. Mostrem que les inestabilitats del creixement poden conduir a la formació de superfícies de doble terminació. Allunyar-se del comportament ideal de creixement constitueix una forma d’obtenir nanoestructures formades espontàniament amb propietats funcionals locals en superfícies. Les propietats de transport i la composició d’aquestes superfícies s’analitzen utilitzant tècniques de sonda d’escaneig i microscopia electrònica de fotoemissió amb resolució lateral. Aquestes tècniques sensibles a la superfície són adequades per caracteritzar les propietats a la nanoescala d’aquest tipus de sistemes.
Lanthanum manganite-based oxides conform an extensive family of compounds deriving from LaMnO3: a perovskite with general formula ABO3. Its physical properties can be drastically modified by cation substitution. In this thesis we explore two compounds obtained by A-site and B-site substitutions. On the one hand, La2Co0.8Mn1.2O6 (LCMO), obtained by substitution of Mn by Co (B-site substitution), leads to a double perovskite structure with ferromagnetic insulating properties. On the other side, the La0.7Sr0.3MnO3 (LSMO) compound resulting from the partial substitution of La by Sr (A-site substitution) turns the material into a ferromagnetic half-metal. The development of techniques for the growth of oxide materials in the form of thin films ease their integration in semiconductors technology and enable the design of micro and nanoscale devices with potential in spintronics and non-volatile memory applications. Nowadays, there is an increasing interest in the study of double perovskite thin films combining ferromagnetic and insulating properties due to their relatively high transition temperature and their integration on top of other perovskite oxides. The implementation of ferromagnetic insulators (FM-I) as substitutes of ferromagnetic metal-based devices can lead to much higher switching eficiency and less dissipative current transport. For instance, ultrathin films of (FM-I) might be used as spin-selective active tunneling barriers acting as spin-filters. Therefore, the main objective of the first part of this thesis has been to give a comprehensive understanding of LCMO physical properties and propose a guide to potential applications. Firstly, we present a detailed study of the growth of the LCMO films as well as their structural and magnetic characterization. We demonstrate that we can obtain high quality thin films, with transition temperatures up to 230 K and with good cationic order despite a certain degree of off-stoichiometry. Then, we analyze magnetic anisotropy effects, focusing on the following aspects: i) the study of the appearance of strong perpendicular magnetic anisotropy (PMA) with large coercive and anisotropy fields, and ii) the relation of magnetic anisotropy strength and sign with lattice mismatches. In particular, we show that PMA appears for the tensile strain case while compressive strain produces in-plane easy axis. We also give a more detailed understanding of the origin of magnetic anisotropy using a simple atomistic model based on first-order perturbation theory calculations.We relate our predictions with X-ray magnetic circular dichroism (XMCD) experiments and evidence that magnetic anisotropy in LCMO has a magnetocrystalline origin due to the strong spin-orbit coupling of Co2+ ions. With the aim of integrating ferromagnetic insulating properties and PMA in a device, we have fabricated tunnel junctions using LCMO as a magnetically active barrier and have explored its spin-derived functionalities. We have found that the device provides high spin-filtering effciency (of almost 100% of spin-polarization) as well as anisotropic sensing and memory functionalities. This is, the strong straininduced PMA provokes a large difference between magnetoresistance curves measured with the magnetic field applied in the perpendicular or parallel directions, this phenomenon is the so called tunneling anisotropic magnetoresistance (TAMR). TAMR values as high as 20-30% have been found. Finally, we have proven that the device can be used as a magnetic memory as we can detect the existence of two nonvolatile resistive state that switches depending on the direction of the magnetic field used to write it. The last part of thesis presents results focused on the A-substituted manganite, LSMO, thin films. We show that growth instabilities can lead to the formation of double-terminated surfaces. Indeed, deviations from the ideal growth behaviour constitute a way to obtain spontaneously formed nanostructures with modulated local functional properties at the surface. The transport properties and the composition of these films have been analyzed by making use of scanning probe techniques and space-resolved photoemission electron microscopy, which are surface-sensitive techniques suitable to characterize properties at the nanoscale of this type of systems.

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20

Lim, Joshua A. "The role of local versus itinerant magnetism : studies of dilute magnetic semiconductors and multi-k magnets". Thesis, University of Birmingham, 2013. http://etheses.bham.ac.uk//id/eprint/4639/.

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The electronic properties of the materials studied in this thesis: a dilute magnetic semiconductor and a member of the actinide family, are thought to lie on the edge of local and itinerant behaviours. The role of localised versus itinerant magnetism is indirectly explored by characterising the magnetism through a range of experimental techniques. Reports of magnetism in dilute magnetic semiconductors have been largely conflicting, with most focusing on thin films. This work characterises high quality bulk single crystals of Cr-doped TiO\(_2\) and finds no magnetic ordering down to 4K. This suggests that the observed thin film magnetisation is a result of non-equilibrium or impurity phases and lattice strains. In the canonical 3-k magnet, USb, the spin waves soften at a temperature, T* (well below T\(_n\)), with no change in magnetic or structural symmetries. It had been suggested that this was due to de-phasing of the different Fourier components making up the 3-k state: this was tested using inelastic polarised neutrons and found not to be the case. Instead, the effects at T* are likely linked to a change in itinerancy. The magnetic domain dynamics are probed using X-ray photon correlation spectroscopy and find changes to the domains near T\(_n\) and also T*.

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21

Chakraborty, Arnab. "Magnetism of Nanocrystallized Amorphous Fe75B10Si15". Thesis, KTH, Materialvetenskap, 2012. http://urn.kb.se/resolve?urn=urn:nbn:se:kth:diva-107191.

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Amorphous ribbons of alloy composition Fe75B10Si15 are cast by melt spinning and annealed to partially nanocrystalline states. The magnetic properties are investigated by VSM and MTGA. Structure is examined using XRD and SEM. Results obtained show nanostructured material with excellent soft magnetism in samples annealed at temperatures below the crystallization temperature as well as enhancement of magnetic hardness for annealing at high temperatures. This validates Herzer’s Random Anisotropy model of magnetism in nanostructured materials and provides basis for further inquiry into tweaking alloy compositions and/or manipulating annealing parameters. Also, increase of Curie temperature is noted with respect to increasing annealing temperatures arising from stress relaxation, validating a study on the relationship between the two.

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22

Edström, Alexander. "Theoretical and Computational Studies on the Physics of Applied Magnetism : Magnetocrystalline Anisotropy of Transition Metal Magnets and Magnetic Effects in Elastic Electron Scattering". Doctoral thesis, Uppsala universitet, Materialteori, 2016. http://urn.kb.se/resolve?urn=urn:nbn:se:uu:diva-304666.

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In this thesis, two selected topics in magnetism are studied using theoretical modelling and computational methods. The first of these is the magnetocrystalline anisotropy energy (MAE) of transition metal based magnets. In particular, ways of finding 3d transition metal based materials with large MAE are considered. This is motivated by the need for new permanent magnet materials, not containing rare-earth elements, but is also of interest for other technological applications, where the MAE is a key quantity. The mechanisms of the MAE in the relevant materials are reviewed and approaches to increasing this quantity are discussed. Computational methods, largely based on density functional theory (DFT), are applied to guide the search for relevant materials. The computational work suggests that the MAE of Fe1-xCox alloys can be significantly enhanced by introducing a tetragonality with interstitial B or C impurities. This is also experimentally corroborated. Alloying is considered as a method of tuning the electronic structure around the Fermi energy and thus also the MAE, for example in the tetragonal compound (Fe1-xCox)2B. Additionally, it is shown that small amounts (2.5-5 at.%) of various 5d dopants on the Fe/Co-site can enhance the MAE of this material with as much as 70%. The magnetic properties of several technologically interesting, chemically ordered, L10 structured binary compounds, tetragonal Fe5Si1-xPxB2 and Hexagonal Laves phase Fe2Ta1-xWx are also investigated. The second topic studied is that of magnetic effects on the elastic scattering of fast electrons, in the context of transmission electron microscopy (TEM). A multislice solution is implemented for a paraxial version of the Pauli equation. Simulations require the magnetic fields in the sample as input. A realistic description of magnetism in a solid, for this purpose, is derived in a scheme starting from a DFT calculation of the spin density or density matrix. Calculations are performed for electron vortex beams passing through magnetic solids and a magnetic signal, defined as a difference in intensity for opposite orbital angular momentum beams, integrated over a disk in the diffraction plane, is observed. For nanometer sized electron vortex beams carrying orbital angular momentum of a few tens of ħ, a relative magnetic signal of order 10-3 is found. This is considered realistic to be observed in experiments. In addition to electron vortex beams, spin polarised and phase aberrated electron beams are considered and also for these a magnetic signal, albeit weaker than that of the vortex beams, can be obtained.

Felaktigt ISBN i den tryckta versionen: 9789155497149

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23

Villuendas, Pellicero Diego. "Magnetic deflagration in Mn₁₂-ac and Nd₅Ge₃ : new techniques and phenomena". Doctoral thesis, Universitat de Barcelona, 2016. http://hdl.handle.net/10803/396118.

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The goal of this work is to investigate the magnetic deflagration phenomena in two very different magnetic systems, using two very different experimental techniques. In the first chapter of this dissertation I introduce the concept of magnetic deflagra-tion, together with a description of the state of the art of the field. In the second chapter of this thesis I present the study of magnetic deflagration in single crystals of the prototypical single molecule magnet Mn12—ac, using the magneto-optical imaging method; never done before. In the chapter the reader will find out that, effectively, the deflagration process can be explored using this technique. The second part of the thesis include the other five chapters. Such a difference in length between both parts is due two factors. The first factor is that the Mn12—ac is a very well known system, with few new experiments to study; while on the contrary, the Nd5Ge3 intermetallic compound has been barely studied in its single crystalline form, and therefore there are many experiments to conduct to understand its dynamics. The second factor is the rich phenomena that this intermetallic compound presents directly related with the abrupt magnetic changes that possesses. In this system sudden changes occur in every studied physical property when the magnetization changes in a stepped manner. Two fundamental properties of the magnetic dynamics of the Nd5Ge3 system make it so remarkable. First, it is one of few systems with a large irreversible AFM-+FM transition induced by the magnetic field, and second, the magnetic changes during its magnetization processes occur in a stepped manner. Therefore, the motivation to investigate the possibility of finding for the first time magnetic deflagration phenomena in a ferromagnet, and also for the first time in two different magnetic phases of a given material, was very high. In the first two chapters dedicated to the Nd5Ge3 I explore the magnetic properties, the heat capacity and the electric resistivity. Some of the measurements were not previously reported in a single crystal. In the fifth chapter I explore explicitly the spon-taneous field-induced avalanches in magnetism, heat capacity and resistivity. The reason of this three chapters is to have as much ingredients as possible to be able to answer the questions that the possible magnetic deflagrations would raise. I say possible because it is not until the fifth chapter of this thesis when we get the experimental confirmation that the field-induced spontaneous avalanches that the compound present correspond to magnetic deflagration phenomena. In this chapter the magnetic deflagrations were studied in detail in the AFM-+FM transition and in the FM reversal, as a function of the magnetic field and the temperature. The spontaneous and induced deflagrations are explored, obtaining good estimations of the propagation speed and the temperature of the deflagration flame. Using the theoretical framework of the magnetic deflagrations we fit the speed propagation of the front to the experimental data using only one pa-rameter, the heat diffusivity of the material; obtaining a value within the range of heat diffusivities found in other intermetallic compounds.
El objetivo que persigue esta tesis es impulsar el estudio de las deflagraciones magnéti-cas gracias, por una parte al descubrimiento del fenómeno en un sistema nuevo y pro-metedor como es el compuesto intermetálico Nd5Ge3, y por otra a la presentación de un método nuevo de medición de las dependencias espacio-temporales de las mismas utilizando técnicas magneto-ópticas. Manteniendo el hilo conductor del fenómeno de las deflagraciones magnéticas, esta tesis doctoral se divide en dos partes. En la primera parte presento mis investigaciones en el estudio del sistema Mn12—ac. A partir de los tratamientos de los videos obtenidos se confirma la presencia de deflagraciones magnéticas. La segunda parte de la tesis está dedicada al compuesto intermetálico Nd5Ge3. Éste compuesto se trata de uno de los pocos sistemas en los que mediante un campo magnéti-co externo se induce espontáneamente un estado ferromagnético (FM) con gran irre-versibilidad proviniendo de un estado antiferromagnético (AFM). Además, los cambios magnéticos que experimenta el sistema, tanto dicha transición AFM—>FM como la in-versión de la magnetización en el estado FM, ocurren de forma muy abrupta, siendo también uno de los escasos sistemas que presenta esta propiedad. Dedico tres capítulos al estudio de sus propiedades magnéticas, térmicas y eléctri-cas, tanto estáticas como dinámicas. En esas medidas encuentro fenómenos interesantes, desde generación espontánea de voltaje durante las deflagraciones magnéticas, hasta la aparición de saltos espontáneos de la magnetización con el tiempo (manteniendo la tem-peratura y el campo magnético constantes), pasando por la obtención de términos de origen antiferromagnético en la dependencia térmica de la capacidad calorífica del estado ferromagnético saturado, o una magnetorresistencia gigante entre ambos estados, entre otros. En el sexto capítulo, las medidas experimentales confirman la existencia del fenómeno de la deflagración magnética en ambas fases, AFM y FM. La velocidad de propagación del frente obtenida en la teoría de deflagraciones se ajusta bien a los datos experimen-tales. Utilizando la bondad del ajuste, extrapolamos la velocidad teórica hacia campos magnéticos elevados y encontramos la posibilidad de que ésta iguale o supere la velocidad del sonido en el material. Lo más remarcable es que esta posible transición se observa en la extrapolación para campos menores de 50 kOe. Por lo que, en principio, reduciendo la temperatura podríamos ser capaces de obtener medidas de dicha transición. Sin em-bargo, el estudio de las deflagraciones espontáneas en función de la temperatura llevado a cabo en un criostato de dilución resultó un claro ejemplo de serendipia. En vez de alcanzar velocidades supersónicas, lo que encontré fueron unas discontinuidades de salto en los campos de deflagración espontánea no predichas. Por lo tanto, el capítulo pasa a enfocarse en su estudio, concluyendo que su origen está relacionado con propiedades intrínsecas del Nd5Ge3.

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24

Quintana, Puebla Alberto. "Enhanced magnetoelectric effects in electrolyte-gated nanoporous metallic alloy and dense metal oxide films". Doctoral thesis, Universitat Autònoma de Barcelona, 2018. http://hdl.handle.net/10803/663838.

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Esta tesis abarca el estudio de propiedades magnetoeléctricas en aleaciones magnéticas y metálicas nanoporosas, y en capas densas de óxidos de metales de transición. La naturaleza interfacial de los procesos magnetoeléctricos ha hecho que históricamente el estudio de estos fenómenos se abordara en sistemas de elevada relación superficie/volumen, limitándose muchas veces a capas ultradelgadas (1-2 nm). En esta tesis, se postula una nueva forma de afrontar el estudio de estos procesos, basada en el uso de materiales nanoporosos los cuales se caracterizan por tener una relación superficie/volumen muy elevada y una pared de poro o ligamento de pocos nanómetros. De esta forma, se han obtenido efectos magnetoeléctricos en materiales cuyo grosor global supera el centenar de nanometros. La síntesis de los materiales de interés se ha llevado a cabo por métodos de deposición electroquímica. Específicamente, se ha sacado partido de la formación de micelas al disolver copolímeros bloque en soluciones acuosas por encima de la concentración micelar crítica. Estas micelas quedan atrapadas durante el proceso de electrodeposición, actuando como agente moldeador. Usando este enfoque, se han podido sintetizar diferentes muestras de distintas morfologías y composiciones de la aleación cobre-níquel. La aplicación de voltaje se ha realizado haciendo uso de electrolitos, aprovechando la formación de una doble capa eléctrica. Con la intención de obtener efectos magnetoeléctricos puros (acumulación de carga) y evitar procesos oxidativos simultáneos, se ha utilizado un electrolito orgánico aprótico. Con este método, se pueden obtener campos eléctricos del orden de centenares de MV/cm. Gracias a este elevado campo eléctrico, junto con la enrome relación superficie/volumen de los materiales nanoporosos, se ha obtenido una disminución de la coercitividad de una muestra nanoporosa de Cu25Ni75 en un 32 %. Simulaciones ab-initio atribuyen estos cambios a modificaciones en la energía de anisotropía magnética adscritos a la acumulación de cargas electrostáticas en la aleación. En una segunda aproximación, se han realizado estudios de procesos de oxidación-reducción en medios acuosos (1M NaOH) controlados por voltaje, en este tipo de aleaciones. Después de aplicar potenciales positivos, se ha visto una modificación de un 33 % en la magnetización, debido a la oxidación selectiva del cobre en una muestra nanoporosa de Cu20Ni80. La oxidación resulta en una aleación enriquecida en níquel y, por ende, en una aleación con mayor momento magnético. En esta tesis, también se ha demostrado la idoneidad de la técnica de deposición por capas atómicas para producir recubrimientos conformales en materiales nanoporosos. Se ha visto que esta técnica permite preservar la integridad morfológica y estructural de la capa activa, asentando así las bases para aplicaciones en estado sólido. En la última parte de esta tesis, se ha demostrado la posibilidad de inducir ferromagnetismo mediante la aplicación de voltaje eléctrico en capas densas de Co3O4. El campo eléctrico aplicado da lugar a una migración iónica controlada, resultando en regiones ricas en oxígeno y otras en cobalto, estas últimas originando el ferromagnetismo. Este experimento es una de las primeras evidencias de movimiento iónico inducido por voltaje a temperatura ambiente y sin la necesidad de utilizar capas donadoras/aceptores de oxígeno (en otras palabras, sin fuentes o sumideros de oxígeno).
This Thesis covers the study of the magnetoelectric response in nanoporous metallic alloy and transition metal oxide dense films. The interfacial nature of magnetoelectric processes, independently of its origin, has limited its study to ultrathin film configurations (usually 1-2 nm). Here we propose a novel approach to overcome this thickness limitation, thus achieving magnetoelectric response in materials whose overall thickness is larger than 100 nm. To accomplish this, we have employed nanoporous materials, with pore walls and ligands of very few nanometers, which are characterized by a large surface to volume ratio. These materials have been synthesized by micelle assisted electrodeposition. Micelles get trapped during the electrodeposition process thus acting as a soft templating agent, allowing us to synthesize nanoporous copper-nickel alloy films with tunable composition and morphology. Voltage application has been performed through electrolyte-gating, taking advantage of the generation of an electrical double layer in aprotic organic electrolytes which helps to avoid spurious oxidation processes. This method allows for the application of electric fields as high as hundreds of MV/cm. Thanks to the high electric field achieved, together with the ultrahigh surface area of nanoporous materials, a 32 % reduction in the coercivity of a Cu25Ni75 nanoporous film has been achieved. Ab-initio simulations attribute this large effect to changes in the magnetic anisotropy energy due to charge accumulation in the sample|electrolyte interface. In a second approach, the voltage control of redox processes has been studied in aqueous electrolytes (1M NaOH). After positive bias application up to a 33 % reduction in the magnetization has been achieved in a Cu20Ni80 nanoporous sample thanks to the selective Cu oxidation. The controlled oxidation process resulted in an enriched Ni alloy which possesses a larger magnetic moment. Moreover, we have demonstrated the suitability of atomic layer deposition to conformally coat the nanoporous alloys, preserving the morphology and structure, thus setting the basis for future solid state applications. In the last part of this Thesis, it has been demonstrated that, upon electric field application, a ferromagnetic response arises in a paramagnetic single Co3O4 layer, at room temperature. The applied voltage promotes the ionic diffusion, resulting in oxygen rich and cobalt rich regions, being the latter the responsible of the induced magnetic signal. This experiment is one of the first evidences of ionic motion at room temperature without the assistance of oxygen buffer layers such as Gd2O3 or HfO2.

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25

Brandl, Ana Lucia. "Propriedades magnéticas de sistemas nanocristalinos". [s.n.], 2004. http://repositorio.unicamp.br/jspui/handle/REPOSIP/278234.

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Orientador: Marcelo Knobel
Tese (doutorado) - Universidade Estadual de Campinas, Instituto de Fisica Gleb Wataghin
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Resumo: Sistemas magnéticos granulares são constituídos de pequenas partículas magnéticas imersas numa matriz não magnética. Essas partículas têm formas e tamanhos variados, eixos de anisotropia variados e orientados aleatoriamente e, dependendo do tipo de matriz (isolante ou condutora) e da concentração do material magnético, diferentes tipos de interações magnéticas podem estar presentes. Esses materiais apresentam diversas propriedades físicas interessantes, como magneto-resistência gigante e efeito Hall gigante. Devido à complexidade desses sistemas, a sua magnetização só pode ser calculada analiticamente em dois casos limites: quando a temperatura é zero (modelo Stoner-Wohlfarth) ou quando a temperatura é alta (modelo de Langevin). Embora o modelo de Langevin seja aplicado com bastante sucesso para temperaturas acima da temperatura de bloqueio média (TB) do sistema, mostramos nesse trabalho que os resultados podem ser enganosos, fornecendo parâmetros estruturais muito diferentes dos reais. Essas discrepâncias podem ser atribuídas a efeitos de interações magnéticas e a efeitos de anisotropia, ambos desconsiderados no formalismo de Langevin. Os principais resultados experimentais apresentados nesta dissertação foram obtidos de um conjunto de filmes granulares do tipo metal-isolante, com partículas nanocristalinas de Co imersas numa matriz amorfa de SiO2, fabricados por evaporação catódica. A caracterização magnética foi realizada através de medidas de magnetização em função do campo, susceptibilidade resfriada com e sem campo magnético aplicado e magnetização termo-remanente. A caracterizção estrutural foi realizada através de medidas de microscopia de transmissão de elétrons, difração de raio-x e espalhamento de raio-x a baixo ângulo
Abstract: Granular magnetic systems are formed by magnetic grains whose size is of the order of a few nanometers, embedded in a non-magnetic (insulating or metallic) matrix. These ultrafine particle systems present size, shape, and anisotropy distributions, besides randomly orientated easy directions. Magnetic interactions always exist, being stronger or weaker according to the volume concentration and the matrix type. These systems have shown interesting magnetotransport properties, as giant magnetoresistance and giant Hall effect. Owing to the inherent complexity of the nanostructure, the magnetization can be analytically calculated only in two limiting cases: when T = 0 (Stoner-Wohlfarth model) or for high temperatures (Langevin model). The Langevin model presents very good results when applied at temperatures higher than the mean blocking temperature (TB) of the system. However this adequacy can be just apparent: the obtained structural parameters are very different from the real ones, as we show in this work. These discrepancies can be attributed to magnetic interactions andanisotropy effects, both unconsidered in the Langevin formalism. The main results presented in this thesis were obtained from a set of metal-insulator granular films, composed of Co nanoparticles immersed in an amorphous SiO2 matrix. The films were produced by magnetron co-sputtering. The magnetic characterization was perfomed with magnetization loops, zero-field cooled and field cooled susceptibilities, and thermoremanent magnetization. The microstructural characterization was done by transmission electron microscopy, x-ray diffraction, and small angle x-ray scattering
Doutorado
Física
Doutor em Ciências

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26

Igarashi, Ricardo Noboru. "Estudo teórico de nanoestruturas magnéticas em superfícies metálicas". Universidade de São Paulo, 2012. http://www.teses.usp.br/teses/disponiveis/43/43134/tde-26032013-144125/.

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Neste trabalho, utilizamos o método de primeiros princípios RS-LMTOASA (Real Space - Linear Muffin-Tin Orbital - Atomic Sphere Approximation), baseado na Teoria do Funcional da Densidade (DFT Density Functional Theory) e implementado para o cálculo de estruturas magnéticas não colineares, para investigar as propriedades magnéticas de nanoestruturas adsorvidas em superfícies metálicas. Primeiramente, devido ao aparecimento de estruturas magnéticas complexas, foram estudadas nanoestruturas magnéticas depositadas em substratos ferromagnéticos. Foram consideradas nanoestruturas de Mn tais como nanofios adsorvidos, pirâmides e aglomerados adsorvidos e embebidos, com o tamanho destas nanoestruturas variando de dois até nove átomos depositados em Fe(001) e Fe(110). Nossos cálculos indicam uma interação de troca magnética de longo alcance entre os átomos de Mn-Mn e Mn-Fe. Além disto, a presença de uma forte dependência destas interações de troca magnética com o meio local, frustrações magnéticas e o acoplamento spin-órbita forneceram a possibilidade da presença de estruturas magnéticas complexas tais como, por exemplo, spin espiral e half-skyrmion. Por fim, com o objetivo de estudar nanoestruturas que podem apresentar altos valores de momento magnético, foram investigados nanofios de FexCo1-x adsorvidos em uma superfície de Pt(111) . Nossos resultados indicam que os momentos magnéticos de spin dos átomos de Fe e Co são independentes da concentração de Fe e apresentam valores superiores quando comparados aos das ligas FeCo bcc, enquanto que o momento magnético médio de spin do nanofio FexCo1-x varia linearmente com a concentração de Fe, comportamento este que é diferente da curva de Slater-Pauling observada na liga FeCo bcc. O momento magnético orbital médio do nanofio de FexCo1-x é monotonicamente decrescente com a concentração de Fe que se apresenta bastante diferente a monocamada FexCo1-x sobre Pt(111).
We use the first principles RS-LMTO-ASA (Real Space - Linear Muffin- Tin Orbital - Atomic Sphere Approximation) method, in the framework of the Density Functional Theory and implemented to calculate noncollinear magnetic structures, to investigate the magnetic properties of nanostructures adsorbed on metallic surfaces. First, due to presence of the complex magnetic properties, we investigated magnetic nanostructures deposited on a ferromagnetic substrate. We have considered a variety of nanostructures such as adsorbed wires, pyramids, at and intermixed clusters with sizes varying from two to nine atoms deposited on Fe(001) and Fe(110). Our calculations reveal the long-range nature of exchange interactions between Mn-Mn and Mn-Fe atoms. Moreover, the presence of the strong dependence of these interactions on the local environment, the magnetic frustration, and the effect of spin-orbit coupling lead to the possibility of realizing complex noncollinear magnetic structures such as helical spin spiral and half-skyrmion. Finally, we also investigated FexCo1-x nanowires deposited on Pt(111) surface aiming to investigate materials with large local magnetic moment. Our results reveal that the Fe and Co spin magnetic moment are independent of the Fe concentration with the enhancement of the spin magnetic moment when compared with the FeCo bcc alloys, while the average spin magnetic moment is a linear function of the Fe concentration. This is in contrast to the Slater-Pauling model observed in the FeCo bcc alloys. The average orbital magnetic moment shows a linearly decreasing behavior with the Fe concentration which is in contrast to the behavior of FexCo1-x monolayer on Pt(111) surface.

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Martins, Cezar Soares. "Magnetização e Magnetoresistência Gigante em Ligas Granulares CuNiFe". Universidade de São Paulo, 2000. http://www.teses.usp.br/teses/disponiveis/43/43133/tde-02122013-185104/.

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Neste trabalho, estudamos as propriedades magnéticas e a magnetoresistência gigante (GMR) em fitas de Cu IND.80 Ni IND.20-xFe IND.x (x = 2,5 ; 5; 10; 17, 5; 20) produzidas por melt-spinning. As fitas foram estudadas como função da temperatura de tratamento térmico T IND.an 500°C, usando-se um magnetômetro SQUID. Uma grande variedade de estruturas granulares foram obtidas para diferentes razões de Ni/Fe e condições de tratamento térmico. Para Cu80 Ni10 Fe10, a magnetização não apresentou histerese mensurável para T 50K. Este comportamento é consistente com as curvas de susceptibilidade que indicam uma temperatura de bloqueio abaixo de 50K. Nesta temperatura, o maior valor da GMR(19%) foi obtido para as fitas tratadas a 400°C por duas horas. As curvas de magnetização foram comparadas a um modelo teórico que assume uma distribuição de momentos magnéticos. Para Cu80Ni15Fe5,a microestrutura e as propriedades magnéticas são muito sensíveis ao tratamento térmico. Para as ligas com composições 10; 15; 17,5% de Fe, a magnetização de saturação apresentou uma redução com o tratamento a 400°C. Esta redução foi explicada através da formação de partículas perto da região Invar. Um comportamento anômalo linear da amplitude da magnetoresistência foi observado e explicado pelo espalhamento dependente do spin, que acontece quando um elétron se move de uma partícula superparamagnética para uma partícula bloqueada termicamente.
In this work, we study the magnetic properties and giant magnetoresistance (GMR) in ribbons of Cu80 Ni20-xFex (x = 2.5, 5, 10, 17.5, 20) prepared by melt-spinning. The ribbons were studied as a function of annealing temperature Tan 500°C, using a SQUID magnetometer. A wide variety of granular structures é obtained for different Ni/ Fe ratios and annealing conditions. In Cu80 Ni10 Fe10 , the magnetization shows no static hysteresis for T 50K. This behaviour is consistent with the susceptibility curve which indicates a blocking temperature below 50K. At this temperature, the largest GMR value was obtained for the ribbons annealed at 400°C for two hours. The magnetization curves were compared with a theoretical model that takes into account the magnetic moment distribution. In Cu80 Ni15Fe5, the microstructure and magnetic properties are much more sensitive to annealing. For t he ribbons with 10, 15, 17.5 % Fe composition, the magnetization presented a reduction with annealing at 400°C. This redution may be explained in terms of particle formation near the Invar region. An anomalous linear behaviour of the MR was observed and can be explained in terms of spin-dependent scattering when an electron passes from a superparamagnetic particle to a thermally blocked particle.

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28

Dalponte, Alessandro. "Melhoramento de sensores magnetostrictivos de deformação". reponame:Repositório Institucional da UCS, 2017. https://repositorio.ucs.br/handle/11338/2146.

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Fitas magnéticas amorfas vêm demonstrando uma ampla variedade de aplicações sensoriais devido às suas propriedades magnéticas. A capacidade de serem interrogadas remotamente permite sua aplicação em locais onde limitações físicas impedem o uso de sensores com conexões cabeadas. A capacidade de adquirirem uma anisotropia magnética transversal através de tratamentos térmicos, sob campo magnético e/ou tensão, é o que torna esses materiais atraentes para a aplicação em sensores magnetostrictivos de deformação com acesso remoto. Os materiais avaliados neste trabalho, Metglas 2826MB3, Yeke 1k501, Yeke 1k501a, Vitrovac 7600, Vitroperm 800 e as fitas de EAS Tags, apresentavam curvas de magnetização características de materiais que não possuem uma anisotropia magnética transversal. Isto serviu de motivação para a realização de tratamentos termomagnéticos. Os tratamentos foram realizados sob campo magnético transversal de aproximadamente 500 Oe e nas temperaturas de 235, 275, 315 e 350˚C por tempos de 15, 30 e 60 minutos, com um forno construído em laboratório. Isto resultou em melhorias consideráveis na forma de suas curvas ΔE, tornando-as mais assimétricas e aumentando as variações de frequência que estes materiais podem apresentar. Os tratamentos resultaram também em curvas M-H lineares, susceptibilidade magnética constante até a saturação. Foi construída uma bancada para aplicação de tensões de até 30 MPa nas amostras. A maior assimetria das curvas ΔE para o par ressonador e transdutor (partes componentes do sensor de deformação) permitiu maior resposta de variação de frequência de ressonância do sensor durante testes de tração. No melhor dos casos, uma variação de frequência superior a 5 kHz foi detectada para deformações de aproximadamente 260 ppm, ao se utilizar um par ressonador e transdutor de Metglas 2826MB3 tratados termomagneticamente. Medindo-se as curvas ΔE de uma fita de Metglas 2826MB3, tratada, sobre substratos de aço 1010, material magneticamente mole, observou-se que as mesmas ainda estavam presentes. Porém, apresentam sua forma alterada devido à interação com aço. Simulações realizadas com o software FEMM, auxiliaram na compreensão de fenômenos ocorridos nos testes de deformação e na interação entre a fita e os substratos magnéticos moles.
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Coordenação de Aperfeiçoamento de Pessoal de Nível Superior, CAPES.
Magnetic amorphous ribbons have a great variety of sensorial applications due to their magnetic properties. The possibility of being remotely queried allows their application in cases where sensors with wired connections present a physical limitation. The capability of acquiring a transverse magnetic anisotropy through thermal treatments, under applied magnetic field and/or stress, is what makes these materials attractive for application in magnetostrictive strain-gauges. The materials studied in this work, Metglas 2826MB3, Yeke 1k501, Yeke 1k501a, Vitrovac 7600, Vitroperm 800 and a commercial EAS Tags, presented magnetization curves with characteristics of materials with reduced transverse magnetic anisotropy. This was the motivation to carry out thermomagnetic treatments. The treatments were carried out under a magnetic field of approximately 500 Oe, using temperatures of 235, 275, 315 and 350˚C, for periods of 15, 30 and 60 minutes, in a home-made oven. The treatments resulted in considerable improvements on the shape of the ΔE curves, making them more asymmetric and increasing the frequency variations that these materials can present. The treatments also resulted in linear M-H curves, with constant magnetic susceptibility until saturation. A system for the application of stresses up to 30 MPa on the samples was built. ΔE curves with greater asymmetry for the pair resonator transducer (components of the strain-gauge) allowed a greater resonance frequency variation response of the sensor during strain tests. In the best case, a frequency variation above 5 kHz was detected for deformations of approximately 260 ppm, when the pair resonator transducer made of thermomagneticaly treated Metglas 2826MB3 was used. The measurements of the ΔE curves of a treated Metglas 2826MB3 ribbon, placed over 1010 steel substrates, soft magnetic material, showed that they were still detectable. However, their form was altered due to the magnetic interaction with the steel. Simulations made using the software FEMM, helped to understand the observed phenomena during the strain tests and the interaction between the ribbon and the soft magnetic substrates.

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29

Buz, Jennifer. "Recent lunar magnetism". Thesis, Massachusetts Institute of Technology, 2011. http://hdl.handle.net/1721.1/69470.

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Thesis (S.M.)--Massachusetts Institute of Technology, Dept. of Earth, Atmospheric, and Planetary Sciences, 2011.
Cataloged from PDF version of thesis. Page 37 blank.
Includes bibliographical references (p. 36).
The magnetization of young lunar samples (<1.5 Ga) is a mystery because common sources of magnetic fields (e.g. core dynamo and long-lived impact plasma fields) have not been present within the last 1.5 Ga. To better characterize the source of magnetization in young lunar samples, we conducted paleomagnetic measurements on several subsamples of lunar glass 12017, which has a formation age of -9 ka, as well as on the underlying basalt from the same sample. Various methods of magnetization were tested, including possible contamination from Earth's magnetic field, exposure to transient magnetic fields such as on the Apollo module, and interaction fields from underlying magnetized rocks. The magnetic field emanated by the rock underlying the 12017 glass was determined to be -150 nT, comparable with the noise associated with paleomagnetic techniques. This opens up a new possible source of magnetization for samples- the laboratory. It is likely that other young lunar samples' paleointensities are overestimates, and that the strengths of magnetic fields on the Moon in the last 1.5 Ga are more similar to the strengths we observed from the 12017 glass. Underlying rocks and laboratory techniques are viable sources of magnetic fields, therefore, young lunar sample magnetization can no longer be used as evidence against an ancient core dynamo. Furthermore, the basalt portion of the 12017 is unidirectionally magnetized, suggesting an ancient core dynamo on the Moon when it formed at 3.2 Ga.
by Jennifer Buz.
S.M.

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30

Robertson, Laura y A. Lowery. "Electricity and Magnetism". Digital Commons @ East Tennessee State University, 2016. https://dc.etsu.edu/etsu-works/780.

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Mishra, Subodha. "Theory of photo-induced ferro-magnetism in dilute magnetic semiconductors". Diss., Columbia, Mo. : University of Missouri-Columbia, 2006. http://hdl.handle.net/10355/4413.

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Thesis (Ph. D.) University of Missouri-Columbia, 2006.
The entire dissertation/thesis text is included in the research.pdf file; the official abstract appears in the short.pdf file (which also appears in the research.pdf); a non-technical general description, or public abstract, appears in the public.pdf file. Title from title screen of research.pdf file (viewed on August 6, 2007) Includes bibliographical references.

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32

Venkataramana, Vikash. "Neutrons to probe nanoscale magnetism in perpendicular magnetic recording media". Thesis, University of St Andrews, 2012. http://hdl.handle.net/10023/3187.

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Magnetic recording media refers to the disc shaped thin film magnetic medium present inside the hard disk drive of a computer. Magnetic recording is an important function of the hard disk drive by which information such as text, pictures, audio and videos are stored. Information is broken down to a simple binary format and is stored as magnetised bits along the tracks of the disk forming the hard drive. Over the years advancements in research on the type of magnetic materials used has allowed increased data storage capacities by reducing magnetic bit sizes. It is with this advancement in magnetic data storage, that we have today's hard disk drive technology, which uses a perpendicular magnetic medium to store data. A perpendicular magnetic medium is a multi-layered magnetic thin film structure with the topmost layer comprising nanoscale magnetic grains of high perpendicular anisotropy. The topmost recording layer (RL) is mapped into individual bits of 80-100 nm² area that consist of 5-10 nm diameter CoCrPt grains, embedded in an oxide matrix. A bit area is defined to ensure a significant number of stable grains allowing data to be stored in each bit as a ‘0' or a ‘1' depending on its switched magnetic state. The magnetic grains if sputtered below a threshold grain size tend to suffer from thermal fluctuation and instability due to super-paramagnetic effects, hence bringing limitations to grain size. As a result of this, research in recent years has been directed at introducing a softer magnetic exchange coupled composite (ECC) layer above the recording layer. This layer facilitates the delicate balance of switching smaller grains with strong magneto-crystalline anisotropy at lower magnetic fields, by exchange coupling with the CoCrPt grains in the recording layer. However this technique of increasing the efficiency in the perpendicular magnetic medium by introducing ‘facilitating' layers is an area that is still being widely researched and understood. Although numerous surface and bulk analysis techniques exist to study magnetic and surface properties of these materials, there is limited information on the structural and magnetic properties of these materials at the nanoscale level. The reported work investigates the structural and magnetic properties of the magnetic grains and multi-layers in the perpendicular magnetic medium using polarised neutron scattering and reflectivity techniques. The work investigates the structural and magnetic properties of the CoCrPt grains, apart from understanding the CoCrPt magnetic grain switching. The work also investigates the magnetisation in the layers of the thin film perpendicular media structure using polarised neutron reflectivity (PNR). Using polarised small angle neutron scattering (PolSANS), it has been shown that ferromagnetic ordered core region of the CoCrPt grain in the recording layer is smaller than the physical CoCrPt granular structure. The magnetic switching behaviour of the CoCrPt grain at different magnetic fields is also analysed and the experimental PolSANS data is fitted with non-interacting size-dependent analytical grain switching models. This result provides significant evidence that the magnetic anisotropy increases with grain size, with larger magnetic grains having larger magnetic anisotropy. Polarised neutron scattering experiments are carried out with the magnetically softer exchange coupled composite (ECC) layer included in the thin film magnetic structure. The first experiments investigate if the ECC layer contributes to the nuclear and magnetic interference scattering term in the experimenting scattering data. The experiments clearly show that there is no contribution from the ECC layer in the nuclear and magnetic scattering interference term. The role of the ECC layer in the magnetic switching process is then investigated at different magnetic fields. The ECC layer was found to influence the size-dependent magnetic grain switching of the CoCrPt grains in the recording layer and a detailed investigation is presented in the reported work. Polarised neutron reflectivity (PNR) experiments have also been carried out with the ECC layer on the perpendicular magnetic media samples. These experiments investigate the composition and thickness of the thin film structure, while also providing information on the magnetic state of the thin films under the influence of an in-plane magnetic field. The in-plane magnetisation in the recording and ECC layer is determined at different in-plane magnetic fields. The magnetisation values determined for the ECC layer and the recording layer (RL) at different in-plane magnetic fields help better understand the differences in their magnetic properties.

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33

Svensson, Jacob N. "A Study of the Magnetic Properties of Yb4LiGe4: Unusual Magnetism". Thesis, Boston College, 2010. http://hdl.handle.net/2345/1376.

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Thesis advisor: Michael Graf
The R5T4 compounds (R = rare earth, T = Ge or Si) are interesting because the magnetic properties are very sensitively dependent on slight changes in the crystalline structure. Yb5Ge4 is one such compound, with (presumed) antiferromagnetic order occurring at TN = 1.7 K. We are interested in the effects of substituting Li in place of one Yb atom. Previous measurements of the magnetic properties of polycrystalline Yb4LiGe4 using NMR, specific heat, and resistance measurements at temperatures down to 0.5 K and in magnetic fields up to 4 Tesla were made to compare results with the parent compound. The resistance measurements showed a maximum at 1.1 K, which may indicate the onset of magnetic order. Thus we performed μSR measurements on Yb4LiGe4 and Yb5Ge4, and analysis of the data confirmed magnetic ordering (possibly antiferromagnetic) at 1.1 K. The μSR measurements also revealed a dependence on the magnetic history of the sample. Currently we are studying the pressure dependence of the (presumed) Néel Temperature in order to explore whether increased pressure can drive the TN to 0 K, and results will be discussed
Thesis (BS) — Boston College, 2010
Submitted to: Boston College. College of Arts and Sciences
Discipline: Physics Honors Program
Discipline: Physics

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Lima, Valquiria Fernanda Gonçalves de. "Preparação e caracterização de nanopartículas magnéticas de Sm-Co, Nd-Fe-B, Fe-Pt e Co-Pt pelo método de agregação gasosa". Universidade de São Paulo, 2013. http://www.teses.usp.br/teses/disponiveis/43/43134/tde-26112014-135553/.

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Atualmente, nanopartículas (NPs) são utilizadas em todos os ramos da tecnologia. Suas promissoras aplicações envolvem entre outros, o campo dos sensores e transdutores, mídia de gravação magnética, carreadores magnéticos de drogas medicinais. Com o objetivo de produzir NPs pelo método físico, um gerador de nanopartículas foi adaptado usando um dos canhões do sistema de magnetron sputtering, baseando-se no método de agregação gasosa. Com o gerador somos capazes de produzir NPs de diversos materiais e codepositá-las em matrizes dielétricas ou metálicas. Neste trabalho apresentamos o desenvolvimento da metodologia para a produção de nanopartículas de materiais magnéticos duros, usando alvos de SmCo5, Sm2Co17, Nd2Fe17B, FePt e CoPt. Investigamos a influência dos parâmetros de deposição (pressão, fluxo de gás e potência de sputtering), tipo de substrato e a existência de buffer e/ou codeposição, na obtenção das propriedades estruturais e magnéticas desejadas para esses materiais. As NPs produzidas são analisadas magneticamente pelo VSM e SQUID, sua morfologia e tamanho por TEM e SEM, a sua estequiometria pelo RBS, e a sua estrutura cristalina por XRD, a fim de obter nano-ímãs de alta anisotropia magnética. Da caracterização morfológica, através de microscopia eletrônica, encontramos para as NPs produzidas e estudadas diâmetros entre 5 e 17 nm. Através de análises de RBS obtemos para composição das NPs que as mesmas possuem estequiometria diferente dos alvos usados. Estudos estruturais e magnéticos mostram que para Sm-Co, Fe-Pt e Co-Pt é possível obter NPs cristalinas e com coercividade da ordem de 1 kOe.
In the recent years, nanoparticles (NPs) are being in all fields of technology. Their promising applications involve among others, the field of sensors and transducers, magnetic recording media, magnetic carriers of medicinal drugs. Aiming to produce NPs by physical method, a generator of nanoparticles was adapted using a system of guns \"magnetron sputtering\", based on the aggregation gas method. With the generator we are able to produce NPs with different types of material. In this work, we present the development of the methodology for the production of nanoparticles of hard magnetic materials, using targets of SmCo5, Sm2Co17, Nd2Fe17B, FePt and CoPt. We investigated the influence of the deposition parameters (pressure, gas flux and sputtering power), substrate type and the existence of the buffer and/or codeposition layers, to obtain the desired structural and magnetic properties for the nanoparticles. The produced NPs were magnetically analyzed by VSM and SQUID, the morphology and size by TEM and SEM, the stoichiometry by RBS and the crystal structure by XRD. The main objective of this work is to obtain nano-magnet with high magnetic anisotropy. Through the morphological characterization by electron microscopy, we found for NPs produced and studied have diameters between 5 and 17 nm. Through RBS analysis we have obtained the composition of the NPs, and also that they have different stoichiometry in relation to the used targets. Structural and magnetic studies have show that for Sm-Co, Fe-Pt and Co-Pt it is possible to obtain crystalline NPs with coercive field around 1 kOe.

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35

Stanley, Daniel C. "MAGNETIC DAMPING IN FE3O4 THROUGH THE VERWEY TRANSITION FOR VARIABLE AG THICKNESSES". Miami University / OhioLINK, 2013. http://rave.ohiolink.edu/etdc/view?acc_num=miami1376500586.

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36

González, Alonso David. "Magnetisme i estructura en aliatges funcionals = Magnetism and structure in functional alloys". Doctoral thesis, Universitat de Barcelona, 2014. http://hdl.handle.net/10803/145681.

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Bé sigui el canvi de temperatura que experimenta un material a l’aplicar-li un camp magnètic en condicions adiabàtiques, o bé sigui el canvi d’entropia del material com a conseqüència de la modiﬁcació del camp magnètic en condicions isotermes, en tots dos casos obtenim una desposta tèrmica que es coneix com a Efecte Magnetocalòric (MCE). De fet, qualsevol variació de les variables tèrmiques es pot classificar segons el camp extern que s’hi apliqui. Així doncs, depenent de l’estímul extern obtindrem un efecte calòric diferent, com per exemple l’ esmentat efecte magnetocalòric quan s’aplica un camp magnètic extern. Als anys trenta del passat segle XX, aquest efecte magnetocalòric va oferir la possibilitat d’atansar temperatures pròximes al zero kelvin mitjançant la desmagnetització de salts paramagnètiques. Avui dia, el descobriment de nous materials juntament amb els avenços tecnològics ens ha permès reduir la distància entre la recerca i la refrigeració en estat sòlid a temperatura ambient. En resum, dins d’aquesta tesi hem estudiat una varietat de materials magnètics que presenten diferents efectes calòrics gegants i que estan demostrant ser materials molt prometedors per a la refrigeració ecològica d’estat sòlid.
Depending on whether the magnetic ﬁeld is applied adiabatically to the sample or in isothermal conditions, the thermal response of the material will result in a temperature change or in an entropy change, respectively. In both cases, one obtain a thermal response which is known as Magnetocaloric Effect (MCE). In fact, any variation of the thermal variables can be classiﬁed based on the external applied ﬁeld. Thus, depending on the nature of the external stimulus we will obtain a different caloric effect, such as the aforementioned magnetocaloric effect when applying a magnetic ﬁeld. In the 1930s this magnetocaloric effect, indeed, showed up the possibility to reach temperatures well below 1 K through the adiabatic demagnetization of paramagnetic salts. Today, the discovery of new materials together with technological improvements are bridging the gap between research and solid-state refrigeration at room temperature. In summary, within this thesis we have studied a variety of magnetic materials exhibiting different giant Caloric effects that are proving to be very promising materials for eco-friendly solid-state refrigeration purposes.

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37

Liu, Mingde. "Magnetization-steps spectroscopy in dilute magnetic semiconductors and in molecular magnetism /". Thesis, Connect to Dissertations & Theses @ Tufts University, 1998.

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Thesis (Ph.D.)--Tufts University, 1998 .
Adviser: Yaacov Shapira. Submitted to the Dept. of Physics. Includes bibliographical references. Access restricted to members of the Tufts University community. Also available via the World Wide Web;

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38

Staneva, Maya. "Theoretical study of dilute magnetic semiconductors : Properties of (Ga,Mn)As". Thesis, Uppsala universitet, Institutionen för fysik och astronomi, 2010. http://urn.kb.se/resolve?urn=urn:nbn:se:uu:diva-126096.

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The dilute magnetic semiconductor (Ga,Mn)As , which is the most interesting and promising material for spintronics applications, has been theoretically studied by using Density Functional Theory. First of all, calculations on GaAs were done and it was found that GaAs is a semiconductor with a direct band gap. The calculated value of the band gap is ~ 0.5eV. Secondly, the material iron was considered and it was confirmed that iron is a ferromagnetic metal with 2.2µB net magnetic moment. Then a magnetic impurity of manganese, Mn was introduced in the nonmagnetic GaAs and it became ferromagnetic with a net magnetic moment of 4µB. The origin of the ferromagnetic behaviour is discussed and also the Curie temperature TC of the material. It appeared that (Ga,Mn)As is a suitable material for DMS but TC has to be increased before (Ga,Mn)As could be used for spintronics applications and on that account some methods of increasing TC are considered at the end.
Den magnetiska halvledaren (Ga,Mn)As som är det mest intressanta och lovande materialet för spinelektroniska tillämpningar har teoretiskt undersökts med hjälp av Täthetsfunktionalteorin. Först gjordes beräkningar på GaAs och det visade sig att GaAs är en halvledare med direkt bandgap. Det beräknade värdet på bandgapet är ca 0.5eV. Sedan var det järn som undersöktes och det blev bekräftat att järn är en ferromagnetisk metall med netto magnetisk moment lika med 2.2μB. Då magnetiska störningar i form av mangan atomer, Mn, infördes i det omagnetiska GaAs blev halvledaren ferromagnetisk med netto magnetisk moment lika med 4μB. Orsakerna till den ferromagnetiska ordningen diskuteras och även Curie temperaturen TC för materialet. Det visade sig att (Ga,Mn)As är ett lämpligt material för tillverkning av magnetiska halvledare men TC måste ökas innan (Ga,Mn)As skulle kunna användas i spinntroniska tillämpningar och av det skälet anges i slutet vissa metoder för att öka TC.

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39

Szczech, Yolande Helen. "Magnetism in Hubbard models". Thesis, University of Oxford, 1996. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.318681.

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40

Cruz, H. R. da. "Critical phenomena in magnetism". Thesis, University of Oxford, 1985. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.355743.

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41

Walton, Stephanie Katharine. "Magnetism in frustrated nanostructures". Thesis, Imperial College London, 2014. http://hdl.handle.net/10044/1/25964.

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Artificial Spin Ice (ASI), comprised of ferromagnetic nanobars in a honeycomb geometry, attracts much attention since it is a directly imageable frustrated system which exhibits rich physics including ice-rule behaviour and magnetic monopole excitations. ASI's nanobars undergo domain wall mediated magnetic reversal in external fields. Understanding and indeed controlling the trajectories of field driven domain walls and hence the order in which ASI's nanobars are reversed is a crucial step towards mapping out ASI's full functionality for potential applications. In this thesis, trajectories of domain walls during the early stages of ASI's magnetic reversal are studied. Data showing domain walls executing non-random walks in the transverse domain wall regime and random walks in the vortex domain wall regime is presented. The former behaviour is linked to the domain wall's chirality, and as such, attempts to control a domain wall's initial chirality via triangular injection pads are discussed. In addition, ways in which a vortex domain wall's chirality may be controlled with ellipsoidal hole obstructions are shown. Artificial Dipolar 2D-XY, a complementary system to ASI, also promises interesting behaviour. In this new frustrated architecture, individual nanobars are replaced with single domain nanodiscs whose magnetisations can point in any in-plane direction. In this thesis, intriguing results from preliminary experiments on this new system are presented. One of the best techniques used to study the magnetisations of nanostructures such as those described above is Lorentz Transmission Electron Microscopy (LTEM). Since the contrast yielded for unusual magnetic states was not well documented, software called Micromagnetic Analysis to Lorentz TEM Simulation (MALTS) was developed in order to aid in analysis of LTEM images. MALTS can simulate the LTEM contrast of any magnetic object of any size, shape or state. A description of its full functionality is also included in this thesis.

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42

Schmehr, Julian Leonard. "Incommensurate magnetism in UAu2". Thesis, University of Edinburgh, 2015. http://hdl.handle.net/1842/17958.

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The aim of this thesis was to identify a candidate material for spin-triplet superconductivity with a two-component order parameter. This unconventional superconducting state is thought to allow for exotic quantum states such as Majorana fermions. A wide survey was taken into potential candidate materials, and UAu2 was chosen for in-depth investigation. This little-studied hexagonal heavy fermion compound's unusual resistivity behaviour, combined with a series of interesting features in magnetisation and heat capacity, make it an extremely interesting material to study. The phase diagram of UAu2 was determined with measurements of heat capacity, resistivity, magnetisation and magnetoresistance on the first single crystalline samples of this material. No superconductivity was detected. Instead, a range of magnetic phase transitions were observed, which were further investigated with muon-spin relaxation experiments and time-of- ight neutron powder diffraction. UAu2 was found to undergo a transition to an incommensurate antiferromagnetic state (q1 = (1=3; 1=3; δ)) below TN = 43:5 K, but then develops signatures of weak ferromagnetism below T = 20 K. The ferromagnetism coincides with a 2q magnetic structure, with a coexistence of q1 and q2=(1/3,1/3,0). The magnetic structures of both phases were found to be most likely amplitude-modulated, with moments aligned along the crystallographic c-axis. A transition to a ferromagnetic state was observed in magnetic fields applied parallel to the c-axis. TN was found to remain almost constant in applied magnetic fields up to 9 T, while hydrostatic pressures of up to 6 kbar weakly suppress the antiferromagnetic transition temperature. The field-induced transition was found to be strongly pressure-dependent, shifting to higher applied fields with increasing pressure. The residual resistivity of UAu2 samples prepared by both the Czochralski method and quenching from the melt is relatively large, which may inhibit Cooper pairing and hence may be the reason for the absence of superconductivity in the samples investigated. Solid-state electrotransport (SSE) equipment was developed, which can induce the motion of a crystal's constituents and thereby lead to vastly improved sample quality. Refinement of UAu2 samples with SSE could be a further step in the search for spin-triplet superconductivity in this material.

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43

Conlon, Peter Hugh. "Aspects of frustrated magnetism". Thesis, University of Oxford, 2010. http://ora.ox.ac.uk/objects/uuid:66d30c85-3ccf-4887-b0de-b3f2ee983ffe.

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Models of magnetism show complex collective behaviour which arises from simple interactions among microscopic degrees of freedom. Upon cooling from high temperatures conventional magnets typically undergo a phase transition to a magnetically ordered phase due to microscopic interactions which favour an ordered state. In frustrated magnets however, competing microscopic interactions place non-trivial constraints on the allowed configurations at low temperature, without selecting a unique ordered state. In this thesis, we investigate the collective behaviour of a paradigmatic frustrated magnet, the classical Heisenberg model on the pyrochlore lattice with antiferromagnetic nearest neighbour interactions. Within a self consistent Gaussian approximation, we derive analytic expressions for correlation functions which match Monte Carlo simulations extremely well at all temperatures. We study the precessional dynamics of the model and provide a comprehensive description of the dynamics by constructing an analytically tractable stochastic model by extending the self-consistent Gaussian approximation to include dynamics. We relate these results to other highly constrained models. Real experimental systems often have features that go beyond the phenomenology afforded by the simplest models; we investigate the effects of further neighbour interactions on paramagnetic spin correlations, and propose further neighbour interactions as the mechanism underlying experimentally observed patterns of scattering in frustrated spinel compounds. In the dynamics linearized around a ground-state, the macroscopic degeneracy of the classical model leads to modes with zero frequency. Small perturbations stabilize ordered states and lift such zero modes to finite frequency. The ordered state has two widely separated energy scales both of which affect the dynamics, the leading scale of nearest neighbour exchange coupling, and the much smaller scale of the perturbation which relieves the frustration. We investigate the interplay between these widely different energy scales in setting the mode frequencies in states ordered by weak interactions.

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44

Walsh, James Paul Slater. "Anisotropy in molecular magnetism". Thesis, University of Manchester, 2014. https://www.research.manchester.ac.uk/portal/en/theses/anisotropy-in-molecular-magnetism(11474b91-0d3d-4b0a-97cd-214d1713674e).html.

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A collection of studies are reported that focus on the examination of exchange interactions in complexes containing paramagnetic ions with a large magnetic anisotropy. A number of complementary techniques are used to analyse the complicated systems that arise, including high-field high-frequency electron paramagnetic resonance, inelastic neutron scattering, SQUID magnetometry, and ab initio calculations. The nuclearity of the complexes ranges from dimetallic, to trimetallic, to octametallic. A family of five water- and carboxylate-bridged nickel(II) dimetallics are the focus of a magneto-structural correlation study that succeeds in measuring the magnitude of the exchange interaction despite dominating effects from large zero-field splitting effects. Similar work is reported for four cobalt(II) analogues of these compounds, with the relationship between exchange interactions and geometry also being probed by pressure INS. Charge density studies that combine high resolution X-ray and neutron diffraction studies are reported on cobalt and nickel analogues from the same family of dimetallics, revealing strong evidence for non-direct exchange. A family of four trimetallic triangle complexes containing two nickel(II) ions and one chromium(III) ion bridged by a central fluoride and a total of six carboxylates are reported, and the exchange interactions are elucidated from a global model that accounts for the low-field magnetic, heat capacity, and EPR data. Two new octametallic vanadium(III) wheels—where each pair of adjacent metals are bridged by a fluoride and two carboxylates—are reported along with preliminary results from magnetic measurements and solid state proton NMR spectra, which reveal significant field-dependent effects arising from level crossings at high fields.

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45

Hagelberg, Frank. "Magnetism in Carbon Nanostructures". Digital Commons @ East Tennessee State University, 2017. https://dc.etsu.edu/etsu_books/151.

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Magnetism in carbon nanostructures is a rapidly expanding field of current materials science. Its progress is driven by the wide range of applications for magnetic carbon nanosystems, including transmission elements in spintronics, building blocks of cutting-edge nanobiotechnology, and qubits in quantum computing. These systems also provide novel paradigms for basic phenomena of quantum physics, and are thus of great interest for fundamental research. This comprehensive survey emphasizes both the fundamental nature of the field, and its groundbreaking nanotechnological applications, providing a one-stop reference for both the principles and the practice of this emerging area. With equal relevance to physics, chemistry, engineering and materials science, senior undergraduate and graduate students in any of these subjects, as well as all those interested in novel nanomaterials, will gain an in-depth understanding of the field from this concise and self-contained volume.
https://dc.etsu.edu/etsu_books/1164/thumbnail.jpg

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46

Piccin, Rafael. "Interações magnéticas dipolares entre fios e microfios magnéticos". [s.n.], 2004. http://repositorio.unicamp.br/jspui/handle/REPOSIP/278210.

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Orientador: Marcelo Knobel
Dissertação (mestrado) - Universidade Estadual de Campinas, Instituto de Fisica Gleb Wataghin
Made available in DSpace on 2018-08-03T22:09:03Z (GMT). No. of bitstreams: 1 Piccin_Rafael_M.pdf: 28985729 bytes, checksum: afa3d6ef86c868ccfd0239d4dfa72438 (MD5) Previous issue date: 2004
Resumo: Nesta dissertação de mestrado são apresentados resultados do efeito do campo dipolar magnético em conjuntos formados por fios e microfios ferromagnéticos amorfos de composição Fe77,5Sil2,5B15. Devido à estrutura de domínios peculiar destes fios magnéticos, eles podem ser, em princípio, aproximados a dipolos magnéticos, permitido a análise destes campos magnetostáticos nestas estruturas macroscópicas. Os estudos foram realizados em conjuntos de fios colocados lado a lado. As curvas de magnetização foram medidas variando-se o número de elementos. Além disso, no caso de dois fios, medidas em função da separação entre os dois elementos foram realizadas. Estas medidas magnéticas foram feitas empregando um magnetômetro desenvolvido durante o projeto e também utilizado um magnetômetro SQUID. Os resultados são explicados considerando o campo dipolar gerado por cada fio do conjunto, que altera os valores do campo de inversão da magnetização e faz com que platôs sejam observados nas curvas de magnetização. Os efeitos do campo dipolar também foram observados em medidas de magnetoimpedância, onde a presença do fio rico em Fe desloca as curvas de magnetoimpedância de um fio amorfo base Co, de modo análogo a um campo bias
Abstract: The results of the magnetic dipolar field in an array of amorphous ferromagnetic wires and microwires of composition Fe77,5Si12,5B15 are presented in this dissertation. Due to their peculiar domain structure, in principle, they can be approximated to magnetic dipoles, a11owing the analysis of the magnetostatic field among these macroscopic entities. The studies were carried out in arrays of wires placed side by side. The magnetic loops were measured changing the number of elements. Moreover, in the case of two parallel wires, measurements as function of the distance between the wires were performed. The magnetic measurements were carried out in an inductive magnetometer developed during this project and a1so using a SQUID magnetometer. The results are explained considering the dipolar field created by each wire in the array. The dipolar field changes the reversal field of the wires, a11owing the appearance of plateaus during the demagnetization. The effects of the dipolar field were also observed in magnetoimpedance measurements when a Fe-rich wire in placed near an amorphous Co-based wire, its presence dislocates the Co-based wire magnetoimpedance curves, ana1ogously as a bias field
Mestrado
Física
Mestre em Física

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47

Landi, Gabriel Teixeira. "Desenvolvimento de um gerador de nanopartículas e caracterização de nanopartículas de cobalto". Universidade de São Paulo, 2009. http://www.teses.usp.br/teses/disponiveis/43/43134/tde-12052009-092829/.

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Neste trabalho, desenvolvemos um gerador de nanopartículas (NPs) como uma adaptação para um sistema de magnetron sputtering. Com ele, somos capazes de produzir NPs de materiais diversos e codepositá-las em matrizes dielétricas ou metálicas. A adaptação consiste em incluir uma região de alta pressão relativa de Ar no caminho do vapor atômico removido do alvo. A aglomeração ocorre termodinamicamente devido a diminuição da energia cinética após colisões com o gás. Desenvolvemos também, uma metodologia para colimar o fluxo de NPs dentro da região de alta pressão. A deposição é feita no substrato na forma de uma mancha com alguns milímetros de diâmetro e o tempo de preparação da amostra é significativamente curto. Desenvolvemos um modelo fenomenológico para explicar a condensação e a colimação do nosso sistema. Este, apesar de não sofisticado, explica bem ambos os fenômenos e consegue prever o diâmetro das nanopartículas para certas condições. Em paralelo ao desenvolvimento, produzimos e caracterizamos nanopartículas de cobalto. Da caracterização morfológica, através de microscopia eletrônica, concluímos que as NPs produzidas tem diâmetros médios de 10 nm com uma dispersão de 13 %. Através de análises de retro espalhamento Rutherford estudamos a distribuição do material sobre o substrato e observamos que este segue uma distribuição Gaussiana de espessuras. Além disso, devido a colimação, observamos que as taxas de deposição são da ordem de 50 vezes maiores que as taxas usuais de um sistema de sputtering. Estudos estruturais através de difração de raios X mostraram que as nanopartículas são nanocristalinas e imagens em alta magnificação de microscopia eletrônica de transmissão comprovaram esta hipótese. Finalmente, estudos magnéticos mostraram que as NPs não possuem eixos preferenciais de magnetização. Desenvolvemos condições padrões de operação e estabilizamos o sistema que atualmente produz amostras confiáveis e reprodutíveis. Além do Co, nanopartículas de Cu e SmCo foram produzidas em condições parecidas. A morfologia destas partículas foi investigadas por microscopia eletrônica e seus tamanhos se mostraram próximos dos das NPs de Co. Estes resultados ilustraram a universalidade do nosso sistema de deposição de nanopartículas.
We have developed a nanoparticle (NP) generator by adapting one of the sputtering guns on a magnetron sputtering system. With it, we are able to produce nanoparticles with different types of material. The adaptation consists of including a high-pressure region in the path of the atomic vapor removed from the sputtering target. The condensation happens thermodynamically through the loss of kinetic energy that the atomic vapor suffers after collisions with the gas. We have also developed a methodology to collimate the flow of nanoparticles inside the high pressure region. The deposition on the substrate is in the form of a stain with a few millimeters in diameter. The sample preparation time is also relatively short. We created a phenomenological model to explain both the condensation and collimation phenomena in our system. Despite being relatively simple, this model explain both quite well. In parallel to the development of the system, we produced and characterized cobalt nanoparticles. From a morphological analysis, carried out using electron microscopy, we determined that the nanoparticles mean diameter is of about 10 nm with a dispersion of 13 %. Through Rutherford back-scattering analysis, we studied the thickness distribution of the sample along the substrate. We observed that it follows a Gaussian distribution. Also, because of the collimation of the material, the deposition rates are about 50 times higher than in a regular sputtering system. Using X ray diffraction we were able to determine that the NPs are nano-crystalline which is corroborated with high resolution transmission electron microscopy images. Finally, magnetic measurements showed that the nanoparticles do not have any preferential magnetization axis. We developed standards of operations and stabilized the system. The samples we produce are trustworthy and reproducible. Besides Co, Cu and SmCo NPs were produced using this system with conditions similar to the ones used on the Co NPs. Through morphological analysis, we determined that their sizes are also similar. These results illustrate the universality of our system.

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48

Coaquira, José Antonio Huamaní. "Propriedades Magnéticas e Hiperfinas das Ligas Zr(FexCr1-x)2 e seus Hidretos". Universidade de São Paulo, 1998. http://www.teses.usp.br/teses/disponiveis/43/43133/tde-11032014-164100/.

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Investigamos as propriedades magnéticas e as interações hiperfinas das ligas \'ZR\'\'(\'FE IND. x\'\'CR IND. 1-x\') IND. 2\' (0.3 < x < 0.7) e seus hidretos, através de medidas de magnetização em campos até 9 T, suscetibilidade AC e espectroscopia Mossbauer. A difração de raios X confirmou a estrutura de fase de Laves hexagonal Cl4 para todas as amostras. Os espectros Mossbauer das ligas à temperatura ambiente apresentaram dois dubletos quadrupolares, que podiam ser atribuídos de duas maneiras aos sítios cristalográficos 2a e 6h do Fe. Esta ambigüidade foi resolvida por um cálculo de cargas pontuais, com as posições atômicas determinadas por refinamento Rietveld. Medidas magnéticas e de Mössbauer a baixas temperaturas indicaram que todas as ligas da faixa de concentrações estudada apresentaram comportamento de vidros de spin, sem ordem magnética de longo alcance. As temperaturas de congelamento de spins observadas puderam ser descritas em seu conjunto pela equação de Vogel-Fulcher. A concentração crítica de Fe para a formação de momentos magnéticos foi determinada. O efeito da absorção de hidrogênio foi investigado: a) com o teor máximo de H em todas as ligas, b) com teores intermediários na liga \'ZR\'\'(\'FE IND. 0.5\'\'CR IND. 0.5\') IND. 2\'. Nesta última, os espectros Mössbauer mostraram a coexistência de uma fase pobre e uma fase rica em H. O desdobramento quadrupolar do Fe no sítio 6h apresentou variação com a temperatura, atribuída a efeitos de difusão de hidrogênio. O principal efeito do hidrogênio sobre o magnetismo das ligas foi um incremento notável dos momentos magnéticos, porém sem aparecimento de ordem ferromagnética; ao contrário, observou-se um pequeno abaixamento das temperaturas de congelamento de spins.
We have investigated the magnetic properties and hyperfine interactions in the \'ZR\'\'(\'FE IND. x\'\'CR IND. 1-x\') IND. 2\' (0.3 < x < 0.7) alloys and their hydrides, by means of magnetization measurements in fields up to 9 T, AC susceptibility and Mõssbauer spectroscopy. X-ray diffraction confirmed the Laves phase Cl4 hexagonal structure for all samples. The room temperature Mõssbauer spectra of all samples showed two quadrupolar doublets, which could be assigned in two ways to the Fe crystallographic sites 2a and 6h. This ambiguity was solved by a point-charge calculation using atomic positions determined by Rietveld refinement. Magnetic and Mõssbauer measurements at low temperatures indicated that all investigated samples showed spin-glass behavior, without long-range magnetic order. The set of observed spin freezing temperatures could be described with the Vogel-Fulcher equation. The critical Fe concentration for magnetic moment formation has been determined. The effect of hydrogen absorption was investigated: a) with the maximum H content for all alloys, b) with intermediate H contents for the \'ZR\'\'(\'FE IND. 0.5\'\'CR IND. 0.5\') IND. 2\' alloy. Mõssbauer spectra for the latter samples showed the coexistence of a H-poor and a H-rich phase. The Fe quadrupole splitting at the 6h site exhibited a temperature dependence which was attributed to hydrogen diffusion effects. The main effect of hydrogen on the magnetism was a significant increase in magnetic moments, yet without ferromagnetic ordering; a slight lowering of spin freezing temperatures was actually observed.

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49

Costa, Marcio. "First-principles Studies of Local Structure Effects in Magnetic Materials". Doctoral thesis, Uppsala universitet, Materialteori, 2012. http://urn.kb.se/resolve?urn=urn:nbn:se:uu:diva-179223.

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This thesis focus on the magnetic behavior, from single atoms to bulk materials. The materials considered in this thesis have been studied by computational methods based on ab initio theory, density functional theory (DFT), including treatment of the spin-orbit coupling, non-collinear magnetism, and methods capable of treating discorded systems. Furthermore strongly correlated materials have been investigated using the dynamical mean field theory (DMFT). The uniaxial magnetic anisotropy energy (MAE) of the Fe2P was investigated using the full-potential linear muffin tin orbital (FP-LMTO) method. Based on a band structure analysis, the microscopical origin of the large magnetic anisotropy found for this system is explained. It is also shown that by straining the crystal structure, the MAE can be enhanced further. This opens up for the possibility of obtaining a room temperature permanent magnet based on the Fe2P. The spectral properties of Fe impurities in a Cs host have been investigated, for both surface and bulk systems, by means of combination of density-functional theory in the local density approximation and the dynamical mean-field theory (LDA+DMFT), using two different impurity solvers, the Hubbard I approximation (HIA) and the Exact Diagonalization (ED) method were used. It is shown that noticeable differences can be seen in the unoccupied part of the spectrum for different positions of Fe atoms inside the host. The calculations show good agreement with the experimental photoemission spectra. The stability of the 12-fold metal-phosphorous coordination, existing in the meteorite mineral melliniite has been investigated trough total energy calculations using the coherent potential approximation (CPA) combined with an analysis of the chemical bonds, performed by balanced crystal overlap population (BCOOP). It was shown that its uniquely high metal–phosphorous coordination is due to a balance between covalent Fe–P binding, configurational entropy and a weaker nickel–phosphorus binding. Supported clusters have drawn a lot of attention as possible building blocks for future data storage applications. This topic was investigated using a real space noncollinear formalism where the exchange interactions between Co atoms were shown to be tuned by varying the substrate surface composition. Furthermore the spin dynamics of small Co clusters an a Cu(111) surface have been investigated and a new kind of dynamics, where magnetization switching can be accelerated by decreasing the switching field, has been found. A method for calculating the electronic structure for both ordered and disordered alloys, the augmented space recursion (ASR) method, have been extended to treat non-collinear magnetic order. The method has been used to investigate the energy stability of non-collinear arrangements of MnPt and Mn3Rh alloys.

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Thanaboonsombut, Buncha. "An investigation of magnetically induced defect recovery in ferromagnetic nickel". Thesis, Georgia Institute of Technology, 1990. http://hdl.handle.net/1853/17041.

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Tesis sobre el tema "Magnetism"

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