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  • About
  • The Global ETD Search service is a free service for researchers to find electronic theses and dissertations. This service is provided by the Networked Digital Library of Theses and Dissertations.
    Our metadata is collected from universities around the world. If you manage a university/consortium/country archive and want to be added, details can be found on the NDLTD website.
51

Microwave Components Based on Magnetic Wires

Sizhen, Lan, Lian, Shen January 2010 (has links)
With  the  continuous  advances  in  microwave  technology,  microwave  components  and  related magnetic materials become more important in industrial environment. In order to further develop the microwave components, it is of interest to find new kinds of technologies and materials. Here, we  introduce  a  new  kind  of  material  --  amorphous  metallic  wires  which  could  be  used  in microwave  components,  and  use  these  wires  to  design  new  kinds  of  attenuators.  Based  on  the fundamental  magnetic  properties  of  amorphous  wires  and  transmission  line  theory,  we  design  a series of experiments focusing on these wires, and analyze all the experimental results.    Experimental  results  show  that  incident  and  reflected  signals  produce  interference  and  generate standing  waves  along  the  wire.  At  given  frequency,  the  insertion  attenuation  S21 [dB]  of  an amorphous wire increases monotonically with dc bias current. The glass cover will influence the  magnetic  domain  structure  in  amorphous  metallic  wires.  Therefore,  it  will  affect  the circumference  permeability  and  change  the  signal  attenuation.  It  is  necessary  to  achieve  the impedance  matching  by  coupling  to  an  inductor  and  a  capacitor  in  the  circuit.  The  impedance matching  makes  the  load  impedance  close  to  the  characteristic  impedance  of  transmission  line. The magnetic wire-based attenuator designed in this thesis work are characterized and compared to conventional pin-diode attenuator.
52

Synthesis and characterization of magnetic thin films--exchange bias systems

Pang, Wenjie January 2005 (has links)
[Truncated abstract] Although exchange bias was discovered more than four decades ago, a satisfactory understanding of every instance of exchange bias observed in experiment has not yet emerged. Understanding exchange bias is complicated by many factors. For example, details of the antiferromagnet interface structure set up during the initial field cooling, thermal activation processes in the ferromagnet and antiferromagnet, and domain formation and domain wall movement in the antiferromagnet are all important in determining features associated with exchange bias. Two exchange bias systems are investigated in this thesis. One is a disordered system: a single layer Co/CoO film with random interfaces prepared by a reactive RF sputtering technique. The other is a ‘model’ system of Fe/KFeF 3 bilayers with compensated interfaces prepared by molecular beam epitaxy (MBE). The central theme of this work is to understand exchange bias and other related magnetic properties in these two very different systems. The Co/CoO exchange bias system studied here is different in structure from conventional exchange bias systems such as bilayer and multilayer systems where interfaces between ferromagnet and antiferromagnet are reasonably well defined. In this Co/CoO system, the Co and CoO is in the form of particles distributed randomly in a sputtered film. The interfaces between the Co and CoO are randomly distributed and may not be continuous over a large length scale. More importantly, the interface area is dependent on the shape and size of the particles and on their distribution. Many unique magnetic properties are related to the random interface in this system. For example, exchange bias and coercivity obtained at low temperatures are very large due to the large interface area between Co and CoO particles. The interface area can be controlled by changing the Co/CoO mass ratio in the film. Unlike in bilayer systems, film thickness in this single layer Co/CoO system turns out not to be critical for exchange bias and coercivity. The independence of film thickness may be technically important. More interestingly, because the interface is random, exchange bias can be setup by field cooling in any direction. Both training and magnetic viscosity effects were studied and provided evidence of thermal activation processes in this Co/CoO system. Training is explained as formation of a domain wall in the CoO with motion limited locally due to limited continuity of Synthesis and Characterization of Magnetic Thin Films - Exchange Bias Systems interfaces between the Co and CoO. Specific magnetization measurements over time were made and studied using viscosity theory. The magnetic viscosity was found to be strongly temperature dependent. There is a broad distribution of blocking temperatures which might be due to a broad distribution of Co particle sizes
53

Studium feroelektrických materiálů / Study of ferroelectric materials

Kos, Lukáš January 2012 (has links)
This work is focused on the study of perovskite ferroelectric materials group and monitoring changes their dielectric parameters in temperature and frequency dependence Is described scrystallographic systems of barium titanate and their influence on material properties. The measured values are mathematically interpreted using the Curie – Weiss law and discussed about the influence of strontium titanite on important dielectric parameters.
54

Studium spinové struktury a dynamiky magneticky uspořádaných tenkých filmů / Investigation of spin structure and dynamics in magnetically ordered thin films

Janda, Tomáš January 2021 (has links)
Title: Investigation of spin structure and dynamics in magnetically ordered thin films Author: Tomáš Janda Abstract: This thesis is devoted to the development of methods for imaging and ultrafast manipulation of magnetic textures, such as magnetic domains and domain walls (DWs), and to the investigation of the corresponding magnetiza- tion dynamics. We focused on techniques that are, besides ferromagnets (FMs), applicable also to antiferromagnets (AFs), in particular, AF thin films. The employed excitation techniques were mostly based on direct or indirect effects of ultrashort laser pulses. We studied the DW motion induced by a transfer of angular momentum from circularly polarized femtosecond laser pulses in a FM GaMnAsP and we found that the observed macroscopic DW displacement is only possible due to its inertia. In a distinct experiment, picosecond current pulses were generated by an absorption of the ultrashort laser pulses in a verti- cal PIN diode-based photoconductive switch and used to excite a magnetic film deposited directly on top of the diode. For imaging of AF domain patterns, we developed a simple table-top laser-scanning technique, which is based on a magneto-thermoelectric response of the AF textures in the presence of a local laser-induced heating. We first used the method to image the...
55

Численное моделирование динамических доменных структур в магнитных средах : магистерская диссертация / Numerical simulation of the dynamic domain structure in magnetic media

Nesterenko, A. A., Нестеренко, А. А. January 2014 (has links)
Master's thesis is devoted to the issues of stability spiral domains. It gives an overview of previously submitted behaviors spiral domains. Their features and some discrepancy with experi-ments were marked. For an explanation the experimental results in this paper proposed a model in which were introduced additional internal degrees of freedom - bending deformation of the domain boundary, resizing the inner core and changing the width of the domain. The equations describing the change of geometric parameters of the spiral domain were ob-tained as part of the dissipative model. In the model insulated spiral domain were considered on an endless film. The equations were solved by numerical calculation. The study identified two scenar-ios for the destruction of spiral domains. Discovered scenarios correspond more closely to the ex-perimentally observed than in models with only one degree of freedom. The work consists of four chapters, introduction and conclusion with 63 pages. The paper contains 41 figures and one application. / Магистерская диссертация посвящена вопросам стабильности спиральных доменов. В ней приводится обзор ранее представленных моделей поведения спиральных доменов. Вы-деляются их особенности и некоторое несоответствие экспериментам. Для объяснения экс-периментальных результатов в данной работе была предложена модель, в которой введены в рассмотрение дополнительные внутренние степени свободы – изгибное искажение доменной границы, изменение размера внутреннего ядра и изменение ширины домена. В рамках диссипативной модели были получены уравнения, описывающие изменение всех геометрических параметров спирального домена. В модели рассматривался изолированный спиральный домен на бесконечной пленке. Для решения уравнений применялся численный расчет. В ходе исследования определили два сценария разрушения спиральных доменов. Обнаруженные сценарии в большей степени соответствуют наблюдаемым на опыте, чем в модели только с одной степенью свободы. Работа состоит из четырех глав, введения и заключения общим объемом 63 страницы. В работе содержится 41 рисунок и одно приложение.
56

A study of magnetic properties of hard and soft magnetic materials by Lorentz transmission electron microscopy and magnetic x-ray circular dichroism

Pickford, Rachael Anne January 2001 (has links)
No description available.
57

Estudo magnético de objetos geométricos microscópicos de Co e CoFeSiB amorfo / Magnetic study of geometric microscopic objects of Co and amorphous CoFeSiB

Huamani, Rodrigo Mario Calle 10 April 2019 (has links)
Neste trabalho procura-se compreender o comportamento das configurações magnéticas presentes em objetos micrométricos e filmes finos. Estes estudos foram realizados principalmente através de microscopia de força magnética (MFM) e do efeito Kerr magneto-ótico (MOKE). Foi também desenvolvido um procedimento para a obtenção de curvas de histerese a partir de imagens de microscopia MOKE. Para definir o tamanho e a geometria dos objetos empregaram-se as técnicas de litografia por feixe de luz e por feixe de elétrons, e depois para a deposição do material de Co e CoFeSiB amorfo, se fez uso do magnetron sputering. O material assim obtido foi analisado para determinar suas propriedades magnéticas por meio das técnicas do MOKE, magnetrometria de amostra vibrante (VSM), MFM, e microscopia de força atômica (AFM) para o estudo da morfologia dos objetos. Os resultados do MFM e sua interpretação foram apoiados por simulações da estrutura magnética usando o programa Mumax3. Além disso foram analisados as relações entre as propriedades magnéticas e cristalografias de um material de Fe6%Si por médio das imagens MOKE e as imagens de EBSD. As medidas das curvas de histereses por VSM foram feitas em filmes preparados nas mesmas condições dos objetos litografados. Elas mostram que não há eixo anisotropia preferencial, portanto, não apresentam uma direção preferencial de magnetização. Com base nas medidas do MFM se encontrou que os objetos de Co policristalino apresentam configurações de estrutura de domínios magnéticos complexas, diferentes comparativamente do material magneticamente mais mole de CoFeSiB amorfo. Isto se deve as diferenças entre os dois materiais, na anisotropia magnética e magnetostricção. As diferentes geometrias dos objetos estudados apresentaram configurações magnéticas diferentes entre si, fato este, determinado principalmente pela anisotropia de forma. As semelhanças entre objetos da mesma geometria, mas de diferentes materiais mostraram fortes semelhanças, porém com a presença de perturbações locais no caso de Co. O material mole, amorfo, apresenta estados magnéticos ideais e simples, isto é, as tensões mecânicas residuais não geram perturbações magnéticas neste material de baixa magnetostricção. Além disto, nos objetos quadrados a presença de paredes de domínio do tipo cross-tie tem lugar sob certas condições, tendo a espessura do objeto como principal parâmetro. Estes resultados para a parede do tipo cross-tie foram vistos também em simulações micromagnéticas feitas no programa Mumax3. As estruturas magnéticas foram simuladas, com boa similaridade para a amostra de CoFeSiB, mas encontrou-se discordâncias no caso da amostra de Co, estas discordâncias foram analisadas e se estabeleceu as possíveis causas em cada geometria. / In this work the aim is to understand the behavior of the magnetic configurations present in micrometric objects and thin films. This study was developed through the use of magnetic force microscopy (MFM) and magneto-optical Kerr effect (MOKE). Another development was the use of MOKE microscopy images to obtain hysteresis curves. To define the size and geometry of the objects were used laser beam and electron beam lithography techniques, and for the deposition of Co and amorphous CoFeSiB materials was used magnetron sputtering. The produced samples were analyzed to determine its magnetic properties by MOKE, vibrating sample magnetometer (VSM), MFM, and atomic force microscopy (AFM) technique for the study of the object morphology. The results of the MFM and its interpretation were supported by simulations of the magnetic structure using the Mumax3 software. The measurements of the hysteresis loops by VSM were made on films produced under the same conditions of the lithographed objects. They show that there was no overall preferential anisotropy on the surface, this shows the absence of anisotropy in the deposition of the grains, in the case of Co, and therefore do not present a preferential direction of magnetization. Based on the MFM measurements, the polycrystalline Co objects were found to have different configurations compared to the amorphous CoFeSiB magnetically soft material. This is due to the differences in magnetic anisotropy and magnetostriction. The different geometries of the studied objects presented different magnetic configurations between them, this fact is determined mainly by the shape anisotropy. The similarities between objects of the same geometry but of different materials showed strong similarities with the presence of local perturbations in the case of Co. The magnetically soft amorphous material presents ideal and simple magnetic states, due to the fact the residual mechanical stresses do not generate disturbances in this low magnetostriction material. By the other side, in the square objects the presence of cross-tie domain walls takes place under certain conditions, where the thickness of the magnetic object is the main parameter. This kind of domain wall is also seen on the micromagnetic simulations developed using the Mumax3 software. The magnetic structures were simulated with good similarity to the amorphous CoFeSiB sample, but disagreements were found in the case of the Co sample. These disagreements were analyzed and the possible causes in each geometry were established.
58

Magnetoimpedância como ferramenta para a caracterização magnética de microfios amorfos / Magnetoimpedance as a tool to investigate the magnetic properties of microwires

Sossmeier, Kelly Daiane 22 September 2006 (has links)
Coordenação de Aperfeiçoamento de Pessoal de Nível Superior / The possibility of exploring the magnetic properties of amorphous microwires in technological applications has attracted the attention of the scientific community in the last years. Beyond, the magnetoimpedance effect has been established as a powerful tool to study these properties. In this work we show a study of the magnetic properties of Joule heated glasscovered amorphous microwires, CoFeSiB, under applied stress. The stress modifies the anisotropy and the domain structure of the sample being studied by the magnetoimpedance measurements and the FMR dispersion relations. It was established a method to determine the permeability from the magnetoimpedance measurements, valid for a wide frequency range. From the study of the permeability curves parameters associated to the domain walls dynamic can be determined. From the FMR dispersion relations, the value and the direction of the transverse anisotropy field, as well as an estimative of the magnetic domain structure in these microwires were obtained. It can be concluded that the longitudinal anisotropy has the main role in the magnetic behavior of the microwire without stress. Also, it was verified the presence of an inner core with longitudinal anisotropy surrounded by an outer shell with circumferential anisotropy under applied stress. These magnetic configurations can be explained in terms of the frozen stress in the production process of the wire and the additional applied stress. / A possibilidade de explorar as propriedades magnéticas de microfios amorfos em aplicações tecnológicas tem atraído a atenção da comunidade científica nos últimos anos. Além disso, a magnetoimpedância foi estabelecida como uma ferramenta poderosa para estudar estas propriedades. Neste trabalho apresentamos um estudo das propriedades magnéticas de microfios amorfos, CoFeSiB, recobertos por vidro, tratados termicamente e sob aplicação de tensão. A tensão modifica a anisotropia e a estrutura de domínios presente na amostra de tal maneira que podem ser estudadas a partir das medidas de magnetoimpedância e da relação de dispersão de FMR (Ressonância Ferromagnética) extraída delas. Foi estabelecido um método para a determinação da permeabilidade a partir de medidas de magnetoimpedância, válido para uma ampla faixa de freqüências. A partir do estudo dessas curvas de permeabilidade pode-se determinar, por exemplo, parâmetros relativos à dinâmica de paredes de domínios. Do ajuste das relações de dispersão de FMR foram obtidos o valor e a orientação do campo de anisotropia transversal, bem como uma estimativa da estrutura de domínios dos microfios. Destes estudos, concluiu-se que a anisotropia longitudinal domina o comportamento magnético do microfio sem aplicação de tensão. Aplicando-se tensão aos microfios, verificou-se a presença de um núcleo interno com anisotropia longitudinal envolto por uma casca externa com anisotropia circunferencial. Estas configurações magnéticas podem ser explicadas em termos da tensão residual que surge no processo de produção do fio e da tensão adicional aplicada.
59

Visualização de estrutura de domínios em cerâmicas e nanoestruturas ferroelétricas via microscopia de piezoresposta

Gonçalves, André Marino 27 February 2013 (has links)
Made available in DSpace on 2016-06-02T20:16:51Z (GMT). No. of bitstreams: 1 5333.pdf: 5685389 bytes, checksum: 082b6b7ff9988cce9196ad87a26521b6 (MD5) Previous issue date: 2013-02-27 / Universidade Federal de Sao Carlos / In this work, the domain structure of a transparent ferroelectric ceramic of (Pb0,79La0,21)TiO3 (PLT 21) was investigated in detail by piezoresponse force microscopu (PFM) and a protocol of measurements and analysis of the piezoresponse for the tridimensional reconstruction of the polarization in ferroelectric domains, including from mono and polycrystalline materials was developed. With this protocol, properties of domains and domain walls of the PLT 21 ceramic and of a PbTiO3 (PT) thin film were investigated. Three types of domains were recognized in the PLT 21 ceramic: domains separated by 180° walls, and domains separated by 90° walls in two scales, one in wich the domains have dimensions of about 1 μm and another with dimensions lower than 100 nm. Classical domain structures of lamellae and herringbones, and even more exotic structures as quadrants could be observed in PLT 21. This last one might suggest the existence of flux closure states of polarization in ferroelectric ceramics. In our knowledge this is the first time that quadrant structures are observed in ceramics. We could also observe in PLT 21 ceramic, ferroelectric domains that run through de grain boundary. Analysis of the reorientation of polarization by applying a localized BIAS field, revealed a strong electromechanical coupling in the sample, with the appearance of new 90° domain structure as a way to compensate local deformations generated by the poling process. The domain structure of the PT thin film revealed grains with monodomain structure and grains with polydomain structure, being the latter preferentially formed by 90° walls. The study of the reorientation of domains in the thin films of PT, showed the formation of a monodomain structure in the majority of the grains after the poling process, what significantly differs from the results of the PLT 21 ceramics. Lastly, thin films of Pb(Fe0,5Nb0,5)O3 (PFN) with good structural, microstructural and electrical properties were produced by radio frequency sputtering (R.F. Sputtering) with different thickness (50 nm 950 nm). The values of remnant polarization and coercive field of the hysteresis loop were 7 μC/cm2 and 70 kV/cm, respectively, which are bigger than many results found in the literature. Ferroelectric local properties were investigated in monolithic thin films (i.e., films that have a single grain in the thickness) of PFN and compared with the properties obtained in polycrystalline thin films of PT. / Neste trabalho, a estrutura de domínios de uma cerâmica transparente de (Pb0,79La0,21)TiO3 (PLT 21) foi detalhadamente investigada por microscopia de piezoresposta e um protocolo de medida e análise da piezoresposta para a reconstrução tridimensional da polarização de domínios em ferroelétricos, incluindo materiais mono e policristalinos, foi desenvolvido. Com este protocolo, as propriedades de domínios e paredes de domínios da cerâmica de PLT 21 e de um filme fino de PbTiO3 (PT) foram investigadas. Três tipos de domínios puderam ser reconhecidos na cerâmica de PLT 21: domínios separados por paredes 180° e domínios separados por paredes de 90° em duas escalas, uma em que os domínios têm dimensão de aproximadamente 1 μm e outra com dimensão menor que 100 nm. Estruturas de domínios clássicas como as lamelas e espinhas de peixes, até estruturas mais exóticas como a de quadrantes, puderam ser observadas no PLT 21. Essa última estrutura, pode sugerir a existência de estados de polarização de flux closure em cerâmicas ferroelétricas. Em nosso conhecimento, esta é a primeira vez que estruturas de quadrantes são observadas em cerâmicas. Foi possível observar na cerâmica de PLT 21, domínios ferroelétricos que transpõe a barreira do contorno de grão. A análise da reorientação da polarização com a aplicação de campo localizado revelou um forte acoplamento eletromecânico na amostra, com o aparecimento de novas estruturas de domínios de 90° como forma de compensar as deformações locais geradas pelo processo de polarização. A estrutura de domínios de filme fino de PT apresentou grãos com estrutura de monodomínio e grãos com estrutura de polidomínios, sendo o último preferencialmente formado por paredes de 90°. Os estudos de reorientação dos domínios nos filmes de PT mostraram a formação de uma estrutura de monodomínios na maioria dos grãos após o processo de polarização, o que difere significativamente dos resultados obtidos cerâmicas de PLT 21. Por fim, filmes finos de Pb(Fe0,5Nb0,5)O3 (PFN) com boas propriedades estruturais e microestruturais e elétricas foram produzidos por sputtering em radiofrequência (R.F. Sputtering) com diferentes espessuras (50 nm até 950 nm). Os valores obtidos por histerese ferroelétrica foram de 7 μC/cm2 de polarização remanescente e 70 kV/cm de campo coercitivo, maiores do que a maioria dos resultados encontrados na literatura. Propriedades ferroelétricas locais foram investigadas nos filmes finos monolíticos (i.e., filmes formados por um único grão na espessura) de PFN e comparadas com as propriedades obtidas em filmes finos policristalinos de PT.
60

O efeito grau máximo sobre os domínios: como \'todo\' modifica a relação argumento-predicado / The maximal degree effect: how todo modifies the predication

Gomes, Ana Paula Quadros 19 February 2009 (has links)
Esta tese investiga o modo de organização dos domínios nominal, verbal e dos adjetivos em Português do Brasil (PB), tendo como guia a aceitabilidade de sentenças com todo. Para o inglês, a natureza do parâmetro orienta a seleção de argumentos por operadores; já para o PB, o que importa é a oposição entre tipos de escala. O PB não tem determinantes que distingam entre nome contável e massivo, como much e many. O operador aspectual progressivo não modifica estados em inglês, mas em PB sim. Em inglês, very seleciona adjetivos de parâmetro relativo. Em PB, muito + adjetivo tem parâmetro relativo, e todo + adjetivo tem parâmetro absoluto. Todo é um operador interdomínios, sensível aos tipos de escala. Todo modifica a relação de predicação. Todo impõe condições (quantitativas) sobre como a saturação de um predicado por certo argumento deve ocorrer. Todo não é nem um modificador nominal, nem um quantificador canônico como cada. Todo não cria, apenas modifica uma relação existente. A distribuição que ocorre em sentenças com todo é uma entre as muitas formas de saturação de um predicado por um argumento: uma relação incremental. Se o argumento for quantizado, o predicado necessariamente também se tornará quantizado. Analisamos uma descrição definida (DD) como um sintagma de medida (SM). O artigo definido torna um predicado nominal em denotação quantizada, mas todo não. Relacionamos ser quantizado a ser argumental, e ser cumulativo a ser predicativo. E associamos sentidos diferentes às posições de todo na sentença. / This thesis takes the distribution of todo as a probe for the structure of nominal, verbal and adjective domains in Brazilian Portuguese (BP). Todo is a Degree Modifier (DM) and is sensitive to scale structure. English DMs (e.g., very) select adjectives by their standards; the BP DMs select adjectives only by their scale structure. However, they produce phrases with standard specialization. Todo + adjective shows absolute standard interpretation. We claim that the domains show the same properties in both languages, but the nature of scale standard matters in a distinct level for each one. We claim that todo is neither a noun modifier nor a true quantifier. Todo is a relation modifier. Todo modifies the way the argument saturates the predicate. A quantized incremental argument will make the predicate quantized as well. Todo is not the true source of distributivity, since incremental relations occur even in its absence. Definite Descriptions are treated as measure phrases. The definite article relates noun predicates to situations. So it will change a bare noun into a quantized denotation, which todo cannot do. Each land site corresponds to a different meaning for floating todo.

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