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S?ntese e caracteriza??es estrutural e magn?tica das ferritas de cobalto-mangan?s (Co1-xMnxFe2O4 E Co1,2Fe1,8-xMnxO4)Amorim, Bruno Ferreira 07 December 2015 (has links)
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Previous issue date: 2015-12-07 / Coordena??o de Aperfei?oamento de Pessoal de N?vel Superior (CAPES) / Conselho Nacional de Desenvolvimento Cient?fico e Tecnol?gico (CNPq) / As ferritas de cobalto-mangan?s (Co1?xMnxFe2O4 e Co1,2Fe1,8?xMnxO4) possuem uma
estrutura do tipo espin?lio mista, e t?m sido consideradas como um das candidatas competitivas
para grande variedade de aplica??es em dispositivos, desde a gera??o e detec??o de
ultra-som, sensores, transformadores, bem como na ind?stria m?dica. Ferritas de cobaltomangan?s
nanoestruturadas foram produzidas via moagem de alta energia, com subsequente
tratamento t?rmico, e foram caracterizados por difra??o de raios X, fluoresc?ncia de raios
X, microscopia eletr?nica de varredura e magnetiza??o. Amostras do tipo Co1?xMnxFe2O4 e
Co1,2Fe1,8?xMnxO4 foram obtidas a partir dos p?s precursores Fe3O4, Co3O4 e Mn3O4, os quais foram
estequiometricamente misturados e mo?dos por 10h e tratados termicamente ? 900?C por 2h.
A difratometria confirma a forma??o das fases nanocristalinas puras para s?rie Co1,2Fe1,8?xMnxO4
com umdi?metro m?dio de cerca de 94nm. Verificou-se que o par?metro de rede aumenta com
a substitui??o do Fe3? pelo Mn3?. A fluoresc?ncia de raios X revelou que as por??es de metais nas
amostras estavam pr?xima das composi??es estequiom?tricas nominais. As caracter?sticas microestruturais
observadas nas micrografias demonstraram que as part?culas formadas apresentam
morfologia e granulometria bastante distintas. As medidas de histerese magn?ticas
realizadas em baixa temperatura, mostraram que a magnetiza??o de satura??o e reman?ncia
aumentaram com a concentra??o de mangan?s, enquanto que o campo coercivo diminuiu.
A constante de anisotropia (Ke f ), foi calculada a partir dos ajustes dos dados pela lei de
aproxima??o de satura??o. Verificou-se que a anisotropia diminui substancialmente com a
substitui??o do ferro pelomangan?s. / The cobalt-manganese ferrites (Co1?xMnxFe2O4 and Co1,2Fe1,8?xMnxO4) has a mixed
structure of spinel type and it has been regarded as one of candidates for petitive wide variety of
applications in devices from ultrasonic generation and detection, sensors, transformers, as well
as in medical industry. Ferrites cobalt-manganese nanostructured were produced via mechanical
alloying with subsequent heat treatment and were characterized by X-ray diffraction, X-ray
fluorescence, scanning electron microscopy and magnetization. Samples of Co1?xMnxFe2O4 and
Co1,2Fe1,8?xMnxO4 were obtained from the precursor powders Fe3O4, Co3O4 and Mn3O4 which were
stoichiometrically mixed and ground by 10h and heat treated at 900?C for 2h. The diffraction
confirmed the formation of the pure nanocrystalline phases to series Co1,2Fe1,8?xMnxO4 with an
average diameter of about 94nm. It was found that the lattice parameter increases with the
substitution of Fe3? by Mn3?. The x-ray fluorescence revealed that the portions of metals in
samples were close to the nominal stoichiometric compositions. The microstructural features
observed in micrographs showed that the particles formed show very different morphology and
particle size. The magnetic hysteresis measurements performed at low temperature showed
that the saturation magnetization and remanence increased as the concentration of manganese,
while the coercive field decreased. The anisotropy constant (Ke f ), was estimated from the data
adjustments the law of approaching saturation. It was found that the anisotropy decreases
substantially with the substitution of Fe by Mn.
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Nanoestruturas magn?ticas do tipo n?cleo-casca: um estudo do impacto do campo dipolar / Core-shell magnetic nanostructure: a study of impact of dipolar fieldOliveira, Leonardo Linhares 23 June 2016 (has links)
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Previous issue date: 2016-06-23 / Nanopart?culas bi-magn?ticas t?m se mostrado promissores em v?rias aplica??es
tecnol?gicas, tais como produ??o de ?m?s permanentes, desenvolvimento de geradores
de micro-ondas, nano osciladores e sistemas para grava??o magn?tica. Apresentamos um
estudo te?rico acerca de nanoestruturas bimagn?ticas do tipo n?cleo@casca constitu?da
de materiais ferromagn?ticos de alta e baixa anisotropia. O presente trabalho analisou
nanopart?culas com geometria esf?rica e cil?ndrica. Part?culas com formato esf?rico pode
ser empregada como pe?a fundamental na constru??o de im?s permanentes de alto desempenho,
pois podem apresentar melhorias expressivas no produto energ?tico m?ximo,
(BH)max, do sistema. O (BH)max ? um par?metro chave, pois determina se um material
? considerado bom para im? permanente. Nossos resultados mostram que o (BH)max
pode ser melhorado significativamente, uma part?cula SmCo5 com 3,5 nm de di?metro
recoberta por uma casca de Ferro de 2,5 nm de espessura pode apresentar (BH)max cerca
de 4 vezes maior que a part?cula n?o recoberta. No entanto, para um n?cleo de mesmo
material, com di?metro muito superior e cascas de Ferro relativamente espessas h? uma
redu??o do (BH)max que inviabiliza seu uso para a fabrica??o de ?m?s permanentes. Discutiremos
nesse trabalho o comportamento do produto energ?tico destes sistemas. Nanoestruturas
com geometria cil?ndrica apresentam diversas aplica??es, como nano osciladores
e mem?rias magn?ticas. Dessa forma, conhecer o perfil magn?tico e o comportamento da
magnetiza??o no processo de desmagnetiza??o ? de grande relev?ncia. Um cilindro de
Permalloy pode com di?metro de 57,0 nm e altura de 21,0 nm apresentar ao longo de sua
curva de magnetiza??o o estado v?rtice. A inibi??o deste estado ? relevante para algumas
aplica??es e pode ser alcan?ada com a presen?a de um anel externo de um material com
momento magn?tico elevado. Do mesmo modo, pode apresentar v?rtice na curva de magnetiza??o
devido a presen?a do anel magn?tico. Estudamos ainda os estados magn?ticos
presentes em an?is devido a intera??o magn?tica dipolar com o n?cleo. / Bi-magnetic anoparticles has been shown promises in several tachnological applications,
such that permanent magnets, microwave generators devices, nanooscilators and
magnetic record system for example. We presents a theoretical study about bi-magnetic
core@shell nanoparticles consisting of high and low anisotropy ferromagnetic materials.
The present work has analyzed nanoparticles with spherical and cylindrical geometries.
Spherical particles can be employed as building block for high performance magnets,
because can presents a expensive improvement in high energy product, (BH)max, of the
system. The (BH)max is a key parameter, because it?s indicate if a material is good to permanent
magnets. Our results show that (BH)maxcan be improved significantly, a particle
of SmCo5 with 3.5 nm recovered by iron shell with 2.5 nm thickness can presents (BH)max
thereabout 4 times great then uncovered particle. In other way, a core of the same material,
with major diameter s relative thick shell there is a reduction in (BH)max that unfeasible
their use in production of permanent magnets. We discuss in the present work the
behavior of energy product these systems. Nanostructures with cylindrical geometries
presents several applications, such that nano-oscillators and magnetic memory. In this
way, know the magnetic profile and behavior of magnetization in demagnetizing process
is relevant. A permalloy cylinder can, with 57.0 nm diameter and 21.0 nm height, presents
along your magnetization curve, a vortex state. The inhibition of this state is relevant for
some applications and can be reached with a presence of an external ring with elevated
magnetic moment material. In the same way, can present vortex in magnetization curve
by magnetic ring presence. We study further the magnetic states existing in magnetic ring
due to magnetic dipolar interaction with a core.
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