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Efeitos dipolares sobre fases magn?ticas de aglomerados superparamagn?ticosPedrosa, Silas Sarmento 15 September 2017 (has links)
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Previous issue date: 2017-09-15 / Conselho Nacional de Desenvolvimento Cient?fico e Tecnol?gico (CNPq) / Coordena??o de Aperfei?oamento de Pessoal de N?vel Superior (CAPES) / H? presentemente grande interesse de pesquisa em aglomerados de nanopart?culas superparamagn?ticas,
devido em parte ? alta demanda para aplica??es biom?dicas, e ao mesmo
tempo ao grande interesse, do ponto de vista fundamental, em novas fases magn?ticas. A suscetibilidade
magn?tica inicial e o campo de fuga, s?o fatores essenciais para otimiza??o de sistemas
para aplica??es biom?dicas. H?, ao mesmo tempo, grande interesse em confirmar a exist?ncia
de ferromagnetismo dipolar, em sistemas onde a energia de troca n?o ? fator dominante.
Desenvolvemos um estudo te?rico do impacto da intera??o dipolar sobre as fases magn?ticas de
nanopart?culas superparamagn?ticas, confinadas em aglomerados esf?ricos e elipsoidais. Consideramos
nanopart?culas de Fe3O4 com tamanhos no intervalo de 9 nm a 12 nm, arranjadas
com densidade uniforme em aglomerados de tamanho da ordem de centenas de nan?metros.
Mostramos que as fases magn?ticas, e a suscetibilidade inicial, s?o controladas pela intera??o
dipolar, e que a topologia do arranjo de nanopart?culas, o tamanho das nanopart?culas e a densidade
de empacotamento s?o fatores que controlam as propriedades magn?ticas. Mostramos que
a intera??o dipolar pode estabilizar fases magn?ticas cl?ssicas, conhecidas apenas para sistemas
com alto conte?do de energia de troca e de anisotropia. Al?m disso, as fases magn?ticas em
reman?ncia t?m uma caracter?stica peculiar: a m?dia t?rmica do momento de cada nanopart?cula
pode se aproximar do valor de satura??o, mantendo o aglomerado superparamagn?tico.
Aglomerados elipsoidais de alta excentricidade s?o os sistemas de escolha para aplica??es biom?dicas
porque podem exibir expressivo aumento de suscetibilidade magn?tica, mantendo um
campo de fuga de baixa intensidade em reman?ncia. O modelo te?rico reproduz satisfatoriamente
resultados experimentais de aglomerados esf?ricos de Fe3O4, e de sistemas de part?culas
de Fe e Co de baixa dimensionalidade. / Superparamagnetic nanoparticles clusters are currently driving considerable research
attention. The interest stems from chances of designing systems with promising potential for
technological applications, and from the fundamental viewpoint, tailoring new magnetic phases.
The initial magnetic susceptibility and the stray field, at remanence, are key features for the
optimization of magnetic systems for biomedical applications. Also, the existence of dipolar
ferromagnetism, in the absence of exchange energy, has been one of the focus of magnetism
for decades. We report a theoretical discussion of the impact of the dipolar interactions on the
magnetic phases of superparamagnetic nanoparticles confined in spherical and ellipsoidal clusters.
We consider Fe3O4 nanoparticles, with size ranging from 9 nm to 12 nm, arranged with
uniform density in hundreds nanometer size volumes. We show that the magnetic phases, and
the initial susceptibility, are controlled by the dipolar interaction. Also, the topological nanoparticle
arrangement, the nanoparticle size, and the packing density, are key features. We show that
the dipolar interaction alone may stabilize classical magnetic phases, well known for systems
with large content of exchange and anisotropy energies. In addition, we have found that at remanence
the nanoparticles clusters magnetic phase have a unique property. The dipolar energy
leads to thermal stabilization of the individual nanoparticles moments. Large nanoparticles densities
may allow nearly full thermal value of the nanoparticles magnetic moments. Despite this,
the nanoparticles cluster is superparamagnetic, with a rather small stray field at remanence, as
required for biomedical safety. Nanoparticle clustering in large eccentricity ellipsoidal volumes
are promising systems for both low field and large field biomedical applications. For low field
applications, there is a large increase in the initial susceptibility, with enhancement in the efficacy
of vector targeting and also for hyperthermia absorption rate. For high field applications,
the enhancement of the stray is much stronger than that for spherical clusters. Our theoretical
model reproduces typical properties of Fe3O4 nanoparticles spherical clusters, as well as
intriguing results for Fe and Co quasi-one-dimensional systems.
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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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