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This dissertation describes a novel methodology to prepare, functionalize, and assemble polymer-coated ferromagnetic cobalt nanoparticles (PS-CoNPs) and cobalt oxide nanowires. This research demonstrated the ability to use dipolar nanoparticles as `colloidal monomers' to form electroactive 1-D mesostructures via self- and field-induced assembly. The central focus of this dissertation is in developing a novel methodology termed as `Colloidal Polymerization', in the synthesis of well-defined cobalt oxide nanowires as nanostructured electrode materials for potential applications in energy storage and conversion.Ferromagnetic nanoparticles are versatile building blocks due to their inherent spin dipole, which drive 1-D self-assembly of colloids. However, the preparation and utilization of ferromagnetic nanoparticles have not been extensively examined due to the synthetic challenges in preparing well-defined materials that can be easily handled. This dissertation has overcome these challenges through the hybridization of polymeric surfactants with an inorganic colloid to impart functionality, colloidal stability and improved processing characteristics. This modular synthetic approach was further simplified to prepare ferromagnetic nanoparticles in gram scale, which enabled further investigations to develop new chemistry and materials science with these materials. These polymer-coated magnetic nanoparticles self-assembled into extended linear chains due to strong dipolar attractions between colloids. Additionally, novel dipolar assemblies, such as, flux-closure nanorings and lamellae type mesostructures were demonstrated by controlling the interparticle of attractive forces (dipolar versus van der Waals).The research presented herein focused on utilizing polymer-coated ferromagnetic cobalt nanoparticles as `colloidal molecules' to form interconnected 1-D mesostructures via `Colloidal Polymerization'. This process exploited the magnetic organization of dipolar colloids into 1-D mesostructures followed by a facile oxidation reaction to form interconnected electroactive cobalt oxide nanowires. This facile and template free approach enabled the large scale synthesis of semiconductor cobalt oxide nanowires, in which the electronic and electrochemical properties were confirmed for potential applications for energy storage and conversion. This work served as a platform in fabricating a wide range of semiconductor heterostructures, which allowed for structure-property investigation of new nanostructured electrodes.

This thesis deals with the studies of magnetization and electrical transport properties of iron nanoparticles embedded in the carbon matrix. The synthesis and characteristics of the nanoparticle systems studied, are also presented. Carbon-iron (C-Fe) based systems are of growing interest due to their improved magnetic properties as well as in their potential application as sensors, catalysts, and in various other applications. In particular, nanocomposites of iron carbide, such as the cementite phase Fe3C, are further suited to diverse technological exploitations due to their enhanced mechanical properties and importance in ferrous metallurgy. The recent interest in magnetic nanostructures lies in the emergence of novel magnetic and transport properties with the reduction of size. As the dimension approaches the nanometer length scale, interesting size-dependent properties like enhanced coercivity, enhanced magnetic moment, super paramagnetism etc. are seen. Thermal assisted chemical vapour deposition (CVD) is used to decompose and chemically react the introduced precursors, maleic anhydride and ferrocene. This method provides relative size control over the individual particles by varying C/Fe concentration in precursors and the pyrolysis temperature during the co-deposition process. Ferrocene has been used actively for the production of nanoparticle composites and in the production of nanostructured carbon. The temperature of preparation, reaction rate, and the time duration of annealing directly effects the nanoparticle compositions. The catalytic effect of transitional elements are well documented in literature. This thesis is an effort to understand the growth of ferromagnetic nanocrystallites in carbon matrix, which undergo partial graphitization due to the catalytic effect of transitional elements. The effect of transitional metal on the degree of graphitization of the carbon matrix, morphology of the nanoparticle and the carbon matrix are studied. The phase of the ferromagnetic iron nanoparticles and the structural investigation forms part of the study. Here X-Ray diffraction (XRD) is employed to study the presence of different phases of iron in the partially graphitized carbon matrix. The matrix morphology and the particle size distribution were studied using Transmission Electron Microscopy (TEM) and High-Resolution TEM (HRTEM). The ferromagnetic states of the iron nanoparticles are investigated using Mossbauer spectroscopy. The results from these studies, are used to correlated the macroscopic properties to the microscopic studies. The enhanced magnetization, coercivity and the temperature dependence of the magnetization value is understood within the frame work of ferromagnetic Bloch law and surrounding carbon spins. The logarithmic temperature dependence of conductivity of the nanoparticle composites is analyzed in the framework of interference models as well as the many-body Kondo interaction effect. This thesis contains seven chapters: In chapter 1, a brief introduction to mesoscopic physics and the size-dependent phenomenon are given. Special attention is paid to magnetic nanoparticle and its composites, and the various finite-size effects exhibited by them are discussed in detail. The relevance of carbon matrix and its importance on the growth of iron nanoparticles with high thermal stability is also discussed. The ballistic and diffusive transport phenomena observed in low-dimensional systems are briefly discussed. The interplay of localization and various interaction effects at nanoscale are examined. In disordered metals the low temperature conductivity is dominated by the interference effects. A brief discussion is made on the conductivity in disorder systems, with the presence of magnetic impurities and how the classic many-body Kondo problem, is effected by various interactions. Chapter 2, mainly deals with the experimental techniques employed in the thesis. The thermal-assisted chemical vapour deposition setup used to decompose and chemically react the introduced organometallic precursors, for the preparation of C:Fe composites are discussed and its advantage over other preparation methods are emphasized. The method is optimized to provide relative size control over the nanoparticles composites and the phase compositions by varying C/Fe concentration in precursors and the pyrolysis temperature, during the co-deposition process. The various structural characterization tools used in the present study are summed up concisely in this chapter. The SQUID magnetometer system; its working principle and the various protocol used for the low temperature magnetization measurements are elaborated. Further, details regarding superconducting magnetic cryostat, utilized for the low temperature conductivity and magneto resistance measurements, are discussed. Films of C:Fe composites are grown on substrates to study the effect of disorder and sample size on the conductivity behaviour of the composites at low temperature. Chapter 3, presents the outcome of the structural studies undertaken on the C:Fe composites using XRD, TEM, and HRTEM. X-ray diffraction measurements performed on the powder composites reveal that, in addition to the presence of sharp diffraction peak from nanographite, peaks corresponding to the different phases of Fe are also seen. The effect of preparation temperature on the matrix morphology is revealed from the estimation of degree of graphitization. Iron carbide is the predominant phase in all the prepared composites. For low concentration of iron, iron carbide alone is present but as the percentage of iron in the samples increased other phases of iron are also seen. The microscopic studies on the prepared compositions revealed the presence of nanosized iron particles well embedded in the partially graphitized matrix. Here again, with the increase in iron percentage, agglomeration of ferromagnetic nanoparticles are seen. The kinetics of the particle growth and the filamentous nature of the carbon matrix are also discussed. Mossbauer investigation on C:Fe composites are presented in chapter 4. The measurements revealed the iron atom occupation in the crystal lattice. In the lower Fe concentration samples, the room temperature Mossbauer spectrum revealed the presence of sextet from Fe3C (cementite) phase. As the percentage of iron increased, sextet from α-Fe, Fe3O4 are also seen in some of the prepared compositions. Effect of carbon atoms on the structure and magnetic properties of the nanoparticle species are obvious from the isomer shift measurements. Chapter 5 comprises of the various magnetic properties and interactions present in small particle system such as magnetic anisotropy, coercivity, enhanced magnetization, inter-and intra-particle interactions etc. Magnetization measurements carried out in SQUID magnetometer on the C:Fe composites and carbon flakes (prepared from organic precursor, maleic anhydride alone) are presented. The enhanced magnetic properties of the nanoparticle assembly is discussed in detail. The hysteresis loops trace, with a finite coercivity at room temperature, indicates the ferromagnetic nature of the samples. At room temperature the magnetization value saturates at high magnetic field, indicating negligible effect from super paramagnetic particles on the hysteresis loop. The squareness ratio, saturation magnetization, coercivity and remanence magnetization values are analyzed in detail. The temperature dependence of magnetization shows a combination of Bloch law and Curie-Weiss behaviour, consistent with the picture of ferromagnetic clusters embedded in a carbon matrix. The Bloch’s constant is found to be larger by an order of magnitude compared to the bulk value, implying stronger dependence of magnetization with temperature. Effort to understand the enhanced magnetic moment in the light of magnetism in carbon was taken up. The proximity effect of ferromagnetic metal on the carbon and the hydrogen bonding with the dangling bonds, both studied in detail in literature, in connection with the induced magnetic moments in carbon, are invoked. In chapter 6, the different conductivity regimes are identified, to study the conduction mechanisms in composites and films. For the transport measurements pelletized samples are used for the resistivity and magneto resistance measurements. The conductivity data are analyzed based on the interplay of localization and Kondo effect in the ferromagnetic disordered system. In order to understand the effect of disorder and thickness on the Kondo problem, transport measurements are carried on thin films of C:Fe composites grown on quartz and alumina substrate. Disorder induced metal-insulator transition is observed in the prepared samples. The zero-field conductivity and magneto resistance data is fitted to variable range hopping (VRH) in strong localization regime. Chapter 7 summarizes the thesis and presents some perspectives for the future.

En raison de leur constante d’anisotropie magnétocristalline particulièrement élevée,les nanoparticules de FePt cristallisant dans la phase « chimiquement » ordonnée L10présentent un grand intérêt pour la réalisation de média magnétiques discrets à très hautedensité (>1 Tb/in2) jusqu’à un diamètre limite de 3,5 nm. Nos travaux portent sur la synthèsepar voie chimique (thermolyse) de nanoparticules de FePt-ɣ, calibrées en taille (4 ≤ Ø ≤ 8 nm)et de composition chimique proche de Fe50Pt50. Par la suite, leur transition vers la variété L10est réalisée afin de leur assurer un comportement ferromagnétique fort à 300 K. En dépitd’une composition non homogène en fer au sein de chaque nanoparticule (coeur riche enplatine et surface davantage riche en fer), la phase L10 est obtenue après un recuit sousatmosphère réductrice (Ar/H2 5%) à des températures supérieures à 650°C. Par ailleurs, afinde prévenir la coalescence des nanoparticules lors du recuit, trois méthodes de protectionsdistinctes ont montré leur efficacité : une matrice de NaCl, des écorces de silice amorphe etde MgO cristallisé. Cette dernière méthode de protection a permis, une fois les recuitsréalisés, de redisperser les nanoparticules de FePt-L10 par le biais d’une modification de leursurface par des chaînes de Polyoxyde d’éthylène-thiol (Mn =2000 g.mol-1). Une encremagnétique est obtenue une fois ces nanoparticules mises en solution avec desmacromolécules de copolymères à blocs Polystyrène-b-Polyoxyde d’éthylène. Le dépôt decette encre sur un substrat permet de former, après auto-assemblage supramoléculaire desmacromolécules, un film hybride contenant les nanoparticules ferromagnétiques FePt-L10localisées sélectivement dans les domaines cylindriques de POE. / Nanoparticles made of FePt alloy in a face-centered-tetragonal (fct) structure have agreat interest for the enhancement of data density (> 1 Tbit/in²) in magnetic recordingmedia due to their high magneto-crystalline anisotropy and low critical diameters (3.5 nm).Our works lie in the synthesis of ɣ-FePt nanoparticles controlled in size (4 ≤ Ø ≤ 8 nm) andchemical composition (≈ Fe50Pt50) by thermal decomposition of organometallic precursors.Following ɣ-FePt NPs synthesis, annealing at high temperature is required for a completetransition from fcc to fct structure (L10) that ensure a ferromagnetic behavior at ambient.Despite a non-homogenous chemical composition on each nanoparticles (platinum-rich coreand iron-rich surface), L10 structure has been obtained after annealing under atmosphereAr/H2 (5%), at temperature up to 650°C. To prevent coalescence of FePt NPs duringannealing, tree distinct protection routes have shown their effectiveness: an inert NaClmatrix, an amorphous silica shell or a crystalline MgO shell. This last method shows bestresults in redispersion of L10-FePt nanoparticles after annealing via surface modification ofnanoparticles by PEO-thiol chains (Mn =2000 g.mol-1). A magnetic ink is then formulated inpresence of PS-b-PEO macromolecules. At least, this as-made ink is deposited on a substrateto obtain, after copolymer self-assembly, a hybrid film containing ferromagnetic L10-FePtnanoparticles selectively located into PEO cylindrical domains.

Submitted by Renata Lopes (renatasil82@gmail.com) on 2017-06-28T14:29:11Z No. of bitstreams: 1 damiaodesousavieirajunior.pdf: 10265456 bytes, checksum: 8b1ceaeb4c4be0e91a46c2d30add7349 (MD5) / Approved for entry into archive by Adriana Oliveira (adriana.oliveira@ufjf.edu.br) on 2017-08-07T21:17:55Z (GMT) No. of bitstreams: 1 damiaodesousavieirajunior.pdf: 10265456 bytes, checksum: 8b1ceaeb4c4be0e91a46c2d30add7349 (MD5) / Made available in DSpace on 2017-08-07T21:17:55Z (GMT). No. of bitstreams: 1 damiaodesousavieirajunior.pdf: 10265456 bytes, checksum: 8b1ceaeb4c4be0e91a46c2d30add7349 (MD5) Previous issue date: 2016-02-03 / CAPES - Coordenação de Aperfeiçoamento de Pessoal de Nível Superior / O contínuo desenvolvimento das técnicas de fabricação de estruturas em escala nanométrica, com considerável precisão e reprodutibilidade, tem permitido e estimulado a investigação científica em torno das propriedades básicas e novas aplicações tecnológicas desses sistemas. Especialmente a partir dos anos 90, é crescente o interesse da comunidade científica no comportamento de sistemas magnéticos nano-estruturados. Nestes, a quebra da simetria espacial devido às pequenas dimensões faz com que exibam comportamentos completamente distintos dos observados em amostras macroscópicas. A anisotropia de forma resultante das interações clássicas entre os dipolos magnéticos permite a formação de estruturas magnéticas exóticas em nanomagnetos como vórtices, skyrmions, paredes de domínio individuais e, até mesmo, excitações topológicas similares a monopolos magnéticos. A compreensão e controle do comportamento magnético estático e dinâmico dessas estruturas é fundamental para o desenvolvimento de novos dispositivos tecnológicos baseados em spintrônica. Neste trabalho foram estudadas nanopartículas planares, alongadas na forma elíptica, de material ferromagnético macio, especificamente o Permalloy-79. Tais nanopartículas tem atraído atenção devido ao seu potencial de aplicação prática no desenvolvimento de novos sensores, dispositivos de lógica, mídias de armazenamento de dados de alta densidade e dispositivos MRAM (Magnetic Random Access Memory). Pelo viés do interesse científico básico, tais nano-ilhas ferromagnéticas são a unidade fundamental em arranjos magnéticos bidimensionais geometricamente frustrados, como sistemas de gelo de spin artificiais. Nestes sistemas o arranjo geométrico das ilhas quebra a degenerescência do estado fundamental da rede, caracterizando um estado de frustração geométrica que permite excitações de comportamento análogo ao de monopólos magnéticos. Sob tais aspectos, é essencial caracterizar as configurações magnéticas no estado fundamental e os processos de reversão da magnetização em nanopartículas individuais. A forma elíptica planar gera uma forte anisotropia magnética, definindo duas configurações fundamentais para a magnetização do estado fundamental das nanopartículas: o estado de vórtice ou o estado alinhado ao longo do maior eixo — estado tipo C. A partir de uma razão de aspecto limite, a magnetização do estado fundamental é confinada no plano e ao longo do eixo maior de cada nano ilha, definindo um nanomagneto monodomínio com dois estados degenerados de magnetização, útil às aplicações previamente descritas. Partindo desse intuito estudamos inicialmente, através de simulação por dinâmica de spin, a competição entre os estados de vórtice e os estados alinhados tipo C como uma função da forma de cada nano-ilha elíptica, construindo um diagrama de fases de estados vórtice - tipo C. Cada nanopartícula magnética é modelada por momentos magnéticos que interagem via interação de troca entre primeiros vizinhos e por interação dipolar clássica de longo alcance. Nossos resultados mostram que é possível fabricar nano-ilhas alongadas com estado fundamental alinhado tipo C em razões de aspecto menores que dois. Este é um resultado interessante do ponto de vista tecnológico, pois permite usar ilhas menores que as atuais em pesquisas com gelos de spin e MRAM. Geralmente, os arranjos experimentais são feitos com nanopartículas de razão de aspecto próximas a três para garantir o estado fundamental alinhado da magnetização. Acrescentando ao modelo um termo de interação Zeeman com um campo magnético externo, estudamos o comportamento da reversão da magnetização nas nanopartículas. Consideramos espessuras diferentes e duas razões de aspecto distintas: uma do tamanho experimental usual e outra menor proposta a partir de nossos resultados. Aplicando campo magnético senoidal em diferentes frequências e em direções distintas no plano das nanoilhas, observou-se a dependência dos processos de reversão em função da espessura das partículas e com a direção e frequência do campo aplicado. Os resultados permitem traçar linhas gerais acerca do comportamento da reversão da magnetização nas nanopartículas individuais sob campo magnético externo. Evidentemente para o desenvolvimento das possíveis aplicações tecnológicas, inclusive o controle de excitações como monopólos magnéticos em gelos de spin, é crucial entender os processos ultra rápidos de reversão da magnetização, o que envolve a aplicação de campo externo de alta frequência em direções cuidadosamente definidas. Com esse objetivo, também estudamos a reversão da magnetização nas nano-ilhas por pulsos curtos de campo magnético (da ordem de nanosegundos) aplicados em diferentes direções. Observamos uma forte dependência da coerência da reversão da magnetização com a direção do campo aplicado e uma significante diferença na dependência angular da coercividade em relação ao observado em trabalhos prévios para campos aplicados na condição quase-estática. Finalmente, baseado em nossos resultados, propomos um método para o controle da reversão coerente da magnetização de nanopartículas individuais em matrizes quadradas de gelos de spin artificiais. Acreditamos que nossos resultados poderão ser úteis no desenvolvimento ulterior de arranjos magnéticos artificiais geometricamente frustrados e no controle das excitações topológicas destes sistemas. / The continuous development of structures fabrication techniques at the nanometer scale with considerable precision and reproducibility has allowed and encouraged scientific research around the basic properties and new technological applications of these systems. Especially from the 90's, there is growing interest of the scientific community in the behavior of nanostructured magnetic systems. In these, the breaking of spatial symmetry due to small dimensionality causes quite different behaviors from those observed in the bulk. The resulting shape anisotropy of the classical interaction between magnetic dipoles allows the formation of exotic magnetic structures in nanomagnets as vortices, skyrmions, single domain walls and even topological excitations similar to magnetic monopoles. The understanding and control of static and dynamic magnetic behavior of these structures is essential for the development of new technological devices based on spintronics. In this work we studied planar elongated nanoparticles in the elliptical shape of soft ferromagnetic material, specifically the Permalloy-79. Such nanoparticles have attracted attention because of their potential to practical application in the development of new sensors, logic devices, high density data storage media and MRAM (Magnetic Random Access Memory) devices. By the bias of basic scientific interest, such ferromagnetic nano-islands are the fundamental unit in two-dimensional magnetic arrangements geometrically frustrated as artificial spin ice systems. In these systems, the geometric arrangement of islands break the degeneracy of the network ground state featuring a state of geometrical frustration that allows excitations with analogous behavior of magnetic monopoles. Under these aspects, it is essential to characterize the magnetic configurations in the ground state and the magnetization reversal processes in individual nanoparticles. The elliptical planar shape generates a strong magnetic anisotropy which defines two basic configurations for the magnetization of the ground state of the nanoparticles: the vortex state or the aligned state along the major axis - type C state. As from an aspect ratio limit value, the magnetization of the ground state is confined in the plane and along the major axis of each nano-island defining mono-domain nanomagnet with two degenerate states of magnetization, useful for the applications previously described. Starting from this purpose we study initially, through simulation by spin dynamics, the competition between the vortex states and aligned type C states as a function of the shape of each elliptical nano-island to build a states diagram. Each magnetic nanoparticle is modeled by magnetic moments that interact by exchange interaction between nearest neighbors and by the classical long-range dipolar interaction. Our theoretical results indicate the possibility to manufacture elongated nano-islands with ground state like aligned C state for aspect ratios less than two. This is an interesting result from the technological point of view because it will be possible to use smaller islands in researches on spin ice and MRAM. Generally, the experimental arrangements are made with nanoparticles of aspect ratio close to three to ensure aligned magnetization in the ground state. Adding to the model a Zeeman interaction term between the magnetic moments and an external magnetic field we study the behavior of the magnetization reversal in nanoparticles. We consider different thickness and two different aspect ratios: one in the usual experimental size and a smaller proposed from our results. Applying sinusoidal magnetic field at different frequencies along the anisotropy axis in directions of ten and forty-five degrees from this, we observed the dependence of the reversal processes on the thickness of the particles and with the direction and frequency of the applied field. The results allow to establish general guidelines about the magnetization reversal behavior of the individual nanoparticles under external magnetic field. Evidently, for the development of possible technological applications, including the control of excitation like magnetic monopoles in spin ice, it is crucial to understand the ultrafast magnetization reversal processes which involves the application of high frequency magnetic fields in carefully defined directions. With this aim, we also studied the magnetization reversal of the nano-islands by short pulses of magnetic field (of the nanosecond order) applied in different directions. We observed a strong dependence on the coherence of the magnetization reversal with the direction of the applied field and a significant difference in the angular dependence of the coercivity compared to those seen in previous studies with applied magnetic fields in quasistatic conditions. Finally, based on our results we propose a method for the control of the coherent magnetization reversal of individual nanoparticles in square artificial spin ice arrays. We believe that our results may be useful in further developments of geometrically frustrated magnetic artificial arrangements and in the control of the topological excitations of these systems.