Orientador: Marcelo Knobel / Tese (doutorado) - Universidade Estadual de Campinas, Instituto de Fisica Gleb Wataghin / Made available in DSpace on 2018-08-04T01:56:42Z (GMT). No. of bitstreams: 1 Brandl_AnaLucia_D.pdf: 5290468 bytes, checksum: 32290a7675f89cf9c2a2ea53be70c6fb (MD5) Previous issue date: 2004 / 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

Central to the practical use of nanoscale materials is the controlled growth in technologically meaningful quantities. Many of the proposed applications of the nanomaterials potentially require inexpensive production of the building blocks. Solution-based synthetic approach offers controllability, high throughput, and scalability, which make the process attractive for the potential scale-up. Growth kinetics could be readily influenced by chemical interactions between the precursor and the solvent. In order to fully utilize its benefits, it is therefore pivotal to understand the decomposition chemistry of the precursors used in the reactions. Supercritical fluids were used as solvent in which high temperature reactions could take place. Silicon nanowires with diameters of 20~30 nm was synthesized in supercritical fluids with metal nanocrystals as seeds for the nanowire growth. To unravel the effect of silicon precursors, several silicon precursors were reacted and the resulting products were investigated. The scalability of the system is discussed based on the experimental data. The nanowires were characterized with various characterization tools, including high-resolution transmission electron microscopy and electron energy loss spectroscopy. The crystallographic signatures were analyzed through the transmission electron microscopic study, and fundamental electrical and optical properties were probed by electron energy loss spectroscopy. Carbon nanotubes were prepared by reacting carbon-containing chemicals in supercritical fluids with organometallic compounds that form metal seed particles in-situ. A batch reaction, in which the temperature control was relatively poor, yielded a mixture of multiwall nanotubes and amorphous carbon nanofilaments with a low selectivity of nanotubes in the product. When reaction parameters were translated into a continuous flow-through reaction, nanotube selectivity as well as the throughput of the total product significantly improved. Magnetic properties of various metal nanocrystals were also studied. Colloidal synthesis enables the growth of FePt and MnPt3 nanocrystals with size uniformity. The as-synthesized nanocrystals, however, had compositionally disordered soft-magnetic phases. To obtain hard magnetic layered phase, the nanocrystals must be annealed at high temperatures, which led to sintering of the inorganic cores. To prevent sintering, the nanocrystals were encapsulated with silica layer prior to annealing. Interparticle magnetic interactions were also explored using particles with varying silica thickness. / text