Título:	Efeitos de viés de troca e treinamento em dispersões de nanopartículas bimagnéticas : características intrapartícula e interpartícula
Outros títulos:	Exchange bias and training effects in dispersions of bimagnetic nanoparticles : intraparticle and interparticle characteristics
Autor(es):	Viana, Márcio Gomes
E-mail do autor:	marcio.viana@ifpi.edu.br
Orientador(es):	Depeyrot, Jérôme
Assunto:	Nanopartículas magnéticas Fluidos magnéticos Interações interpartículas Interações intrapartículas
Data de publicação:	21-Jul-2023
Data de defesa:	9-Dez-2022
Referência:	VIANA, Márcio Gomes. Efeitos de viés de troca e treinamento em dispersões de nanopartículas bimagnéticas: características intrapartícula e interpartícula. 2022. 116 f. il. Tese (Doutorado em Física) — Universidade de Brasília, Brasília, 2022.
Abstract:	In the last decades, magnetic nanoparticles have been the object of increasing scientific research, due to their unique properties, not observed in bulk materials, which allow their use in biomedical, environmental, and energy applications. In this context, the objective of this thesis is to investigate the intraparticle and interparticle characteristics in exchange bias and training effects observed in dispersions of bimagnetic nanoparticles. Thus, we synthesized magnetic fluids based on cobalt ferrite nanoparticles with sizes of 3.5 nm, 4.2 nm, 4.7 nm, and 6.0 nm, covered with a shell of maghemite. Samples with different states of interaction between particles were obtained, by varying the volumetric fraction of the colloidal dispersions φ between 0.10% and 2.65% and also in powder. The structural characterization of the nanoparticles was performed using X-Ray Diffraction measurements and indicate a spinel-like cubic structure. Transmission Electron Microscopy results confirmed the formation of approximately spherical particles with low polydispersion. Using Atomic Absorption Spectroscopy measurements, the stoichiometry and morphochemical characteristics of the particles were extracted, as well as the volumetric fraction of particles in the dispersions. Measurements of high field DC magnetization as a function of temperature showed the existence of an intraparticle interface between a ferrimagnetically ordered core (FI) and a spin glass disordered shell (SGL). From this type of bimagnetic structure, the phenomenon of exchange bias arises, when magnetization measurements are performed after cooling with an applied field (field cooling - FC). In a dilute fluid regime, in which the individual behavior stands out, it was observed that the particles of smaller diameter and with a higher fraction of spins frozen in the SGL shell present a more intense exchange bias when compared to particles of larger diameter and with smaller proportion of disordered spins. Thamm-Hesse analysis showed, both in concentrated dispersions and powder sample, a demagnetizing contribution, in which interparticle dipolar interactions predominated, which intensifies when the interparticle distance decreases. Over the entire range of the investigated cooling field, the exchange bias is greater when the dipole interactions are more intense. The investigation of the training effect indicates two populations of disordered spins. Rotatable spins always reorganize faster than frozen ones and this is enhanced by dipole interactions. However, in powder regime, interparticle exchange interactions make more difficult to rearrange disordered surface spins.
Unidade Acadêmica:	Instituto de Física (IF)
Informações adicionais:	Tese (doutorado) — Universidade de Brasília, Instituto de Física, Programa de Pós-Graduação em Física, 2022.
Programa de pós-graduação:	Programa de Pós-Graduação em Física
Aparece nas coleções:	Teses, dissertações e produtos pós-doutorado

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