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| Author(s): |
Fabiana Rodrigues Arantes
Total Authors: 1
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| Document type: | Doctoral Thesis |
| Press: | São Paulo. |
| Institution: | Universidade de São Paulo (USP). Instituto de Física (IF/SBI) |
| Defense date: | 2014-12-10 |
| Examining board members: |
Daniel Reinaldo Cornejo;
Marcos de Abreu Avila;
Giancarlo Esposito de Souza Brito;
Gabriel Teixeira Landi;
Elis Helena de Campos Pinto Sinnecker
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| Advisor: | Daniel Reinaldo Cornejo |
| Abstract | |
In this thesis we present a study of the behavior of a system of magnetic nanoparticles by means of experimental measurements and Monte Carlo simulations. We experimentally study the role of the interactions between particles at low temperatures in commercial samples of ferrofluids through ZFC-FC, delta m curves, and FORC diagrams. We observed the phenomenon of supercooling and phase transitions from solid to liquid states in the ZFC-FC curves of ferrofluids. For the samples of liquid crystal doped with magnetic nanoparticles, we saw the transition between the isotropic and nematic phases. We detected in the samples of ferrofluids and in micellar solutions doped with nanoparticles an increase of the viscosity in the presence of an applied magnetic field, the so-called magnetoviscous effect, which arises due to interactions between particles. In the Monte Carlo simulations, we found that the critical temperature (Tc) decreases with particle size, a behavior that is described well by a scaling law. The simulations also showed that a dead layer on the surface of the nanoparticles causes a slight decrease in the critical temperature value, what does not occur when we add a hard layer, which increases Tc significantly. For simulations of a system of interacting nanoparticles, we paid special attention to interpret how the magnetizing and demagnetizing interactions manifest themselves in FORC diagrams for a set of nanoparticles with size distribution. We observed that demagnetizing interactions is associated with a displacement of the peak of the FORC diagram to positive values of the local field interaction Hb , and that the presence of a magnetizing interaction can shift this peak to larges values of the Hc field, related to the distribution of coercivities. (AU) | |
| FAPESP's process: | 10/01655-2 - Systems of magnetic nanoparticles: experimental studies and Monte Carlo simulations |
| Grantee: | Fabiana Rodrigues Arantes |
| Support Opportunities: | Scholarships in Brazil - Doctorate |