Banca de DEFESA: Márcio Gomes Viana

Uma banca de DEFESA de DOUTORADO foi cadastrada pelo programa.
STUDENT : Márcio Gomes Viana
DATE: 09/12/2022
TIME: 09:30
LOCAL: On-line
TITLE:

Exchange bias and training effects in dispersions of bimagnetic nanoparticles: intraparticle
and interparticle characteristics

KEY WORDS:

Keywords: Magnetic nanoparticles, Magnetic fluids, Interparticle and intraparticle interaction,
Exchange bias, Training effect.

PAGES: 117
BIG AREA: Ciências Exatas e da Terra
AREA: Física
SUMMARY:

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, covered with a shell of maghemite, with
sizes of 3.5 nm, 4.2 nm, 4.7 nm, and 6.0 nm. 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 spinellike
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.

BANKING MEMBERS:
Externa ao Programa - 2691265 - CYNARA CAROLINE KERN BARRETO
Externo ao Programa - 404882 - GERALDO JOSE DA SILVA
Presidente - 1218541 - JEROME DEPEYROT
Externo à Instituição - RAFAEL CABREIRA GOMES - UFSC
Externo à Instituição - RODOLPHO CARVALHO LEITE - IFPI