Banca de DEFESA: ANTONIA VERUSKA BENEVIDES PINHEIRO

Uma banca de DEFESA de DOUTORADO foi cadastrada pelo programa.
STUDENT : ANTONIA VERUSKA BENEVIDES PINHEIRO
DATE: 18/12/2020
TIME: 09:00
LOCAL: Plataforma Google Meet
TITLE:

Study of magnetic couplings in CoFe₂O₄@CoFe₂ nanostructures dispersed in Ag matrix

KEY WORDS:

Nanocomposite, core-shell, exchange spring, Plots de Henkel, exchange bias.

PAGES: 136
BIG AREA: Ciências Exatas e da Terra
AREA: Física
SUMMARY:

Using the coprecipitation method, samples (precursors) of the CoFe₂O₄-Ag nanocomposite were synthesized, varying the concentration of CoFe₂O₄. These samples were subjected to thermal reduction, under H₂ atmosphere at 350 °C, for different time intervals in order to obtain a core-shell nanostructure of CoFe₂O₄@CoFe₂ dispersed in the Ag matrix. Through X-ray diffractometry, it was observed the phases CoFe₂O₄, CoFe₂ and Ag,, where the peak of CoFe₂ (shell) becomes more evident for samples reduced for a longer time. Rietveld refinements provide values for the size of CoFe₂O₄ nanoparticles (core) which decrease size with the increase in the reduction time, from 30 to 90 minutes. This decrease was also observed in Transmission Electron Microscopy images, and is around 0.5 nm. Meanwhile, the shell thickness (δ_CF) changes from 2.5 nm to 3 nm between samples with a lower concentration of CoFe₂O₄ reduced by 30 and 90 minutes. The nanoparticles of Ag, in the precursor samples, have diameters of 11 nm, but this value increases to ≈ 15 - 20 nm after the heat treatment. Magnetization measurements, at 300 K, reveal a decrease in the coercive field (H_c) as δ_CF increases, and the behavior of the curves suggests an exchange spring coupling. A phenomenological model was developed to describe the behavior of magnetic interactions in relation to the distance between the core shell nanoparticles, D_nc, based on the analysis of  Henkel and δm plots which are obtained through the relationship between the remaining reverse and direct magnetizations. For small D_nc distances, there is a greater contribution of dipolar interactions. When D_nc increases, the exchange interactions are more pronounced, as the contribution due to coupling at the core-shell interface is evidenced. Magnetization measurements as a function of the field in 4 K by zero field cooling and field cooling under a 1 T cooling field exhibit a horizontal shift in the curves, which can be attributed to freezing spins at the core-shell interface. It was found that the field of exchange bias (H_EB) in samples with lesser δ_CF is approximately double that presented in samples with thicker δ_CF. This increase in H_EB is related to the ratio of the surface area on one disordered layer to the volume of the ferromagnetic layer.

BANKING MEMBERS:
Externo à Instituição - RAIR MACÊDO - GCU
Interno - 1519258 - DORY HELIO AIRES DE LIMA ANSELMO
Interno - 1508681 - FELIPE BOHN
Externo à Instituição - JOÃO MARIA SOARES - UERN
Presidente - 1675199 - SUZANA NOBREGA DE MEDEIROS