Banca de DEFESA: CLAUDIVAN MOREIRA DE SOUZA

Uma banca de DEFESA de DOUTORADO foi cadastrada pelo programa.
DISCENTE : CLAUDIVAN MOREIRA DE SOUZA
DATA : 25/10/2019
HORA: 09:00
LOCAL: Auditório do DFTE
TÍTULO:

THERMOMAGNETIC EFFECTS ON MAGNETIC NANOPARTICLES AGGLOMERATES

PALAVRAS-CHAVES:

Thermal Hysteresis, Agglomerates, Dipolar Interaction, Fe₃O₄ Magnetic Nanoparticles and Gd, Magnetocaloric Effect.

PÁGINAS: 135
GRANDE ÁREA: Ciências Exatas e da Terra
ÁREA: Física
RESUMO:

Recent research has studied the magnetic properties of nanoparticle superparamagnetic clusters with the possibility of applications in drug delivery control systems, magnetic resonance imaging, magnetic hyperthermia, etc. At the same time, several researchers have studied the phenomenon of thermal hysteresis under different circumstances and systems. Another object that has concentrated efforts in current research is the magnetocaloric effect on different structures focusing on the application in magnetic cooling technology. The objective of this work is to investigate the phenomenon of thermal hysteresis in superparamagnetic clusters of magnetite (Fe₃O₄) and Gadolinium (Gd) with spherical and ellipsoidal geometry and to analyze the impact of dipolar interaction on the thermal stability of as well as to investigate the magnetocaloric effect on Gd nanoparticle clusters. For this, we investigated the thermal magnetization curve of clusters with different eccentricities, with the size of the order of hundreds of nanometers composed of particles of Fe₃O₄ from 9 nm to 12 nm in diameter and Gd particles of 5.5 nm to 20 nm, with

variable density and evenly distributed in the clusters. We consider a temperature range from 200 to 1200 K, calculate the cooling and heating curves, as well as analyze the magnetic phases of the system under the effect of a low and constant external magnetic field. The entropy range  was calculated for an external field range for Gd ellipsoidal cluster systems. In our model, we do not consider effects of magnetocrystalline anisotropy. The anisotropy present in the system comes from the shape of the clusters and the dipolar interaction that naturally produces an anisotropic effect. We observe that the dipolar field

has a relevant contribution in the formation of thermal hysteresis. In spherical clusters we do not observe the formation of thermal hysteresis, in these clusters the sequence of magnetic phases in the cooling and heating branch are identical. In ellipsoidal clusters of eccentricity 0.97, the formation of thermal hysteresis associated with a sequence of magnetic phases characteristic of the ellipse was strongly influenced by the action of the dipolar field of the nanoparticles. The inverse magnetocaloric effect was observed in high eccentricity ellipsoidal clusters that have the Antiferromagnetic state as the natural state of magnetization. The results indicate that the magnetic phases that lead to the emergence of thermal hysteresis result from the competition between Zeemann, thermal and dipolar energies associated with the effect of the cluster topology. Therefore, in the systems analyzed in this study, the above parameters can control the appearance and characterization of thermal hysteresis and magnetocaloric effect.

MEMBROS DA BANCA:
Externa à Instituição - ANA LUCIA DANTAS - UERN
Presidente - 6345702 - ARTUR DA SILVA CARRICO
Interno - 1508681 - FELIPE BOHN
Externo à Instituição - LEONARDO LINHARES OLIVEIRA - UERN
Interna - 1675199 - SUZANA NOBREGA DE MEDEIROS