Efeitos termomagnéticos em aglomerados de nanopartículas magnéticas
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...
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| Formato: | doctoralThesis |
| Idioma: | pt_BR |
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Brasil
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| Endereço do item: | https://repositorio.ufrn.br/jspui/handle/123456789/28782 |
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| 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 ∆S 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 Zeeman, 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. |
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