Nanoestruturas magnéticas de ferritas: análise das propriedades magnéticas e estruturais

Abstract

The enhancement of magnetic properties in core-shell nanoparticle structures and the e ect of heating magnetic nanoparticles through the application of an alternating magnetic eld have sparked signi cant scienti c and technological interest. Advances in this eld have been driven by progress in techniques for producing magnetic materials on the nanoscale, enabling various applications such as permanent magnets, biomedical applications, and magnetic hyperthermia. This thesis addresses two distinct nanostructured systems. The rst focuses on the synthesis of CoFe2O4@CoFe2 nanocomposites with a core-shell structure, using a method that does not involve special reagents or gases. The process involved the preparation of glutaraldehyde-crosslinked chitosan spheres containing CoFe2O4 nanoparticles, followed by high-temperature and vacuum heat treatment. The CO gas released during this process facilitated the reduction of Fe3+ and Co2+ ions to their zero-valent states. Upon production of the samples, their structural, morphological, and magnetic properties were investigated. X-ray di raction analysis, transmission electron microscopy (TEM), and Mössbauer spectroscopy revealed the presence of the desired magnetic phases. TEM images con rmed the core-shell structure con guration. Magnetic characterization indicated a magnetic exchange coupling between the phases under certain synthesis conditions, resulting in a maximum energy product (BH)max = 0.67 MGOe. The thickness of the CoFe2 phase (∼ 9.0 nm) aligns with the theoretical limit expected by the Kneller-Hawig theory, which is 10.2 nm for exchange coupling at the interface. The second system addresses the synthesis and study of the magnetic properties of Manganese Ferrite (MnFe2O4) nanoparticles. In this part of the work, MnFe2O4 nanoparticles were synthesized using the same method as the previous system, but with conventional heat treatment in the presence of air. Particles with various sizes were obtained by heat treatments at di erent temperatures, ranging from 7.8 to 13.3 nm. At 300 K, the saturation magnetization of the nanoparticles varied from 16.2 to 35.8 emu/g. Low-temperature Mössbauer spectroscopy showed the presence of monophasic MnFe2O4. For all samples, Mössbauer results at 300 K suggest the samples are in the superparamagnetic regime. AC and DC susceptibility measurements indicated that below the blocking temperature, the system behaves as a superspin glass. Speci c loss power (SLP) measurements were conducted at a frequency of 74 kHz and an AC eld amplitude of 247 Oe. Our results for both systems were promising, suggesting potential applications in permanent magnets and magnetic hyperthermia. Speci cally, we highlight the CoFe2O4@CoFe2 and MnFe2O4 systems.