CONICET | Buscador de Institutos y Recursos Humanos

The understanding of the magnetic properties of iron oxide nanoparticles, specifically the ferrites, has a strong significance, since they are present in a broad spectrum of applications, from electronics to biomedicine. The magnetic behavior in reduced size particles is expected to be dominated by surface effects as was evidenced in many ferro-, ferri- and antiferromagnetic nanocrystalline materials. At the same time, size reduction in nanoparticles enhances the presence of defects, such as broken bonds and vacancies within the particle, which affects their magnetic properties.In the present work, we have systematically studied the magnetic properties of magnetite (Fe3O4) nanoparticles with 3, 7 and 11 nm of diameter with very narrow grain size distributions. Samples were prepared by the thermal decomposition of Fe(acac)3 in the presence of surfactants and the final nanoparticles are covered by oleic acid. High Resolution Transmission Electron Microscopy images and x-ray diffraction patterns confirm that all samples are composed by crystalline nanoparticles with the spinel structure expected for the iron ferrite. Ac and dc magnetization measurements, as well in-field Mössbauer spectroscopy, indicate that the magnetic properties of nanoparticles with larger sizes (11 nm and 7 nm) are different from those nanoparticles of 3 nm. The larger nanoparticles present high magnetic order degree, with large MS (close to the bulk). In another way, the smallest nanoparticles system is composed by a magnetically ordered region and a magnetically disordered one (containing 67 % of Fe atoms) as a consequence of the reduced grain size. We have observed that the magnetically disordered component of the 3 nm nanoparticles corresponds to the last two atomic layers in the surface of the particle. We observe a strong increase of magnetization as consequence of the surface ordering at low temperatures, which induces frustration and larger effective anisotropy. At higher temperatures both regions have independent magnetic relaxations.

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