Size-dependent Magnetic Properties of Ferrite Fe3O4 Nanoparticles Chemically Synthesized

ENIO LIMA JR.; E DE BIASI; M VASQUEZ MANSILLA; M E SALETA; F EFFENBERG; L M ROSSI; R COHEN; H R RECHENBERG; R D ZYSLER

Manizales, COlombia

Workshop; LAW3M 2010 – IX Latin American Workshop on Magnetism, Magnetic Materials and Their Applications; 2010

The understanding of the magnetic properties of iron oxide nanoparticles, specifically the ferrites, has a strong significance, since they present 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. In the present work, we have systematically studied the magnetic properties of ferrite (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. HRTEM images and XRD diffraction patterns confirms 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 bigger size (11 nm and 7 nm) are different from those nanoparticles of 3 nm. The bigger nanoparticles present high magnetic order degree, with large M_S (close to the bulk). In another way, the smallest nanoparticles are 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 of the particle. We observe a strong increase in the couple of the disordered and ordered regions at low temperatures (2 K) and at higher temperatures (~ 10 K) both thermally fluctuates in independent ways.