Size-Dependent Magnetic Properties of Ferrite Fe_3 O_4 Nanoparticles Chemically Synthesized

E. LIMA JR.; E. DE BIASI; M. VÁSQUEZ MANSILLA; M. SALETA; F. EFFENBERG; L. M. ROSSI; R. COHEN; H. R. RECHEMBERG; R. D. ZYSLER

Workshop; X Latin American Workshop on Magnetism, Magnetic Materials and their Applications (LAW3M2010); 2010

The understanding of the magnetic properties of iron - oxide nanoparticles, specifically the ferrites, has a embedded intodifferent diameter zigzag carbon strong significance, since they are present in a broad nanotubes (CNT), by means of ab-initio calculations. spectmm of applications, from electronics to we find that the easy axis changes abruptly froin biomedicine. The magnetic behavior in reduced size paralle1 to perpendicular to the symmetry axis at certain particles is ex~e c t edto be dominated by surface rfiects CNT indexes as shows in Table 1. as was evidenced in many ferro-, ferri- and antiferroma-g netic nanocrystalline materials. At the saine time. size reduction in nanoparticles enhances the Table 1: the MAE (in meVper Fe atom) for a nanowire embedded into a CNT with the given index. The first field refers to the pristine preSence of defects3 as broken and nanowire. A positive (negative) MAE value rneans an parallel vacancies within tlie particle, which affects their (perpendicular) easy axis. magnetic properties. In the present work, we have systematically studied the magnetic properties of magnetite (Fe304) nanoparticles with 3, 7 and 1 I nm of diameter with very narrow grain size distributions. Samples were prepared by the thermal decomposition of F e ( a ~ a c )in~ the presence of surfactants and the final nanoparticles are covered by oleic acid. High Resolution Transmission Electron Microscopy images and x-ray diffraction pattems confírm that al1 samples are composed by crystalline O nanoparticles with the spinel structure expected for the 1 iron ferrite. Ac and dc magnetization measurements, as well in-fíeld Mossbauer 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.