Size-Dependent Magnetic Structure of MFe2O4 (M=Fe, Co, Mn) Nanoparticles

ENIO LIMA JR.; A D ARELARO; L M ROSSI; P K KIYOHARA; R D ZYSLER; H R RECHENBERG

Rio de Janeiro, Brasil

Conferencia; IX International Conference on Nanostructured Materials - NANO2008; 2008

Spin structure and magnetic properties of noninteracting nanoparticles are determined between surface and core magnetic behavior. The role of the surface on the magnetic behavior is associated to the lack of symmetry and broken bonds and it becomes more significant with decreasing the grain size. In this way the synthesis method of magnetic nanoparticles is of paramount importance to technological application since it will define the morphological characteristics that establish the magnetic properties of the system, such as the mean diameter, grain size distribution, crystallinity degree, number of deffect centers in the surface, chemistry of surface, etc. In this work, we have performed a systematic investigation of the size-dependence of the magnetic structure in MFe2O4 (M = Fe, Co, Mn) nanoparticles. Samples were prepared by the thermal decomposition of Fe(acac)3 and the corresponding M ion compound at high temperature and in the presence of oleic acid, oleilamine, and 1,2-octanediol. Particle diameter was tailored by the molar ratio of the surfactants and the precursors. TEM images show that our samples are constituted by nanoparticles with narrow grain size distribution with mean diameter changing from 3 nm up to 10 nm. Nanoparticles with 5 nm or bigger diameter present high crystallinity degree. Magnetic porperties of the systems were investigated by magnetization measurements as function of temperature and field, ac susceptibility measurements, in-field Mössbauer spectroscopy and FMR measurements. According to our results, the spin structure, and consequently the magnetic porperties, depends of d. For nanoparticles with 5 nm or bigger diameter, we have osberved an ordered structure and the magnetig behavior is the expected for a monodomain. In contrast, samples smaller than 5 nm present a distinct magnetic behavior, corresponding to a cluster-glass-like magnetic structure within the nanoparticles.