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

E. LIMA JR.; A. D. ARELARO; L.M. ROSSI; P. KYOHARA; R.D. ZYSLER; H.R. RECHENBERG

Rio de Janeiro

Congreso; 9th International Conference on Nanostructured Materials - NANO 2008; 2008

Nano conferences

Spin structure and magnetic properties of non-interacting nanoparticles are determined by the competition between surface and core magnetic behaviors. Concerning the magnetic behavior of the surface, it is associated to the lack of symmetry and break bonds and it becomes more significant with decreasing the grain size. In this way, the synthesis method of magnetic nanoparticles is decisive with respect to technological applications since it will determine the morphological characteristics that establish the magnetic properties of the system, such as the mean diameter <d>, grain size distribution, crystallinity degree, number of defects 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 and Mn) nanoparticles. Samples were prepared by the thermal decomposition of Fe(acac)3 at high temperature (265°C) in the presence of oleic acid, oleilamine and 1,2 octanediol. Particle diameter was tailored by the molar relation of the surfactants and the precursor. TEM images show that our samples are constituted by nanoparticles with narrow grains size distribution with diameter (<d>) changing from 3 nm to 10 nm. Particles with <d> > 5 nm present a high crystallinity degree.Magnetic properties of the samples were investigated by magnetization measurements as function of applied field and temperature (ZFC and FC modes), in-field Mössbauer spectroscopy (Happ ≤ 120 kOe) and FMR measurements (ZFC and FC modes). According to our results, the spin structure, and consequently the magnetic properties, depends of <d>. For nanoparticles with <d> ≥ 5 nm, we have observed an ordered structure and the magnetic behavior is the expected for non-interacting monodomains with no major contribution from a disordered surface. In opposition, samples with <d> ≤ 5 nm present a distinct magnetic behavior, corresponding to a cluster-glass-like magnetic structure within the nanoparticles.