Influence of the nanoparticle concentration on the magnetic and structural properties in Fe₃O₄-PVA nanocomposites

Full text: Our aim is to design magnetic nanocomposites in which the dipolar inter-nanoparticles interactions can be tuned. To achieve this, a set of magnetic gels were synthesized, with different nanoparticle concentrations. Polyvinyl alcohol (PVA) was chosen as a containing non-magnetic matrix due to its biocompatible properties. To ensure a correct dispersion of the fine particles within the PVA matrix, these were coated with citric acid (CA), thus the hydrogen bonds formed between the carboxylic (CA) and hydroxyl (PVA) groups promotes a better dispersion and prevent percolation of the nanoparticles. Magnetic results indicated that when the nanoparticles concentration rises, a shift in the blocking temperature (TB) towards higher values is observed. According to the superparamagnetic theory, such behavior is related to an increase in the magnetic anisotropy of the system or an increase in dipolar magnetic interaction between nanoparticles. To discard any structural distortion in the internal structure of the nanoparticles, which could lead to changes in the magnetic anisotropy of the studied systems, we decided to carry out measures of X-ray absorption fine structure (EXAFS and XANES). XAFS results are indicating that the internal structure of the NPs is not affected by the increase in nanoparticle concentration (or inter-nanoparticles interactions). Changes in TB, and in magnetic properties in general are associated with the increase in magnetic interactions, possibly due to the formation of non-compact aggregates. In order to confirm this hypothesis, were performed measures of small angle X-ray scattering (SAXS). SAXS experimental data were fitted using the expression postulated in fractal aggregate model [4]. Fitting results suggest that the increase in the nanoparticle concentration promotes the formation of denser and compact aggregates. We acknowledge the support of CONICET and ANCPyT (Argentina); LNLS, FAPESP and UNICAMP (Brazil); and IFW (Germany). (author)