Effect of submicrometer clustering on the magnetic properties of free-standing superparamagnetic nanocomposites

Superparamagnetic polymer nanocomposites with well differenced distribution of the nanofiller in the matrix were obtained by using oleic acid-coated maghemite nanoparticles. Both homogeneously dispersed magnetic films and dispersions constituted by a distribution of subrnicrornetric nanoparticle (NP) clusters could be obtained starting from the same batch of magnetic NPs. We demonstrated that differences in the magnetic behavior of these materials are solely clue to changes in the NP spatial distribution not involving changes in their size or total volume fraction. The experimental behavior of random distributions of NPs was consistent with Monte Carlo (MC) simulations performed on equivalent systems. On the contrary, the presence of aggregates produced higher values of the blocking temperatures (T-B) due to an increase in the strength of dipolar interactions produced by an increase of the local concentration of NPs in the aggregates. In this way, we determined that clustering, even in the submicrometric range, strongly controls the magnetic behavior of nanocomposites. This is of fundamental importance in the design of new magnetic materials with technological applications.