Experimental evidence of exchange forces between nanoparticles in a superparamagnetic system

Abstract

Although interacting superparamagnetic systems have been widely explored, in most cases the correlations are only assigned to dipolar interactions. Here, we perform a systematic investigation of the properties of superparamagnetic MgFe2O4 nanoparticles, a system with characteristics that led us to believe they are associated with the existence of exchange interactions in the system. We first confirm that our sample consists of pure MgFe2O4 nanoparticles, having an average size of ∼12 nm and superparamagnetic behavior at room temperature with an irreversibility temperature of 250 K. Nevertheless, we reveal that the magnetic response is not well described by the Langevin function, even when taking into account the size distribution of the nanoparticles. In order to address the reasons for such deviation from the non-interacting behavior of a superparamagnetic system, we focus our attention on equilibrium and dynamic magnetization measurements, and consider a theoretical approach in the context of mean-field approximation, in which the effects of interactions in the sample are investigated. From the results, we find a positive value of 243 K for the parameter Θ associated with the mean field. Such a condition imposes the existence of magnetizing effects due to interactions in the system, which, according to Weiss’ mean field theory, are a fingerprint of the presence of exchange forces between nanoparticles.