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The magnetic properties of the double perovskites Sm 2 Mn 1+x Co 1−x O 6 (x = 0, 0.05, 0.12 and 0.26) were investigated. It was found that the Curie temperature, the lattice parameters and the net magnetic moments increased for increasing amounts of Co. An irreversible behavior was observed by measuring the magnetization after cooling the sample with and without applied magnetic fields (H). The temperature below which the irreversibility was observed is H dependent and the data were nicely fit to de Almeida-Thouless lines. The ac magnetic susceptibility was measured for frequencies f in the range 0.03-10 kHz yielding X 0.003 for the shifting in the freezing temperature per decade of f . The spin-dynamics were found to follow a power-law with a product of the critical exponents zν of about 4.99. The overall results are understood within a framework where the variation in the bonding angle associated to the super-exchange interactions are taken into consideration.

Although interacting superparamagnetic systems have been widely explored, in most of cases the correlations are only assigned to the dipolar interactions. Here we perform a systematic investigation of the properties of superparamagnetic MgFe2O4 nanoparticles, a system having characteristics that led us to the belief they are associated with the existence of exchange interactions into the system. We first confirm our sample does consist of pure MgFe2O4 nanoparticles, having average size of ∽12 nm and superparamagnetic behavior at room temperature with irreversibility temperature of 250 K. Nevertheless, we reveal the magnetic response is not well described by the Langevin function, even 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 yet consider a theoretical approach in the context of the mean-field approximation, in which the effects of interactions into the sample are investigated. From the results, we find a positive value of 243 K for the parameter Θ associated to the mean field. Such 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.