Thermal dependence of the saturation magnetisation of Mn0.6Fe0.4Fe2O4 nanoparticles in a ferrofluid

The saturation magnetization of non-interacting surfacted nanoparticles of Mn0.6Fe0.4Fe2O4 has a relative increase of 55.80% when the temperature decreases from 300 to 90 K. This increase is significantly higher than that corresponding to bulk ferrite, for which the increase is of only 20.98% in the same temperature range. Considering the size distribution in the real system, we have shown that the difference between the two values of the magnetisation is a result of the increase from 10.87 to 11.85 nm of the average diameter of the nanoparticles cores that are ferrimagnetically ordered. This behaviour is determined by the modification of the superexchange interaction in the layer that is formed at the nanoparticles surfaces as a result of the adsorption of the surfactant molecules. The sample examined was a ferrofluid containing Mn0.6Fe0.4Fe2O4 particles with an average diameter of 12.14 nm (established by TEM), coated with oleic acid (surfactant) and dispersed in kerosene (solvent).