Magnetic behaviour of Mn0.6Fe0.4Fe2O4 nanoparticles in ferrofluid at low temperatures

The temperature dependence of saturation magnetisation of non-interacting Mn0.6Fe0.4Fe2O4 nanoparticles, coated with oleic acid and dispersed in kerosene, differs significantly from that of bulk ferrite. When the temperature decreases from 300 to 90 K, the relative increase of the saturation magnetisation of the nanoparticles system is 55.80% as opposed to only 20.98%, the relative increase of the saturation magnetisation of the bulk ferrite. The difference of approximately 35% is due to the increase of the mean magnetic diameter (attached to the core of aligned spins), from 10.87 to 11.85 nm, as a result of the modification of the superexchange interaction from the nanoparticle shell. Compared to the nanoparticles without surfactant, the T-3/2 law is verified down to the room temperature, but having a parameter dependent on temperature. In this case, we suggest a core-shell model in agreement with the experimental results. At low temperatures, the magnetisation shows hysteresis as a result of magnetic relaxation processes. (C) 2002 Elsevier Science B.V. All rights reserved.