Brownian relaxation of magnetic nanoparticles in fluid: the effect of the solvent

The magnetic contribution of the solvent on the RT ac magnetic susceptibility behaviour of Fe3O4 magnetic nanoparticles in solution at different concentrations, is presented and discussed. The room temperature complex susceptibility of the solvent water was studied as a function of several parameters, namely different ac fields, superimposed dc fields, different pHs and temperature: in any case a transition from a diamagnetic behaviour to a paramagnetic one at a characteristic frequency (~4,500 Hz) was observed. These results were compared with the data obtained for other common organic solvents. A dependence of the complex magnetic susceptibility from the polarity of the solvent was observed. The results were discussed in terms of a paramagnetic contribution of polarization to the whole measured susceptibility proportional to the applied frequency. For the real and imaginary susceptibility of water at room temperature, empirical polynomial equations as a function of the frequency were obtained. On the basis of these fits, the observed complex susceptibilities of the different nanoparticles solutions were corrected from the solvent contribution and the results are presented.