Size-independent residual magnetic moments of colloidal Fe3O4-polystyrene nanospheres detected by ac susceptibility measurements

Complex ac susceptibility of magnetic colloids of nanospheres, each consisting of Fe(3)O(4) nanoparticles densely and uniformly embedded in a polystyrene matrix, is measured as a function of frequency. A data analysis based on a model and the directly measured size distribution shows that each spherical aggregate of nanoparticles carries a small residual magnetic moment, whose average is basically independent of the sphere size. The mentioned model assumes that magnetic spheres undergo rotational Brownian motions obeying Debye's theory and nanoparticles undergo Neel magnetization rotations. A discussion is made on the superparamagnetism and the nanoparticle interactions in the present case, in order to justify the applicability of the model and to look for the mechanism of the detected sphere-size independent magnetic moment. (c) 2008 American Institute of Physics. [DOI: 10.1063/1.3005988]