| We have studied the slow dynamics of Fe3O4 nanoparticles (superspin-glass) and Au-coated Fe3O4 nanoparticles (superparamagnets) and have found a few similarities and differences in their low temperature magnetic properties. Origin of interaction in the former is attributed to dipole-dipole interactions among particle moments (superspins); while the latter is ascribed to a broad distribution of relaxation times which in turn comes from the anisotropy in energy barriers. We have studied here analytically two models of non-interacting magnetic nanoparticle systems (superparamagnets). Also experimentally, we have studied the Fe3O4 and Au-coated Fe3O4 nanoparticle systems using a superconductive quantum interference device (SQUID) magnetometer. Certain phenomena such as time dependence of relaxation rate, temperature dependence of genuine zero-field-cooling, rejuvenation, and unique memory effect have been found singular to superspin-glass systems while others such as aging and memory effects are common to both. We have also calculated approximately the number of nanoparticles, their size, inter-planar distance, and their interaction energy. |
