Memory effects in a nanoparticle system: Low-field magnetization and ac susceptibility measurements

A dilute magnetic liquid with Co nanoparticles with an average diameter of 5 nm in hexane has been studied systematically following the experimental approach proposed recently for observing memory effects in magnetic nanoparticles. All phenomena reported previously have been observed, which were earlier ascribed to the memory effect. However, the standard experiments for observing memory effects (low frequency as susceptibility and low field dc magnetization measurements) do not show the memory effect. To understand those observations, very detailed physical pictures, based on the relaxation of the individual particle, are proposed here.