Dipolar Attraction of Superparamagnetic Nanoparticles

Superparamagnetic nanoparticles (SMNPs) are essential components in a number of medical diagnostic- and drug transport techniques, and cancer therapies. It is often claimed that the rapid thermal reversals of SMNP magnetic moments negates their dipolar attraction, hence facilitating colloidal stability. We find that this is a misconception. Using Langevin dynamics, we simulate SMNP pairs and the dimer clusters they form. To quantify the tendency to aggregate, we introduce the dimer debonding time and calculate the average magnetic force of attraction which results from correlations in the fluctuating moments─a magnetic analogue of the van der Waals interaction. We find that the magnetocrystalline anisotropy, which determines the rate of superparamagnetic reversals, has no influence on debonding time, and comparing with computed Néel relaxation times this holds for both blocked and superparamagnetic particles. Hence superparamagnetism does not affect aggregation, which explains the success of previous simplified models in describing the equilibrium structure of SMNP systems. Because the key dimensionless parameter for the Néel relaxation of a lone SMNP and the one for magnetic attraction have the same size and temperature scaling, there is a strong correlation with colloidal stability, as observed experimentally, but no causal relation.