Experimental investigation of dipolar interaction in suspensions of magnetic nanoparticles

The structure and the magnetisation behaviour of two different systems of magnetic nanoparticles (MNP), namely Resovist (R) with a wide core size distribution (diameter, sigma=0.3) and SHP-20 with a rather narrow distribution (sigma=0.1), were investigated by magnetorelaxometry (MRX) and magnetisation measurements in a wide concentration range. MRX on fluid and solid suspensions yielded the distribution of hydrodynamic diameters and effective magnetic anisotropy energies (E-A), where towards higher iron concentrations the spatial particle correlation, i.e. aggregation, and also the width of the E-A distribution were increased significantly. It was further found that these effects quantitatively depend on the suspension medium, where an increased salt concentration enhanced the aggregate size distribution and E-A dispersion. At mentioned higher MNP concentrations, the quasistatic magnetisation, normalised with respect to their on content, decreased by upto 40%. In the case of SHP-20, where single core MNPs dominate, the maximum of this drop down of the magnetisation occurred at a field strength that corresponds to the strength of meansquared dipolar interaction. (C) 2010 Elsevier B.V. All rights reserved.