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® with a wide core size distribution (diameter, σ=0.3) and SHP-20 with a rather narrow distribution (σ=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 (EA), where towards higher iron concentrations the spatial particle correlation, i.e. aggregation, and also the width of the EA 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 EA dispersion. At mentioned higher MNP concentrations, the quasistatic magnetisation, normalised with respect to the iron content, decreased by up to 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 mean squared dipolar interaction.

Research highlights
► Magnetorelaxometry measures the size of MNP aggregates and its magnetic anisotropy energy.
► Increase in the size of aggregates leads to increased magnetic anisotropy energies.
► Increased anisotropy energy barriers attenuate the magnetisation at a distinct field range.
► Interaction energy between single core particle’s dipoles matches the increased anisotropy.