No abstract

We present a new method for the characterization of magnetic nanoparticles based on the analysis of the dependence of the Néel relaxation signal on the sample temperature. In contrast to the established characterization methods, the new method directly delivers the energy barrier distribution of the magnetic system (in the case of ferrofluid particles or their aggregates). A water based ferrofluid consisting of magnetic nanoparticles with an iron oxide core and a shell of carboxydextran has been magnetically fractionated and immobilized and the fractions have been investigated in a temperature range from 77 to 350 K. The influence of the fractionation process on the distribution of the energy barriers of the particle system has been studied qualitatively.

A recently introduced method for the characterization of magnetic nanoparticles (MNPs) based on the analysis of the temperature-dependent Néel relaxation signal (TMRX) has been applied to characterize maghemite particles with different particle size distributions. The samples were made using an improved magnetic fractionation method for a ferrofluid with a broad particle size distribution. The temperature range of the measurement set-up has been extended from 315 K down to 4 K to detect even the smallest particles in the fractions. A mean magnetically relevant particle size has been derived from TMRX and low temperature coercivity measurements and has been compared to the physical size determined by atomic force microscopy (AFM) investigations.