A mixture of ferritin and magnetite nanoparticles mimics the magnetic properties of human brain tissue

Magnetic properties of a two-component system, consisting of horse spleen ferritin (HoSF) which contains a 5-8 nm sized antiferromagnetic ferrihydrite (5Fe(2)O(3)center dot 9H(2)O) core and ferrimagnetic magnetite (Fe3O4) nanoparticles (MNP) with an average size of 10-20 nm, have been investigated by using four different methods: induced magnetization versus (1) temperature and (2) field; (3) AC susceptibility; and (4) first-order reversal curves (FORC). All measurements were done on a mixed system of HoSF and MNP, as well as separately on the individual components. The average blocking temperature (T-B) of the mixed system at 50 mT is 15.6 K, which is a shift towards higher temperatures compared to pure HoSF (T-B=12 K). The contribution of the MNP component to magnetic ordering is evident only as a separation of the zero-field-cooled and field-cooled measurement curves. ac susceptibility is dominated by the ferrimagnetic MNP and shows strong frequency dependence. The peak ac susceptibilities can be described by the Vogel-Fulcher law, indicating the influence of interactions within the system. Hysteresis measurements at 5 K show a wasp-waisted shape due to the mixture of a high coercivity phase (HoSF) with a low coercivity phase (MNP). Initial magnetization curves above T-B can be fitted by a sum of Langevin functions, showing superparamagnetic behavior of both components. FORC diagrams are effective in illustrating the change from that of blocked MNP particles together with the superparamagnetic HoSF at 20 K to purely superparamagnetic behavior in both components above 50 K. We conclude that the mixed nanoparticle system is a good model for complex natural samples, such as human brain tissue.