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

Abstract

Magnetic properties of a two-component system, consisting of horse spleen ferritin (HoSF) which contains a $5--8\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$ sized antiferromagnetic ferrihydrite $(5{\mathrm{Fe}}_{2}{\mathrm{O}}_{3}∙9{\mathrm{H}}_{2}\mathrm{O})$ core and ferrimagnetic magnetite $({\mathrm{Fe}}_{3}{\mathrm{O}}_{4})$ nanoparticles (MNP) with an average size of $10--20\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$, 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 $({T}_{\mathrm{B}})$ of the mixed system at $50\phantom{\rule{0.3em}{0ex}}\mathrm{mT}$ is $15.6\phantom{\rule{0.3em}{0ex}}\mathrm{K}$, which is a shift towards higher temperatures compared to pure HoSF $({T}_{\mathrm{B}}=12\phantom{\rule{0.3em}{0ex}}\mathrm{K})$. 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 $5\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ 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 ${T}_{\mathrm{B}}$ 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 $20\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ to purely superparamagnetic behavior in both components above $50\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. We conclude that the mixed nanoparticle system is a good model for complex natural samples, such as human brain tissue.