Complex magnetic dynamics in an akaganéite-based iron deficiency drug

Magnetic dynamics of iron oxyhydroxide akaganéite nanoparticles, functional building blocks of the newest generation iron deficiency drug Ferinject, has been studied. While magnetism of the involved nanoparticles relies in general on bulk properties of akaganéite, the low temperature magnetic dynamics of Ferinject differs a lot from large number of other iron-oxides-based intravenous drugs in pharmaceutical practice. Clear separation of the high-temperature (Curie) and the low-temperature (spin-freezing) regimes has been identified. Instead of standard single superparamagnetic blocking temperature we show that the spin freezing relies on the two dominant and superimposed relaxations, characterizing the temperature ranges 50–100 K and 4.2–10 K, respectively. Complex magnetic dynamics is interpreted to rely on presence of the two assemblies of nanoparticles differing significantly in their average sizes. Bimodal distribution of particle sizes has been confirmed using atomic force microscopy. The knowledge of the drug’s physicochemical characteristics has been thus significantly improved.