Mössbauer studies of stoichiometry of Fe3O4: characterization of nanoparticles for biomedical applications

The iron oxide Fe3O4, the mineral magnetite sometimes called ferrosoferric oxide, is notoriousy non-stoichiometric even in bulk form so its formula may be written Fe3−δO4. In nanoparticle form, where it has applications in medicine and information technology, it is even more susceptible to oxidation. In this paper we report synthesis and studies of superparamagnetic Fe3O4 nanoparticles with controlled diameters of 5.3, 10.6 and 11.9 nm. In room temperature spectra, departures from stoichiometry δ of up to 0.02 were estimated from the relative amounts of Fe 3+/ Fe 2+ and from their isomer shifts. This cannot be used for very small particles of diameter 10.6 nm and less as they are superparamagnetic at room temperature and do not show hyperfine splitting owing to fast relaxation. Such particles have promise for use in enhancing MRI signals. The magnetic spectrum is restored by the application of a relatively small magnetic field (10 kG). As the temperature is lowered the relaxation slows down and 6-line magnetic hyperfine patterns appear below a blocking temperature TB. The values of TB obtained are lower than those of many other researchers reported in the literature, suggesting that our particles are less affected by magnetic interactions between them. At low temperatures all the spectra are similar and closely resemble that of bulk Fe3O4 confirming that departures from stoichiometry are small.