Nanoparticles of Fe3O4 (magnetite) and gamma-Fe2O3 (maghemite) have been studied by Mössbauer spectroscopy. These nanoparticles have applications as contrast agents for Magnetic Resonance Imaging. At room temperature, they are superparamagnetic and the magnetic hyperfine fields were averaged to zero. The spectra of Fe3O4 were composed of two lines corresponding to two crystal sites, identified from their isomer shifts as Fe3+ on the tetrahedral A site and Fe2.5+ (mixed Fe2+ and Fe3+) on the octahedral B site. The relative intensity of the two lines shows that the samples are almost stoichiometric with the formula Fe3-xO4, where x is less than 0.04 at room temperature. The x that was obtained was compared to those obtained in a magnetic field and at room temperature. The lines are broad and the measurements as a function of temperature were analyzed using Néel’s theory of superparamagnetism to yield values of the relaxation times. The same nanoparticles that have oxidized into gamma-Fe2O3 have been studied for comparison
