Determining Magnetite/Maghemite Composition and Core-Shell Nanostructure from Magnetization Curve for Iron Oxide Nanoparticles

Iron oxide magnetic nanoparticles produced by chemical synthesis are usually composed of both magnetite and maghemite phases. Information about the phase composition is typically obtained using Mössbauer spectroscopy. A method that can provide information about the magnetite versus maghemite phase composition of the nanoparticles and the organization of the phases simply from magnetization curve is still missing. Here we present a simple and elegant method that for nanoparticles with a known size distribution can give information about the magnetite and maghemite phase composition and suggests a magnetite core and a maghemite shell structure for all the nanoparticles sizes. The method is based on fitting of the room-temperature magnetization curve using a Brillouin function, while considering dipolar interactions. The model predicts that the nanoparticles are composed of a single magnetic domain for sizes below 14 nm. The model is validated by Mössbauer spectroscopy.