Oxidation of wüstite rich iron oxide nanoparticles via post-synthesis annealing

Abstract

When forming magnetic nanoparticles, the decomposition of organo-metallic precursors causes a reduction of Fe(III) to Fe(II) which leads to the formation of an antiferromagnetic rock salt phase of FeO. The antiferromagnetic phase reduces the nanoparticle magnetization, so a new method of oxidation was developed that can convert FeO rich particles to Fe3O4 particles. Iron oxide nanoparticles with different sizes were synthesized to validate the oxidation method. We demonstrate that iron oxide nanoparticles can be oxidized by post synthesis annealing without addition of oxidizing agents. The oxidized particles were measured with XRD, VSM and AC calorimetry to show the effective oxidation by comparing to the as prepared sample. The resulting 20 nm oxidized particles have a saturation magnetization of 72 Am2/kg at 300 K and a specific absorption rate of 181 W/g under a 212 kHz, 33 mT AC field.