Effects of Calcination on the Magnetic Properties of Iron Oxide Nanoparticles

Magnetite $(Fe3O4)$ and maghemite $(γ‐Fe2O3)$ nanoparticles are two different phases of iron oxide which are considered to be among the best candidates for biomedical applications such as site specific drug delivery, hyperthermia and magnetic resonance imaging. Preventing the oxidation of magnetite to maghemite phase at room temperature and to hematite phase at higher temperatures remains a key issue to be addressed in the synthesis of magnetite nanoparticles. In this study, magnetite nanoparticles of two different sizes were prepared by the conventional co‐precipitation method using sodium hydroxide (NaOH) as the alkaline base. TEM characterization indicated the sizes of the nanoparticles to be $10.5±1.40 nm$ and $7.5±1.32 nm$ at stirring rates of 400 and 1000 rpm respectively. From the EDX characterization, the 7.5 nm particles had higher impurity level as compared to the 10.5 nm sample. VSM analysis of the as‐synthesized nanoparticles showed that the 7.5 nm particles had a lower saturation magnetization $(Ms)$ value as compared to the 10.5 nm particles due to spin disorder effects and higher impurity level. Calcination treatments at moderate temperatures of 200° C and 275° C helped to increase the Ms values of both the samples while at higher temperatures of 600° C and 675° C, the $Ms$ values were decreased. This may be attributed to the partial and complete transformation to hematite for the 10.5 nm and 7.5 nm sample respectively.