Controlling magnetic properties of iron oxide nanoparticles using post-synthesis thermal treatment

Changes in morphological and magnetic properties of Fe3O4 nanoparticles before and after annealing are investigated in the present work. The nanoparticles are synthesized in a standard capacitively coupled plasma enhanced chemical vapour deposition system with two electrodes using ferrocene as the source compound. Post annealing, due to the sintering process, the particles fuse along with recrystallization. This results in increased size of the nanoparticles and the interparticle interaction, which play a major role in deciding the magnetic properties. X-ray diffraction patterns of the samples before and after annealing indicate a phase change from Fe3O4 to Fe2O3. Annealing at 200 C-a similar to causes the apparent saturation magnetization to increase from 6 emu g(-1) to 15 emu g(-1). When annealed at 500 C-a similar to, the magnetic properties of the nanoparticles resemble those of the bulk material. The evidence for the transition from a superparamagnetic state to a collective state is also observed when annealed at 500 C-a similar to. Variation of the magnetic relaxation data with annealing also reflects the change in the magnetic state brought about by the annealing. The correlation between annealing temperature and the magnetic properties can be used to obtain nanocrystallites of iron oxide with different sizes and magnetic properties.