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Using microwave low-pressure discharge, we synthesised magnetic iron-oxide nanopowder from the iron pentacarbonyl precursor. We were able to vary the size and chemical composition (especially the ratio between various iron oxides) by careful control of the process parameters. The nanoparticulate product was analysed by X-ray diffraction (XRD) and Raman spectroscopy. However, the XRD cannot reliably distinguish between the size-broadened peaks of γ-Fe₂O₃ (maghemite) and Fe₃O₄ (magnetite) due to their nearly identical crystalline structure. Hence we used a chemical method to determine the presence of Fe(II) and Fe(III) ions in the nanopowder samples. The results agree with those from the Raman spectroscopy.