Distinguishing nanostructured Al-doped maghemite-like Fe2-yAlyO3 from spinel-like Fe3-yAlyO4

Abstract

We present an investigation of a nanostructured Fe-Al-O system with x=Al/Al+Fe ratios of 0.17, 0.33 and 0.50, prepared by the autocombustion method. The main phase found in all samples is the cubic spinel phase. The inclusion of Al in the spinel structure resulted in a progressive decrease in both the cell parameter and the grain size, with the former approaching the cell parameter of maghemite rather than magnetite. Although the XRD patterns match those reported for AlFe2O4, Mössbauer spectroscopy and magnetic measurements allows us to determine that the formed spinel phase can be identified as Aldoped maghemite-like Fe2-yAlyO3. The room temperature Mössbauer spectrum of sample x=0.17 consists of magnetic components belonging to iron at the iron (III) oxide spinel sites, plus a central doublet. For higher Al content, the relative area of the magnetic components decreases, and the spectra mainly consists of a Fe3+ broad doublet corresponding to superparamagnetic iron (III) oxide particles. The 13 K spectrum of x=0.33 sample (nominally AlFe2O4) is mainly composed of magnetic sites belonging to blocked particle moments of Al-doped maghemite-like particles. The magnetization loops are consistent with a ferro/ferrimagnetic behaviour, whose saturation magnetization decreases with the incorporation of Al.