Cationic ordering and magnetic properties of rare-earth doped NiFe<inf>2</inf>O<inf>4</inf> probed by Mössbauer and X-ray spectroscopies

Cationic distribution and magnetic properties of partially rare-earth (R) doped NiFe2O4 (NFO) compounds, i.e. NiFe2-xRxO4 (x = 0 and 0.075; R = Gd, Dy and Ho) were investigated, and the results are discussed and presented in this paper. All the compounds were found to stabilize in the cubic inverse spinel structure with the space group Fd3-m. The analysis of the results based on X-ray absorption spectroscopy (XAS) and X-ray magnetic circular dichroism (XMCD) measurements are primarily focused on the valence state information of constituting rare-earth or transition-metal ions, cationic distribution, and the coupling between their magnetic moments. The XAS data revealed that nickel is in Ni2+ valence state while iron is mainly in the Fe3+ valence state though some perceptible traces of Fe2+ were observed. The rare-earth ions are in trivalent states. The partial doping causes differences in the cationic distribution of Ni2+ and Fe3+ ions which obviously results in differences in the net Fe or Ni magnetic moments. Furthermore, 57Fe Mössbauer spectra revealed altered hyperfine field parameters upon the Gd3+, Dy3+ and Ho3+ substitution and all the compounds were found to exhibit collinear ferrimagnetic structures. The results are relevant for the investigation of orientation of individual magnetic moments and to obtain site-specific information related to all the cations involved in the rare-earth doped nickel ferrite structures.