The influence of ruthenium on the magnetic properties of γ-Fe2O3 (maghemite) studied by Mossbauer spectroscopy

Ruthenium-doped gamma-Fe2O3 has been synthesized and examined by x-ray powder diffraction, XANES, EXAFS and by Fe-57 Mossbauer spectroscopy. Ruthenium K-edge x-ray absorption spectroscopy shows that ruthenium adopts a fully occupied octahedral site in the spinel related gamma-Fe2O3 structure as Ru4+. The Fe-57 Mossbauer spectra recorded in the presence of a longitudinal magnetic field of 6 T confirmed the octahedral coordination of the tetravalent ions and canting angles for the Fe3+ ions were determined as 24degrees for those in octahedral sites and 33degrees for those in tetrahedral sites. The Fe-57 Mossbauer spectra recorded in situ from ruthenium-doped gamma-Fe2O3 showed parameters typical of maghemite up to 600 K but with a magnetic hyperfine field distribution suggesting an inhomogeneous distribution of ruthenium within particles of varied size around about 15 nm. At 700 K a phase transition from gamma-Fe2O3 to alpha-Fe2O3 was observed and further studies showed the ruthenium-doped alpha-Fe2O3 to have a Morin transition temperature of about 400 K.