High-temperature Mossbauer spectroscopy of mechanically milled NiFe2O4

Oxide spinels, in particular those containing iron, often exhibit technically important electrical- and magnetic-properties. We report here on X-ray powder diffraction and Mossbauer studies of nanostructured NiFe2O4 particles prepared by high-energy ball milling from bulk NiFe2O4, which is an inverse spinel. The Mossbauer spectra were recorded in situ at different temperatures in the range of 300-850 K. The Mossbauer spectra of the milled samples show a broad distribution of magnetic hyperfine fields together with a paramagnetic state at room temperature. Initially, at 700 K the spectrum is mainly paramagnetic, but during the process of annealing, magnetic sextets emerge. The treatment results in a significant change in the B/A area ratio of the ferrite. The Neel temperature of the samples is estimated from the B(T) relation to be in the range of 800-850 K.