Evolution of structure and magnetic properties with annealing temperature in nanoscale high-energy-milled nickel ferrite

Metastable nanocrystalline nickel ferrite (NiFe2O4) with crystallite size similar to9 nm produced by high-energy milling the bulk material has been annealed at elevated temperatures to investigate the change induced in the microstructure and the magnetic properties. High-temperature X-ray diffraction, thermogravimetric measurements, Mbossbauer spectroscopy, electron microscopy, and magnetisation experiments have been used for this purpose. The results show that the range of the thermal stability of the mechanically induced defects in the structure of milled NiFe2O4 extends up to 600K. Owing to the thermally induced variability of the cation arrangement and the spin configuration, the milled NiFe2O4 annealed at temperatures above 600 K exhibits a wide range of magnetic behaviour. The observed magnetic softening and an increase of both the saturation magnetisation and the Neel temperature with increasing annealing temperature are attributed to the cation reequilibration process and to the disappearance of the canted spin arrangement. (C) 2002 Elsevier Science B.V. All rights reserved.