SYNTHESIS AND CHARACTERIZATION OF ULTRAFINE NICKEL(II)-BEARING FERRITES (NIXFE3-XO4,X=0.14-1.0)

Ultrafine Ni-II-bearing ferrites (UNFs) with 14-100% Ni2+-substitution (x in NixFe3-xO4) for Fe2+ in magnetite have been synthesized by hydrolysis of Ni2+, Fe2+ and Fe3+ ions at 60 degrees C, followed by aging of the co-precipitates at 60 degrees C and heating to 300 degrees C. The aged co-precipitates (ACPs) obtained at an Ni-II/Fe-total mole ratio r=0.05-0.30 (x=0.14-0.69) in the starting solution gave the relatively sharp peaks of the spinel structure in the XRD patterns. At r>0.40 (x greater than or equal to 0.85), the spinel peaks were broader as r increased. The IR spectra of the ACPs showed that, with increasing the r value to r greater than or equal to 0.30, the ACP approached an amorphous phase in which OH- groups and/or coordinated water remained, indicating that the Fe-II ions enhance the dehydration of the ACP. The ACPs were the precursors of the UNFs, and were transformed into the UNFs by heating to 300 degrees C. The Ni2+-substitution of the UNFs was corroborated by a linear decrease in the lattice constant with an increase in the Ni2+ content of the product. The average crystallite sizes ranged from 8 to 17 nm. The transition from a magnetically split pattern to a doublet in the room-temperature Mossbauer spectra of the UNFs corresponded to a decrease in the average crystallite size, indicating superparamagnetic behaviour of ultrafine ferromagnetic particles. The UNFs gave a room-temperature saturation magnetization (M(s)) much lower than the theoretical M(s) values.