MAGNETIC PROPERTIES OF NANOCRYSTALLINE NiFe2O4 POWDERS PREPARED BY AN AQUEOUS OXIDATIVE PRECIPITATION PROCESS

Nanocrystalline NiFe2O4 powder has been prepared by an oxidative precipitation process at room-temperature and the crystallite size dependent magnetic properties have been studied. The NiFe2O4 powders prepared at room-temperature had an average crystallite size of 6 nm and showed a reduced saturation magnetisation (Ms) of 3 emu · g-1. The crystallite size was increased by heating the samples to increasingly higher temperatures. The Ms value increased from 3 to 40 emu·g-1 as the crystallite size was increased from 6 to 120 nm. The samples having crystallite sizes from 6 to 20 nm were superparamagnetic at room-temperature. The Mossbauer spectrum of the 6 nm sized sample showed a broad quadruple doublet, whereas, the 40 nm sized sample showed a clear sextet pattern, with a hyperfine field values of 466 and 504 kOe for A(tetraheral) and B(octahedral) sublattices, respectively. It is shown that the saturation magnetisation and magnetic hyperfine field values are significantly reduced in nanocrystalline NiFe2O4 as a function of crystallite size.