Magnetic properties of nanocrystalline NiFe2O4 powders prepared by an aqueous oxidative precipitation process

Nanocrystalline NiFe2O4 powder has been prepared by an oxidative precipitat ion process at room-temperature and the crystallite size dependent magnetic properties have been studied. The NiFe2O4 powders prepared at room-tempera ture had an average crystallite size of 6 nm and showed a reduced saturatio n magnetisation (M-s) of 3 emu g(-1). The crystallite size was increased by heating the samples to increasingly higher temperatures. The M-s value inc reased from 3 to 40 emu (.) g(-1) as the crystallite size was increased fro m 6 to 120 nm. The samples having crystallite sizes from 6 to 20 nm were su perparamagnetic at room-temperature. The Mossbauer spectrum of the 6 nm siz ed 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 50 4 kOe for A(tetraheral) and B(octahedral) sublattices, respectively It is s hown that the saturation magnetisation and magnetic hyperfine field values are significantly reduced in nanocrystalline NiFe2O4 as a function of cryst allite size.