  Magnetization and Mossbauer studies have been performed on the polymer coated
magnetite nanoparticles with particle size from 5.1 to 14.7 nm. The maximum in
the temperature dependence of magnetization (TM) is found to be inconsistent
with the particle size (DTEM). The effective magnetic anisotropy (Kan) is found
to increase with the decrease of DTEM, which is attributed to the increase of
surface anisotropy. The absence of coercivity and remanence of magnetization
noticed well above TM indicates the superparamagnetic behaviour, which has also
been observed in the temperature dependent Mossbauer results. The temperature
dependence of hyperfine field is found to follow similar dependence of
saturation magnetization for bulk magnetite.
