Magnetism of amorphous Fe2O3 nanopowders synthesized by solid-state reactions

Monodispersed amorphous Fe2O3 nanoparticles with the size ranging from 1 to 4 nm have been prepared by thermally induced oxidative decomposition of Prussian Blue, Fe-4[Fe(CN)(6)](3) in air. Amorphous nanopowders exhibit specific magnetic behaviour strongly affected by interparticle interactions and extremely low particle size with a high contribution of the surface anisotropy as illustrated by temperature dependent and external field Mossbauer spectroscopy and magnetization measurements (4-300 K). Some of the properties are in close similarity to those of spin glasses including the fast thermal variation of superparamagnetic fraction in the Mossbauer spectra, or the particle size independence of the blocking temperature, and non-saturation of magnetization even at 5 K and magnetic field of 10 T. The reduced value of internal magnetic hyperfine field (similar to49 T at 5 K) and no indications of the tetrahedral Fe3+ sites in the 5T/5K external field Mossbauer spectra are the principal found differences in comparison with the magnetic behaviour of nanocrystalline gamma-Fe2O3. (C) 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim