Dynamics of the magnetization in antiferromagnetic nanoparticles

Ultrafine particles of hematite (alpha-Fe2O3) with an average size of about 15 nm have been prepared by controlled heating of ferric nitrate. The fluctuations of the sublattice magnetization directions have been studied by combining Mossbauer spectroscopy and magnetization measurements. Two different modes of superparamagnetic fluctuation of the particle moment have been observed. This shows that the behavior of these antiferromagnetic hematite particles are more complicated than the usual ferri- and ferromagnetic particles which exhibit only one such mode. The particles have also been studied by neutron scattering, and for the first time it has been shown that one can obtain information about magnetic fluctuations in nanoparticles by use of this technique. This was done by inelastic neutron scattering on an antiferromagnetic reflection. Even smaller hematite particles have now been prepared and the study of these particles is currently in progress. A similar study on nickel oxide (NiO) nanoparticles are now in progress.