Formation of magnetic nanocrystals in a glass ceramic studied by small-angle scattering

The formation of nanosized crystallites of magnetite, Fe3O4, by heat treatm ent of a glass containing iron oxide was investigated. The magnetic propert ies of the glass ceramic manufactured strongly depend on the heat treatment conditions. The evolution of size distribution and volume fraction of the nanocrystallites formed was studied by small-angle x-ray scattering (SAXS). The size distribution of the nanocrystalline phase turned out to show bimo dal shape. The possibility of magnetic contrast variation offered by small- angle neutron scattering (SANS) was utilized in order to distinguish the sm all-angle scattering of magnetite from the scattering contributions of nonm agnetic iron containing crystallites that can additionally be formed during the heat treatment. The results obtained reveal that both size grades of p articles observed in the size distribution are superparamagnetic consisting of magnetite. The evolution of the volume fraction of magnetite in depende nce on the heat treatment was found to be correlated with the magnitude of the specific saturation magnetization of the glass ceramic. The volume size distributions derived from magnetic SANS revealed peaks at smaller radii i n relation to those from nuclear SANS and SAXS data. Therefore, the existen ce of a nonmagnetic surface layer is suggested that surrounds the magnetica lly active core of the magnetite nanocrystals. (C) 1999 American Institute of Physics. [S0021-8979(99)08604-1].