Nano-crystalline powder was synthesized by ball milling. By varying the duration of mechanical attrition, powder with grain sizes ranging from 8 to 25nm was produced. The nano-crystalline material exhibited a 2-component Mössbauer spectrum, due to the presence of 2 phases. These were the crystallites, and the grain-boundary regions between them. The grain boundaries gave rise to a distribution of hyperfine magnetic fields which was typical of an amorphous-type configuration. A marked variation in the temperature dependence of the magnetization above 580K was observed; as compared with that of coarse-grained material. The effect was attributed to the order-disorder transition of spins located at the grain boundary. Experiments which were performed on 8nm-sized samples that had been previously annealed at high temperatures revealed an increase in the grain dimensions, and atomic rearrangement at the grain boundaries.

Grain-Boundary Structure and Magnetic Behavior in Nano-Crystalline Ball-Milled Iron L.Del Bianco, A.Hernando, E.Bonetti, E.Navarro: Physical Review B, 1997, 56[14], 8894-901