Grain-boundary structure and magnetic behavior in nanocrystalline ball-milled iron

Nanocrystalline Fe powder was synthesized by mechanical-energy high-transfer technique (ball milling). By varying the duration of the mechanical attrition, powder with grain sizes in the 8-25 nm range was produced. The nanocrystalline material exhibits a two-components Mossbauer spectrum due to the presence of two phases: the crystallites and the grain-boundary region among them. In particular, the grain boundaries give rise to a distribution of hyperfine magnetic fields typical of an amorphouslike configuration. The structural features of nanocrystalline iron affect strongly the magnetic behavior. A marked variation of the temperature dependence of magnetization above 580 K is observed, compared to that of coarse-grained iron. The effect has been connected with the order-disorder transition of the spins located at the grain boundary. Experiments performed on 8-nm-sized iron previously annealed at high temperature indicate an increase of the grain dimensions and anatomic rearrangement at the grain boundaries.