Spin-glass-like behaviour in nanocrystalline Fe

Pure Fe powder was milled down to about 10 nm crystal size. In the as-milled sample, Mossbauer spectroscopy reveals the presence of two phases: crystallites and grain boundaries, the latter with a strongly disordered atomic configuration. The microstructural features affect the magnetisation behaviour, as measured in the 5-250 K range at low and saturating magnetic field. Low field measurements indicate a transition from a high-temperature ferromagnetic state to a low-temperature spin-disordered state. This has been explained as a consequence of the spin freezing at the grain boundaries which drastically decreases the exchange coupling between the grains.