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 Mössbauer 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 an atomic rearrangement at the grain boundaries.

The magnetic properties of the ball-milled nanocrystalline fcc phase of FeRh alloys are reported and discussed. The observed features (maximum of ac and zero-field cooling susceptibility at a temperature $T_{\max }$ weakly dependent on the frequency and on the applied field) suggest a progressive blocking of moments of nanograins with decreasing temperature, followed by their collective freezing in random directions at $T_{\max }.$ The shifted hysteresis loop observed after field cooling as well as the remarkable irreversibility in high fields indicates that $T_{\max }$ corresponds to the freezing temperature of the strongly deformed grain boundaries, which behave as a spin-glass-like phase.

The coercive field of ${\mathrm{Fe}}_{87.2}$ ${\mathrm{Zr}}_{7.4}$ ${\mathrm{B}}_{4.3}$ ${\mathrm{Cu}}_{1.1}$ amorphous ribbons obtained by melt spinning becomes 1 A ${\mathrm{m}}^{\mathrm{-}1}$, in the relaxed state, which is achieved after annealing at 410 °C during 1 h. After annealing for the same time at 480 °C the coercive force abruptly increases up to 100 A ${\mathrm{m}}^{\mathrm{-}1}$. For increasing annealing temperatures the coercivity drops again to 10 A ${\mathrm{m}}^{\mathrm{-}1}$. X-ray diffraction and Mössbauer spectroscopy show that the magnetic hardening is due to the appearance of a few nanocrystals of Fe which are separated a distance that in average is longer than the exchange correlation length of the amorphous matrix. As the number of Fe nanocrystals increase, the intergranular distances decrease and the grains become exchange coupled giving rise to the subsequent magnetic softening.