Measurements of grain growth in nanocrystalline Fe reveal a linear dependence of the grain size on annealing time, contradicting studies in coarser-grained materials, which find a parabolic (or power-law) dependence. When the grain size exceeds approximately 150 nm, a smooth transition from linear to nonlinear growth kinetics occurs, suggesting that the rate-controlling mechanism for grain growth depends on the grain size. The linear-stage growth rate agrees quantitatively with a model in which boundary migration is controlled by the redistribution of excess volume localized in the boundary cores.

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We present an experimental study of the magnetic microstructure in the nanocrystalline hard magnet Tb. Field-dependent small-angle neutron scattering ͑SANS͒ data are analyzed quantitatively in terms of the correlation function of the spin misalignment. We find that up to applied fields of several tesla the magnetization remains ''locked in'' to the basal planes of the hcp crystal lattice of each individual crystallite, but that the in-plane orientation of the spins is highly nonuniform within each grain. This spin disorder at the nanoscale can be suppressed by a large applied field, but in the remanent state the disorder reduces the magnetization to values considerably below the Stoner limit. In field-dependent SANS, the intragrain spin disorder gives rise to a crossover of the scattering curves, and to the unusual finding that the scattering cross section at small scattering vector increases with increasing magnetic field. As the origin of the internal spin disorder within the grains, we propose an extra magnetic anisotropy energy at small grain size, presumably due to microstrain, a suggestion which is supported by analysis of ac-susceptibility data in the paramagnetic state. Our finding of a reduced remanence at small grain size is contrary to the remanence enhancement that is observed in other nanocrystalline hard magnets. We also report an unusual logarithmic field dependence of the magnetization over wide ranges of the applied field and temperature.

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