Magnetic after-effect and approach to ferromagnetic saturation in nanocrystalline iron

Nanocrystalline iron with grain sizes of 10–15 nm was produced by gas-phase condensation and compaction under high pressure. Magnetic after-effect measurements exhibit a broad relaxation spectrum between 300 and 500 K with annealing effects in the same temperature region. These relaxations can be attributed to irreversible rearrangements of the local atomic structure in the grain boundaries governed by activation enthalpies in the range 1.0–1.4 eV. Additionally, after annealing at 600 K the mean magnetic moment per atom increases, indicating a change in the atomic coordination and in the nearest-neighbour distances in the grain boundaries. Approaching the ferromagnetic saturation, the polarization increases according to a 1/(μ 0H) 1 2 law which can be assigned to the presence of nanovoids with a diameter of about 2.2 nm. This size is nearly unaffected by the annealing at 600 K.