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 an d 500 K with annealing effects in the same temperature region. These r elaxations can be attributed to irreversible rearrangements of the loc al atomic structure in the grain boundaries governed by activation ent halpies 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 polari zation increases according to a 1/(mu(0)H)(1/2) law which can be assig ned to the presence of nanovoids with a diameter of about 2.2 nm. This size is nearly unaffected by the annealing at 600 K.