On the origin of stress-anneal-induced anisotropy in Finemet-type nanocrystalline magnets

It has recently been shown that the anisotropy in nanocrystalline Finemet-type magnets, induced by their crystallization under tensile stress, can originate from the magnetoelastic coupling within the crystallites (G. Herzer, IEEE Trans. Magn. 30 (1994) 4800) or can have its source in directional diatomic ordering which occurs also within the volume of nanocrystalline phase (Hofmann and Kronmüller, J. Magn. Magn. 152 (1996) 91). In the present work the temperature dependence of this anisotropy has been investigated for a series of initially amorphous Fe 73.5Cu 1Nb 3Si 15.5B 7-ribbon samples (Vacuumschmelze, GmbH), nanocrystallized at various temperature-time-stress conditions. It has been assumed that the obtained results will allow to judge which of the mechanisms mentioned above governs in reality the considered anisotropy. An analysis of the results obtained shows that diatomic directional ordering within the volume of the nanocrystalline phase seems to be very likely the origin of the stress-anneal-induced anisotropy in FINEMET-type magnets. This analysis has been performed assuming that the anisotropy originating from the atomic ordering should scale with the square of saturation magnetization in the high-temperature range (in the vicinity of critical temperature-Curie point of nanocrystalline phase) and with the cube of this quantity at low temperatures. Quite satisfactory agreement has been obtained for the best fit of the experimental data to the polynomial consisting of the two above terms.