The effects of the partial substitution of Fe by Co in an FeSiBCuNb alloy on the crystallization process and on the magnetic properties have been analyzed. The crystallization of the amorphous melt-spun alloys, gives rise to an ultrafine structure composed by a residual amorphous matrix and crystals (tens of nm in size). Saturation magnetization and Curie temperature are analyzed in the nanocrystalline state in terms of the estimated compositional dependence of both structural phases. Coercivity behavior is interpreted within the framework of the random anisotropy model, where the structure and magnetic nature of the precipitated crystalline phase plays the dominant role. While a soft magnetic behavior is ascribed to the magnetic coupling between the $\alpha -\mathrm{FeSiCo}$ crystals for low Co concentrations, the precipitation of Co rich crystals in the Co base samples gives rise to a drastic magnetic hardening. The main interest of these new soft magnetic FeCoSiBCuNb alloys lies in applications at elevated temperatures, due to the improvement of the soft magnetic response with respect to the conventional FeSi based nanocrystalline alloys.

• Received 22 January 2001

DOI:https://doi.org/10.1103/PhysRevB.65.024433