Structural and thermal dependence of soft magnetic properties of FeCo-based nanocrystalline materials

Structural and magnetic investigations of three families of alloys were carried out, derived from Finemet (FeSiNbBCu) type and Nanoperm type (FeNbBCu) alloys by cobalt addition, which is supposed to increase the thermal stability (higher crystallization and Curie temperature) of the material. In-situ hysteresis measurements during nanocrystallization of various selected alloys is presented and interpreted. These invetigations pointed out that the precipitating nanocrystalline phase can be in différent magnetic states during isothermal annealing depending on several parameters. It is determinative whether the amorphous phase is still ferromagnetic or already in paramagnetic state when crystallization occurs. An experimental method was developped in order to measure the longitudinally induced anisotropy. This method allows for the first time direct measurement of the longitudinal induced anisotropy in amorphous or nanocrystalline ribbons and wires. Lastly, the tensile back-stress model is applied to explain the mechanism of stress-induced anisotropy in two-phase nanocrystalline soft magnetic materials.