Soft Magnetic Materials—Nanocrystalline Alloys

Nanocrystalline Fe-based alloys combine the advantageous properties of various soft magnetic materials, that is, the high saturation induction of Fe-based alloys, the high permeability and low magnetostriction of permalloys or amorphous Co-based alloys and the favorable high-frequency behavior of ferrites or amorphous metals. The materials to be highlighted are devitrified glassy Fe–Cu–Nb–Si–B alloys, which, owing to their outstanding properties, have meanwhile successfully entered into application. The microstructure consists of randomly oriented, about 10–15-nm small bcc-FeSi grains embedded in a ferromagnetic amorphous minority matrix. The article surveys the key factors from which the extraordinary properties of this new class of soft ferromagnets derive. Thus, the local magnetocrystalline anisotropies are largely averaged out since their orientation is randomly fluctuating on a scale much smaller than the domain wall width. The structural phases formed upon crystallization lead to a low or vanishing saturation magnetostriction. Like in other soft magnetic materials, annealing-induced anisotropies finally allow to tailor the hysteresis loop according to the various demands of application. Keywords: soft magnetic properties; nanocrystalline materials; random anisotropy model; magnetic anisotropies; magnetostriction; grain size dependence of coercivity/permeability; magnetocrystalline anisotropy; field induced anisotropy; creep-induced anisotropy; annealing-induced anisotropy; Fe-Cu-Nb-Si-B; exchange coupling