Nanocrystalline materials for high temperature soft magnetic applications:A current prospectus

Conventional physical metallurgy approaches to improve soft ferromagnetic p roperties involve tailoring chemistry and optimizing microstructure. Alloy design involves consideration of induction and Curie temperatures. Signific ant in the tailoring of microstructure is the recognition that the coercivi ty, (H-c) is roughly inversely proportional to the grain sine (D-g) for gra in sizes exceeding similar to 0.1-1 mu m (where the grain size exceeds the Bloch wall thickness, delta), In such cases grain boundaries act as impedim ents to domain wall motion, and thus fine-grained materials are usually har der than large-grained materials, Significant recent development in the und erstanding of magnetic coercivity mechanisms have led to the realization th at for very small grain sizes D-g < similar to 100 nm, H-c decreases sharpl y with decreasing grain size, This can be rationalized by the extension of random anisotropy models that were first suggested to explain the magnetic softness of transition-metal-based amorphous alloys. This important concept suggests that nanocrystalline and amorphous alloys have significant potent ial as soft magnetic materials. In this paper we have discussed routes to p roduce interesting nanocrystalline magnets. These include plasma (arc) prod uction followed by compaction and primary crystallization of metallic glass es. A new class of nanocrystalline magnetic materials, HITPERM, having high permeabilities at high temperatures have also been discussed.