Architecture and magnetism of alnico

A rare-earth supply crisis has stimulated an intensive search for alternative permanent magnets. Alnico materials, alloys containing Al, Ni, Co and Fe, are functional nanostructured alloys, which show great potential for replacing the best commercial Nd-based rare-earth alloys for applications above 200 °C. However, their coercivity is ∼2-3× below theoretical limits. The coercivity of alnico depends on the nanostructure developed during spinodal decomposition. In this work, atom probe tomography, combined with advanced electron microcopy, indicate that the microstructure of alnico is sensitive to the introduction of alloying elements such as Ti and Cu, as well as the crystallographic orientation of the parent phase with respect to the direction of the imposed magnetic field during spinodal decomposition. The alnico coercivity mechanism involves interplay of size, chemistry and possibly stress at interfaces. Control of these parameters should allow reduction of the spatial dimension of the FeCo-rich precipitates and the interaction between them, which should in term increase the coercivity of alnico alloys.