Hard and semi-hard magnetic materials based on cobalt and cobalt alloys

Abstract

Cobalt (Co) is one of the most important 3d transition metal elements used in the development of magnetic materials, especially in hard magnetic materials, from the early Co-based steel magnets to the rare-earth permanent magnets based on the 3d-4f compounds. Like iron (Fe), Co possesses a high saturation magnetization (MS) and a high Curie temperature (TC). Unlike Fe, Co has high magnetocrystalline anisotropy due to its spin-orbit coupling. As a result, it can be used to achieve a high degree of magnetic hardening in its elemental, alloy or compound materials. For instance, Co-based magnetic recording media is based on its elemental material. On the other hand, nano-structuring of Co-based alloys gives important hard magnetic materials including Alnico magnets and Fe–Cr–Co magnets, which are probably the earliest nanostructured hard magnetic materials. Recent progress in the development of Co-containing nanoscale hard magnetic materials has been very encouraging and promising based on the outstanding magnetic properties we have seen in L10 CoPt, tetragonally distorted FeCo and Co-based intermetallics. Particularly, the morphology of the nanoparticles has also been modulated to develop high coercivity via utilizing shape anisotropy, leading to extraordinarily high coercivity in Co nanowire assemblies. This review presents an overview of the development of Co-based hard and semi-hard magnetic materials and their intrinsic and extrinsic magnetic properties in view of fundamental understanding and technological applications.