Materials with hard magnetic properties: industrial materials

One-sixth of the market for magnetic materials is dedicated to permanent magnets, and is currently growing at a rate of 10% per year. Permanent magnet materials fall into three main families: alnico metals, hard ferrite ceramics and rare-earth-based intermetallics, plus small quantities of metal alloys (less than 1%), such as the expensive platinum-cobalt.

As a general rule, a magnet is an integral part of a system. As a result, both mechanical and magnetic constraints have to be considered simultaneously. Depending on the application, one range of materials will be better suited than another, for example, depending on the volume and shape of the space available for the magnet (an elongated space allows alnico, a flat space requires hard ferrites or rare-earth alloys, a reduced volume imposes rare-earth alloys). Then, depending on the other conditions likely to be encountered in operation — adverse magnetic fields, shocks, extreme temperatures, irradiation, corrosion —, this choice will be refined or settled on one of the other families, with, of course, a new conception of the authorized space. But we must not forget the constraints of shaping (machining is generally difficult), price, availability, etc.

The aim of this article is to present the main characteristics, manufacturing processes and causes of magnetic hardening of permanent magnet materials belonging to the three families mentioned above. It should thus help permanent magnet users to select and make the best use of the materials required for their applications.