Magnetism: it is permanent

Permanent magnet materials have one of the longest histories of any technological phenomenon, stretching from before 4000 BC to the 20th century. The early history of lodestones (naturally occurring magnets), their applications and their use to make artificial magnets is described. In the 20th century, three major families of permanent magnet materials (metal, ceramic and rare earth) have been developed. The composition, properties and the method of manufacturing these metal (aluminium-nickel-cobalt-iron), ceramic (barium or strontium ferrite) and the three generations of the rare earth (RCo₅, R₂Co₁₇ and NdFeB) magnets are described, and the possible applications of permanent magnetic materials are briefly tabulated. Using performance indicators that have been defined earlier, it is shown that the performance of permanent magnet materials have increased two-hundred-fold in the 20th century. The past and future of these important commercial engineering materials is permanent.