Materials with hard magnetic properties: the basics

Materials with hard magnetic properties, or permanent magnets, are ferromagnetic or ferrimagnetic solids at room temperature. Once magnetized, they are likely to retain a certain magnetic state indefinitely, characterized by significant remanent magnetization and high resistance to demagnetization. Few branches of technology have progressed as rapidly over the course of the twentieth century, yet have as long a history as permanent magnets. Today's best permanent magnets are almost 130 times more powerful than those used in 1900, while their resistance to demagnetization can be 250 times greater.

Making a magnet means arranging all the magnetic moments in a material in parallel and holding them there by forces acting at the level of the atoms inserted in the crystal. Creating a permanent magnet requires energy. But once completed, the modern magnet is an energy converter of exceptional performance which itself creates — under certain appropriate conditions of use — a magnetic field with no energy consumption or loss of intensity.

Hard magnetic materials are primarily characterized by their remanent magnetization and coercive field values. The remanent magnetization is determined by the spontaneous magnetization of the material, which requires the presence of transition elements. Coercivity is linked to magnetic anisotropy. But optimum magnetic properties can only be achieved in the presence of a heterogeneous microstructure or nanostructure capable of effectively and favorably controlling the magnetization mechanisms involved. The aim of this first article is to review the main magnetic characteristics of materials and to highlight the conditions required for the emergence of hard magnetic properties.