New permanent magnet materials

Modern permanent magnet devices require the presence of large coercive forces and in turn this requires the presence of magnetocrystalline anisotropies. Favourable candidates for sufficiently large anisotropies are rare-earth-base-materials, where this property originates from a combination of the crystal-field interaction of the 4f electrons with electrostatic charge of the surrounding ions and the relatively strong spin-orbit interaction of the 4f electrons. A sufficiently high magnetization and magnetic ordering temperature is guaranteed by combining rare-earth elements with 3d transition metals. More than a decade ago high-performance permanent magnets were based on Sm and Co, (SmCo₅). Recently and even more powerful permanent magnet material was discovered which is based primarily on the ternary intermetallic compound Nd₂Fe₁₄B and which has procreated considerable scientific and technological interest. In this review a description will be given of the basic properties of rare-earth compounds of the type R₂Fe₁₄B,R₂Co₁₄B and several related intermetallic compounds. This description comprises crystal structure, phase relationships, magnetization, magnetic structure and magnetic anisotropy. The properties of all these materials will be compared and discussed in terms of magnetic exchange interaction and crystal-field theory. A substantial part of the paper will be devoted to permanent magnet fabrication and includes a discussion of the various coercivity mechanisms and their relation to the microstructure.