THE EXCHANGE-SPRING MAGNET - A NEW MATERIAL PRINCIPLE FOR PERMANENT-MAGNETS

The best available hard magnetic materials have a lower saturation polarization J(s) than many soft magnetic materials. They are chemically very reactive and also expensive on account of a substantial content of a rare earth. Therefore, it is proposed to make permanent magnets of composite materials consisting of two suitably dispersed ferromagnetic and mutually exchange-coupled phases, one of which is hard magnetic in order to provide a high coercive field, while the other may be soft magnetic just providing a high saturation J(s) and should envelop the hard phase regions in order to prevent their corrosion. A general theoretical treatment of such systems shows that one may expect besides a high energy product (BH)max two uniquely characteristic features, a reversible demagnetization curve (exchange-spring) and, in certain cases, an unusually high isotropic remanence ratio B(r)/J(s), while the required volume fraction of the hard phase may be very low, of the order 10%. The technological realization of such materials is shown to be based on the principle that all phases involved must emerge from a common metastable matrix phase in order to be crystallographically coherent and consequently magnetically exchange coupled. Finally it is shown that a corresponding composite material thus produced exhibits the characteristic magnetic behavior as predicted by the theory.