The exchange-spring magnet: a new material principle for permanent magnets

It is proposed that permanent magnets can be made 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, 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, on the order of 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.