Giant energy product in nanostructured two-phase magnets

Exchange hardening of nanostructured two-phase systems composed of an aligned hard phase and a soft phase with high magnetization is investigated using an approach which yields analytic nucleation fields from the micromagnetic vector equation, and accounts for interactions between the soft regions. In suitable structures the nucleation field is proportional to the volume-averaged anisotropy constant. For example, a multilayer composed of alternating 2.4 nm hard-magnetic ${\mathrm{Sm}}_2$${\mathrm{Fe}}_{17}$${\mathrm{N}}_3$ layers and 9 nm ${\mathrm{Fe}}_{65}$${\mathrm{Co}}_{35}$ layers can have an energy product as high as 1 MJ/${\mathrm{m}}^3$ (120 MG Oe), with a rare-earth content of only 5 wt %. Giant energy products may also be achieved in suitable cellular and disordered structures.