Magnetism of ultrathin intergranular boundary regions in Nd–Fe–B permanent magnets

The magnetism of a thin grain-boundary (GB) phase that envelopes the Nd2Fe14B grains in optimally annealed Nd–Fe–B sintered magnets was investigated by electron holography. The phase shift measured from a thin-foil specimen containing a tilted amorphous GB phase (∼3 nm in width) was −0.34 rad, which is substantially smaller than that expected for the nonferromagnetic GB phase of −1.2 rad. Simulations of the phase shift with various magnetization values suggest that the magnetic flux density of the GB phase is ∼1.0 T. The observations imply significant exchange coupling between Nd2Fe14B grains, which can explain the avalanche propagation of magnetization reversal observed in sintered magnets.