Coercivity Limits and Mechanism in Nanocomposite Nd-Fe-B Alloys

The largest coercivity in the Nd–Fe–B system was achieved by systematically tailoring the microstructure from strongly interacting Nd 2 Fe 14 B grains to magnetically isolated Nd 2 Fe 14 B grains in a Nd-rich matrix. The crystal structure, phase purity and magnetic properties of the Nd–Fe–B samples were extensively measured. In particular energy-filtered images, using spatially resolved measurements of inner-shell ionization edges, was critical in evaluating the particle shapes (platelets with the crystallographic c-axis normal to the plate), size (~100  40  25 nm) and distribution. For randomly oriented, non-interacting particles, the largest observed coercivity, μ 0 H c ~ 2.75 T is ~83% of the theoretical limit expected for StonerWohlfarth coherent rotation behavior including demagnetization effects. Initial magnetization curves of thermally demagnetized Nd–Fe–B samples show a systematic increase in susceptibility with an increase in Nd concentration as a result of a competition between contributions from strongly interacting Nd 2 Fe 14 B grains with grain sizes smaller or equal to the magnetic domain size and completely isolated multidomain Nd 2 Fe 14 B grains. The possible coercivity mechanisms in such Nd–Fe–B samples are discussed.