Bulk nanocomposite magnets with enhanced exchange coupling*

Bulk FePt/Fe$_{3}$Pt nanocomposite magnets have been prepared by compaction of chemically synthesized nanoparticles. Advanced compaction techniques, warm compaction, shock-wave compaction, spark plasma sintering and microwave sintering, have been adopted in our research. Density of the bulk samples up to 95{\%} theoretical value has been obtained while the nanostructured morphology is retained. It is found that the density increases with the compaction pressure and temperature and is also affected by the phase transition of the FePt compound from the face-centered cubic structure to the face-centered tetragonal structure. Pressure is more effective in reaching high densities. Comparison of different compaction techniques has been made. It is also found that compaction of the particles and post-annealing lead to enhanced intergrain exchange coupling. Energy products up to 16.3 MGOe of the isotropic bulk nanocomposite magnets have been achieved, which is significantly higher than the theoretical limit for fully dense single-phase FePt magnets.