A comparative study of Nd₁₅Fe₇₈B₇ and Nd₁₅Co₇₈B₇ systems: phase formations and coercivity mechanisms

Both Nd-Fe-B and Nd-Co-B nanocrystalline magnets exhibit coercivity above 1 T. While the microstructure and mechanism of the coercive force of Nd-Fe-B magnets are well known, the Nd-Co-B system is much less studied, and the microstructure and origin of the high coercive state in this material is not well understood. In this work, we selected Nd₁₅Fe₇₈B₇ and Nd₁₅Co₇₈B₇ alloys with simple and stable stoichiometries to reveal the similarities and differences between these two systems. For this study, melt-spun nanocrystalline ribbons, hot-compacted magnets and bulk materials crystallized from the amorphous precursor were used. The coercivity of Nd₁₅Fe₁₈B₇ alloy is in the range of 0.9–1.7 T, while for Nd₁₅Co₁₈B₇ it was of 0.5–1.0 T. Electron microscopy and atom probe tomography analyses indicate that, in contrast to conventional Nd₁₅Fe₇₈B₇, where the ferromagnetic Nd₂Fe₁₄B phase is the dominant matrix phase and the Nd-rich phase distributes along the grain boundaries providing magnetic decoupling, the Nd₁₅Co₁₈B₇ alloys comprise of three randomly redistributed exchange-coupled ferromagnetic phases: Nd₂Co₁₄B, NdCo₅ and NdCo₄B. Single crystals of all constituent phases were grown and their spontaneous magnetization M_s and anisotropy constant K₁ were determined. Using these measured intrinsic parameters and real nanostructure of Nd₁₅Fe₇₈B₇ and Nd₁₅Co₇₈B₇ alloys, micromagnetic simulations were carried out in order to reveal the differences in the coercivity mechanisms in these two distinctly different systems.