Microstructures and Magnetic Properties of Rare-Earth-Free Co-Zr-Mo-B Alloys

The growing demand for rare-earth magnets has raised concerns over their price and the country’s risk of depleting the supply of rare-earth elements. These severe concerns have led to the study of rare-earth-free magnets that do not rely on rare-earth elements. Co-Zr-Mo-B alloys, one of the prospective candidates for rare-earth-free magnets, were produced by the melt-spinning technique and subsequent annealing. It was found that a small substitution of Mo for Zr in the Co-Zr-B alloys increased coercivity. The Co-Zr-Mo-B alloy with a Mo content of 2 at% showed a high coercivity of 6.2 kOe with a remanence of 40 emu/g. SEM studies showed that the annealed Co-Zr-Mo-B alloys had fine, uniform grains with an average diameter of about 0.6 μm. Further studies using STEM demonstrated that the ferromagnetic phase in the annealed Co-Zr-Mo-B alloys with high coercivity was composed of the Co5Zr phase and the long-period stacking ordered (LPSO) phase. That is, the fine grains observed in the SEM studies were found to be ferromagnetic dendrites containing numerous twin boundaries of the Co5Zr phase and its derived LPSO phase. Therefore, the high coercivity of the Co-Zr-Mo-B alloys can be attributed to the presence of ferromagnetic crystals of Co5Zr and the derived LPSO phase.