Development of magnetic hardness in melt-spun Mn₅₀Bi_50-<i>x</i>M<i>_x</i> alloys (M = Mg, In, Sn, Sb, Bi) subjected to magnetic-field annealing

The theoretical maximum energy product (BH)_m of the MnBi compound, 20 MGOe, perfectly fits the "gap" between the inexpensive hard ferrites and the supply-critical rare-earth magnets. Decades of R&D efforts focused on consolidation of fine MnBi single crystals could only produce MnBi magnets exhibiting (BH)_m of 5–8 MGOe. A different approach relying on a melt-spinning, followed by compaction and magnetic-field annealing avoids the highly reactive single crystals, but it does not develop coercivity H_c in the binary MnBi alloys. Here, we report how alloying with M = Mg, In, Sn and Sb affects the phases, the texture and the hard magnetic properties of field-annealed magnets prepared from the melt-spun Mn₅₀Bi_50-xM_x alloys. Sn delays the formation of the MnBi phase while further decreases the already low H_c. 1.5 at.% In decreases the solidus temperature and slightly increases the H_c. In melt-spun alloys modified with 1.5 at.% Sb, a transient metastable phase ensures a submicron size of MnBi grains and leads to a H_c >7 kOe, but suppression of the texture limits the (BH)_m to 5.4 MGOe. Alloying with Mg increases the H_c without undermining the texture; a breakthrough (BH)_m of 9.8 MGOe was realized in the Mn₅₀Bi₄₇Mg₃ magnet.