Preparation of highly pure α-MnBi phase via melt-spinning

High concentration of the magnetically hard α phase in the Mn-Bi alloys is important for the development of these alloys as rare-earth-free permanent magnets. Among several explored manufacturing methods, melt-spinning followed by annealing is known to be suitable of producing the most pure α structure. In this work, a series of melt-spun Mn100-xBix alloys was prepared with x = 43 – 51 at a wheel speed of 67 m/s by ejecting the alloys through orifices 0.17 mm and 0.27 mm in diameter. The smaller orifice diameter favored formation of an amorphous phase in the as-spun alloys as well as a higher saturation magnetization Ms in the alloys subsequently annealed at 300 °C. Although the most pure, 98 vol.%, α phase was obtained for the off-stoichiometric Mn55Bi45 composition, the Ms of this material was lowered, possibly because the excess Mn atoms induced antiferromagnetic coupling in the α phase. The highest Ms of 78 emu/g was obtained for a composition closer to the Mn50Bi50 stoichiometry, despite the slightly lower purity of α phase. Evolution of the room-temperature coercivity with the formation of the α phase in the melt-spun alloys was studied for the Mn55Bi45 ribbons compacted at 275 °C; the coercivity values of around 1 kOe obtained through this simple procedure are not sufficient for permanent magnet applications.