An Approach for Preparing High-Performance MnBi Alloys and Magnets

MnBi alloys were arc-melted and solidified by different cooling rates, which resulted in microstructures with different sizes of Mn grains embedded in the matrix consisting of Bi and MnBi phases. By annealing at an appropriate temperature and duration, the Mn grains become MnBi grains caused by their combination with the surrounding Bi. This process enhances the MnBi content leading to the large spontaneous magnetization (Ms) of alloyed samples. The coercivity (iHc) of alloys is determined mostly by the MnBi grain distribution formed during the annealing process. Consequently, an alloy of large Ms and iHc can be produced by controlling the required cooling rate as it figures␣in the arc-melting process and the appropriate annealing. The high-performance MnBi alloy was cold ball-milled into fine powders that were used for preparing MnBi bulk magnets by aligning in an 18 kOe-field followed by warm-compaction at 280°C and 12 MPa for 10 min. The magnet is highly anisotropic with an energy product of 7 MGOe.