The metallic compound MnBi is a promising rare-earth-free permanent magnet material, unique among all candidates for its high intrinsic coercivity (Hci) and its large positive temperature coefficient. The Hci of MnBi in thin-film or powder form can exceed 12 and 26 kOe at 300 and 523 K, respectively. Such a steep rise in Hci with increasing temperature is unique to MnBi. Consequently, MnBi is a highly sought-after hard phase for exchange coupling nanocomposite magnets. However, the reaction between Mn and Bi is peritectic, and hence Mn tends to precipitate out of the MnBi liquid during the solidification process. As result, when the alloy is prepared using conventional induction or arc-melting casting methods, additional Mn is required to compensate the precipitation of Mn. In addition to composition, post-casting annealing plays an important role in obtaining a high content of MnBi low-temperature phase (LTP) because the annealing encourages the Mn precipitates and the unreacted Bi to react, forming the desired LTP phase. Here we report a systematic study of the effect of composition and heat treatments on the phase content and magnetic properties of Mn-Bi alloys. In this study, 14 compositions were prepared using conventional metallurgical methods, and the compositions, crystal structures, phase content and magnetic properties of the resulting alloys were analyzed. The results show that the composition with 55 at.% Mn exhibits both the highest LTP content (93 wt.%) and magnetization (74 emu g À1 with 9 T applied field at 300 K).
The metallic compound MnBi has high intrinsic coercivity with large positive temperature coefficient. The coercivity of MnBi exceeds 12 kOe and 26 kOe at 300 K and 523 K, respectively. Hence MnBi is a good candidate for the hard phase in exchange coupled nanocomposite magnets. In order to maximize the loading of the soft phase, the size of the MnBi particle has to be close to 500 nm, the size of single magnetic domain. Low energy milling is the common method to reduce MnBi particle size. However, only 3-7 micron size particle can be achieved without significant decomposition. Here, we report our effort on preparing submicron MnBi powders using traditional powder metallurgy methods. Mn 55 Bi 45 magnetic powders were prepared using arc melting method, followed by a series of thermal-mechanical treatment to
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