New anisotropic MnBi permanent magnets by field-annealing of compacted melt-spun alloys modified with Mg and Sb

A method has been developed for manufacturing rare-earth-free MnBi-based magnets capable of filling the existing "gap" between the ferrite and rare-earth permanent magnets. Whereas the earlier approaches relied on sintering of fine and easily degrading single-crystal MnBi particles, the new method achieves refinement of the key alpha-MnBi phase through melt-spinning combined with appropriate alloying. Modification of the MnBi alloys with Mg and Sb generates a high-coercivity nanostructure of the metastable beta' phase. A subsequent compaction at 150 degrees C produces fully dense materials while converting the beta' phase into the stable alpha phase. Finally, a short annealing at 265-300 degrees C in a magnetic field of 3 T increases the fraction of the alpha phase to 97-98% and aligns the c axes of the alpha crystallites. A maximum energy product (BH)(max) of 11.5 MGOe and an intrinsic coercivity H-c of 5.6 kOe have been obtained in a magnet with the nominal composition Mn50Bi46.5Mg3Sb0.5. Because the coercivity increases with temperature, the maximum energy product of this magnet is still as high as 8.9 MGOe at 175 degrees C. Increasing the Sb content to 1.5 at.% increases the H-c to 9.3 kOe, but at the same time inhibits the development of the texture thus decreasing the (BH)(max). The addition of Mg was found to increase the c lattice parameter of the alpha phase resulting in an unusually large ratio c/a approximate to 1.432.