Modifying magnetic properties of MnBi with carbon: An experimental and theoretical study

MnBi and MnBi-based materials have been investigated as prospective rare-earth-free permanent magnets with moderate energy product. One of the main issues with MnBi synthesis is the presence of residual Bi in the sample which reduces the net magnetization. We have found that MnBi synthesized in the presence of carbon substantially reduces the amount of residual Bi, improving its saturation magnetization. We have synthesized Mn55Bi45 and Mn55Bi45C x samples using arc melting and high-vacuum annealing. The room temperature x-ray diffraction patterns indicate that both Mn55Bi45 and Mn55Bi45C x crystallize in the hexagonal NiAs-type structure. The Rietveld analysis of the x-ray patterns shows that the amount of residual Bi reduces from 16 wt.% for Mn55Bi45 to 5 wt.% for Mn55Bi45Cx. The high-field (3 T) magnetizations measured at room temperature are 61 emu g-1 and 66 emu g-1 for Mn55Bi45 and Mn55Bi45C x, respectively. To understand the role of C in enhancing the magnetization of MnBi, we carried out the first-principles calculations of both stoichiometric and nonstoichiometric MnBi alloys, which suggests that the increase of magnetization in Mn55Bi45C x may be due to the coating of MnBi grains with C.