Role of carbon in enhancing the magnetization of MnBi: an experimental and theoretical study

MnBi and MnBi-based materials have been investigated as prospective rare-earth-free permanent magnets with moderate energy product. We have synthesized Mn₅₅Bi₄₅ and Mn₅₅Bi₄₅C samples using arc melting and high-vacuum annealing. The room temperature x-ray diffraction patterns indicate that both Mn₅₅Bi₄₅ and Mn₅₅Bi₄₅C crystallize in the hexagonal NiAs-type crystal structure. The Rietveld analysis of the x-ray patterns shows that the amount of Bi segregation reduces from  16 wt.%  for Mn₅₅Bi₄₅ to 5 wt. % for Mn₅₅Bi₄₅C. The high-field (3T) magnetizations measured at room temperature are 61 emu/g and 66 emu/g for Mn₅₅Bi₄₅ and Mn₅₅Bi₄₅C, respectively. In addition,  the carbon containing sample shows slightly higher value of the anisotropy constant as compared to that of Mn₅₅Bi₄₅. 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 is due to the coating of MnBi grains with C.