Exploring new phases of Fe_3−x Co_x C for rare-earth-free magnets

Structures, magnetic moments, and magnetocrystalline anisotropy energies of the Fe_3−x Co_x C intermetallic compounds are systematically investigated using adaptive genetic algorithm (AGA) crystal-structure predictions and first-principles calculations. Besides reproducing the known cementite (Pnma) structure of Fe₃C, i.e. x   =  0, the AGA searches also capture several new metastable phases within the room-temperature range. In particular, a bainite ( P 6₃22) structure exhibits the largest magnetic moment among all low-energy structures, and its energy is only 4 meV/atom higher than the cementite (Pnma) phase. The atomic structure of the Pnma Fe₂CoC phase, i.e. x   =  1, is also identified, and the calculated x-ray diffraction spectrum, magnetocrystalline anisotropy energy, and saturation magnetization based on the structure from our theoretical study are in good agreement with experiment. (paper)