Structure and magnetic properties of Fe3C and Fe2CoC
Structures, magnetic moments and also magnetocrystalline anisotropy energies of binary and ternary M3C (M = Fe and/or Co) materials are systematically investigated using a combination of adaptive genetic algorithm crystal structure predictions and first-principles calculations. Besides reproducing the known cementite (Pnma) structure of Fe3C, the AGA searches also capture several new meta-stable phases which can be stable within the room-temperature range. In particular, a bainite (P6322) structure exhibits largest magnetic moment among all the structures in the Fe3C pool, yet its energy is only 4 meV higher than the cementite (pnma) phase. Some metastable phases have been found to be potential permanent magnets with magnetocrystalline anisotropy energies of around 1 MJ/m3. We also identify the atomic structures of the Pnma phase for Fe2CoC. The simulated X-ray spectrum and calculated magnetocrystalline anisotropy energy based on the atomic structure of Fe2CoC from our theoretical study are in very good agreement with experiment.