First-principles study on surface structural, magnetic and electronic properties of alloyed cementite with Co or Ni

In order to explore the influence of Co/Ni doping on the cementite surface stability and provide the basic data for the following studies on interfaces, a first-principles study on the surface structural, magnetic and electronic properties of Fe2CoC and Fe2NiC was carried out. The calculated results show that Co or Ni doping enhances the stability of cementite surfaces. And the excellent surface stability may result from the strengthened surface Fe–C covalent bond. Both Fe2CoC and Fe2NiC surface stabilities gradually decrease from (0 0 1) and (0 1 0) to (1 0 0). All of the Fe2CoC or Fe2NiC (0 0 1) surface atoms relax towards the bulklike layer, and the relaxation-caused normal-direction displacement (RCNDD) of the (0 0 1) surface C atom is maximal. And the Fe–C covalent bond of the (0 0 1) surface layer is stronger. These may lead to the excellent stability of the (0 0 1) surface. The magnetic moments (MMs) of (0 0 1), (0 1 0) and (1 0 0) surface of Fe2CoC (Fe2NiC) are 5.08 (4.22), 5.17 (4.36), and 5.14 (4.20) μB/f.u, respectively. And the MMs of Fe2MC (M = Co/Ni) surfaces are mainly contributed by 3d electrons of metal atoms, and all bigger than those of their bulk.