Electronic structure and magnetic properties of bulk and (001) surfaces of γ′-Fe4C from first principles

We report first principles investigation of the electronic structure and magnetic properties of bulk γ ′ -Fe ₄ C and its (001) surfaces within density functional theory. The bulk equilibrium lattice parameter ( a 0 = 3.81 Å ) and bulk modulus ( B 0 = 141 . 2 GPa) were determined by first relaxing the cell volume while keeping the atoms located at their ideal positions. The corresponding magnetic moments were found to be 3.10 μ B and 1.77 μ B on the two inequivalent iron sites. The magnetic exchange parameters between the magnetic spin moments were computed as functions of inter-atomic distances up to four times the lattice parameter, from which the mean-field estimates of the Curie temperature were obtained. Effects of disorder, due in one case to substitution of carbon atoms by nitrogen atoms, and in an other to a small concentration of vacancies on the iron sites, on the Curie temperature were also investigated within the coherent potential approximation. We found that disorder has a dramatic effect on the transition temperature, especially in the second case. The electronic structure and magnetic profile of the γ ′ -Fe ₄C (001) surface with Fe ₂ C termination was studied in the supercell geometry. Substantial surface relaxations were found to occur, but they do not induce significant changes in the magnetic moments when compared to those of the unrelaxed surface. No notable charge transfers were found either in the bulk or at the surface.