Magnetism in bcc and fcc Fe with carbon and manganese

Abstract

Density functional theory calculations were performed to study the structure and magnetic properties ofbcc (α)and fcc (γ) Fe with 3 at.% carbon and manganese impurities. We find that all bcc-based Fe, Fe–C andFe–Mn–C phases exhibit a ferromagnetic (FM) ground state, while the antiferromagneticdouble-layer (AFMD) state is lowest in energy within the collinear spin approach in fcc Fe,Fe–C and Fe–Mn–C phases. However, the carbon and manganese impurities affect the localmagnetic interactions significantly. The states with opposite manganese magnetic momentsare quasi-degenerate in bcc Fe–Mn alloy, whereas octa-site carbon stabilizes ferromagneticcoupling of the nearest manganese atom with the Fe host. We demonstrate that theantiferromagnetic (AFM) fcc Fe–C and Fe–Mn–C alloys are intrinsically inhomogeneousmagnetic systems. Carbon frustrates the local magnetic order by reorientation ofmagnetic moments of the nearest Mn and Fe atoms, and favors their ferromagneticcoupling. The competition between ferromagnetic and antiferromagnetic Fe–Fe andFe–Mn interactions and the local magnetovolume instability near carbon maygive rise to the spin-glass-like regions observed in austenitic Fe–Mn–C alloys.