A first-principles study of the impact of paramagnetism on grain boundary segregation in FeMn alloys*

To understand the impact of paramagnetism on defect energetics and kinetics is, though conceptually and computationally challenging, important for designing Fe-based alloys. Since magnetic degrees of freedom change faster than atomic degrees of freedom in the high-temperature paramagnetic state, the atoms move according to an averaged force instead of instantaneous forces attained from each spin configuration.

Therefore, a new computationally efficient method based on spin-space averaging [1] has been developed to handle magnetic disorder next to defects, which uses the spin constraint tool developed in the DFT code S/PHI/nX. First, we focus on vacancies in the FeMn system, for which we demonstrate that paramagnetism significantly affects atomic relaxations as well as vacancy diffusion barriers and thereby explain why Mn diffusion shows a different temperature dependence than Fe-self diffusion in α-Fe. Next, we expand our method to extended defects and show that paramagnetism has a significant effect on the grain boundary segregation of Mn. Finally, we combine our results to reveal the chemo-magneto-structural coupling underlying Mn segregation to grain boundaries in Fe alloys.

[1] Körmann et al., Phys. Rev. B 85(12):125104, (2012)