Study of αʺ-phase Fe₁₆X_2-nY_n (X,Y=N, C) Alloys Crystal Structures using Molecular Dynamics Modeling

Atomic structures of αʺ-Fe₁₆N₂, Fe₁₆C₂, and “Minnealloy” with C-doped Fe₁₆N_2-nC_n (n=(0,2)) are investigated using Molecular Dynamics (MD) energy minimization (EM) method. Based on simulated annealing (SA) principles, a fast MD search method is developed to provide highly efficient estimation to the crystal structure of “global” minimum energy state by sampling a series of local energy minimal within the continuously divided minimization spaces. Minnealloy is a promising soft magnetic material that has experimentally shown high saturation flux density and low magnetic anisotropy. Theoretical calculation of the crystalline structure has been desirable to further advance this experimental effort. In this study, we developed a highly efficient crystal structure prediction method using the MD simulation in studying αʺ-Fe₁₆N_2-nC_n (n=(0, 2)) crystal structure related to their magnetic properties. We developed required MD modules including the initial lattice construction process, quad-cell superlattice, SA-based volume-division-step (VDS) EM process together with Modified-Embedded-Atom Methods and Tersoff interatomic potentials. The VDS-EM method has demonstrated high computation efficiency. It is found that in Fe₁₆N_2-nC_n alloys the ranges of C-Fe and N-Fe bond lengths according to their Wyckoff positions are consistent with those in Fe₁₆C₂ and Fe₁₆N₂ phases. There are also existing Fe₁₆N_2-nC= alloys with high-C compositions showing higher stability than the intrinsic phases.