Theoretical investigation into the possibility of very large moments in Fe16N2

We examine the mystery of the disputed high-magnetization alpha ''-Fe16N2 phase, employing the Heyd-Scuseria-Ernzerhof screened hybrid functional method, perturbative many-body corrections through the GW approximation, and on-site Coulomb correlations through the generalized gradient approximation (GGA) + U method. We present a first-principles computation of the effective on-site Coulomb interaction (Hubbard U) between localized 3d electrons employing the constrained random-phase approximation (cRPA), finding only somewhat stronger on-site correlations than in bcc Fe. We find that the hybrid functional method, the GW approximation, and the GGA + U method (using parameters computed from cRPA) yield an average spin moment of 2.9 mu(B), 2.6-2.7 mu(B), and 2.7 mu(B) per Fe, respectively.