Theory of giant saturation magnetization in α-Fe₁₆N ₂: Role of partial localization in ferromagnetism of 3d transition metals

A new model is proposed for the ferromagnetism associated with partially localized electron states in the Fe-N2 system that explains its giant saturation magnetization. It is demonstrated that an unusual correlation effect is introduced within the Fe-N octahedral cluster region and the effective onsite 3d-3d Coulomb interaction increases due to a substantial 3d electron charge density difference between the cluster and its surroundings, which leads to a partially localized electron configuration with a long-range ferromagnetic order. The first-principles calculation based on the LDA + U method shows that giant saturation magnetization can be achieved at sufficiently large Hubbard U values. The feature of the coexistence of the localized and itinerant electron states plays a key role in the formation of giant saturation magnetization.