Theory of giant saturation magnetization in α-Fe16N2: 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 Fe16N2 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 on-site 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.