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 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. The search for material with higher saturation magnetization becomes more crucial for high magnetic-energy-product permanent magnets and extremely high areal density magnetic recordings. It is well known that the greatest saturation magnetization value that has been achieved so far is Ms = 2.45 T with Fe65Co35 alloy [1]. Decades ago, it was claimed that one of 4 Author to whom any correspondence should be addressed. New Journal of Physics 12 (2010) 063032 1367-2630/10/063032+08$30.00 © IOP Publishing Ltd and Deutsche Physikalische Gesellschaft