University of Minnesota
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Jianping Wang

Theory of giant saturation magnetization in α''-Fe16N2: role of partial localization in ferromagnetism of 3d transition metals
N. Ji, X. Q. Liu, and J. P. Wang, New Journal of Physics 12, 063032 (2010)

Download from doi: 10.1088/1367-2630/12/6/063032


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. GENERAL SCIENTIFIC SUMMARY Introduction and background. The search for materials with higher saturation magnetization becomes more crucial for high magnetic-energy-product permanent magnets and extremely high areal density magnetic recordings. The greatest saturation magnetization value that has been achieved so far is Ms = 2.45 T with Fe65Co35 alloy, which was well predicted by quantum magnetic theory. Whether an iron nitride, α ''-Fe16N2, possesses a giant saturation magnetization far beyond the iron cobalt alloy system has been a 40-year mystery for the magnetic materials and condensed matter communities, since it was first reported in 1972. One of the key missing pieces for this unsolved puzzle is the absence of a convincing magnetic theory. Main results. We propose for the first time an insight physics/model that some Fe atoms in Fe16N2 are more influenced by Coulomb repulsion (Hubbard U) than others. This scenario is consistent with Fe16N2's unique crystal structure, consisting of the chemically ordered octahedral Fe6–N cluster +Fe atoms. Careful calculations made using the LDA+U method gave a theoretical justification for the possible giant magnetism observed in Fe16N2, which exceeds that of the Fe65Co35 alloy. In particular, isolated Fe6–N octahedral clusters induce non-uniform charge distribution and cause an increase of the effective U value that creates both localized and itinerant 3d electrons. Wider implications. It was proposed that dual-electron behavior is a necessity at a fundamental level for rationalizing giant saturation magnetization. This opens up a new direction in identifying promising candidates. More importantly, we believe that in this proposed scenario, clusters (providers of local electrons) + atoms (providers of itinerant electrons), can be applied to understand and design other material systems. New physical properties can also be expected.