University of Minnesota researchers claim that Fe16N2 crystals are more magnetic than the most magnetic material previously known, and its magnetism exceeds the predicted limit of magnetism for a material. FE16N2 is approximately 18% more magnetic than the predicted limit
Can localized 3d electron exist in strong ferromagnetic metal because of some unusual correlation effect? This question is related to the controversy on whether α”- Fe16N2 has giant saturation magnetization which has been debated for decades since its first observation. Here we report the synthesis of α”- Fe16N2 thin films. The highest moment is obtained to be 3.0μB/Fe. XMCD experiment is systematically performed on a series of iron nitrides samples. Among all the iron nitrides phases, it is found that there exist highly localized 3d electrons only in chemically disordered Fe8N and ordered F16N2 phases. This discovery hints at the origin of the giant magnetic moment is correlated with the 3d electron localization in such system. First principle calculation (LDA+U) further verifies that the d electron localization is the key element to rationalize the high moment formation in iron nitrides system. We also provide a speculative outlook on the giant saturation magnetization formation based on “cluster + atom” concept.
A new model is proposed for the strong ferromagnetism associated with partially localized orbitals in the Fe16N2 metallic system which draws substantially from models of heavy fermion metals. We demonstrated that an unusual correlation effect is brought up within the Fe-N octahedral cluster region and the effective on-site 3d-3d Coulomb interaction increases due to a substantial 3d electrons charge density difference between the clusters and its surroundings, which leads to a partially localized high spin electron configuration with a long range ferromagnetic order. First principle calculation based on LDA+U method shows that giant magnetic moment can be achieved at sufficiently large Hubbard U value. The feature of the coexistence of the localized and itinerant electron states plays a key role on the formation of the giant saturation magnetization.
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