Origin of Giant Saturation Magnetization in Fe$_{16}$N$_{2}$ thin film

Can localized 3d electron exist in strong ferromagnetic metal because of some unusual correlation effect? This question is related to the controversy on whether $\alpha $''- Fe$_{16}$N$_{2}$ has giant saturation magnetization which has been debated for decades since its first observation$^{1,2}$. Here we report the synthesis of $\alpha $''- Fe$_{16}$N$_{2}$ thin films. The highest moment is obtained to be 3.0$\mu _{B}$/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 Fe$_{8}$N and ordered F$_{16}$N$_{2}$ phases$^{3}$. 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$^{4}$. We also provide a speculative outlook on the giant saturation magnetization formation based on ``cluster + atom'' concept. 1.Kim, T.K. and Takahashi. M, Appl. Phys. Lett., 20, 492 (1972) 2.Sugita, Y., et al., J. Appl. Phys. 76, 6637 (1994) 3.Liu, X. Q. et al., arxiv: 0909.4480 (2009) 4.Ji, N. et al., arxiv: 0909.4478 (2009)