Clarification of Coercive Mechanism in Rare Earth Iron Nitride Sm-Fe-N magnet

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For clarifying the role that the interstitial N atoms played on the microscopic scale for basic magnetism in R_2Fe_<17>N_3, we determined the enhancements of Fe magnetic moments on the crystallographically different atomic sites and crystalline electric field parameter A_2^0 upon nitrogenation by neutron diffraction studies of the rhombohedral Y_2Fe^<17> and Y_2Fe_<17>N_<3.1> and high-field magnetization measurements of single crystalline Nd_2Fe_<17> and Nd_2Fe_<17>N_3, respectively. The results obtained are compared with those estimated by the band structure calculation. For clarifying the coercivity mechanism in Sm_2Fe_<17>N_3, we also examined the particle (powder) size and surface cleaning effects on the coercive field μ_0H_c for the Sm_2Fe_<17>N_3 single crystal powder. The results indicate that μ_0H_c monotonically decreases with increasing the particle size, suggesting that the coercivity mechanism is of a nucleation type. When there are many nucleation sites on the surface of Sm_2F_<17>N_3 powder before surface cleaning, the value of μ_0H_c is relatively small and reveals strong temperature dependence, while μ_0H_c is strongly enhanced by surface cleaning, leading to relatively weak temperature dependence. In this work, we obtained the resultant maximum energu product (BJH)_<max> = 330 kJ/m3, which is the highest as far as we know.